A TARGET FOR MO-99 MANUFACTURE AND METHOD OF MANUFACTURING SUCH A TARGET

Abstract

A method of manufacturing particles of UO.sub.2 for a porous matrix of a target for use in the manufacture of .sup.99Mo, comprising: infiltrating a solution of uranyl nitrate into a polymer template (62); either (i) introducing an alkali chemical to the uranyl nitrate infiltrated polymer template (64), causing precipitation of uranium oxide/hydroxide, and converting the uranium oxide/hydroxide to U.sub.3O.sub.8 (68) and concurrently removing the polymer template (70), by heating the infiltrated polymer template; or (ii) converting the uranyl nitrate to U.sub.3O.sub.8 (66) and concurrently removing the polymer template (74), by heating the infiltrated polymer template; and reducing the U.sub.3O.sub.8 to UO.sub.2 via heating in a reducing atmosphere (72).

Claims

1. A method of manufacturing particles of UO.sub.2 for a porous matrix of a target for use in the manufacture of .sup.99Mo, the method comprising: infiltrating a solution of uranyl nitrate into a polymer template; either (i) introducing an alkali chemical to the uranyl nitrate infiltrated polymer template, causing precipitation of uranium oxide and uranium hydroxide, and converting the uranium oxide and uranium hydroxide to U.sub.3O.sub.8 and concurrently removing the polymer template, by heating the infiltrated polymer template; or (ii) converting the uranyl nitrate to U.sub.3O.sub.8 and concurrently removing the polymer template, by heating the infiltrated polymer template; and reducing the U.sub.3O.sub.8 to UO.sub.2 via heating in a reducing atmosphere.

2. A method as claimed in claim 1, comprising heating the infiltrated polymer template to a maximum temperature of 400 C. or 600 C.

3. A method as claimed in claim 1, comprising reducing the U.sub.3O.sub.8 to UO.sub.2 at a maximum temperature of 1000 C.

4. A method as claimed in claim 1, comprising manufacturing particles of CeO.sub.2 for the porous matrix, the method comprising: infiltrating a solution of a cerium salt into a further polymer template; and either (i) introducing an alkali chemical to the infiltrated further polymer template, causing precipitation of cerium oxide and cerium hydroxide; and converting the cerium oxide/hydroxide to CeO.sub.2 and concurrently removing the further polymer template, by heating the infiltrated further polymer template; or (ii) converting the cerium salt to CeO.sub.2 and concurrently removing the further polymer template, by heating the infiltrated further polymer template.

5. A method as claimed in claim 4, wherein the further polymer template is in the form of polyacrylonitrile (PAN) beads.

6. A method as claimed in claim 4, comprising forming the particles of UO.sub.2 and the particles of CeO.sub.2 sequentially, and mixing the particles of UO.sub.2 and the particles of CeO.sub.2.

7. A method as claimed in claim 4, comprising controlling a ratio of cerium and uranium by controlling the amount or amounts of infiltration of the cerium salt and uranyl nitrate.

8. A method of manufacturing particles of UO.sub.2 and CeO.sub.2 for a porous matrix of a target for use in the manufacture of .sup.99Mo, the method comprising: infiltrating a solution of uranyl nitrate and cerium nitrate into a polymer template; precipitating uranium oxide and uranium hydroxide and cerium oxide and cerium hydroxide by introducing an alkali chemical to the uranyl nitrate and cerium nitrate infiltrated template; converting the uranium oxide and uranium hydroxide, and cerium oxide and cerium hydroxide, to U.sub.3O.sub.8 and CeO.sub.2 respectively and concurrently removing the template, by heating the infiltrated template; and reducing the U.sub.3O.sub.8 and CeO.sub.2 to U.sub.xCe.sub.1-xO.sub.2 via heating in a reducing atmosphere, where x is the initial molar mixing ratio of uranium and cerium.

9. A method as claimed in claim 8, comprising controlling a ratio of cerium and uranium by controlling the amount or amounts of infiltration of the cerium salt and uranyl nitrate.

10. A method as claimed in claim 1, wherein the polymer template is in the form of polyacrylonitrile (PAN) beads.

11. A method as claimed in claim 1, wherein the reducing atmosphere is approximately 3.5% hydrogen in nitrogen gas.

12. A method of manufacturing particles of UO.sub.2 for a porous matrix of a target for use in the manufacture of .sup.99Mo, the method comprising: creating a template comprising polymer beads; infiltrating the polymer beads of the template with UO.sub.2; and calcinating the infiltrated polymer beads of the template.

13. A method as claimed in claim 12, comprising: infiltrating the polymer beads additionally with cerium, such that the polymer beads are infiltrated with both UO.sub.2 and cerium.

14. A method as claimed in claim 13, wherein the cerium for infiltration is in the form of a cerium salt.

15. A method as claimed in claim 13, comprising selecting the ratio of infiltrated cerium and uranium and the enrichment of the uranium so as to provide a desired ultimate molar ratio of .sup.235U to Ce and .sup.238U.

16. A method as claimed in claim 12, wherein the polymer beads are polyacrylonitrile (PAN) beads.

17. A particle for a porous matrix of a target for use in the manufacture of .sup.99Mo, manufactured with the method of claim 1.

18. A porous matrix of a target for use in the manufacture of .sup.99Mo, the porous matrix comprising a plurality of particles manufactured with the method of claim 1.

19. A target for use in the manufacture of .sup.99Mo, comprising the porous matrix of claim 18.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0106] In order that the invention be better understood, embodiments will now be described, by way of example, with reference to the accompanying drawing in which:

[0107] FIG. 1 is a schematic view of a reactor model used to model the performance of a reusable target;

[0108] FIG. 2 is a schematic view of the reactor model of FIG. 1 with a reusable target;

[0109] FIG. 3 is a plot of effective neutron multiplication factor, k.sub.eff, versus core UO.sub.2 core density, as simulated for the reactor model of FIG. 1;

[0110] FIG. 4 is a plot of .sup.99Mo, .sup.95Zr, .sup.133Xe, .sup.131I and .sup.135Xe yield versus reusable UO.sub.2 target density, as simulated for the reactor and target models of FIG. 2, using a 20% .sup.235U enriched target and a 2 day irradiation;

[0111] FIG. 5 is a plot of .sup.99Mo, .sup.95Zr, .sup.33Xe, .sup.131I and .sup.135Xe yield versus reusable UO.sub.2 target density, as simulated for the reactor and target models of FIG. 2, using a 20% .sup.235U enriched target and a 5 day irradiation;

[0112] FIG. 6 is a plot of .sup.99Mo, .sup.95Zr, .sup.33Xe, .sup.131I and .sup.135Xe yield versus reusable UO.sub.2 target density, as simulated for the reactor and target models of FIG. 2, using a 20% .sup.235U enriched target and a 10 day irradiation;

[0113] FIG. 7 is a plot of .sup.99Mo, .sup.95Zr, .sup.133Xe, .sup.131I and .sup.135Xe yield versus reusable UO.sub.2 target density, as simulated for the reactor and target models of FIG. 2, using a 1% .sup.235U enriched target and a 2 day irradiation;

[0114] FIG. 8 is a plot of .sup.99Mo, .sup.95Zr, .sup.33Xe, .sup.131I and .sup.135Xe yield versus reusable UO.sub.2 target density, as simulated for the reactor and target models of FIG. 2, using a 1% .sup.235U enriched target and a 5 day irradiation;

[0115] FIG. 9 is a plot of .sup.99Mo, .sup.95Zr, .sup.133Xe, .sup.131I and .sup.135Xe yield versus reusable UO.sub.2 target density, as simulated for the reactor and target models of FIG. 2, using a 1% .sup.235U enriched target and a 10 day irradiation;

[0116] FIG. 10 is a plot of .sup.99Mo production target efficiency .sub.targ versus UO.sub.2 target density, for a 20% .sup.235U enriched target and a 1% .sup.235U enriched target and 2, 5 and 10 day irradiations, derived from the plots of FIGS. 4 to 9;

[0117] FIG. 11 is a plot of .sup.235U percentage burnup versus UO.sub.2 target density, for a 20% .sup.235U enriched target in the configuration of FIG. 2, for various irradiations;

[0118] FIG. 12 is a plot of .sup.235U percentage burnup versus UO.sub.2 target density, for a 1% .sup.235U enriched target in the configuration of FIG. 2, for various irradiations;

[0119] FIG. 13 is a three-dimensional plot of the modelled .sup.99Mo target total output (A.sub.T) plotted versus UO.sub.2 density (D) and versus irradiation time (t), for a 1% .sup.235U enriched target in the configuration of FIG. 2;

