Material made of uranium, gadolinium and oxygen and use thereof as consumable neutron poison

Abstract

The present invention relates to a novel material made of uranium, gadolinium and oxygen, having a crystalline phase having cubic crystallographic structure, having an atomic ratio Gd/[Gd+U] of 0.6 to 0.93, the uranium being present in an oxidation state of +IV and/or +V. The invention further relates to the use of such a material as a consumable neutron poison of a fuel element.

Claims

1. A material consisting of uranium (U), gadolinium (Gd) and oxygen (O) exhibiting a crystalline phase with a crystallographic structure of cubic type, with a Gd/[Gd+U] atomic ratio between 0.6 and 0.93, the uranium being present therein in the +IV and/or +V oxidation state.

2. The material as claimed in claim 1, exhibiting a crystalline phase referred to as cubic 1 phase, the Gd/[Gd+U] atomic ratio of which is between 0.79 and 0.93.

3. The material as claimed in claim 2, in which the crystallographic structure of cubic type exhibits a unit cell parameter between 10.8 and 10.9 .

4. The material as claimed in claim 1, exhibiting a crystalline phase referred to as cubic 2 phase, the Gd/[Gd+U] atomic ratio of which is between 0.6 and 0.71.

5. The material as claimed in claim 4, in which the crystallographic structure of cubic type exhibits a unit cell parameter between 5.3 and 5.5 .

6. The material as claimed in claim 1 of two-phase type, exhibiting (i) a cubic 1 phase, the Gd/[Gd+U] atomic ratio of which is between 0.79 and 0.93, and (ii) a cubic 2 phase, the Gd/[Gd+U] atomic ratio of which is between 0.6 and 0.71.

7. The material as claimed in claim 1, in which the uranium is uranium isotopically enriched in .sup.235U, uranium isotopically depleted in .sup.235U or natural uranium.

8. The material as claimed in claim 1, in which the gadolinium is natural gadolinium or gadolinium isotopically modified in its .sup.155Gd/Gd.sub.total and/or .sup.157Gd/Gd.sub.total ratio.

9. A process for the preparation of a material defined according to claim 1, comprising a stage of sintering, at a temperature ranging from 1200 to 2200 C. and under a reducing atmosphere, a powder formed of a mixture of uranium oxide and gadolinium oxide Gd.sub.2O.sub.3 in proportions such that the gadolinium is present in a Gd/[Gd+U] atomic ratio ranging from 0.6 to 0.93.

10. The process as claimed in claim 9, in which the sintering is carried out under an argon atmosphere to which 5 mol % of hydrogen has been added.

11. The process as claimed in claim 9, in which the sintering is carried out for a period of time of greater than or equal to 1 hour.

12. A burnable neutron poison of a nuclear fuel element, which comprises the material as claimed in claim 1.

13. A nuclear fuel pellet, comprising a material as defined according to claim 1.

14. A nuclear fuel rod comprising at least one fuel pellet as defined according to claim 13.

15. A nuclear fuel assembly comprising at least one fuel rod as defined in claim 14.

16. A heterogeneous nuclear fuel pellet formed of at least an internal part comprising at least one fissile material, the internal part being coated with an annular external part that is formed in whole or part of a material as defined according to claim 1.

17. The pellet as claimed in claim 16, in which said annular external part exhibits a thickness ranging from 0.05 to 7.5% of the radius of said pellet.

18. The pellet as claimed in claim 16, in which said internal part is formed in whole or part of uranium oxide, plutonium oxide, thorium oxide or their mixtures.

19. A process for manufacturing a heterogeneous nuclear fuel pellet defined according to claim 16, comprising at least the following steps: (i) providing a powder comprising a material based on uranium (U), gadolinium (Gd) and oxygen (O) exhibiting a crystalline phase with a crystallographic structure of cubic type, with a Gd/[Gd+U] atomic ratio between 0.6 and 0.93, the uranium being present therein in the +IV and/or +V oxidation state; or providing a powder formed of a mixture of uranium oxide and gadolinium oxide Gd.sub.2O.sub.3 in proportions such that the gadolinium is present in a Gd/[Gd+U] atomic ratio ranging from 0.6 to 0.93; (ii) preparing a slip from the powder of stage (i); (iii) depositing the powder in the slip form on the surface of a pellet comprising at least one fissile material; and (iv) sintering the pellet obtained on conclusion of stage (iii) under a reducing atmosphere and at a temperature between 1200 C. and 2200 C.

