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

The use of fine distributed moderating material to enhance the feedback effects in sodium cooled fast reactors and to reduce the sodium void effect is described. An overview on possible materials is provided and the relationship between hydrogen content and thermal stability is described. A solution for the problem of the limited thermal stability of hydrogen bearing moderating material ZrH_1.6 is developed by the use of Yttrium-mono-hydride and the similarity to the effects reached by ZrH is demonstrated by comparison calculations. The topic will be closed by an overview on material properties, manufacturing issues, experience in fast reactors and a comparison of raw material costs. The simulation results for the influence of the moderating material on the fuel assembly geometry, the neutron spectrum, the feedback effects, the power distribution, the burnup distribution, and the transmutation performance is given. (author)