Patent Number: 047568712
Section: description

SPECIFIC DESCRIPTION As can be seen from the drawing, after removal of spent nuclear fuel elements at 10 from a nuclear reactor core 11, the spent fuel is sprayed with a solution 22 or with an emulsion 13 or is immersed in either or is coated with a melt 14 containing gadolinium or some other substance with a high neutron cross section to form an absorbent coating which is dried at 15 and 16 or solidified by cooling as shown at 17. The coated spent fuel particles are then further coated with a water-repellent film 18 before they are subjected to storage at 19 for decay. After the desired storage term and before reprocessing at 20, the coating is removed at 21. Fresh nuclear fuel is shown at 22 can also be coated with a foil, this time containing the absorbing substance as shown at 23 and thus can be stored and handled at 24 without the danger of reaching criticality. Before the fresh fuel is introduced into the nuclear reactor, however, the coating is removed at 25. SPECIFIC EXAMPLES EXAMPLE 1 Spent metal sheathed nuclear fuel elements are removed from the nuclear reactor core into a tank containing cooling water in the form of a solution of gadolinium nitrate and gadolinium chloride although other water soluble gadolinium salts or water-soluble salts of other neutron-absorbing elements can also be used. The solution concentrations can be such that the neutron-absorbing substance is present in an amount between substantially 0.5% to the solubility limit of the water-soluble salt of the absorbing substance. Deposition of gadolinum on the metallic shell, effected galvanically by polarizing the metal shell negatively, i.e. as a cathode against an inert (e.g. stainless steel) anode and applying a low-voltage direct current, e.g. about 24 volts. Depending upon the activity of the nuclear fuel, coating was continued to build up thicknesses sufficient to reduce the neutron emission below any level which can sustain criticality. EXAMPLE 2 Spent nuclear fuel elements are coated by spraying or dipping in a single component or multicomponent lacquer, preferably an epoxy resin lacquer in which gadolinium oxide powder is dispersed in the lacquer. The lacquer is permitted to set or solidify. EXAMPLE 3 The fuel elements are coated by gadolinium acetyl acetonate by immersing them in a melt of this substance or in an aqueous solution thereof or by spraying them with this solution or pouring this solution over them. Upon drying of the coating, the fuel elements are found to be water repellent and incapable of assembling into a critical mass. EXAMPLE 4 Spent graphitic fuel elements are introduced into a melt of gadolinium acetyl acetonate and are stored in a solution of the melt at least until gases begin to be driven off. The gases appear to be air which is expelled by heat from the pores to promote penetration of the melt and the solution so that these graphite pores are penetrated and at least partially filled with flowable material. After drying, electron microscopy, autoradiography and neutron activation analysis in the nuclear reactor shows a penetration of the neutron-absorbing substance into the deepest levels of the graphite fuel elements. The surface of the fuel elements were then immersed in hot saturated aqueous gadolinium acetate solution and then cooled and dried. They are found to be filled with gadolinium acetate crystallites. When the graphitic fuel elements are heated to a temperature above about 750.degree. C. after this treatment, the gadolinium acetate is decomposed, leaving behind gadolinium oxide which is water insoluble and is immobile in the graphitic material even at high temperatures. Gadolinium oxide thus becomes an integrated component of the spent fuel element and resists loss therefrom even with major traumatic insults to the fuel elements during accidents of transport or storage by the effects of water and heat and even upon rupture of the fuel elements. EXAMPLE 5 Comparative tests with Example 4 were made with graphitic fuel elements treated with aqueous gadolinium acetate at room temperature and atmospheric pressure. The amount of the neutron-absorbing substance taken up by the fuel element increased with the residence time of the fuel element in the solution. EXAMPLE 6 Nuclear fuel elements are coated with a synthetic resin foil. The foil is a polyvinyl chloride and is applied in a melt which contains as neutron-absorbing substance europium oxide. The concentrations of this material in the foil range from 0.5% to 25% by weight. Foil wraps of polyester, (Mylar) containing europium oxide as well as gadolinium and calcium oxide also were used effectively. Before introduction of the fuel element into the reactor or reprocessing, the foil was removed as previously described.