20. The process as claimed in claim 19, in which stage (iii) includes the drying of the slip layer deposited at the surface of the pellet.

21. A nuclear fuel element of plate-type geometry comprising one or more fissile regions covered, at least in part, with a material as defined according to claim 1.

Description

FIGURES

(1) FIG. 1: Diagrammatic representation of a heterogeneous pellet according to a specific form of the invention.

(2) For the purposes of clarity, the various elements in FIG. 1 are not drawn to scale, the true dimensions of the different parts not being observed.

(3) FIG. 2: Photographs obtained with an optical microscope of the observation in cross section of a heterogeneous pellet exhibiting a fissile core (1) of UO.sub.2 coated with a peripheral layer (2) rich in gadolinium of phase C1, obtained according to example 4;

(4) FIG. 3: Diagrammatic representation of a section of an assembly modeled as 1717 rods, incorporating 52 rods (denoted GD) formed of pellets in accordance with the invention, the rods denoted U being composed of homogeneous UO.sub.2 pellets enriched to 4.9% .sup.235U, and TG denoting guide tubes. The left-hand image is simply an illustrative example of such an assembly.

(5) FIG. 4: Change in the infinite multiplication factor (K.sub.inf) of a reactor as a function of the mean burnup (modeling using the APOLLO2 computing code), for: an imaginary reactor with an assembly composed of homogeneous UO.sub.2 pellets enriched in .sup.235U to 4.9% (curve 1); a critical reactor as defined in example 4.i, for which K.sub.true=1.00 (curve 7); an ideal reactor as defined in example 4.i, for which the true reactivity, .sub.true, is +2000 pcm (curve 2), until the end of the cycle is reached (between the operating points (3) and (4)); an imaginary reactor with an assembly composed of homogeneous UO.sub.2 pellets enriched in .sup.235U to 4.9% and taking into account the effect of a concentration of 2000 pcm of dissolved boron in the heat-exchange fluid (curve 5).

(6) FIG. 5: Change in the multiplication factor K.sub.inf of an imaginary 1717 assembly as a function of the mean burnup for an assembly composed of homogeneous UO.sub.2 pellets enriched in .sup.235U to 4.9% (modeling using the APOLLO2 computing code);

(7) FIG. 6: Change in the multiplication factor K.sub.inf of a standard UO.sub.2 assembly comprising 4.9% of U.sup.235, of an ideal assembly and of a critical assembly as defined in example 4;

(8) FIG. 7: Change in the multiplication factor K.sub.inf of imaginary assemblies as a function of the mean burnup (modeling using the APOLLO2 computing code) for:

(9) FIG. 7.a: different assemblies employing 40 or 52 rods formed of heterogeneous pellets in accordance with the invention, the annular coating of which exhibits a thickness of 50, 60 or 150 m and different Gd isotropic vectors; and

(10) FIG. 7.b: different assemblies employing from 8 to 40 rods formed of homogeneous pellets composed of a Gd.sub.2O.sub.3 and UO.sub.2 solid solution comprising 8% by weight of Gd.sub.2O.sub.3;

(11) FIG. 8: Change in the infinite multiplication factor of an imaginary reactor employing management by quarter and the assemblies illustrated in FIG. 7.a and comparison (modeling using the APOLLO2 computing code);

(12) FIG. 9: Diagrammatic representation of the fuel assembly of plate type (FIG. 9.a), and views along different cross sections (FIGS. 9.b and 9.c), it being possible for the plates to be slightly curved and the number of plates being only an example;

(13) FIG. 10: Diagrammatic representation of a nuclear fuel element of plate type (FIG. 10.a), and view in cross section (FIG. 10.b).

