GB2062343A

GB2062343A - A Fuel Element for Material Test and Research Reactors

Info

Publication number: GB2062343A
Application number: GB8033377A
Authority: GB
Original assignee: Nukem GmbH
Current assignee: Nukem GmbH
Priority date: 1979-10-16
Filing date: 1980-10-16
Publication date: 1981-05-20
Also published as: FR2468187A1; GB2062343B; FR2468187B1; DE2941878A1; DE2941878C2; JPS5663293A

Abstract

Fuel plates which may be used with lower enrichment levels of uranium 235 (around 20%) have recently been required for material test and research reactors having high neutron density. According to the invention, this result is achieved in that the individual fuel plates consist of an assembly of a plurality of flat, non-sub-divided chambers tightly surrounding the fuel and/or fertile material which preferably have an internal height of less, 2 mm and which are filled with non-metallic uranium and/or thorium compounds preferably having a density of greater than 4g/cc.

Description

SPECIFICATION A Fuel Element for Material Test and Research Reactors This invention relates to a fuel element for material test and research reactors comprising fuel plates containing nuclear fuel and/or fertile material and retaining elements for these fuel plates.

The fuel element used for conventional material test reactors (MTR) is made up of flat or curved fuel plates. Each individual fuel plate represents a lamina structure, the actual fuel, normally aluminium-uranium alloys or uranium aluminides dispersed in an aluminium matrix, being tightly surrounded on all sides by aluminium. The consitutuent fuel plates of the fuel element may be assembled in a boxlike or tubular arrangement by means of retaining elements. The standard fuel elements contain between 12 and 23 plates arranged parallel to and equidistant from one another to enable water to flow through between them for cooling and moderation.

The fuel plates are generally 1.27 mm thick and approximatlely 72 mm wide. The actual fuel zone or "meat" is 0.51 mm thick and approximately 63 mm wide for an active length of 600 mm.

MTR elements constructed in this way are favourably distinguished by a large heat-transfer surface.

The fuel plates are normally produced by the sealed assembly rolling process (so-called picture-frame technique).

The use of uranium heavily enriched with the isotope 235U as fuel has proved to be optimal for MTR fuel elements because, in this case, the large stock of fissile material required may readily be obtained for a relatively low fuel density.

On the other hand, uranium heavily enriched with 235U is a sensitive material of which the dissemination has to be closely controlled and limited. For reasons of proliferation, therefore, there is a need to be able to use uranium less heavily enriched with 235U (at most 20% by weight) for MTR reactors.

The effect of changing the MTR reactors over to lower 235U enrichment levels is that, to compensate for the increased losses of nuetrons attributable to the higher 238U-content, the stock of fissile material has to be increased. This coupled with the use of less heavily enriched uranium requires fuel densities higher by a factor of about 5 for the same plate geometry. This means that the limits which can still be controlled in terms of processing technique for the current MTR fuels based on uranium aluminides are exceeded.

Accordingly, it has been proposed to use fuel plates which contain uranium dioxide as fuel in the form of small, thin sintered platelets accommondated in a Zircaloyjacket. Fuel plates of this type are suitable in principle for the changeover of MTR reactors from high to lower enrichment levels. However, there is the disadvantage that the UO2-platelets have to have a minimal thickness of more than 2.0 mm for reasons associated with processing technique.

This leads to fuel plates in which the ratio of heattransfer surface to fuel volume is considerably less favourable than before. This in turn gives rise to considerable disadvantages in regard to cooling. In addition, the fairly considerable thickness of the plates leads to poorer moderation and to a high fuel temperature. Furthermore, the maintenance of strict dimensional tolerances in the thin platelets gives rise to very high manufacturing costs.

Accordingly, an object of the present invention is to provide a fuel element for material test and research reactors comprising fuel plates containing nuclear fuel and/or fertile material and retaining elements which does not give rise to any complications in regard to the operational behaviour of the reactors when they are switched from high to relatively low 235U-enrichment levels.

More particularly, the invention seeks to gurantee as large as possible a ratio of heat-transfer surface to fuel volume by minimising the thickness of the plates.