[0120] FIG. 14 is a three-dimensional plot of the modelled .sup.99Mo target total output (A.sub.T) plotted versus UO.sub.2 density (D) and versus irradiation time (t), for a 3% .sup.235U enriched target in the configuration of FIG. 2;

[0121] FIG. 15 is a three-dimensional plot of the modelled .sup.99Mo target total output (A.sub.T) plotted versus UO.sub.2 density (D) and versus irradiation time (t), for a 7% .sup.235U enriched target in the configuration of FIG. 2;

[0122] FIG. 16 is a three-dimensional plot of the modelled .sup.99Mo target total output (A.sub.T) plotted versus UO.sub.2 density (D) and versus irradiation time (t), for a 10% .sup.235U enriched target in the configuration of FIG. 2;

[0123] FIGS. 17A and 17B are three- and two-dimensional plots respectively of the modelled sustainability index (S.sub.targ) plotted versus UO.sub.2 density (D) and versus irradiation time (t), for a 1% .sup.235U enriched target in the configuration of FIG. 2;

[0124] FIGS. 18A and 18B are three- and two-dimensional plots respectively of the modelled sustainability index (S.sub.targ) plotted versus UO.sub.2 density (D) and versus irradiation time (t), for a 3% .sup.235U enriched target in the configuration of FIG. 2;

[0125] FIGS. 19A and 19B are three- and two-dimensional plots respectively of the modelled sustainability index (S.sub.targ) plotted versus UO.sub.2 density (D) and versus irradiation time (t), for a 7% .sup.235LU enriched target in the configuration of FIG. 2;

[0126] FIGS. 20A and 20B are three- and two-dimensional plots respectively of the modelled sustainability index (S.sub.targ) plotted versus UO.sub.2 density (D) and versus irradiation time (t), for a 10% .sup.235U enriched target in the configuration of FIG. 2;

[0127] FIG. 21 is a plot of sustainability index (S.sub.targ) versus initial UO.sub.2 target volume (V), for 4, 5, 6 and 7 day irradiations and a target average density of 2 g/cm.sup.3, for a 1% .sup.235U enriched target in the configuration of FIG. 2;

[0128] FIG. 22 is a plot, from the same simulation as that of FIG. 21, of total .sup.99Mo output (A.sub.T) versus initial UO.sub.2 target volume (V), for 4, 5, 6 and 7 day irradiations and a target average density of 2 g/cm.sup.3, for a 1% .sup.235U enriched target in the configuration of FIG. 2;

[0129] FIG. 23A is a plot of modelled plutonium production Pu (mg) for an exemplary UO.sub.2 target and various .sup.235U/.sup.238U enrichments, a 6 day irradiation and a target density of 2.6 g/cm.sup.3, for a target in the configuration of FIG. 2;

[0130] FIG. 23B is a plot of modelled normalized plutonium production Pu for an exemplary UO.sub.2 target and various target .sup.235U/.sup.238U enrichments, shown both relative to enrichment and relative to .sup.99Mo production, normalized to plutonium production with 20% .sup.235U enrichment, with a 6 day irradiation and a target density of 2.6 g/cm.sup.3, for a target in the configuration of FIG. 2;

[0131] FIG. 24A is a plot of a simulation of the stopping and range of 90 MeV .sup.99Mo ions in UO.sub.2, modelled with SRIM (trade mark);

[0132] FIG. 24B is a plot of a simulation of the stopping and range of 90 MeV .sup.99Mo ions in CeO.sub.2, modelled with SRIM;

[0133] FIG. 25 is a schematic view of the reactor model of FIG. 1 with a reusable UO.sub.2 target that includes CeO.sub.2, according to an embodiment of the present invention;

[0134] FIG. 26 is a plot of modelled plutonium production for exemplary UO.sub.2 targets with 1% .sup.235U, for various values of Ce content (%), the balance comprising .sup.238U, for a 6 day irradiation and a target density of 2 g/cm.sup.3, for a UO.sub.2/CeO.sub.2 target in the arrangement of FIG. 24;

[0135] FIG. 27A is a flow diagram of a method of manufacturing particles of UO.sub.2 for the porous matrix of a target for use in the manufacture of .sup.99Mo, according to an embodiment of the present invention;

[0136] FIG. 27B is a flow diagram of a method of manufacturing particles of UO.sub.2 and particles of CeO.sub.2 for the porous matrix of a target for use in the manufacture of .sup.99Mo, according to an embodiment of the present invention;

[0137] FIG. 27C is a flow diagram of a method of manufacturing particles of UO.sub.2 and CeO.sub.2 for the porous matrix of a target for use in the manufacture of .sup.99Mo, according to an embodiment of the present invention;

[0138] FIG. 28A is an TG-DSC trace obtained while heating CeO.sub.2@PAN to 600 C. under an atmosphere of 20% O.sub.2 in N.sub.2 (compressed air);

[0139] FIG. 28B is an TG-DSC trace obtained while heating CeO.sub.2@PAN to 600 C. under an Ar atmosphere;

[0140] FIGS. 29A and 29B are SEM images of fractured air-calcined CeO.sub.2 beads after calcination at 400 C.;

[0141] FIGS. 30A and 30B are SEM images of two different pore locations within the air-calcined CeO.sub.2 bead of FIG. 29A, exhibiting pore wall thicknesses of from 4 m to 12 m;

[0142] FIG. 31 is an SEM image of a fractured air-calcined UO.sub.2 bead;

[0143] FIGS. 32A and 32B are SEM images of fractured Ar-calcined CeO.sub.2 beads after calcination at 600 C.;

[0144] FIGS. 33A, 33B and 33C are SEM images of pore locations within a 600 C. Ar-calcined CeO.sub.2 bead exhibiting pore wall thicknesses of from 1.5 m to 2.9 m;

[0145] FIGS. 34A and 34B are SEM images of pore locations within 800 C. Ar-calcined CeO.sub.2 beads exhibiting pore wall thicknesses of from 2.2 m to 3.5 m;

[0146] FIGS. 35A, 35B, 35C and 35D are SEM images of pore locations within 1200 C. Ar-calcined CeO.sub.2 beads exhibiting pore wall thicknesses of from 3.3 m to 8.4 m;

[0147] FIGS. 36A and 36B are SEM images of fractured Ar-calcined U.sub.0.05Ce.sub.0.95O.sub.2 beads after calcination at 800 C.;

[0148] FIG. 37 is an EDS spectrum of a point within the material of the beads of FIGS. 36A and 36B; and

[0149] FIGS. 38A and 38B are plots of N.sub.2 sorption isotherms at 77 K of CeO.sub.2 heated at, respectively, 400 C. under air and 1200 C. under Ar.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0150] FIG. 1 is a schematic view of a simple reactor model 10 used to model the performance of a reusable target. The reactor model 10 includes a cylindrical heavy water reflector vessel 20, and a cylindrical UO.sub.2 core 30 located at the centre of reflector vessel 20.

[0151] Reflector vessel 20 has a diameter of 200 cm and a height of 120 cm. UO.sub.2 core 30 has a diameter of 30 cm and a height of 60 cm.

[0152] FIG. 2 is a schematic view of reactor model 10 of FIG. 1 with a (modelled) reusable target 40 (not shown to scale). Reusable target 40 is cylindrical, with a height of 3 cm, a radius of 1.13 cm and hence a volume of 12.03 cm.sup.3. Reusable target 40 was modelled as being located with its central axis 60 cm from and parallel to the central axis of UO.sub.2 core 30, to simulate a potential position of a target rig in a reactor. This configuration was the basis of the following modelling and analysis, unless stated otherwise.

[0153] For reactor model 10 to simulate a practical reactor, the amount of uranium in UO.sub.2 core 30 is adapted to allow a self-sustaining nuclear reaction. The sustainability of a nuclear reaction is given by the reactor's effective neutron multiplication factor, k.sub.eff:

[00004]$k_{\mathrm{eff}}=\frac{\mathrm{Rate}\mathrm{of}\mathrm{neutron}\mathrm{production}}{\mathrm{Rate}\mathrm{of}\mathrm{neutron}\mathrm{absorption}+\mathrm{rate}\mathrm{of}\mathrm{leakage}}$

where k.sub.eff>1 indicates supercriticality: the number of neutrons produced by fission is greater than the number lost; [0154] k.sub.eff=1 indicates criticality: the number of neutrons produced by fission equals the number lost, the desired configuration for reactor operation; and [0155] k.sub.eff<1 indicates subcriticality: the number of neutrons produced by fission is less than the number lost.

