Patent Number: 052727408
Section: summary

TECHNICAL FIELD The present invention concerns an agent for trapping the radioactivity of fission products which appear in a nuclear reactor fuel element in the course of combustion, that agent comprising a stable oxygenated compound of the fission products. Such a trapping agent is in particular adapted to long-life radioactive fission products such as Cs, Sr . . . which are generated in the course of irradiation in a nuclear reactor. STATE OF THE ART Reactors of the PWR, BWR or fast neutron type which use fuel pellets based on sintered oxide UO.sub.2 or mixed oxides generate `in situ` fission products of which some are not specifically gaseous in the core of the reactor. In normal operation those solid fission products remain generally in place in the pellets although in regard to some thereof migration phenomena may occur, which are due to the temperature differences between the core and the periphery of a pellet, towards the outside of the pellet. Even in that case however the major part thereof remains confined in the fuel pellets. The fission products appear in the pellets in elementary form and may form compounds, which are relatively stable at the temperature of the core of the reactor (300.degree. to 900.degree. C.), with the nuclear fuel oxides which form the pellets. However, in the case of a major accident which causes an excessive rise in temperature in the core of the reactor, followed by damage to or even fusion of the core, such compounds are insufficiently stable and the fission products are then liberated, with serious risk of dissemination into and contamination of the environment; that risk is more especially severe as such fission products have long lives (some tens of years). That is the case for example with Cs 137 and Sr 90. An arrangement which makes it possible to trap caesium in normal operation in a fast neutron reactor has been proposed in patent FR 2438319 (Westinghouse); it comprises interposing between the fissile and fertile fuel elements Cs captors which are formed by pellets of low density and particular shape and which consist of TiO.sub.2 or Nb.sub.2 O.sub.5. Those oxides fix Cs at the usual temperature in the core of the reactor and the shape of the pellets makes it possible to avoid any stress, due to swelling which occurs in the course of normal operation of the reactor, on the sheathing of the fuel element. In that arrangement, it appears that the caesium has to reach the pellets of captors in order to be trapped and that only Cs which has migrated to a sufficient degree is actually trapped. In the event of a major accident such an arrangement would be found to be insufficiently effective to prevent all dissemination of the Cs; in fact, all the free Cs which has not yet been trapped but which is present in the fuel pellets could escape from the sheathing and contaminate the environment, as the trapping speed is not fast enough. In addition compounds such as CsNbO.sub.3 or Cs.sub.2 Ti.sub.2 O.sub.5 formed in the trapping operation seem to be of insufficient stability at very high temperature (for example above 1600.degree. C.). That is why the applicants sought a way of trapping the dangerous fission products, as far as possible as soon as they appear in the course of irradiation, in particular in the mass of the fuel pellets. The applicants also sought a trap which is sufficiently stable and effective so that the fission products are not removed again at elevated temperatures (which can attain or exceed 1600.degree. C. or in the event of core fusion or melt-down) and which is in any case more stable than the compounds which are formed in situ between uranium oxide and the fission products (for example Cs). The applicants also sought to provide a trap which does not give rise to fusion or premature incipient fusion of the fuel pellets in the event of a major accident, in other words a trap which does not perform a fusioning function in relation to the pellets which will thus retain adequate refractoriness.