METHOD AND TARGET FOR MO-99 MANUFACTURE

Abstract

A UO.sub.2 target for use in the manufacture of .sup.99Mo, the target comprising: a porous matrix; wherein the matrix comprises particles of UO.sub.2 or of UO.sub.2 and CeO.sub.2 with a size of less than 7.15 ?m; and a molar ratio of .sup.235U to Ce and .sup.238U is less than 3%. The particles may comprise UO.sub.2 and the UO.sub.2 comprise uranium with a .sup.235U to .sup.238U ratio of less than 3% .sup.235U enrichment. Also, a method of producing .sup.99Mo, comprising: (a) irradiating such a UO.sub.2 target with thermal neutrons, with an irradiation time of between 3 and 7 days; then (b) extracting 99Mo from the target. The method includes performing steps (a) and (b) 2 or more times.

Claims

1. A UO.sub.2 target for use in the manufacture of .sup.99Mo, the target comprising: a porous matrix; wherein the matrix comprises particles of UO.sub.2 or of UO.sub.2 and CeO.sub.2 with a size of less than 7.15 ?m; and a molar ratio of .sup.235U to Ce and .sup.238U is less than 3%.

2. The target as claimed in claim 1, wherein the particles comprise UO.sub.2 and the UO.sub.2 comprises uranium with a .sup.235U to .sup.238U ratio of less than 3% .sup.235U enrichment.

3. The target as claimed in claim 2, wherein the matrix has an average density of between 50% and 70% of the density of the UO.sub.2, or an average density of between 50% and 60% of the density of the UO.sub.2.

4. The target as claimed in claim 2, wherein the UO.sub.2 comprises uranium with a .sup.235U to .sup.238U ratio of between 0.3% and 3% .sup.235U enrichment, or between 0.5% and 3% .sup.235U enrichment, or between 0.7% and 3% .sup.235U enrichment.

5. The target as claimed in claim 2, wherein the uranium has a .sup.235U enrichment of between 0.75% and 2.8%, or of between 0.8% and 2.0%, or of between 0.9% and 1.4%, or of approximately 1%.

6. The target as claimed in claim 2, wherein the .sup.235U to .sup.238U ratio is an initial .sup.235U to .sup.238U ratio.

7. The target as claimed in claim 1, wherein the matrix comprises particles of UO.sub.2 and CeO.sub.2, and the molar ratio of .sup.235U to Ce and .sup.238U is between 0.3% and 3%, or between 0.5% and 3%, or between 0.7% and 3%, or between 0.75% and 2.8%, or between 0.8% and 2.0%.

8. The target as claimed in claim 7, wherein the molar ratio of .sup.235U to Ce and .sup.238U is between 0.9% and 1.4%.

9. The target as claimed in claim 7, wherein the molar ratio of .sup.235U to Ce and .sup.238U is approximately 1%.

10. The target as claimed in claim 1, wherein the matrix has an average density of less than or equal to 75% of the average density of the particles, or of less than or equal to 65% of the average density of the particles, or of less than or equal to 55% of the average density of the particles, or of less than or equal to 45% of the average density of the particles.

11. The target as claimed in claim 1, wherein the target is configured to yield a maximum amount of .sup.99Mo and a maximum amount of burnup from a lowest initial amount of .sup.235U.

12. The target as claimed in claim 1, wherein the average density of the matrix is an initial average density.

13. The target as claimed in claim 1, wherein the target is configured to maximize a sustainability index S.sub.targ, where: $S_{\mathrm{targ}}=\frac{A_T^2}{{}^{235}{U}_T.Math.{}^{235}{U}_b}\left({\mathrm{Bq}}^2.Math.g^{-2}\right),$ where A.sub.T is a predefined amount of .sup.99Mo desired to be produced in the irradiation, .sup.235U.sub.T is the total amount of .sup.235U in the target before the irradiation, and .sup.235U.sub.b is the amount of .sup.235U burned up in the irradiation.

14. The target as claimed in claim 1, wherein the target is doped with .sup.237Np or with one or more minor actinides.

15. The target as claimed in claim 14, wherein an amount of doping is approximately 1% by mole relative to .sup.235U content.

16. A method of producing .sup.99Mo, the method comprising: (a) irradiating a UO.sub.2 target according to claim 1 with thermal neutrons, with an irradiation time of between 3 and 7 days; then (b) extracting .sup.99Mo from the target; wherein the method includes performing steps (a) and (b) 2 or more times.

17. The method as claimed in claim 16, comprising: performing steps (a) and (b) 3 or more times; or performing steps (a) and (b) 4 or more times; or performing steps (a) and (b) 2 to 6 times; or performing steps (a) and (b) 3 to 5 times.

18. The method as claimed in claim 16, comprising a delay between an instance of step (a) and a next instance of step (a), sufficient to allow in combination with a time required to perform step (b) one or more by-products in the target to decay to a predefined level.

19. The method as claimed in claim 18, wherein the predefined level is less than 50% of an amount of a specified by-product present at the end of step (a), or less than 25% of the amount of a specified by-product present at the end of step (a).

