Patent Number: 051494930
Section: summary

FIELD OF THE INVENTION The invention concerns an installation and method to regenerate cold traps loaded with the hydride (possibly loaded with the trituride and oxide) of a liquid metal, such as sodium, used in fast neutron reactors, especially for the secondary circuits. BACKGROUND OF THE INVENTION It is well-known that the liquid is polluted during the functioning of a power plant by the hydrogen which diffuses through the walls of the stream generators, as well as by the oxygen and hydrogen which are able, in particular, to originate from sodium/water reactions resulting from a leak in the steam generator. It is essential to remove these impurities in the sodium so as to limit corrosion of the structures by oxygen, the embrittlement of steel caused by hydrogen, but also to facilitate detection of the sodium/water reaction by maintaining a low hydrogen concentration, and so as to limit the risks of sealing with the hydride and oxide crystals which might form in the cold traps. To avoid this happening, a large number of models of cold traps have been considered, these traps generally comprising a tank where the liquid sodium is cooled below the crystallization temperature of the hydride and the oxide. These crystals become secured to the retention structures inside the trap, such structures being for example filters lined with metal wool. The progressive fouling of the traps may mean that they need to be periodically regenerated by purging from them any solid impurities. The known method, which exists in several variants, has as one of its main characteristics a preliminary dumping of the liquid sodium which fills the cold trap and a heating of the emptied seat so as to thermally decompose the impurities. At 420.degree. C., the sodium hydride contained in the trap is decomposed according to the reaction (1): EQU NaH ---- Na+1/2.H.sub.2 ( 1) The sodium freed by the reaction is in a liquid state and the hydrogen reacts with the sodium oxide according to the reaction (2): EQU Na.sub.2 O+1/2.H.sub.2 ---- Na+NaOH (2) Caustic soda is thus formed which is then able to solidify (below 320.degree. C.) when heating ceases. This method has several drawbacks. First of all, the reaction (2) is slow, it requires a prolonged treatment and, in practice, is impossible to carry out with the trap mounted on the reactor; therefore it would be proper to change the cold trap, if possible at the time of a programmed stoppage of the reactor, and to treat much later the trap full of impurities outside the reactor. Other difficulties arise from the presence of tritium hydrogen which is possible to reject into the atmosphere by only observing very strict rejecting norms, namely at extremely low flowrates; moreover, one part combines so as to form the soda whereas it may be more advantageous to isolate it so as to collect it. The evolution of gaseous hydrogen provokes an excess pressure (which moreover often needs to be maintained by a current of hydrogen in order to complete the chemical decomposition of the sodium oxide), which thus generates the need to continously control the pressure. With regard to the high temperature, the presence of soda results in a rapid generalized corrosion of the stainless steel wall of the trap and its lining. The gaseous hydrogen may also embrittle the steel. Finally, all the cold traps do not lend themselves to an easy emptying of the liquid sodium and the soda. It is possible to avoid rejecting the tritium into the atmosphere by securing it to certain solid bodies. Unfortunately, the other drawbacks still remain. However, the invention is seeking to regenerate the effectively and rapidly cold traps by totally eliminating impurities without having to unload the cold trap from the reactor. One essential object of the invention consists of separating the hydrogenated and oxygenic compounds so as to be able if need be to easily collect the tritium. The formation of soda is avoided, the corrolary of this being the absence of corrosion and which constitutes an essential difference from the European patent 0 012 074 of the same applicant, this patent describing a method to wash the trap with soda. This method facilitates dissolving the impurities in the soda, but the problems of corrosion have led this method to be abandoned. SUMMARY OF THE INVENTION The characteristic installation of the invention includes a circuit filled with liquid metal on which at least one trap to be regenerated is disposed, the circuit being provided with means to establish a circulation of the liquid metal in the circuit and through the trap, means to adjust the temperature of the liquid metal to values sufficient to dissolve the oxide and hydride, a device to draw off the dissolved tritium and hydrogen and a dump equipped with a device for retention of the oxygen. The device to draw off the hydrogen and tritium is envisaged in the form of a permeation membrane with one face communicating with the metal liquid circuit and the other with the partial vacuum pumping circuit, which may be provided with a solid fixing the hydrogen. The transfer of the hydrogen through the membrane is ensured by a partial pressure difference controlled by the operating conditions of the installation. The device for retention of the oxygen is preferably constituted by a cold trap where the oxide is crystallized again. The constitutive method of the invention consists of making the liquid metal circulate through the trap at a temperature sufficient so as to progressively dissolve the hydride and the oxide, to draw off the dissolved hydrogen so as to maintain the dissolved hydrogen concentration below saturation, and to have the liquid metal pass into another cold trap cooled to a temperature lower than the oxide crystallization temperature and greater than the hydride crystallization temperature. The liquid metal may advantageously be reheated before the hydrogen is drawn off so as to improve permeation through the membrane and then be cooled to a temperature greater than the hydride crystallization temperature.