[0156] To determine the density of UO.sub.2 in UO.sub.2 core 30 that will produce a k.sub.eff of approximately 1, a number of different densities of UO.sub.2 core 30 were modelled using the KCODE function in MCNP6 (trade mark), a Monte-Carlo radiation transport code that can be used to track different particle types over a broad range of energies and has user-definable variables such as geometries and timeframes.

[0157] Reactor model 10 was created with an initial value for k.sub.eff of 1.0, and 5000 neutrons per cycle were generated. A total of 250 cycles were run, with data accumulation commencing after the first 50 cycles, resulting in approximately 200 million neutron collisions. These numbers were chosen to make the computing time practical.

[0158] FIG. 3 shows the results, plotted as k.sub.eff versus density (D) of UO.sub.2 core 30 (in g/cm.sup.3). It was found that a UO.sub.2 density of D=2.5 g/cm.sup.3 in UO.sub.2 core 30 yielded a k.sub.eff of 1 (viz. 0.99921 with a standard deviation of 0.00093, as determined by MCNP6). This value of D was then used when subsequently modelling reactor model 10 with reusable target 40 (cf. FIG. 2).

[0159] In order for .sup.99Mo to be ejected from the UO.sub.2 particles in reusable target 40 and into the surrounding material, the density of the UO.sub.2 needs to be adjusted downwards to allow for the presence of other materials or voids that will be used to contain the .sup.99Mo prior to chemical extraction. MCNP6 was used to model different UO.sub.2 densities, with reactor model 10 at 20 MW and using the BURN function of MCNP6. When using the BURN function, the fission products produced are grouped into three tiers. Tier 1 includes the isotopes: .sup.93Zr, .sup.95Mo, .sup.99Tc, .sup.101Ru, .sup.131Xe, .sup.134Xe, .sup.133Cs, .sup.137Cs, .sup.138Ba, .sup.141Pr, .sup.143Nd, 14.sup.5Nd. Tier 2 and tier 3 contain progressively more and more isotopes (which are listed in MCNP6 User's Manual). For calculation simplicity Tier 1 was used with the additional inclusion of .sup.99Mo and .sup.135Xe, as MCNP6 allows the addition of user-selected isotopes to the output. To compare the properties of targets with different .sup.235U to .sup.238U ratios, two types of targets were modelled using MCNP6: 20% enriched, and 1% enriched.

[0160] Firstly, reusable target 40 was modelled with a 20% .sup.235U enrichment, as shown in Table 1:

TABLE-US-00002 TABLE 1 properties of 20% enriched reusable target Material UO.sub.2 Density variable .sup.235U Enrichment (%) 20 Shape Cylindrical Dimensions Radius = 1.13 cm Height = 3 cm Volume = 12 cm.sup.3 Distance from centre of reactor core 60 cm

[0161] FIG. 4 is a plot of the results, shown as total .sup.99Mo yield or activity (A.sub.T) in kBq versus UO.sub.2 density (D) of reusable target 40 in g/cm.sup.3, for a 2 day irradiation. The yields of the next four most abundant radioactive products as given by MCNP6 (viz. .sup.95Zr, .sup.33Xe, .sup.131I and .sup.135Xe) are also plotted. FIGS. 5 and 6 are comparable, but for 5 day and 10 day irradiations, respectively.

[0162] It will be noted from FIGS. 4 to 6 that the .sup.99Mo yield increases relatively linearly from a UO.sub.2 density of 1 g/cm.sup.3 to approximately 5 to 6 g/cm.sup.3 and then appears to flatten out from 6 g/cm.sup.3 to the maximum density of 10.97 g/cm.sup.3 for all of the irradiation times. This suggests that, as the density of uranium increases, the .sup.235U atoms become less accessible to the neutrons and the total number of fissions per .sup.235U atom decreases. Thus, for 20% enriched targets, when considering waste minimization and yield maximization, target design would be optimized for a target density of approximately 5 to 6 g/cm.sup.3 of UO.sub.2. When comparing the different irradiation times it can be seen that the yield increases with irradiation time: there was an approximately 100% increase in the activity with an increase in irradiation time from 2 days to 5 days and a further approximately 30% increase in activity with an increase from 5 days to 10 days irradiation time.

[0163] Secondly, reusable target 40 was modelled with a 1% .sup.235U enrichment, as shown in Table 2:

TABLE-US-00003 TABLE 2 properties of 1% enriched reusable target Material UO.sub.2 Density variable .sup.235U Enrichment (%) 1 Shape Cylindrical Dimensions Radius = 1.13 cm Height = 3 cm Volume = 12 cm.sup.3 Distance from centre of reactor core 60 cm

[0164] FIGS. 7 to 9 are plots of the results, again shown as total .sup.99Mo yield or activity (A.sub.T) in kBq versus UO.sub.2 density (D) of reusable target 40 in g/cm.sup.3, for 2 day, 5 day and 10 day irradiations, respectively. The yields of the next four most abundant radioactive products as given by MCNP6 (viz. .sup.95Zr, .sup.133Xe, .sup.131I and .sup.135Xe) are again also plotted.

[0165] Compared with the 20% enriched target, the 1% enriched target had a relatively linear relationship between activity and density from 1 g/cm.sup.3 to 10.97 g/cm.sup.3, which is higher than that over the density range of 5 to 6 g/cm.sup.3 for the 20% enriched targetconsistent with the idea that, as UO.sub.2 density increases, the amount of fissioning that occurs per .sup.235U atom decreases. Tables 3 compares the amount of .sup.99Mo produced with a UO.sub.2 density of 6 g/cm.sup.3, with 20% .sup.235U enrichment and 1% .sup.235U enrichment respectively:

TABLE-US-00004 TABLE 3 Comparison of .sup.99Mo production with 20% and 1% enriched targets, for 2, 5 and 10 day irradiations using MCNP6 modelling Irradiation time (days) 20% enriched target yield (Bq) 1% enriched target yield (Bq) [00005]$\frac{20\%\mathrm{enriched}\mathrm{target}\mathrm{yield}}{1\%\mathrm{enriched}\mathrm{target}\mathrm{yield}}$ 2 26048 3478 7.5 5 49284 5698 8.6 10 63751 7844 8.1

[0166] Hence, the amount of .sup.99Mo produced is only 7.5-8.6 times higher with the 20% enriched target as compared to the 1% enriched target, despite the fact that the amount of .sup.235U in the 20% enriched target is 20 times greater than in the 1% enriched target. That is, when considering .sup.99Mo produced per quantity of .sup.235U present in the target, the 1% enriched target was found to be 2.3-2.7 times more productive than the 20% target, according to the MCNP6 model used.

[0167] Another parameter to be considered in designing reusable target 40 is the amount of waste produced, which depends on the target efficiency. Target efficiency .sub.targ can be expressed as the total activity of .sup.99Mo produced per total mass of .sup.235U in the target:

[00006]${}_{\mathrm{targ}}=\frac{{}^{99}\mathrm{Mo}\mathrm{produced}\left(\mathrm{Bq}\right)}{{}_{}^{235}{U}^{}\mathrm{in}\mathrm{target}\left(g\right)}=\frac{A_T\left({}^{99}\mathrm{Mo}\right)}{m_T\left({}_{}^{235}U\right)}$

[0168] Target efficiency .sub.targ was thus calculated for both the 20% enriched UO.sub.2 target and the 1% enriched UO.sub.2 target, for 2, 5 and 10 day irradiations and with UO.sub.2 densities ranging from 1 to 10.97 g/cm.sup.3. The results are plotted in FIG. 10, which shows that, the lower the UO.sub.2 density, the more .sup.99Mo per gram of .sup.235U is producedimplying greater target efficiency. Additionally, the efficiency increases by a greater amount at the lower density range and drops off a smaller amount with each increase in density. Increased irradiation time leads to a higher efficiency, but the increase in efficiency from 2 to 5 days irradiation is much larger than the increase from 5 to 10 days irradiation, which suggests thatfrom an efficiency point of view-targets with a low UO.sub.2 density are preferable. When comparing the 20% enriched target with the 1% enriched target, the 1% enriched target outperforms the 20% enriched target in efficiency, with the 1% enriched target producing approximately 4.8-5.7 times the .sup.99Mo at a UO.sub.2 density of 10.97 g/cm.sup.3 and 1.3-1.5 times the amount of .sup.99Mo at a UO.sub.2 density of 1 g/cm.sup.3.