EXAMPLES

Example 1

(14) Preparation of a Material According to the Invention

(15) Different mixed powders formed of a mixture of UO.sub.2 and Gd.sub.2O.sub.3 with a content by weight of Gd.sub.2O.sub.3 varying from 50% to 90% (for example: 55%, 65%, 69%, 80%, 82.4%) are compacted and then sintered under a reducing atmosphere of Ar, 5% H.sub.2, at 1700 C. for 4 hours, in order to give dense pellets.

(16) The results of the analyses, by X-ray diffraction, SEM and energy dispersive analysis (EDS), of the pellets thus obtained are presented in table 1 below.

(17) Crystalline phases with a crystallographic structure of cubic type are detected in the pellets thus obtained and more particularly: a crystallographic structure of cubic type with a unit cell parameter of approximately 5.43 , entitled cubic 2 (C2) phase, for a Gd/[Gd+U] atomic ratio between 0.5 and 0.71; a crystallographic structure of cubic type with a unit cell parameter of approximately 10.8 , entitled cubic 1 (C1) phase, for a Gd/[Gd+U] atomic ratio between 0.79 and 0.93; and a region of phase separation of these two phases for a Gd/[Gd+U] atomic ratio between 0.71 and 0.79.

(18) TABLE-US-00001 TABLE 1 Powders 1 2 3 4 5 Content by weight (%) of 55 65 69 80 82.4 the starting power Gd.sub.2O.sub.3/(Gd.sub.2O.sub.3 + UO.sub.2) Z (Gd/(Gd + U)) atomic 0.66 0.74 0.75 0.85 0.87 ratio of the starting powder Crystallographic structure cubic cubic cubic cubic cubic C2 C2 and C2 and C1 C1 C1 two- C1 two- phase phase Unit cell parameter () .sup.(1) ~5.43 mixed mixed ~10.86 ~10.85 Oxidation state of the +4/+5 +4/+5 +4/+5 +4/+5 +4/+5 uranium State solid solid solid solid solid .sup.(1) obtained by X-ray diffraction analysis

Example 2

(19) Preparation of a Fuel Pellet According to the Invention by Pressing Powders

(20) (i) Powder of Material According to the Invention

(21) A powder of material according to the invention is prepared, as described in example 1, by sintering a mixture of UO.sub.2 and Gd.sub.2O.sub.3, in a Gd.sub.2O.sub.3/(UO.sub.2+Gd.sub.2O.sub.3) ratio by weight of 80%, at 1700 C. and under a reducing atmosphere of Ar, 5% H.sub.2, for 4 hours.

(22) (ii) Preparation of the Heterogeneous Pellet

(23) A pellet is molded according to the structure presented in FIG. 1 with a cylindrical internal part formed from a uranium oxide powder and an annular external part formed from the powder enriched in Gd obtained above. In order to distribute the powders according to FIG. 1, it is possible to use a thin partition made with two concentric rings. On completion of the filling, the thin partition is withdrawn and the pressing is carried out.

(24) The cylindrical core exhibits a radius (R.sub.1) of approximately 4 mm; the annular external part exhibits a thickness (e) of approximately 50 to 250 m as a function of the supply of negative reactivity desired.

(25) The pellet is subsequently sintered under reducing conditions with an atmosphere of Ar, 5% H.sub.2, for 4 hours.

Example 3

(26) Preparation of a Fuel Pellet According to the Invention by Deposition of a Layer Formed of a Powder Having a High Content of Gd

(27) (i) Powder of Material

(28) Two possibilities are selected:

(29) AA powder of material according to the invention is prepared, as described in example 1, by sintering a mixture of UO.sub.2 (indeed even of U.sub.3O.sub.8) and Gd.sub.2O.sub.3 in a Gd.sub.2O.sub.3/(UO.sub.2+Gd.sub.2O.sub.3) ratio by weight of 80%.

(30) BA powder is prepared by mixing UO.sub.2 (indeed even U.sub.3O.sub.8) and Gd.sub.2O.sub.3 in a Gd.sub.2O.sub.3/(UO.sub.2+Gd.sub.2O.sub.3) ratio by weight of 80%.

(31) (ii) Preparation of the Heterogeneous Pellet

(32) A pellet composed of fissile materials (1) is shaped by compaction with a cylindrical geometry.

(33) In order to give cohesion of the powder, presintering of this pellet may be carried out.

