Method for measuring lanthanide content dissolved in uranium oxide

#The present invention relates to a method for measuring lanthanides content dissolved in uranium oxide, more particularly, a method for measuring the lanthanides content of a (U,La)O2 nuclear fuel pellet, wherein the lanthanides content of the nuclear fuel pellet is measured using the thermo gravimetric analysis which measures the weight variation caused by the oxidation and heat treatment of the nuclear fuel pellet.

#Nuclear reactors which generate nuclear fission energy use UO2 as a nuclear fuel material, and further, a mixture of UO2 and materials with strong neutron absorption may be used as well in order to control neutron. Lanthanides (hereinafter referred to as “La”) such as Gd(Gadolinium), Er(Erbium), Eu(Europium) and Sm(Samarium) may be used as materials having such strong neutron absorption. Recently, among these, Gd is the most commonly used. The nuclear fuels containing such lanthanides are named as burnable absorbing nuclear fuel.

#The method for preparing the burnable absorbing nuclear fuel is similar to the method for preparing the uranium oxide nuclear fuel. For example, as for (U,Gd)O2, UO2 powder is mixed with Gd2O3 powder and grinded, the grinded powder is then pre-molded at a pressure of about 1 ton/cm2 to form a slug, and the slug is crushed to prepare granules. The granule is mixed with a lubricant and compression-molded to prepare a cylinder type green pellet. The green pellet is heated in a hydrogen atmosphere to a temperature of 1700-1780° C. and held for 2-6 hours. Such heating and holding are commonly referred as sintering. The nuclear fuel pellet comprising (U,Gd)O2, wherein Gd2O3 is dissolved in UO2, is prepared by the sintering process.

#FIG. 1 is a graph showing the weight variation while the initial sample of the present invention is being heat treated;

#FIG. 2 is a graph comparing the calculated value and the measured value of the relation between the (W1-W2)/W1 (%) value of the present invention and the initial mole fraction (%) of Gd in MO2;

#FIG. 3 is a graph comparing the calculated value and the measured value of the relation between the (W1-W2)/W1 (%) value of the present invention and the initial mole fraction (%) of Gd2O3; and

#Uranium oxide may exist as an oxide in the form of UO2, U4O9 or U3O8 depending on the degree of oxidation of uranium, wherein UO2 and U4O9 have a cubic lattice structure, and U3O8 has a orthorhombic lattice structure. The UO2 pellet is oxidized to U3O8 at a temperature above 300° C. in a oxidizing atmosphere. Since the lattice volume of U3O8 is about 30% larger than that of UO2, a lot of stress is generated while being oxidized, and accordingly, UO2 pellet is spontaneously pulverized and then becomes U3O8 powder.

#(U,La)O2 nuclear fuel is a single phase having a structure wherein a part of the uranium ion is replaced with the La ion in the UO2 lattice structure. Similar to the UO2 pellet, the (U,La)O2 pellet is oxidized to become (U,La)3O8 powder at a certain temperature above 300° C. in an oxidizing atmosphere. If the (U,La)3O8 powder is heat treated at a temperature above 950° C., La ion and U ion are diffused and (U,La)3O8 single phase is separated to two phase, U3O8 and (U,La)O9/4. Whereas pure U3O8 does not display such phase separation even when it is heat treated. This is due to the fact that La ion is so unstable in (U,La)3O8 lattice structure that La ion diffuse out of (U,La)3O8 lattice and forms a new stable phase of (U,La)O9/4 at high temperatures.

#In order to attain the object, the present invention relates to a method for measuring the La content dissolved in a (U,La)O2 nuclear fuel pellet. The said method consists of two combined procedures; One is to measure the weight variation caused by the oxidation of the (U,La)O2 nuclear fuel pellet and the weight variation caused by the heat treatment of the oxidized pellet, and the other is to determine the content of La using the relations between the weight variations. More particularly, (U,La)3O8 powder is obtained when (U,La)O2 nuclear fuel pellet is oxidized, and (U,La)O9/4 phase and U3O8 phase are obtained when such (U,La)3O8 powder is additionally heat treated. That is, the La content can be measured by measuring the weight variation caused by the phase change when (U,La)O2 nuclear fuel pellet is oxidized and heat treated.