JPH0666977A - Fuel assembly containing plutonium and core of atomic reactor - Google Patents

Abstract

PURPOSE: To obtain a fuel assembly for natural water-cooled and moderation type reactor by taking advantage of the fact that the plutonium and uranium of fuel rod takes a form of mixed oxide. CONSTITUTION: In a fuel assembly comprising fuel rods at the nodes of a regulate lattice, these fuel rods are distributed in the form of a central group comprising fuel rods having first content t1 of plutonium, a peripheral row of fuel rods having content t2 of plutonium lower than t1, and a corner fuel rod group 1 having content t3 of plutonium lower than t2. The plutonium takes a form of mixed oxide of a plutonium having identical isotope composition for all fuel rods and a natural uranium or a depleted uranium. The contents t1, t2 and t3 are utilized similarly from the view point of allowable residence time in the reactor and selected to obtain such an average content (t) as an enriched uranium assembly is equivalent to a plutonium assembly.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to natural water cooled and moderated nuclear reactors, and more particularly to fuel assemblies for such nuclear reactors.

[0002]

Reprocessing of irradiated fuel assemblies is construed to refer to depleted uranium and plutonium (this term also refers to plutonium with a low content of americium from the radioactive decay of Pu240). Should be)
Made available in large quantities. Boiling water reactor (U
S-A-4251321) or pressurized water reactors (US-A-4652416) mixed with natural or depleted uranium in the fuel rods that make up a fraction of the fuel assemblies of the reactor. To do so (other fuel rods generally contain enriched uranium), the use of plutonium from the reprocessing of irradiated assemblies has been proposed for some time.

The difference in neutron properties between plutonium and uranium isotopes renders unconditional substitution of U-Pu mixtures for uranium enriched with isotope 235 impossible. Some of these differences have already been reported in the above-referenced US patents and can be referred to. Moreover, the plutonium from the reprocessing depends on the neutron energy spectrum in the irradiated nuclear reactor, depending on the irradiation rate that the fuel receives (somewhat conditioned by the initial enrichment), and thus on the reactor. It has an isotopic composition that varies significantly depending on the residence time within, the subsequent storage time, the reprocessing time and the reuptake time into the aggregate.

In order for the utilization of fuel assemblies containing plutonium to be economically acceptable, the presence of the assemblies containing plutonium and natural or depleted uranium in the form of mixed oxides utilizes only enriched uranium. It is necessary not to require a reduction in equilibrium cycle length in relation to the reactor. Moreover, the presence of U-Pu fuel should not adversely affect the functioning of the reactor. In particular, it should not cause a reduction in the nominal power, especially due to the irregular power distribution. Finally, the use of plutonium should not introduce undue complexity in fuel production and management.

[0005] In order to better understand the problems to be overcome, first of all, the uranium enriched uranium was initially filled and stayed in the reactor for about 2-3 years now in large quantities. I would like to reiterate the main characteristics of plutonium derived from fuel assemblies.

The plutonium from the reprocessing now contains very few fissile elements and uranium. This is accompanied by only a low content of superplutonium element, in particular americium 241. Typical isotopic compositions are (in% by mass): Pu238 2.6% Pu239 53.4% Pu240 23.9% Pu241 11.7% Pu242 7.3% Am241 1.1%.

The odd isotopes of plutonium are fissile. Even isotopes are absorbable and thus interfere with chain reactions. However, Pu238 and Pu240 are parent materials, producing fissile material that may later participate in the production of neutrons.

The various isotopes of the fissile parent substance of plutonium have trapping cross sections that vary with neutron energy in a manner that is significantly different from the corresponding isotopes of uranium.

The effective micro-area of thermal neutron absorption (1
energy below eV) is greater for Pu than for U: absorption cross sections are Pu 239 and Pu compared to U 235.
Almost twice as large for 241;

This cross-sectional area is Pu240 compared to U238.
About 100 times larger. In other words, Pu240 reduces plutonium production due to the conversion of U238 by preferential absorption of neutrons.

The effective fission cross section of U235 is such that the isotopes 239, 240 and 241 of plutonium each exhibit a very significant resonance, namely 0.3 eV for 239 and 241 and about 1 eV for 240, whereas It varies in inverse proportion to energy in the region.

The fission effective cross section of the odd isotopes of plutonium in the thermal region is about three times larger than that of U235.

Therefore, the unconditional utilization of mixed oxides (U, Pu) in all fuel rods of an assembly of the type commonly used in natural water cooling and moderators is the beginning of a functional cycle. At, the presence of water flow will induce a power peak in the environment where the thermalization will increase and will also reduce the nominal power.

