Fine distributed moderating material to the enhance feedback effects in LBE cooled fast reactors

Problem of small feedback effects in fast reactors and some onsets to enhance the negative feedback effects have been discussed last year in an IAEA meeting in Vienna [[1]]. Small negative feedback effects are a common problem in all fast reactors. Especially in lead and lead bismuth eutectic (LBE) cooled reactors this problem is evident, due to the hard neutron spectrum. This hard neutron spectrum is caused by the very weak moderation effect in the heavy coolant even compared to sodium and appears due to the significantly higher atomic mass of lead.
A side view to the community of sodium cooled reactors (SFR) indicates a possible solution for the enhancement of one important negative feedback effect, the fuel temperature effect. The use of moderating material to improve the fuel temperature effect and the sodium void reactivity has already been discussed and investigated in detail for SFR with metallic fuel [[2]]. The concept of zirconium-hydride (ZrH) pins in reactors with metallic fuel has even been investigated before in several publications [[3]], [[4]], [[5]], [[6]]. A first test was already published for the use of ZrH in MOX fuelled, but steam cooled fast reactors [[7]]. In the last time, a new proposal has been analyzed. The negative feedback effects in a sodium cooled fast reactor are significantly enhanced by the introduction of fine distributed moderating material. The absolute value of the fuel temperature effect is significantly increased in combination with a decrease of the positive coolant effect by adding fine distributed moderating material, either in a layer inside the cladding or in the wire wrapper. The study has been focused in the first step on the choice of the ideal moderating material [[8]], [[9]] and in a second step on the optimization of the placing of the zirconium hydride to limit the influence on power distribution and burnup [[10]], [[11]].
The thermal stability of the hydrogen bearing compound is of paramount importance for the use of moderating material in a heavy liquid metal cooled fast reactor (HLMR) where significantly higher temperatures can be accepted during transients than in a SFR. Possible hydrogen bearing materials have already been discussed in a textbook on metal hydrides [[12]] and the effects in SFR has already been analyzed and published [13].
In this work it is demonstrated, that the concept of enhanced feedback coefficients is transferable to LBE cooled fast reactors. The demonstration is based on the fuel assembly design of the CDT project. The effect of the moderating material on the neutron spectrum, on the kinf, and on the fuel temperature feedback and the coolant feedback is shown, discussed and compared to SFRs.