[0169] Another consideration in target design is the amount of .sup.235U burnup, as burnup affects the waste produced and the number of times a target can be reused. Firstly, typical waste from fission based uranium targets is spent uranium containing an isotopic ratio of approximately 19.7% .sup.235U/.sup.238U due to the 2-3% burnup for .sup.99Mo production. A target with a burnup greater than 2-3% thus implies reduced nuclear waste.

[0170] Secondly, as the amount of .sup.235U reduces with target burnup (owing to the destruction of .sup.235U atoms), the amount of .sup.99Mo produced with each subsequent irradiation is reduced. Eventually, .sup.99Mo production is too low to warrant an additional irradiation.

[0171] The burnup percentage of .sup.235U in the 20% and 1% .sup.235U targets was modelled for irradiations of 2 days, 5 days, 10 days, four5 days and ten5 days, for UO.sub.2 densities ranging from 1 to 10.97 g/cm.sup.3 using the BURN function of MCNP6. The four5 (=20) day and ten5 day (=50) day irradiations were modelled to simulate a target being irradiated, .sup.99Mo extracted and the target re-irradiated multiple times, to obtain an indication of how times a target can be profitably reused.

[0172] The results are shown in FIG. 11 (for 20% enrichment) and FIG. 12 (for 1% enrichment), plotted as burnup expressed as FIMA (i.e. fissions per initial metal atom) of .sup.235U (%) versus UO.sub.2 density D (g/cm.sup.3).

[0173] FIG. 11 shows that, with 20% .sup.235U enrichment, .sup.235U burnup increases rapidly as irradiation time increases and density decreases. This would indicate that a lower target density places limitations on the number of times a target can be reused for .sup.99Mo production with the 20% .sup.235U target. FIG. 12 presents a slightly different picture, suggesting thatfor a 1% .sup.235U targetthe burnup of .sup.235U is linear over the density range 1 to 10.97 g/cm.sup.3. That is, the target's UO.sub.2 density has little effect on burnup for a 1% .sup.235U target. It may also be noted that, for all irradiation times, the burnup of the 20% .sup.235U target is lower than that of the 1% .sup.235U target. Furthermore, with 1% .sup.235U enrichment and for low density targets (<5 g/cm.sup.3 UO.sub.2), the burnup is not linear with irradiation time whilst for target densities above 5 g/cm.sup.3 UO.sub.2 the burnup is approximately linear with irradiation time. This may suggest that lower density targets have an insufficient number of .sup.235U atoms to undergo maximum fission as irradiation time increases and .sup.235U atoms are used up.

[0174] These simulations suggest that, for high efficiency and reusability, reusable target 40 advantageously has these characteristics: [0175] i) a target material comprising approximately 1% enriched UO.sub.2, [0176] ii) a UO.sub.2 density as high as necessary to provide sufficient total yield and efficient 99Mo extraction (such as by UAl.sub.x extraction), [0177] iii) an irradiation time of approximately 5 days, and [0178] iv) intended target re-use (i.e. re-irradiation and re-processing) of approximately 2 to 4 times (that is, total target use of 3 to 5 times).

[0179] However, as the overall yield produced with this target design is lower than with a 20% enriched target, a balance must be struck between (a) efficiency and reusability, and (b) total yield, such as by suitable selection of target size and volume, ideally to approach the yield that can be obtained with a 20% enriched target.

[0180] To identify a suitable balance, the maximum output A.sub.T produced per gram of .sup.235U burned up was examinedwhich would allow .sup.99Mo producers to reduce the generation of nuclear waste.

[0181] Current methods of .sup.99Mo production are characterized by the formula:

[00007]$\mathrm{Output}=\mathrm{Total}\mathrm{yield}/\mathrm{Unit}\mathrm{time}$

which is commonly expressed in GBq per week. When designing a target with this formula in mind it is understandable to pack as much .sup.235U into the target as possible to ensure the maximum number of total fissions per unit time. In such cases, the .sup.235U is in a state of saturation as there is significantly greater quantities present in the target than will ever fission. However, the efficiency of .sup.99Mo target 40 may be expressed as the amount of activity produced per gram of .sup.235U burned up, or .sup.235U.sub.b, rather thanas discussed aboveper gram of .sup.235U initially in the target. Hence:

[00008]$\begin{array}{c}{}_{\mathrm{targ}}^{}=\frac{{}^{99}\mathrm{Mo}\mathrm{produced}\left(\mathrm{Bq}\right)}{{}_{}^{235}{U}_b^{}\left(g\right)}\\ =\frac{A_T\left(\mathrm{Bq}\right)}{{}_{}^{235}{U}_b^{}\left(g\right)}.\end{array}$

[0182] A further parameter is then introduced to take into account the total output (A.sub.T), a parameter termed target quality or Q.sub.targ, where:

[00009]$Q_{\mathrm{targ}}={}_{\mathrm{targ}}^{}{A}_T\left({\mathrm{Bq}}^2.Math.g^{-1}\right)$

[0183] Thus, a target with a high Q.sub.targ would produce the highest .sup.99Mo output for the most .sup.235U burned. Next, it is desirable to consider the total amount of .sup.235U originally in the target before irradiation, .sup.233U.sub.T, because the amount remaining in the target after the target's use shouldall things being equalbe minimized, and the amount remaining is the difference between .sup.235U.sub.T and the .sup.235U.sub.b. Hence, a target sustainability index S.sub.targ is proposed,

where:

[00010]$\begin{array}{c}{S}_{\mathrm{targ}}=\frac{{}_{\mathrm{targ}}^{}.Math.A_T}{{}_{}^{235}{U}_T^{}}\\ =\frac{Q_{\mathrm{targ}}}{{}_{}^{235}{U}_b^{}}\\ =\frac{A_T^2}{{}_{}^{235}{U}_T^{}.Math.{}_{}^{235}{U}_b^{}}\left({\mathrm{Bq}}^2.Math.g^{-2}\right)\end{array}$

[0184] Hence, a reusable target 40 with high .sup.99Mo S.sub.targ would produce the maximum output with the highest burnup from the lowest initial amount of .sup.235U, thus minimizing .sup.235U waste.

[0185] MCNP6 was again used to model both .sup.235U burnup in grams and A.sub.T of .sup.99Mo produced. The modelling was conducted with UO.sub.2 target densities of 0.2 to 8 g/cm.sup.3 in 0.2 g/cm.sup.3 intervals, irradiation times of 2, 3, 4, 5, 6, 7, 8, 9, 10, 15 and 20 days, and target enrichments (% .sup.235U/.sup.238U) of 1%, 3%, 7% and 10%.

[0186] FIGS. 13 to 16 are plots of the results for, respectively, 1%, 3%, 7% and 10% .sup.235U target enrichment. In these figures, 99Mo target total output (A.sub.T) in TBq is plotted versus UO.sub.2 density (D) in g/cm.sup.3 and versus irradiation time (t) in days. The results show maximum outputs around highest UO.sub.2 density and longest irradiation timethe focus of existing techniques.

[0187] FIGS. 17A to 20B, however, are corresponding graphs of sustainability index S.sub.targ, plotted as sustainability index (S.sub.targ) in Bq.sup.2.Math.g.sup.2 versus UO.sub.2 density (D) in g/cm.sup.3 and versus irradiation time (t) in days. FIGS. 17A and 17B are 3D and 2D plots respectively for 1% enrichment, FIGS. 18A and 18B are 3D and 2D plots respectively for 3% enrichment, FIGS. 19A and 19B are 3D and 2D plots respectively for 7% enrichment, and FIGS. 20A and 20B are 3D and 2D plots respectively for 10% enrichment.

[0188] From FIGS. 17A to 20B it may be seen that the optimal ranges of the target sustainability index lie in the ranges of 4 to 7 days irradiation time. The highest sustainability index (39.9910.sup.22 Bq.sup.2.Math.g.sup.2) was obtained at 6 days irradiation with a .sup.235U enrichment of 1% and a UO.sub.2 density of 0.2 g/cm.sup.3 (cf. FIG. 17B), yielding a total output of 407 GBqwhich is relatively low and suggests a limitation to the use the sustainability index alone. In contrast, the highest total output was 70818 GBq at 15 days irradiation with a .sup.235U enrichment of 10% and a UO.sub.2 density of 7.8 g/cm.sup.3 (cf. FIG. 20B), with a sustainability index of 88.1610.sup.22 Bq.sup.2.Math.g.sup.2.