20. The method as claimed in claim 16, wherein the irradiation time is between 4 and 6 days, or between 4.5 and 5.5 days, or approximately 5 days.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0096] 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:

[0097] FIG. 1 is a schematic view of a reactor model used to model the performance of a reusable target according to an embodiment of the present invention;

[0098] FIG. 2 is a schematic view of the reactor model of FIG. 1 with a reusable target according to an embodiment of the present invention;

[0099] 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;

[0100] 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;

[0101] FIG. 5 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 5 day irradiation;

[0102] FIG. 6 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 10 day irradiation;

[0103] 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;

[0104] FIG. 8 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 5 day irradiation;

[0105] 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;

[0106] 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;

[0107] 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;

[0108] 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;

[0109] 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;

[0110] 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;

[0111] 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;

[0112] 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;

[0113] 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;

[0114] 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;

[0115] 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.235U enriched target in the configuration of FIG. 2;

[0116] 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;

[0117] 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;

[0118] 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;

[0119] 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;

[0120] FIG. 23B is a plot of modelled normalized plutonium production {tilde over (P)}? 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;

[0121] 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);

[0122] 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;

[0123] 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 another embodiment of the present invention; and

[0124] 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.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0125] FIG. 1 is a schematic view of a simple reactor model 10 used to model the performance of a reusable target according to an embodiment of the present invention. 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.

[0126] 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.

[0127] FIG. 2 is a schematic view of reactor model 10 of FIG. 1 with a (modelled) reusable target 40 (not shown to scale) according to an embodiment of the present invention. 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.

[0128] 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; [0129] k.sub.eff=1 indicates criticality: the number of neutrons produced by fission equals the number lost, the desired configuration for reactor operation; and [0130] k.sub.eff<1 indicates subcriticality: the number of neutrons produced by fission is less than the number lost.

[0131] 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.

[0132] 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.

[0133] 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).

[0134] 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.

[0135] 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

[0136] 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.133Xe, .sup.131I and .sup.135Xe) are also plotted. FIGS. 5 and 6 are comparable, but for 5 day and 10 day irradiations, respectively.

[0137] 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.

[0138] 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

[0139] 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.

[0140] 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

[0141] 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.

[0142] 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]${?}_{\mathrm{targ}}=\frac{{}^{99}\mathrm{Mo}\mathrm{produced}\left(\mathrm{Bq}\right)}{{}^{235}U\mathrm{in}\mathrm{target}\left(g\right)}=\frac{A_T{(}^{99}\mathrm{Mo})}{m_T{(}^{235}U)}$

[0143] 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 viewtargets 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.

[0144] 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.

[0145] 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.

[0146] 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, four?5 days and ten?5 days, for UO.sub.2 densities ranging from 1 to 10.97 g/cm.sup.3 using the BURN function of MCNP6. The four?5 (=20) day and ten?5 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.

[0147] 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).

[0148] 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.

[0149] These simulations suggest that, for high efficiency and reusability, reusable target 40 advantageously has these characteristics: [0150] i) a target material comprising approximately 1% enriched UO.sub.2, [0151] ii) a UO.sub.2 density as high as necessary to provide sufficient total yield and efficient .sup.99Mo extraction (such as by UAl.sub.x extraction), [0152] iii) an irradiation time of approximately 5 days, and [0153] 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).

[0154] 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.

[0155] To identify a suitable balance, the maximum output AT produced per gram of .sup.235U burned up was examinedwhich would allow .sup.99Mo producers to reduce the generation of nuclear waste.

[0156] Current methods of .sup.99Mo production are characterized by the formula:

Output=Total yield/Unit 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:

[00007]$\begin{array}{c}{?}_{\mathrm{targ}}^{}=\frac{{}^{99}\mathrm{Mo}\mathrm{produced}\left(\mathrm{Bq}\right)}{{}^{235}{U}_b\left(g\right)}\\ =\frac{A_T\left(\mathrm{Bq}\right)}{{}^{235}{U}_b\left(g\right)}.\end{array}$

[0157] 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:

Q.sub.targ=?.sub.targ?A.sub.T(Bq.sup.2.Math.g.sup.?1)

[0158] 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.235U.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:

[00008]$\begin{array}{c}{S}_{\mathrm{targ}}=\frac{{?}_{\mathrm{targ}}^{}.Math.A_T}{{}^{235}{U}_T}\\ =\frac{Q_{\mathrm{targ}}}{{}^{235}{U}_T}\\ =\frac{A_T^2}{{}^{235}{U}_T.Math.{}^{235}{U}_b}\left({\mathrm{Bq}}^2.Math.g^{-2}\right)\end{array}$

[0159] 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.

[0160] 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%.

[0161] FIGS. 13 to 16 are plots of the results for, respectively, 1%, 3%, 7% and 10% .sup.235U target enrichment. In these figures, .sup.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.

[0162] 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.

[0163] 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 (39.99?10.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 88.16?10.sup.?22 Bq.sup.2.Math.g.sup.?2.

[0164] 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. 4?t?7 days: cf. the simulations discussed above).

[0165] 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.

[0166] 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.

[0167] 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.

[0168] FIG. 23A is a plot of modelled plutonium production Pu (mg) for various initial target matrix .sup.235U/.sup.238U 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.

[0169] It will be noted that plutonium production decreases essentially monotonically with increasing .sup.235U enrichment.

[0170] FIG. 23B is a plot of modelled normalized plutonium production {tilde over (P)}? 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.

[0171] 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.

[0172] 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 ?. The average radial range of the Mo ions was 1.20 ?m with a straggle of 5983 ?.

[0173] 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.

[0174] 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.

[0175] 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. Channelling 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.

[0176] 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 another embodiment of the present invention. Reusable target 50 is, in most respects, comparable to target 40 of the embodiment 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.

[0177] 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%.

[0178] 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.

[0179] 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.238Uhence 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.

[0180] It is evident that plutonium production can be substantially reduced by, in effect, substituting CeO.sub.2 for .sup.238UO.sub.2. It will be noted thatwith 1% .sup.235U and 99% Ce and hence no .sup.238Uplutonium production is effectively eliminated.

[0181] 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.

[0182] 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.

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