According to the invention the individual fuel plates consist of an assembly of a plurality of flat, non-sub-divided chambers tightly surrounding the fuel and/or fertile material. These chambers, of which the internal height or gap width between the chamber walls for accommodating the fuel and/or fertile material is advantageously less than 2 mm are arranged either only longitudinally of or only transversely of the axis of the fuel element.

The chambers preferably contain uranium and/or thorium in the form of non-metallic compounds although metallic uranium and/or thorium compounds may also be used. It is of particular advantage for the density of these uranium and/or thorium compounds to be greater than 4 g/cc.

The chambers preferably contain the fuel and/or fertile material in the form of discrete spherical particles. It is of advantage to use a loose filling of the particles where the chambers are arranged transversely of the longitudinal axis of the fuel element or to embed the particles in a matrix where the chambers are arranged longitudinally of the longitudinal axis of the fuel element.

By using fuel particles having a diameter of preferably from 0.2 to 0.7 mm, it is possible to keep the overall thickness of the plates to a minimum. This gurantees that the fuel temperature is kept low by virtue of the favorable ratio of fuel surfaces to volume. This applies in particular when the fuel particles are with advantage embedded in a matrix of high thermal conductivity. Another advantage of the fuel element according to the invention lies in the fact that the compact arrangement of the thin plates ensures favourable moderation, i.e. the ratio of hydrogen to 235U atoms.

Another significant advantage of the fuel element according to the invention lies in the fact that the fuel particles can be economically produced in a precisely defined size over a wide range within narrow limits so that the 235Ucontent may readily be adapted with considerable flexibility to the exact requirements of the individual MTR-reactor.

In addition, it is possible partly to replace the uranium isotope 238 by thorium and hence further to restrict the dissemination of bred plutonium through the formation of uranium 233.

Figures 1 to 4 of the accompanying drawings diagrammatically illustrate examples of embodiments of fuel plates according to the invention.

Figure 1 shows a fuel plate (1) consisting of individual chambers (2) which are arranged transversely of the longitudinal axis of the fuel element and which are secured in two lateral retaining elements (3). Figure 2 is a corresponding side elevation. Figure 3 shows two chambers (2) which are joined by weld seams (4) and which contain the fuel particles (5) in the form of a loose filling, whereas in Figure 4 the particles (5) are embedded in a matrix (6). Instead of being joined by weld seams (4), the individual chambers (2) may also be joined together by a tongue-and-groove-like configuration of the chambers (2) or may simply be arranged loosely on top of or alongside one another.

The fuel element according to the invention is illustrated by the following Examples: Example 1 Uranium silicide powder containing 4% by weight of silicon and 96% by weight of uranium was used as the starting powder for producing fuel plates containing platelets of uranium silicide as fuel in the individual chambers. The powder which had a particle size of less than 125 ,um was compressed into platelets under a pressure of 5.5 Kbar in a square cavity, after which the platelets thus obtained were sintered. After sintering, the dimensions of the platelets were as follows: thickness 1.4 mm width 14.8 mm length 14.8 mm The geometric density of the platelets amounted to 12.5, corresponding to a theorectical density of 80%.

Fuel chambers 60 mm long and 15 mm wide with a gap width of 1.5 mm were produced by rolling from aluminium tubes having a wall thickness of 0.3 mm. The chambers were each loaded with four platelests, evacuated, filled with helium and finally sealed by welding. Finally, the fuel-laden chambers were introduced into two side plates transversely of the longitudinal axis of the fuel element and wedged firmly therein by means of a single-roll tool.

Example 2 Spherical particles consisting of UO2 and UC2 in a ratio by weight of 1:1 were used as fuel in the production of fuel plates containing particles of uranium oxide and uranium carbide embedded in an aluminium-silicon matrix in the individual chambers. The particles have a uranium content of 91.45% by weight, an oxygen content of 6.7% by weight, a carbon content of 1.85% by weight and a mean diameter of 300 ptm. The density of particles amounted to 10.5 g/cc, corresponding to a theoretical density of 94%. The particles wee produced in a known manner by casting uranyl nitrate solution. An aluminium-silicon alloy powder containing 12% by weight of Si was used for embedding the fuel particles. For a mean particle size of 50 ym, the powder had a bulk density of 1.1 g/cc.