[0189] In a commercial context, a program for the manufacture of .sup.99Mo will commonly be expressed in terms of the amount of .sup.99Mo to be produced in a specific period. For example, the .sup.99Mo manufacturing plant of the Australian Nuclear Science and Technology Organisation was designed to produce 3000 curie (=111 TBq) per week. Hence, in practical applications it may be important to determine the most sustainable process (viz. with the highest sustainable index) that produces a specified total activity (e.g. A.sub.T=111 TBq) in a specified target irradiation time (e.g. 4t7 days: cf. the simulations discussed above).

[0190] FIG. 21 is a plot of sustainability index (S.sub.targ) in Bq.sup.2.Math.g.sup.2 versus UO.sub.2 target volume (V) in cm.sup.3 (with initial UO.sub.2 target mass (m) in g plotted along the upper horizontal axis), for a .sup.235U target enrichment of 1% and 4, 5, 6 and 7 day irradiations. The UO.sub.2 target density was modelled as 2 g/cm.sup.3.

[0191] FIG. 22 is a plot, for the same simulation as that of FIG. 21, of total .sup.99Mo output (A.sub.T) in Ci (left vertical axis) and TBq (right vertical axis) versus initial UO.sub.2 target volume (V) in cm.sup.3.

[0192] From FIG. 21, it can been seen that the sustainability index per target volume is relatively flat over the range of the plot. (The scatter in the data is merely the result of the Monte-Carlo nature of the MCNP6 modelling.) FIG. 22 shows that .sup.99Mo output increases (for a fixed target density and while maintaining a relatively flat sustainability: cf. FIG. 21) essentially linearly with increasing target volume.

[0193] FIG. 23A is a plot of modelled plutonium production Pu (mg) for various initial target matrix .sup.235U/.sup.235U enrichments, a 6 day irradiation period, a target volume of 12 cm.sup.3 and a target density of 2.6 g/cm.sup.3, for a target in the configuration of FIG. 2. The initial mass of .sup.235U was 0.22 g.

[0194] It will be noted that plutonium production decreases essentially monotonically with increasing .sup.235U enrichment.

[0195] FIG. 23B is a plot of modelled normalized plutonium production {tilde over (P)}u for various initial target matrix .sup.235U/.sup.238U enrichments, shown relative to both .sup.235U enrichment and elemental .sup.99Mo productionnormalized to the plutonium production with 20% .sup.235U enrichment. A 6 day irradiation was again employed, as was a target volume of 12 cm.sup.3, a target density of 2.6 g/cm.sup.3, and an initial mass of .sup.235U of 0.22 g. The configuration was again that of FIG. 2.

[0196] FIG. 24A is a plot of a simulation of the stopping and range of 200 .sup.99Mo ions (with full cascades) of 90 MeV, travelling in the +z direction and hitting a UO.sub.2 substrate at (x, y, z)=(0, 0, 0), plotted as y-axis position y (m) against substrate depth z (m) of the Mo ions. The plots shows the trajectories of both the original .sup.99Mo ions and knock-on ions (the latter being in a slightly lighter shade of grey). The simulation was generated with the SRIM (Stopping and Range of Ions in Matter) computer program package.

[0197] The simulation employed a UO.sub.2 density of 10.97 g/cm.sup.3, and SRIM's standard stopping energies. The average longitudinal range (that is, in the +z direction) of the Mo ions was found to be 7.16 m with a straggle of 6489 A. The average radial range of the Mo ions was 1.20 m with a straggle of 5983 A.

[0198] FIG. 24B is a comparable plot of a simulation of the stopping and range of 200 .sup.99Mo ions (with full cascades) of 90 MeV, travelling in the +z direction and hitting a CeO.sub.2 substrate at (x, y, z)=(0, 0, 0), also modelled with SRIM. The simulation employed a CeO.sub.2 density of 7.22 g/cm.sup.3, and SRIM's standard stopping energies. The average longitudinal range (that is, in the +z direction) of the Mo ions was found to be 8.19 m with a straggle of 4637 . The average radial range of the Mo ions was 0.924 m with a straggle of 4966 . The plot shows the trajectories of both the original .sup.99Mo ions and knock-on ions (the latter being in a slightly lighter shade of grey). There are more knock-on ions in this plot than in that of FIG. 24A because the cerium is more easily displaced than the uranium.

[0199] These plots simulate the travel of the .sup.99Mo within, and hence likelihood of ejection from, UO.sub.2 and CeO.sub.2, respectively. It may reasonably be expected that the range of the .sup.99Mo in a mixture of UO.sub.2 and CeO.sub.2 would be essentially a linear combination of the individual ranges. For example, a target with a UO.sub.2 to CeO.sub.2 ratio of 50:50 may be expected to have a .sup.99Mo range that is approximately the average of the two shown in these plots.

[0200] It is evident from these simulations that Mo ions travel further and deviate less in CeO.sub.2 than in UO.sub.2, as might be expected in view of the lower density of CeO.sub.2. Channeling and other effects are expected to be essentially the same, owing to the similar crystal structures of UO.sub.2 and CeO.sub.2. Thus, from this perspective there should be no disadvantage to the use of CeO.sub.2 in conjunction with UO.sub.2, and the greater range of the Mo ions in CeO.sub.2 willall things being equalincrease the proportion of .sup.99Mo that will be ejected.

[0201] FIG. 25 is a schematic view of reactor model 10 and UO.sub.2 core 30 of FIG. 1 with a (modelled) reusable target 50 (not shown to scale) according to an embodiment of the present invention. Reusable target 50 is, in most respects, comparable to target 40 of FIG. 2 being cylindrical, with a height of 3 cm, a radius of 1.13 cm and hence a volume of 12.03 cm.sup.3. Reusable target 50 was modelled as being located with its central axis 60 cm from and parallel to the central axis of UO.sub.2 core 30, to simulate a potential position of a target rig in a reactor.

[0202] However, reusable target 50 comprises a porous matrix of particles that comprise a mixture of UO.sub.2 and CeO.sub.2 (of natural cerium) in a U:Ce molar ratio of 50%. The particles have a size (viz. mean diameter) of 6 m. In this example, the molar ratio of .sup.235U to Ce and .sup.238U is approximately 1%, so the target contains .sup.235U, .sup.238U and Ce in the (molar) proportions of approximately 1:49:50. This corresponds to a UO.sub.2 feedstock with an .sup.235U enrichment of approximately 2%.

[0203] Target 50 is thus comparable in performance to a UO.sub.2 target of like characteristics (but omitting cerium) of 1% .sup.235U enrichment, such that .sup.235U and .sup.238U are present in the molar ratio of approximately 1:99. However, owing to what is, in effect, the substitution of 49/99=49.5% of the .sup.238UO.sub.2 with CeO.sub.2, the density of target 50 is approximately 17% lower than the density the comparable UO.sub.2 only targetwith the benefit of facilitating .sup.99Mo ejection, as discussed above.

[0204] FIG. 26 is a plot of modelled plutonium production Pu (mg) for exemplary UO.sub.2 targets that include CeO.sub.2, as a function of (natural) Ce content (%) (with 1% .sup.235U, and the balance comprising .sup.235Uhence with effectively varying .sup.235U enrichment), for a 6 day irradiation, a target volume of 32.89 cm.sup.3 (hence larger than that of FIG. 24) and a target density of 2 g/cm.sup.3. The initial mass of .sup.235U was 0.6 g. The percentages are mass percentages. The modelled target includes CeO.sub.2, and the configuration is that of FIG. 24, so also comparable to that of FIG. 2.

[0205] It is evident that plutonium production can be substantially reduced by, in effect, substituting CeO.sub.2 for 2.sup.38UO.sub.2. It will be noted thatwith 1% .sup.235U and 99% Ce and hence no .sup.238Uplutonium production is effectively eliminated.

[0206] FIGS. 27A and 27B are, respectively, a flow diagram of a method 60 of manufacturing particles (e.g. beads) of UO.sub.2 for the porous matrix of a target for use in the manufacture of .sup.99Mo, and a flow diagram of a method 80 of manufacturing particles of UO.sub.2 and particles (e.g. beads) of CeO.sub.2 for the porous matrix of such a target, both according to embodiments of the present invention.

[0207] Referring to FIG. 27A, at step 62 of method 60, a solution of uranyl nitrate is infiltrated into a polymer template (such as a template of PAN, such as in the form of PAN beads). The method 60 can then continue either at step 64 or step 66. If continuing at step 64, a gaseous base or other alkali chemical (such as gaseous ammonia) is introduced to the uranyl nitrate infiltrated polymer template, causing precipitation of uranium oxide/hydroxide.

