Carbon potential in U-Pu-C-O-N system: measurements and calculations

Abstract

The Fast Breeder Test Reactor (FBTR) at the Indira Gandhi Centre for Atomic Research, Kalpakkam, presently employs mixed carbide with a Pu/(U + Pu) ratio of 0.70 as fuel. It is envisaged that the second core of the reactor may use mixed carbide with a Pu/(U + Pu) ratio of 0.55. The fuel pellets are enclosed in AISI 316 stainless steel cladding tubes. Carbides of thorium are presently being considered as alternative fertile materials in breeder reactor systems. Transport of carbon to and from the cladding may occur at the interface between the fuel and the cladding depending upon the prevailing carbonpotential differences between them. In order to maintain favourable mechanical properties, stainless steel must contain an optimum amount of carbon. Hence, any change in carbon content arising as a result of carbon transport would have a detrimental effect on the mechanical integrity of the steel. Carbon transport can also influence the precipitation of chromium carbides, which play an important role in controlling the chemical and physical properties of stainless steel. Therefore, to understand and predict the carbontransport phenomenon, it is essential to have accurate knowledge of the carbon potentials in the fuel and the cladding. As uranium and plutonium undergo fission, a host of fission products are formed. The carbides of these fission products are likely to play a major role in altering the carbon potential of the fuel. Carbonpotential data for various fissionproduct-carbon systems are required to assess fuel-clad compatibility during reactor operation. Among the fissionproduct elements, cerium is an important constituent. Carbonpotential data for the Ce-C system are not available in the literature. In the present study, the carbon potentials of various carbide systems were measured using the methane-hydrogen gas equilibration technique.

Validation of Measurement Technique The free energy of formation of tungsten carbide was measured to validate the methane-hydrogen gasequilibration technique. The measurements were carried out over the temperature range 973-1173 K. The measured freeenergy data agreed well with the evaluated data reported in the literature.

Carbon Potentials of (U,Pu) Mixed Carbides The carbon potentials of (U,Pu) mixed carbides with Pu/(U + Pu) ratios of 0.55 and 0.70 were measured over the temperature range 973-1173 K. The compatibility of these carbides with the stainlesssteel cladding materials was analysed based on the measured carbonpotential values. The carbon potentials of mixed carbides of other compositions were calculated theoretically in order to assess their compatibility with stainless steel. The calculations were carried out assuming idealsolution behaviour for all the solid solutions present in the U-Pu-C-N-O system.

Carbon Potentials of Thorium Carbides Thorium monocarbide and thorium dicarbide samples were prepared by the carbothermicreduction process followed by arc melting and annealing. The carbon potential of this system was measured over the temperature range 973-1173 K.

Carbon Potentials of Additional Carbide Systems The carbon potentials of the following systems were also measured over the temperature range 973-1173 K: a) CrC / CrC b) CrC / CrC c) CeC / CeC The measured carbon potentials of the CrC / CrC and CrC / CrC systems were compared with values reported in the literature. Data on the carbon potential of the CeC / CeC system were generated for the first time.