On the use of fine distributed moderating material to enhance feedback coefficients in fast reactors

Recently, the use of moderating materials in fuel assemblies for Sodium cooled fast reactors has been investigated and published in several papers (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. The validity of the HELIOS results has been demonstrated in a comparison with MCNP and the transferability of the effect to full core calculations has been shown. The fine distribution of the moderating material is very attractive since it causes only a very limited influence on the safety relevant fuel assembly structure and on the operational parameters like power distribution and end of life burnup distribution. In extension to this published work the influence of the use of moderating materials on systems containing fuels for Minor Actinide transmutation (3% Am, 5% Am, and 2% Np – 2% Am) is examined on the basis of detailed lattice calculations based on 112 energy groups and an unstructured mesh geometry modeling of all fuel assembly details. The influence of the insertion of the Minor Actinides on the fuel temperature and the coolant coefficient is investigated for a reference case and the possibilities of enhancing the feedback coefficients by the insertion of ZrH moderating material are analyzed. The changes in the power and burnup distribution due to the use of the moderating material ZrH will be discussed. Further on, the transmutation potential is compared for the cases with and without moderating material. Neptunium and Americium is slightly more reduced during burnup of the fuel with moderating material only the Curium production rises slightly. A detailed analysis demonstrates that the increase of Cm breeding due to the use of moderating material is lower than the additional amount of destroyed Np and Am. Thus, the results demonstrate that even more Am is burnt with the investigated fuel assembly containing moderating material inside the wire wrapper.
Additionally, a first test of the effect of the use of fine distributed moderating material in a lead cooled fast reactor will be shown. The test is based on preliminary data of the fuel assembly in the GUINEVERE facility. The effect of the moderating material on the neutron spectrum, on the kinf, and on the fuel temperature feedback of the zero power facility is shown and discussed.
Over all, the use of fine distributed moderating material has the potential to open the stage for designable feedback coefficients in fast reactors without creating a major influence on the operational parameters and core coolability.