(34) An annular external part formed from the powder enriched in gadolinium, obtained in stage (i) according to mode A or mode B, is deposited, for example in the form of a slip (formed from the powder and ethanol), on the cylindrical surface and then the slip is dried.

(35) The pellet is subsequently sintered under reducing conditions with an atmosphere of Ar, 5% (molar) H.sub.2, for 4 hours.

(36) The cylindrical core, with fissile/fertile elements, exhibits a radius of approximately 4 mm; the annular external part, with the gadolinium, exhibits a thickness of approximately 30 to 250 m as a function of the supply of negative reactivity desired.

(37) FIG. 2 shows the pellet observed in cross section by optical microscopy.

Example 4

(38) Use of a Material According to the Invention as Burnable Poison in Nuclear Reactors for the Supply of Negative Reactivity and/or for the Reduction/Suppression of the Requirements for Boron and Other Neutron Poisons/Absorbing Materials

(39) The neutron performance of different 1717 assemblies of nuclear fuel is modeled using the APOLLO2 computing code.

(40) i. Principles and Definitions of the Notions Used K.sub.infini: multiplication factor of the neutrons in an infinite medium (without taking into account the escapes); K.sub.true: multiplication factor of the neutrons in a finite (true) medium.

(41) The difference between K.sub.infini and K.sub.true is thus related to the amount of neutrons which escape from the reactor, without multiplying, in other words:
K.sub.inifini=K.sub.truefactor.sub.geometric(Eq. 1)

(42) the factor.sub.geometric depending mainly on the geometry of the core but also on the nature of the materials. The reactivity , expressed in pcm (percent mille), is another way of expressing the multiplication factor (infinite or true) mathematically,

(43) $\begin{array}{cc}=\frac{K-1}{K}1.Math.{10}^5& \left(\mathrm{Eq}.2\right)\end{array}$

(44) Thus, equation 1 may be expressed as:
.sub.inf=.sub.true+.sub.escapes(Eq. 3)

(45) Critical Reactor

(46) A critical reactor is a reactor in which the population of neutrons is constant and different from zero (without taking into account external sources), in other words a reactor for which K.sub.true=1.00, or, expressed in terms of reactivity, by employing equation 2, =0 pcm.

(47) The calculations carried out with the abovementioned computing code give us the K.sub.infini. It is found that, for the imaginary reactor, the term .sub.escapes is approximately 2500 pcm.

(48) Thus, a critical reactor within the meaning of the modelings carried out exhibits: .sub.true=0 pcm and .sub.infini=2500 pcm; which is reflected, in multiplication factor, by: K.sub.true=1.00 and K.sub.infini=1.025 (curve 7 in FIG. 4). By way of example, FIG. 4 represents the change in the multiplication factor (K.sub.infini) of an imaginary reactor for an assembly composed of homogeneous UO.sub.2 pellets enriched in .sup.235U to 4.9%, with management by (4 operating cycles, in other words, in each cycle, a quarter of the assemblies [the ones most used] are exchanged for fresh assemblies), modeled using the computing code (curve 1).

(49) It is compared with the reactivity of the same reactor using boron (curve 5) as neutron poison (2000 ppm of boron diluted in the water of the heat-exchange fluid). In the case of the use of boron, the reactor may remain critical starting from the operating point (6) by decreasing the concentration of boron in the heat-exchange fluid (critical boron operating method).

(50) Multiplication Factor of an Ideal Reactor

(51) Curve 2 on the graph of FIG. 4 represents the change in the infinite multiplication factor of an ideal reactor. This ideal core is a reactor without a neutron penalty, with an initial superreactivity of 2000 pcm (thus with a K.sub.infini=1.050) and without penalty over the length of the cycle (operating point (4)).

(52) Thus, as represented in FIG. 4, an ideal reactor within the meaning of the modelings carried out is a reactor which has a true reactivity, .sub.true of +2000 pcm up to the operating point (3). This superreactivity makes it possible to operate the reactor (for example in order to rise in power).