[0208] By heating the infiltrated polymer template, the uranium oxide/hydroxide is converted into U.sub.3O.sub.8 (cf. step 68) and, concurrently, the polymer template is removed (cf. at step 70). The method then continues at step 72, where the U.sub.3O.sub.8 is reduced to UO.sub.2 via heating (such as at a maximum temperature of 1000 C.) in a reducing atmosphere (such as 3.5% hydrogen in nitrogen gas).

[0209] It will be understood that effecting steps 68 and 70 concurrently (and other pairs of steps described and claimed herein as performed concurrently) does not imply that both steps will commence simultaneously (once heating commences) or reach completion simultaneously.

[0210] If, after step 62, the method continues at step 66, then by heating the infiltrated polymer template, the uranyl nitrate is converted into U.sub.3O.sub.8 (cf. step 66) and, concurrently, the polymer template is removed (at step 74).

[0211] The uranyl nitrate may be converted into U.sub.3O.sub.8 (see step 66) by removing the nitrate by, for example, direct denitration. For example, this can be done by heating the sample (e.g. to >300 C., thereby also effecting the concurrent template removal of step 74) in a rotary kiln or a fluidized bed reactor. The rotary kiln is harsher, and may crush the beads owing to their fragility, so it is envisaged that a fluidized bed reactor is likely to be more advantageous in that regard.

[0212] The method then continues at step 72.

[0213] Steps 70 and/or 74 may comprise heating the infiltrated polymer template to a maximum temperature of 400 C.

[0214] Referring to FIG. 27B, steps 64 to 72 for the manufacture of particles of UO.sub.2 proceed as shown in FIG. 27A, and like reference numerals have been used to identify like steps. Subsequently, or concurrently, at step 82 a solution of a cerium salt is infiltrated into a further polymer template (such as a template of PAN, such as in the form of PAN beads). The method 80 can then continue either at step 84 or step 86. If continuing at step 84, a gaseous base or other alkali chemical (such as gaseous ammonia) is introduced to the infiltrated further polymer template, causing precipitation of cerium oxide/hydroxide. By heating the infiltrated further polymer template, the cerium oxide/hydroxide is converted into CeO.sub.2 (cf. step 88) and, concurrently, the further polymer template is removed (cf. step 90).

[0215] If, after step 82, the method instead continues at step 86, by heating the infiltrated further polymer template (such as in a fluidized bed reactor), the cerium salt is converted into CeO.sub.2 (cf. step 86) and, concurrently, the further polymer template is removed (cf. step 92).

[0216] Thus, if the particles of UO.sub.2 and the particles of CeO.sub.2 are formed sequentially, they can then be mixed in readiness for forming the matrix. In addition, the method can include controlling the ratio of cerium and uranium by controlling the amount or amounts of infiltration of the cerium salt (at step 82) and uranyl nitrate (at step 62).

[0217] FIG. 27C is a flow diagram of a method 100 of manufacturing particles (e.g. beads) of UO.sub.2 and CeO.sub.2 for a porous matrix of a target for use in manufacture of .sup.99Mo, according to embodiments of the present invention.

[0218] Referring to FIG. 27C, at step 102, a solution containing uranyl nitrate and cerium nitrate (in known molar ratios) is infiltrated into a polymer template (such as a template of PAN, such as in the form of PAN beads).

[0219] At step 104, a gaseous base or other alkali chemical (such as gaseous ammonia) is introduced to the uranium and cerium nitrate infiltrated polymer template, causing co-precipitation of the uranium oxide/hydroxide and cerium oxide/hydroxide. By heating the infiltrated polymer template, the uranium oxide/hydroxide and cerium oxide/hydroxide are converted to respectively U.sub.3O.sub.8 and CeO.sub.2 (cf. step 106) and, concurrently, the polymer template is removed (cf. step 108).

[0220] At step 110, the U.sub.3O.sub.8 and CeO.sub.2 is reduced to a UO.sub.2/CeO.sub.2 system (U.sub.xCe.sub.1-xO.sub.2, where x is the initial molar mixing ratio of uranium and cerium) in a reducing atmosphere (such as 3.5% hydrogen in nitrogen gas).

[0221] The particles of UO.sub.2 and CeO.sub.2 are formed non-sequentially (concurrently), and method 100 can include controlling the amount or amounts of infiltration of the uranyl nitrate and cerium nitrate (step 102) to achieve a desired molar ratio.

[0222] Subsequently, the size of the particles will depend on (and be controlled by) for how long and/or at how high a temperature sintering is performed when forming the porous matrix.

Manufacture of Porous UO.sub.2 and U.sub.xCe.sub.1-xO.sub.2 Targets Using Nanocasting

[0223] The synthesis of porous CeO.sub.2 (acting as a UO.sub.2 simulant), UO.sub.2 and U.sub.xCe.sub.1-xO.sub.2 were investigated using nanocasting, with the object of making a porous UO.sub.2 system for .sup.99Mo production with a particular focus on materials with a lower density and higher volume compared to conventional smaller volume, high density .sup.99Mo production targets.

Methods and Materials

[0224] In the following examples, simultaneous thermal gravimetric and differential scanning calorimetry analysis (TG-DSC) data were recorded using a Netzsch STA449F3 (trade mark) heating at a rate of 5 C. min.sup.1 under a flow of either Ar or 20% O.sub.2 in N.sub.2 at 30 cm.sup.3.Math.min.sup.1. Gas adsorption studies were carried out using a Quantachrome (trade mark) Autosorb MP instrument and high purity nitrogen gas (99.999%). Surface areas were determined using Brunauer-Emmett-Teller (BET) calculations.

[0225] A Zeiss Ultra Plus (trade mark) scanning electron microscope (SEM, Carl Zeiss NTS GmbH, Oberkochen, Germany) operating at 15 kV equipped with an Oxford Instruments X-Max (trade mark) 80 mm.sup.2 SDD X-ray microanalysis system was used to check the crystal morphology and electron dispersive spectroscopy (EDS) calibrated with a Cu standard for the determination of key elements.

Synthesis of U.sub.xCe.sub.1-xO.sub.2 Beads

[0226] An aqueous solution was prepared by dissolving known amounts of UO.sub.2(NO.sub.3).sub.2.Math.6H.sub.2O and Ce(NO.sub.3).sub.3.Math.6H.sub.2O in H.sub.2O to achieve a desired molar ratios (100% uranium, 100% cerium, 5% uranium in cerium). This solution was then used for infiltration into polyacrylonitrile (PAN) beads. The PAN beads were synthesized using the method described by J. Veliscek-Carolan et al. (2015). The infiltration was achieved by heating the PAN-U/Ce solution in an oven at 60 C. overnight (Ibid).

[0227] Upon infiltration, the beads were removed from the U/Ce solution and vacuum dried at room temperature for 60 minutes. After drying, the beads were placed in an evaporating dish alongside a separate dish containing a solution of a base (e.g. for 100% cerium and 5% uranium in cerium: a 20% ammonia solution). The evaporating dish was covered and left overnight. The beads were collected the next day and washed with H.sub.2O three times over three hours and left to air dry.

Air Calcination of U.sub.xCe.sub.1-xO.sub.2 Beads

[0228] The beads were heated in air at a rate of 1 C./min to 400 or 800 C. and held at this temperature for 5 hours before being cooled to room temperature.

[0229] Pyrolysis of U.sub.xCe.sub.1-xO.sub.2 Beads

[0230] Uncalcined beads were first heated in air at a rate of 1 C./min to 230 C. and held at this temperature for 3 hours before being cooled to room temperature. The beads were then heated under argon at a rate of 1 C./min to 800 C. or 1200 C. and held at this temperature for 3 hours before cooling to room temperature.

Results and Discussion

Material Synthesis

[0231] The synthesis of the uranium-cerium containing beads was achieved in a two-stage process. The first stage involves infiltrating the PAN beads with the desired molar ratio of U/Ce in a concentrated aqueous solution containing known amounts of UO.sub.2(NO.sub.3).sub.2.Math.6H.sub.2O and Ce(NO.sub.3).sub.3. The need for an aqueous solution is evident by the incompatibility of PAN with concentrated amounts of nitrate i.e., a melt reaction.

[0232] Upon infiltration, to convert the NO.sub.3 species to their oxide counterparts, the U/Ce is precipitated as U.sub.xCe.sub.1-xO.sub.2 via vapour diffusion of a base such as NH.sub.3 using a covered evaporating dish. Removal of the nitrate species was achieved by washing the precipitated U.sub.xCe.sub.1-xO.sub.2@PAN with water. The molar ratios explored so far are UO.sub.2, CeO.sub.2 and U.sub.0.05Ce.sub.0.95O.sub.2, with precipitation using gaseous NH.sub.3 used for both the CeO.sub.2 and U.sub.0.05Ce.sub.0.95O.sub.2 samples.