By constructing an assembly such that some of the surrounding fuel rods and the central fuel rods contain uranium enriched by isotope at U235 and some contain depleted uranium and plutonium. Attempts have been made in particular to solve this problem (US-A-425132).
1). Similarly, an assembly having a peripheral portion consisting of fuel rods containing enriched uranium whose neutron flux is approximately thermal neutron flux, and a central portion containing plutonium with a reduced water content so that the neutrons in it have higher energies. Has also been proposed (US-A-4652,416).

These solutions have various drawbacks.
These would place fuel rods containing enriched uranium and fuel rods containing plutonium in the same assembly. These solutions are not suitable for simultaneously utilizing uranium and plutonium assemblies in the same core of a nuclear reactor. By the way, if only some of the assemblies should be produced in the high safety conditions required by the high radioactivity and toxicity of plutonium, the production of fuel will be very easy.

[0016]

The present invention is particularly applicable to natural water cooled and moderated nuclear reactors, which contain plutonium, are free of enriched uranium and can be replaced by aggregates of enriched uranium. Moreover, it is an object of the present invention to provide a fuel assembly that can be used simultaneously with such an assembly in a nuclear reactor.

[0017]

Therefore, the present invention is directed to a fuel assembly having fuel rods distributed at the nodes of a regular grid (fuel rods at some nodes, for example, guide tubes). Or rods can be deleted so that a restraint rod can be installed), these fuel rods are: -a central group consisting of fuel rods having a first plutonium content t1-of fuel rods having a plutonium content t2 lower than t1 Plutonium is distributed in the form of a peripheral row and a group of fuel rods in the corners with a content t3 lower than t3, the plutonium being preferably plutonium and natural uranium or depleted with the same isotopic composition for all fuel rods A fuel assembly in the form of a mixed oxide of uranium is proposed.

The contents t1, t2 and t3 are the average contents t such that the enriched uranium aggregate and the plutonium aggregate which are similarly used in terms of the allowable residence time in the reactor are equivalent. Is selected in such a way that

In the case of an assembly with a square cross section, the corner fuel rods advantageously have not only four corner fuel rods, but also two edge fuel rods surrounding each corner fuel rod.

This concentration distribution allows the power delivered by the different fuel rods to be balanced, despite the neutron energy reduction induced by the moderating effect of the water stream separating the assemblies in the reactor.

In assemblies where some nodes do not have fuel rods and thus constitute water holes, it may be useful to compensate for the local excess of neutron moderation. This is especially the case for fuel assemblies for pressurized water nuclear reactors having a skeleton containing some guide tubes, some of which are located near the edges of the assembly, for example some in the third row from the periphery. That is true. At this time, if the spectrum fluctuation or control rod is not included, between the first row and each guide tube that constitute the zone of strong thermalization of neutrons, 2
It may be useful to replace fuel rods with content t2 for fuel rods with content t1 in the second column.

The invention corresponds to the case of an assembly for a pressurized water thermal neutron reactor in which the fuel rods are distributed according to a square grid of 17 × 17 cells. It will be better understood by reading the following description of a particular embodiment given by way of non-limiting example.

The description refers to the accompanying drawings.

[0024]

EXAMPLE To ensure compatibility with enriched uranium assemblies that are also utilized in the core of the reactor before determining the content distribution between the various fuel rods of an assembly. It is appropriate to select the average content t of plutonium to be given to the aggregate.

In the following, the ratio of the total mass (Pu + Am) in the mixed oxide and the total heavy isotope mass (U + Pu + Am) expressed in percent will be referred to as the plutonium content t of the fuel rod. Similarly, for a complete assembly, the mass ratio including the fuel rods distributed at the nodes of the regular lattice is also represented by t .

Experimental data show that the fuel rods contain only enriched uranium, taking into account the equivalence rate which depends on the isotopic composition of plutonium and the residual U235 content of plutonium-bound natural or depleted uranium. I was able to Uchitateru one relationship capable of determining the content t corresponding to uranium 235 content t of the body.

One-quarter of the aggregate is exchanged every year and one in five
Consider the very typical case of a pressurized water reactor core where individual assemblies are enriched with plutonium.

-The concentration rate T of UO ₂ aggregates is 3.80%.
The average reactor burning rate of the assembly at the time of extraction is 45,000 MWj per ton, and-the plutonium is reprocessed for such enriched uranium fuel after 5 years from recharging to the end of irradiation. The average plutonium content is 0.225 for U235.
% In the mixed oxide state with depleted uranium t = 7.
It was confirmed that it should be 70%.