(53) Multiplication Factor of an Ideal Assembly

(54) FIG. 5 exhibits the change in the multiplication factor in an infinite medium K.sub.infini under hot conditions (that is to say, while considering the effect of the temperature) of an imaginary fuel assembly based on UO.sub.2 as a function of the mean burnup of the assembly, for 1717 assemblies.

(55) The relationship between the change in the K.sub.infini of an assembly (K.sub.assembly.sup.inf) and the change in the K.sub.infini of a reactor (K.sub.core.sup.inf), with management of N cycles, is given by the approximation below:

(56) $\begin{array}{cc}{K}_{\mathrm{core}}^{\inf }\left(x\right)=\frac{1}{N}\underset{i=0}{\overset{i=N-1}{.Math.}}\left[K_{\mathrm{assembly}}^{\inf }\left(x+L_{\mathrm{cycle}}i\right)\right]& \left(\mathrm{Eq}.4\right)\end{array}$
with:
x the burnup of the assemblies in the 1.sup.st cycle,
N the total number of cycles that the assemblies are used in the reactor,
K.sub.core.sup.inf(x) the K.sub.infini of a reactor, with management at N cycles, as a function of the burnup x,
K.sub.assembly.sup.inf(x+L.sub.cyclei) the K.sub.infini of an assembly, as a function of a burnup,
L.sub.cycle the length of a cycle (in burnup units). In particular, L.sub.cycle confirms that K.sub.core.sup.inf(L.sub.cycle)=1.025, in order for the reactor to be critical at the end of the cycle.

(57) By employing equation 4, it is possible to plot (FIG. 6) the change in the K.sub.infini of an ideal assembly and of a critical assembly, so that, employed in a reactor (with management by quarter, thus N=4), they respectively give an ideal reactor behavior and critical reactor behavior, as defined above.

(58) ii. Neutron Effect Obtained with Different Assemblies

(59) The change in the K.sub.infini for various 1717 assemblies are presented in FIG. 7.a: imaginary fuel assembly based on UO.sub.2 (curve 1); assemblies employing 40 or 52 rods formed of heterogeneous pellets in accordance with the invention, the annular coating of which exhibits a thickness of 50, 60 or 150 m and various Gd isotopic vectors. The other rods of the assembly are formed of homogeneous UO.sub.2 pellets enriched in .sup.235U to 4.9% (curves 2 to 5); and ideal and critical assemblies as defined in the preceding point i. (curves 6 and 7).

(60) All these curves consider a boron-free imaginary reactor, that is to say a concentration of 0.0 ppm of boron in the heat-exchange fluid/moderator.

(61) For comparative purposes, the neutron effect obtained for assemblies incorporating rods of conventional homogeneous pellets composed of a (U,Gd)O.sub.2 solid solution comprising 8% by weight of Gd.sub.2O.sub.3 is presented in FIG. 7.b.

(62) It emerges from FIG. 7.a that it is possible, by adjusting the number of rods in accordance with the invention, the thickness of the burnable poison layer of the pellets constituting them and the isotopic vector of the gadolinium, to control the change in the reactivity of the reactor so as to approach an optimum change.

(63) Also, the comparison of FIGS. 7.a and 7.b shows that the change in the reactivity of the assembly may be better controlled with pellets in accordance with the invention than with conventional homogeneous pellets, since the curve of reactivity for an assembly according to the invention more closely approaches the ideal curve at the cycle end.

(64) iii. Reactivity of the Reactor

(65) The behavior of a reactor employing the assemblies of the invention is shown in FIG. 8.

(66) It emerges from FIG. 8 that the effect on the reactivity makes it possible to reduce, indeed even to suppress, the use of boron in the reactor.

REFERENCES

(67) [1] Balestrieri thesis, 1995; [2] Tang and al., Order-to-disorder phase transformation in ion irradiated uranium-bearing delta-phase oxides RE.sub.6U.sub.1O.sub.12 (RE=Y, Gd, Ho, Yb, and Lu), Journal of Solid State Chemistry, 183(4), 844-848; [3] Tang and al., Microstructural evolution in irradiated uranium-bearing delta-phase oxides A.sub.6U.sub.1O.sub.12 (A=Y, Gd, Ho, Yb, and Lu), Journal of Nuclear Materials, 407(1), 44-47.