[0233] Upon precipitation, the PAN was removed by heating the samples under a controlled atmosphere. Two atmospheres have been explored so far. The use of an air atmosphere can be used to completely remove the PAN, with the resulting porous material existing entirely as U.sub.xCe.sub.1-xO.sub.2. The alternative option is to use an Ar atmosphere to pyrolyze the material, resulting in decomposition of the PAN without completely removing the carbon. The purpose of leaving the carbon within the structure is to ideally make the beads more robust and mechanically stable, so that they are suitable for use as a reusable .sup.99Mo production system. Explored below is the characterization of the materials under both atmospheres.

Thermal Characterisation

[0234] TG-DSC was performed on the CeO.sub.2@PAN to determine the temperature at which the PAN can be removed from the structure, and to ensure the remaining CeO.sub.2 remained thermally stable past this point. As the equipment is located in a non-active area, only characterization of the inactive CeO.sub.2 material has been performed thus far.

[0235] FIG. 28A is an TG-DSC trace obtained while heating CeO.sub.2@PAN to 600 C. under an atmosphere of 20% O.sub.2 in N.sub.2 (compressed air). This revealed that the material was stable until around 250 C. A mass loss of 30% was then observed between 250 C. and 400 C. which was coupled with a series of exothermic events in the DSC trace. The exothermic events are characteristic of bond breakage, which when coupled to the mass loss correlate to the decomposition and loss of the PAN from the material.

[0236] The stable mass and return of the DSC trace back to zero after this loss of PAN suggests a completed reaction, and thus the amount of PAN within the material can be totaled as 30% of the total mass. Additionally, this confirms that 400 C. is the minimum target temperature to remove the PAN from the porous beads, leaving behind just CeO.sub.2.

[0237] FIG. 28B is an TG-DSC trace obtained while heating CeO.sub.2@PAN to 600 C. under an Ar atmosphere. This was performed to examine the behaviour of the material during pyrolysis. A small mass loss prior to 200 C. can be attributed to the loss of water, with the otherwise stable trace comparable to that of the sample heated under air (cf. FIG. 28A). Referring to FIG. 28B, two mass loss steps were then observed at 220 C. and 295 C., which were coupled to sharp exothermic events in the DSC trace correlating to bond breakage and a small material loss. Under pyrolytic conditions, the breakdown of PAN should involve the loss of nitrogen and hydrogen, with the carbon remaining within the material. The hydrogen and nitrogen make up 32% of PAN, and therefore a mass loss of 32% of the total amount of PAN within the material is envisaged. If it is assumed that the PAN makes up 30% of the total mass as determined by TG-DSC under air (cf. FIG. 28A), this should therefore result in an approximate mass loss of 10%which closely matches the observed result. The TG trace remains steady after this point, confirming complete reaction of the PAN and suggestive of an Ar calcine temperature of 400 C.

Structural Discussion (SEM)

[0238] SEM-EDS was the primary method chosen to examine the U.sub.xCe.sub.1-xO.sub.2 beads after calcination under both an air and Ar atmosphere, focusing on determining (a) whether the porous structure remains intact upon removal of the PAN, and (b) the resulting pore widths and hierarchical porosity.

SEM of CeO.sub.2 after Air Calcination

[0239] FIGS. 29A and 29B are SEM images of fractured air-calcined CeO.sub.2 beads after calcination at 400 C. FIGS. 30A and 30B are SEM images of two pore locations within the air-calcined CeO.sub.2 bead of FIG. 29A, exhibiting pore wall thicknesses of from 4 m to 12 m. The fields of view of FIGS. 30A and 30B correspond approximately to the boxes superimposed on FIG. 29A: FIG. 30A corresponds to the boxed area towards the upper right of the bead of FIG. 30A, while FIG. 30B corresponds to the boxed area near the centre of the bead (not of the field of view) of FIG. 30A.

[0240] These images reveal an intact bead exhibiting clear hierarchical porosity throughout. The pore wall thicknesses were examined, revealing that the walls were progressively thicker closer to the centre of the bead. The pores near the outer edges of the beads had wall thicknesses of 3.5-4.5 m (see FIG. 30A, in which wall Q1 has a thickness of 3.28 m and wall Q2 has a thickness of 4.63 m); the pores closer to the centre of the porous structure had wall thicknesses of 9-12 m (see FIG. 30B, in which wall Q3 has a thickness of 9.01 m and wall Q4 has a thickness of 12.01 m). With a desired wall thickness around 5 m, these results suggest that these porous CeO.sub.2 beads have the desired properties.

SEM of UO.sub.2 after Air Calcination

[0241] The same calcination procedure was applied to porous UO.sub.2 beads, but the SEM results confirmed that the internal structure of the bead was not intact after PAN removal. FIG. 31 is an SEM image of such a fractured air-calcined UO.sub.2 bead. Owing to the thick, seemingly fused edge of the intact outer shell, it is supposed (but without being bound by theory) thatduring the gaseous NH.sub.3 infiltrationthe UO.sub.2 was precipitating out almost immediately, resulting in clogged pores that prevented further infiltration of the NH.sub.3, such thatduring PAN removalthe inner surfaces of the bead not being in their oxide form resulted in PAN decomposition and loss of the heirarchichal porosity.

[0242] Consequently, weaker gasesous bases are proposed, to allow the base to infiltrate the bead further before precipitation of the UO.sub.2.

SEM of CeO.sub.2 after Ar Calcination

[0243] FIGS. 32A and 32B are SEM images of fractured, pyrolyzed CeO.sub.2 beads after calcination under Ar at 600 C. FIGS. 32A and 32B reveal that structure and hierarchical porosity were maintained.

[0244] FIGS. 33A, 33B and 33C are SEM images of pore locations within a 600 C. Ar-calcined CeO.sub.2 bead (found in the same material as were the beads of FIGS. 32A and 32B). Examination of pore wall thickness revealed much thinner walls compared to the same material under an air calcine, with pore walls of from 1.5 m to 2.9 m being observed.

[0245] FIGS. 34A and 34B are SEM images of pore locations within 800 C. Ar-calcined CeO.sub.2 beads. To produce thicker pore walls, two other heating protocols were applied, with the material heated under Ar to either 800 C. or 1200 C. The CeO.sub.2 sample calcined at 800 C. showed a slight increase in the width of the pore walls, with the observable thickness now in a range of 2.2 m to 3.5 m, as is apparent from FIGS. 34A and 34B.

[0246] FIGS. 35A, 35B, 35C and 35D are SEM images of pore locations within 1200 C. Ar-calcined CeO.sub.2 beads. Increasing the calcination temperature to 1200 C. appears to have had a significant effect on the structure, with pore wall thicknesses of 3.3 m to 8.4 m.

[0247] The achievability of these pore wall thicknesses achievable established that the desired properties for a target could also be achieved under pyrolytic conditions.

SEM-EDS of U.sub.0.05Ce.sub.0.95O.sub.2 after Ar Calcination

[0248] Synthesis and subsequent characterization of a 5% uranium in cerium (U.sub.0.05Ce.sub.0.95O.sub.2) bead was also performed using the gaseous NH.sub.3 precipitation method, and subsequently characterized with SEM-EDS. FIGS. 36A and 36B are SEM images of fractured Ar-calcined U.sub.0.05Ce.sub.0.95O.sub.2 beads after calcination at 800 C. These images suggest that the beads had remained intact, with hierarchical porosity extending throughout.

[0249] FIG. 37 is an EDS spectrum of a point within the material of the beads of FIGS. 36A and 36B, plotted as counts (N) versus energy (E). The spectrum shows that both U and Ce have been incorporated into the structure, with uranium making up the minor component that correlates to the 95:5 molar ratio used.

Mechanical Stability of CeO.sub.2 Beads after Air and Ar Calcination

[0250] These observations suggest that the air calcined material is much more delicate than the same material after Ar calcination. The air calcined beads appear unable to be handled with tweezers without extreme care, whilst the Ar beads are much more robust, increasing in apparent structural integrity as the calcination temperature is increased.

Porosimetry

[0251] Porosimetry was performed on two samples: CeO.sub.2 beads after air calcination at 400 C. and CeO.sub.2 beads after Ar calcination at 1200 C. Thus, FIGS. 38A and 38B are plots of N.sub.2 sorption isotherms at 77 K of CeO.sub.2 heated at, respectively, 400 C. under air and 1200 C. under Ar. In these figures, volume (V) of gas absorbed per gram at STP is plotted against partial pressure (P/P.sub.0).