To produce the chambers for accommodating the fuel, two 0.4 mm thick aluminium plates approximatlely 600 mm long and 70 mm wide were first welded together underneath and laterally in such a way that a gap 1 mm wide was left between the plates. This gap was then filled with a homogeneous mixture consisting of 23.4 g of Al-Si alloy powder and 213.2 g of fuel particles. The method for producing a homogeneous mixture of this type is described in German Offenlegungsschrift No. 2,333,94.

The loaded chambers were then evacuated, sealed by welding at their upper ends and compressed at 5900C under a pressure of 1 Kbar.

This resulted in a reduction in the nominal plate thickness from 1.8 to around 1.6 mm.

Subsequent metallographic examination did not reveal any reaction between the fuel particles and the aluminium alloy powder under the production conditions selected. The approximately 0.8 mm thick fuel-containing zone was uniformly formed and joined to the aluminium surrounding it without any gaps in between. The individual chambers were then fixed at their narrow sides by means of two plates and introduced into the fuel element longitudinally of the longitudinal axis thereof.

Example 3 UO2-particles produced in known manner with a mean diameter of 300 ,um and a density of 10.7 g/cc, corresponding to a theoretical density of 98%, were used as fuel for producing fuel plates containing loose particles of uranium oxide in the individual chambers. Flat chambers with a gap width of 1 mm and a width of 15 mm were first produced by rolling from Zircaloy tubes having a wall thickness of 0.5 mm. Thereafter four chambers were sealed by welding at their lower ends and joined laterally together by welding to form a plate. After the chambers had been loaded with the fuel particles, they were evacuated, filled with helium and sealed by welding.

The packing thickness of the particles amounted to 50% by volume, corresponding to a uranium density in the fuel zonesof 4.7 g/cc.

Example 4 Fuel particles which had been produced by precipitation from a uranyl/thorium nitrate solution were used for producing fuel plates containing loose particles of uranium/thorium oxide in the individual chambers. 80% by weight of the particles consisted of UO2 and 20% by weight of ThO2. The particles has a density of 10.4 g/cc, corresponding to a theroretical density of 97%. The mean diameter amounted to 320 ym. The packing thickness of the fuel zone amounted to 45% by volume, corresponding to a heavy metal density of 4.7 g/cc. Further processing was carried out in the same way as Example 3.

Claims

1. A fuel element for material test and research reactors comprising fuel plates containing nuclear fuel and/or fertile material and retaining elements, the individual fuel plates consisting of an assembly of a plurality of flat, non-subdivided chambers tightly surrounding the fuel and/or fertile material.

2. A fuel element as claimed in Claim 1, wherein the internal height of the chambers is less than 2 mm.

3. A fuel element as claimed in Claim 1 or 2, wherein the chambers are arranged either only transversely of or only longitudinally of the longitudinal axis of the fuel element.

4. A fuel element as claimed in any of Claims 1 to 3, wherein the individual chambers contain uranium and/or thorium in the form of nonmetallic compounds.

5. A fuel element as claimed in Claim 4, wherein the density of the uranium and/or thorium compounds is greater than 4 g/cc.

6. A fuel element as claimed in any of Claims 1 to 5, wherein the chambers contain the fuel and/or fertile material in the form of discrete spherical particles.

7. A fuel element as claimed in Claim 6, wherein, where the chambers are arranged transversely of the longitudinal axis of the fuel element, the particles are present in the form of a loose filling.

8. A fuel element as claimed in Claim 6, wherein, where the chambers are arranged longitudinally of the longitudinal axis of the fuel element, the particles are embedded in a matrix.

9. A fuel element as claimed in any of Claims 1 to 8, wherein the particles have a diameter of from 0.2 to 0.7 mm.

1 0. A fuel element substantially as described with particular reference to any of the accompanying drawings.

11. A fuel element substantially as described with particular reference to any of the Examples.