Patent Number: 052727408
Section: description

DESCRIPTION OF THE INVENTION The invention is an agent for trapping the radioactivity of the fission products which appear in a nuclear fuel based on sintered uraniferous oxides in the course of irradiation, which is stable at elevated temperature, characterised in that it comprises a defined stable oxygenated compound, a combination of at least two metallic oxides and at least one oxide of a non-radioactive isotope of the radioactive fission product or products whose radioactivity is to be trapped. That trapping agent is in general used in the production of the nuclear fuel elements which usually comprise pellets of sintered uraniferous oxide which are surrounded by a metal sheath in the shape of a needle or stick or rod and, according to the invention, the trapping agent. The stable oxygenated compounds which are in general defined, according to the invention, are generally put into fine powder form before being introduced into the fuel element. That introduction operation is generally effected as follows: either by incorporating the powder or the powdery components which make it possible to produce the stable oxygenated compound, in the fuel oxide powder prior to pressing in the form of pellets and sintering; that gives a sintered fuel pellet containing both the fuel oxide and the trapping agent and serving for production of the fuel element; or by coating the pellets of fuel oxides with the powder by any means known to the man skilled in the art (for example by applying a wash, by hot spraying . . . ); or by coating the internal wall of the sheath containing the fuel pellets, the sheath generally being in the form of a needle or rod or stick, using any means known to the man skilled in the art. The term stable oxygenated compounds is generally used to mean those which suffer little or no decomposition at high temperature, that is to say at temperatures which attain or exceed 1600.degree. C. (or better, higher than 2000.degree. C.) which may occur in a nuclear reactor core in a major accident situation, including the situation involving fusion or melt-down of the core. They are in particular more stable than those which may be formed between uranium oxide and caesium. They must also remain inert and non-volatile in the initial sintering of the uraniferous pellet, for example in a reducing atmosphere. They are generally selected from combinations of two at least of the following metallic oxides: Al.sub.2 O.sub.3, CeO.sub.2, Nb.sub.2 O.sub.5, SiO.sub.2, TiO.sub.2, UO.sub.2, V.sub.2 O.sub.3, Y.sub.2 O.sub.3, ZrO.sub.2 and preferably Al.sub.2 O.sub.3, Nb.sub.2 O.sub.5, SiO.sub.2, TiO.sub.2, UO.sub.2 and ZrO.sub.2, with one at least of the oxides of non-radioactive isotopes of the radioactive fission products, for example Cs.sub.2 O and/or SrO. The trapping stable oxygenated compound may involve the addition of another stable defined compound, for example an oxide, of another alkali metal (such as Rb.sub.2 O, Na.sub.2 O or K.sub.2 O) and/or alkaline-earth metal (or assimilated) such as BaO, CaO, SrO, or MgO, to promote the trapping action by a dilution effect. The invention is particularly suited to the trapping of Cs but it may also be extended to Sr or other radioactive fission products. The stable oxygenated compounds are therefore mixed compounds which are at least ternary but also quaternary compounds, that is to say containing at least three or four metallic elements, including for example non-radioactive Cs or stable isotopes of the fission products to be trapped. It is possible for example to use compounds of alumino-silicates (which for example have a high alumina content), alumino-titanates (in particular those based on compounds having a structure of the hollandite type), urano-zirconates, titano-niobates but preferably silico-zirconate, -niobate, -cerate, and non-radioactive Cs, which are obtained by any processes known to the man skilled in the art, including direct reaction of the oxides (of the metals and the non-radioactive isotopes) with each other. It is also possible to melt the stable oxygenated compounds and then solidify them in vitreous phase form before crushing them to give a very fine powder and then introducing them into the fuel element. In particular, as defined stable oxygenated compounds, it is an attractive proposition to use silico-aluminates of the pollucite or zeolite type whose melting point is higher than 1750.degree. C. Thus the stable trapping agent containing a non-radioactive isotope of the fission product may be pollucite of the approximate formula CsAlSi.sub.2 O.sub.6 or zeolite containing Cs, which is referred to as Cs-F. However, Cs having a relatively high neutron capture section, it is possible partially to substitute for or add to the Cs another stable alkali metal compound (preferably in the order Rb, Na or K); it is possible for example to add it in the form of oxide, as already stated, or a stable compound of an approximate formula such as RbAlSi.sub.2 O.sub.6 and of structure of type analcite, or zeolite containing Rb and referred to as Rb-F, such additives improving the trapping action by a dilution effect. As regards the trapping of Sr, the addition could be effected by means of alkaline earth oxide, preferably in the order Ca, Ba and Mg. Trapping of the radioactive fission product occurs in the course of normal operation of the reactor; it is not effected by chemical reaction to form the stable compound, the latter already being present in the fuel, but by isotopic exchange between the radioactive fission product when it appears and its non-radioactive isotope which is present in the stable compound. That gives an equilibrium which results in fixing of the major part of the free fission product which has occurred in the course of irradiation, without having to form the stable compound by chemical reaction in situ. The isotopic exchange reaction tends to render uniform the distribution of the different isotopes (radioactive or non-radioactive) of the same fission product in the fuel element between the different physico-chemical forms in which they occur. The exchange can be schematised in the following fashion: EQU A*+BA.revreaction.A+BA* A* representing a radioactive isotope of a fission product, PA1 A representing a non-radioactive isotope of said fission product, and PA1 B representing a complex metallic oxygenated compound which with A gives a stable oxygenated defined compound BA making it possible to trap A* in the form of the same stable oxygenated defined compound BA*. Distribution of the isotopes being balanced, the ratio of the amount BA initially introduced to the amount of A* which appears in the course of irradiation will determine the effectiveness of the trap; on the other hand the amount BA introduced will have to be compatible with the neutronic characteristics that the fuel element must have. Thus in the case of a 3.5% uranium enriched fuel which is irradiated at 33,000 MWj/t and cooled for a period of 3 years, the mass of caesium generated (Cs134+Cs135+Cs137) is about 0.3% of the total mass of uranium. To trap approximately 75% of that radioactive caesium, it will be necessary to use an amount of trapping agent BA such that the amount of A is about 0.5% (molar) of the total amount of uranium. However that amount of caesium to be introduced can be reduced by the addition to BA of another alkaline compound (B'A') for example based on Rb or Na, which will produce a supplementary dilution effect. To promote such exchanges, it may be appropriate to add to the defined compound a mixture of third-party oxides which would make it possible to stabilise the different alkaline compounds; the third-party oxides may be of the type of mixtures of metallic oxides already referred to above. To illustrate the isotopic exchange, tests were carried out with caesium 137 as a radioactive tracer and non-radioactive Cs 133 in chloride form which is easily available as a radioactivity calibration solution. Using an Inconel crucible, the procedure involved adding, in defined proportions, finely crushed synthetic pollucite to a solution containing a mixture of dissolved 137 CsCl+133CsCl. After slow evaporation of the solvent, a first gammagraphy was effected so as to measure the initial content of 137Cs. The crucible was then closed by means of a cover and the assembly was raised to 800.degree. C. for a period of 15 days in a neutral atmosphere. A second counting operation was then effected to be sure that no caesium 137 has been lost and then the whole was raised in an open alumina crucible in dry air to 1300.degree. C. to evaporate the caesium which is not fixed (radioactive and non-radioactive), the pollucite being stable at that temperature. A last counting operation was then effected, which showed that the pollucite has become radioactive and that its caesium has therefore been exchanged with radioactive caesium 137 which is thus trapped in a stable compound and which will not be given off again in the event of abnormal and accidental heating.