[0252] The N.sub.2 isotherms, measured at 77 K, reveal that both samples remained porous upon calcination and removal of the PAN. The air calcined CeO.sub.2 beads were calculated to have a BET surface area of 57.823 m.sup.2/g, which dropped to 11.212 m.sup.2/g in the Ar calcined beads. One possible reason for this decrease is the carbon remaining in the Ar calcined material, which would reduce the accessible pore space compared to the purely CeO.sub.2 samples made under the air calcination. However, even with the lower observed surface area, the beads remain porous so constitute a reusable platform for .sup.99Mo production.

[0253] It is to be understood that, if any prior art is referred to herein, such reference does not constitute an admission that the prior art forms a part of the common general knowledge in the art in any country.

[0254] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise owing to express language or necessary implication, the word comprise or variations such as comprises or comprising is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

REFERENCES

[0255] Aldawahrah, S., et al., Calculation of fuel burnup and radionuclide inventory for the HEU and potential LEU fuels in the IRT research reactor, Results in Physics, 11 (2018) 564-569. [0256] Bourasseau, E., et al., Experimental and simulation study of grain boundaries in UO.sub.2, Journal of Nuclear Materials, 517 (2019) 286-295. [0257] Boustani, E., et al., Developing a new target design for producing .sup.99Mo in a MTR reactor. Applied Radiation and Isotopes, 147 (2019) 121-128. [0258] Fensin, M. L., Umbel, M., Testing actinide fission yield treatment in CINDER90 for use in MCNP6 burnup calculations, Progress in Nuclear Energy, 85 (2015) 719-728. [0259] Glasstone, S., Sesonske, A. (1994). Nuclear reactor engineering: Reactor design basics, 4th Edition, Vol. 1 Chapman and Hall. Chapter 3. [0260] Brewer, R., (2009) Criticality calculations with MCNP5: A primer, LA-UR-09-00380 [0261] International Atomic Energy Agency, Non-HEU Production Technologies for Molybdenum-99 and Technetium-99m, NF-T-5.4, Vienna, Austria, 2013: pp. 1-75. [0262] Kittel, J. H., Paine, S. H. (1957) Effects of high burnup on natural uranium. Argonne National Laboratory, ANL-5539 [0263] Kostal, M., et al., Comparison of various hours living fission products for absolute power density determination in VVER-1000 mock up in LR-0 reactor, Applied Radiation and Isotopes, 105 (2015) 264-272. [0264] Martin, P. M., et al., Behavior of fission gases in nuclear fuel: XAS characterization of Kr in UO.sub.2, Journal of Nuclear Materials, 466 (2015) 379-392 [0265] Omar, H., Ghazi, N., Time dependent burn-up and fission products inventory calculations in the discharged fuel of the Syrian MNSR, Annals of Nuclear Energy, 3 (2011) 1698-1704 [0266] Pasqualini, E. E., Semi-homogeneous Reactor for .sup.99Mo Production: Conceptual Design, in RERTR 201133rd International Meeting on Reduced Enrichment for Research and Test Reactors. 2011: Marriott Santiago Hotel, Santiago, Chile. p. 10. [0267] Pasqualini, E. E., et al., Irradiation Capsules with Suspended LEU UO.sub.2 Particles for .sup.99Mo Production., in Mo-99 2016 Topical Meeting on Molybdenum-99 Technological Development. 2016: The Ritz-Carlton, St. Louis, Missouri. p. 14 [0268] Pelowitz, D. B., MCNP6 User's Manual, (2013), Los Alamos National Laboratory, LA-CP-13-00634. [0269] Peryoga, Y., et al., Inherent Protection of Plutonium by Doping Minor Actinide in Thermal Neutron Spectra, Journal of Nuclear Science and Technology, 42(5) (2005) 442-450. [0270] Peters, N. J., et al., Using Monte Carlo transport to accurately predict isotope production and activation analysis rates at the University of Missouri research reactor, Journal of Radioanalytical and Nuclear Chemistry, 282 (2009) 255-259. [0271] Raposio, R., et al., Development of LEU-based targets for radiopharmaceutical manufacturing: A review. Applied Radiation and Isotopes, 148 (2019) 225-231 [0272] Rest, J., et al., Fission gas release from UO.sub.2 nuclear fuel: A review. Journal of Nuclear Materials, 513 (2019) 310-345. [0273] Smaga, J. A., et al., Electroplating fission-recoil barriers onto LEU-metal foils for .sup.99Mo-production targets. International Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Oct. 5-10, 1997, Jackson Hole, Wyoming, U.S.A. [0274] Verbeke, J. M., et al., Fission reaction event yield algorithm, FREYAfor event-by-event simulation of fission, Computer Physics Communications, 191 (2015) 178-202. [0275] Werner, C. J., MCNP6.2 Release Notes, Los Alamos National Laboratory, report LA-UR-18-20808 (2018). [0276] Chakravarty, R., et al., Nanoceria-PAN Composite-Based Advanced Sorbent Material: A Major Step Forward in the Field of Clinical-Grade .sup.68Ge/.sup.68Ga Generator, American Chemical Society, 2(7) (2010) 2069-2075, DOI: 10.1021/am100325s. [0277] Lu, P., et al., Photochemical Deposition of Highly Dispersed Pt Nanoparticles on Porous CeO.sub.2 Nanofibers for the Water-Gas Shift Reaction, Adv. Funct. Mater., 25(26) (2015) 4153-4162. [0278] IAEA, IAEA Nuclear Energy Series. No. NW-T-1.14 Status and Trends in Spent Fuel and Radioactive Waste Management, 2018. [0279] International Atomic Energy Agency, Basic Principles Objectives IAEA Nuclear Energy Series Non-HEU Production Technologies for Molybdenum-99 and Technetium-99m, 2013. [0280] M. G. Ruiz et al., Synthesis and characterization of a mesoporous cerium oxide catalyst for the conversion of glycerol, J. Appl. Res. Technol., 1(6) (2018) 511-523. [0281] I. Y. Kaplin et al., Template synthesis of porous ceria-based catalysts for environmental application, Molecules, 25(18) MDPI AG, 1 Sep. 2020. [0282] K. Yoshikawa et al., Synthesis and analysis of CO.sub.2 adsorbents based on cerium oxide, J. CO.sub.2 Util., 8 (2014) 34-38. [0283] R. Srivastava, Eco-friendly and morphologically-controlled synthesis of porous CeO.sub.2 microstructure and its application in water purification, J. Colloid Interface Sci., 348(2) (2010) 600-607. [0284] W. S{umlaut over (t)}ober et al., Controlled growth of monodisperse silica spheres in the micron size range, J. Colloid Interface Sci., 26(1) (1968) 62-69. [0285] X. Zhang et al., Hierarchically porous ceria with tunable pore structure from particle-stabilized foams, J. Eur. Ceram. Soc., 40(12) (2020) 4366-4372. [0286] R. Raposio et al., Modelling of reusable target materials for the production of fission produced .sup.99Mo using MCNP6.2 and CINDER90, Appl. Radiat. Isot., 176 (2021) 109827. [0287] A. M. Seydoux-Guillallallaume et al., Why natural monazite never becomes amorphous: Experimental evidence for alpha self-healing, Am. Mineral., 103(5) (2018) 824-827. [0288] L. Nasdala et al., The absence of metamictisation in natural monazite, Sci. Rep., 10 (2020) 14676. [0289] V. F. Sears, Neutron scattering lengths and cross sections, Neutron News, 3(3) (1992) 26-37. [0290] S. Torrel and K. S. Krane, Neutron capture cross sections of .sup.136,138,140,142Ce and the decays of .sup.137Ce, Phys. Rev. C, 86(3) (2012) 34340. [0291] A. T. Nelson et al., An Evaluation of the Thermophysical Properties of Stoichiometric CeO.sub.2 in Comparison to UO.sub.2 and PuO.sub.2, J. Am. Ceram. Soc., 97(11) (2014) 3652-3659. [0292] J. Veliscek-Carolan et al., Effective Am(iii)/Eu(iii) separations using 2,6-bis(1,2,4-triazin-3-yl)pyridine (BTP) functionalised titania particles and hierarchically porous beads, Chem. Commun., 51 (2015) 11433-11436. [0293] R. Zhao et al., Synthesis of ordered mesoporous uranium dioxide by a nanocasting route, Radiochim. Acta, 104(8) (2016) 549-553.