Thermal stability of moderating material used to enhance the feedback coefficients in SFR cores

Recently, the use of moderating materials in fuel assemblies for Sodium cooled fast reactors has been investigated and published in several papers (e. g. Annals of Nuclear Energy 38, 5, Annals of Nuclear Energy 38, 11 (2011)). Especially the fine distribution of the moderating material in a layer inside the fuel rod or inside the wire spacer has shown very promising results for the enhancement of the feedback coefficients. Additionally, this arrangement is very attractive since it causes only a very limited influence on the safety relevant fuel assembly structure and on the operational parameters. A detailed investigation on the use of moderating material for transmutation fuels has proven that a compensation of the negative influence on the feedback coefficients caused by the Minor Actinides is possible. The critics on the limited thermal stability of the suggested ZrH based moderating material has been a major issue in the discussion of all up to now published publications. In this work the problem will be solved with the help of t change of the hydrogen bearing metal compound. In a first step an overview on possible metal materials will be given and properties as well as manufacturing issues will be discussed. Additionally, an insight will be given into the relationship of the hydrogen content of the compound and the resulting thermal stability. Based on this relation the reasons for the choice of Yttrium are described. In a second step the transferability of the old results gained for ZrH will be proven by a detailed comparison of different important reactor physical parameters for cases using identical amounts of ZrH and YH.
Thermal stability of the moderating material up to more than 1300°C can be ensured by the use of Yttrium-mono-hydride as moderating material. The topic will be closed by a survey on the existing operation experience with YH in fast reactors and a comparison of the raw material costs for Zirconium and Yttrium.