The changes in the fuel thermal conductivity were quantified in simulated high burnup fuel (SIMFUEL) in which additives are introduced in UO2. Some additives are soluble in the lattice and other precipitate as second phases. The advantage of this approach is that the samples are easy to prepare and handle. Single effects can be studied, but the main disadvantage is that only part of the burnup effects can be simulated. This approach was used, for instance, by Ishimoto et al?5 for UO2 and (U, Gd)O2 and by Kang et al40 for UO2. Hartlib et al.41 simulated the effects of fission products in MOX fuel and evaluated the decrease in thermal conductivity, and this work was used by Phi — lipponneau to obtain a thermal conductivity correla­tion for the irradiated fast reactor MOX.42

The most complete work on simulated burnup effects was published by Lucuta et al.,39 who proposed a formula in a form of factors contributing to the degradation of the thermal conductivity 10 of unirra­diated UO2 (eqn [5]).

I95 = ?i(bu)?2 {p)?3(x)?4 (r)1o [5]

In this formula, 11(bu) is the burnup dependence, containing the effect of dissolved fission products as derived from measurements on SIMFUEL, and the effect of precipitates, assessed theoretically with composite materials formulae, including a correction reflecting that precipitation takes place at high tem­perature. 12(p) accounts for the porosity and bubbles contribution, based on the Maxwell-Eucken formula for composite materials. 13(x) refers to the effect of nonstoichiometry and was assessed from SIMFUEL measurements results, but this factor is generally not used as the fuel is assumed to be stoichiometric. 14(r) describes the radiation damage effect presuming that recovery takes place progressively in the range 600-1200 K. This model was used in some fuel perfor­mance codes43 but was replaced44 because its predic­tions were found to be too high.