As mentioned above, in the case where all the fuel rods are the assemblies having the same mass content t of plutonium, the power distribution becomes very irregular. In FIG. 1, as an example, the document is a reference document EP-A-018.
All fuel rods compared to the power distribution of adjacent UO ₂ assemblies with a conventional configuration and 17 × 17 cells, which may be that described in 7578 or in the corresponding patent FR 8418645. It shows the calculated power distribution that would be exhibited by an aggregate with the same plutonium atom content t of .70%. It can be seen that a particular output exhibits a strong peak in the first aggregate crown from the ambient near the water stream 10 separating the two aggregates.

In the case of an assembly containing fuel rods with a relatively low plutonium enrichment associated with an assembly containing a relatively low enrichment of uranium oxide for a nuclear reactor which replaces one third of the core each year. , A similar output peak can be found again.

In order to damp certain power deviations between the crowns, the fuel rods have an average content in the assembly of t = 7.7.
It is distributed in three groups with different plutonium contents chosen depending on the number of fuel rods in each group to be close to 0%.

Satisfactory results are obtained by using the following groups: -a central group of fuel rods with a first plutonium content t1 of, for example 1.13t, -0.73t, for example. A peripheral row of fuel rods with a content t2 smaller than the average content t , and-a content t3 smaller than t2, e.g. equal to 0.51t, to take into account strong thermalization in the zone common to the two water streams. Fuel rod group in the corner.

Since the effect of this intersection of the two water streams is felt over the four corner fuel rods, the location of the fuel rods with content t1 is line shaded to the location of the fuel rods with content t2. It is often advantageous to have three fuel rods in each corner, as shown in Figure 1, where a mesh screen is applied and the locations of the fuel rods with a content t3 are left white. Will. The central location is occupied by instrumentation tubes.

The fuel assembly shown in FIG. 1 is such that in the case of a control rod or a reactor which may be a somewhat epithermal neutron reactor, energy spectrum fluctuations constitute a water hole when not occupied by a fuel rod. Includes 24 guide tubes. To take into account the increased thermalization of neutrons induced by the combined effect of water flow and water holes where they are contained between the circumference and the guide tube 12 closest to the edge, a fuel rod with a content t1 is It can be replaced by fuel rods with a content t2 at these places, such as 14. Similarly, at the location 16 immediately adjacent to the location of the control rod with the content t3, the content t
It is also possible to put two control rods.

Thus, an assembly is obtained whose specific power distribution per crown is that shown in FIG. 3 (numeral 1 indicates the outer crown of the fuel rod).

It can be seen that the output deviation is much lower in the case of FIG. 2 and is acceptable.

The assemblies according to the invention can be regularly distributed in the nuclear reactor, the places between these assemblies being occupied by enriched uranium assemblies. In particular,
Initially does not contain measurable amount of plutonium,
It is possible to construct a core with 80% enriched uranium aggregate and 20% with the above content. Similarly, 30% and 70% aggregate cores were also considered.

[Brief description of drawings]

FIG. 1 is a plan view showing distribution of fuel rods into three groups in an assembly containing plutonium in a plurality of contents.

FIG. 2 is a graph showing the power distribution per fuel rod crown in an assembly where all fuel rods have the same plutonium content and in adjacent enriched uranium assemblies.

FIG. 3 in an aggregate of the type shown in FIG.
3 is a graph similar to FIG. 2 showing the power distribution obtained between the fuel rod crowns.

[Explanation of symbols]

 10-water flow 12-guide pipe 14, 16-replacement place

 ─────────────────────────────────────────────────── ─── Continuation of the front page (71) Applicant 593128116 Campani General de Matiere Nucriere COMPAGNIE GENERAL DES MATIERES NUCLEA IRES France, 78140 Verisi-Villag-Bure, Rue Paul Dautier 2 (72) Inventor Jean・ Francois Mallow France, 01710, Néron, Ampers Koruna 3 (72) Inventor Patrice Mae France, 69003, Lyon, Ryu Paul Bell 79

Claims (2)

[Claims] 1. A fuel assembly for a natural water cooling and moderating nuclear reactor comprising a bundle of fuel rods comprising natural uranium or depleted uranium and plutonium in the oxide state distributed at regular lattice nodes. These fuel rods are: a central group of fuel rods having a first plutonium content t1, a perimeter row of fuel rods having a plutonium content t2 lower than t1, and a corner having a content t3 lower than t2. Of fuel rod groups, and the fuel assembly is characterized in that the plutonium and uranium are in the form of mixed oxides. 2. A fuel rod having a content of t2 having a skeleton containing a plurality of guide tubes, some of which are located in a third row from the periphery, the fuel rod having a content t2 being in the first row and each guide tube. The assembly according to claim 1, wherein the fuel rod is replaced with a fuel rod having a content t1 in the second row from the surroundings.