One of the methods considered for fuel assay in a nuclear safeguards program is analysis of reactivity response. For uranium-plutonium LMFBR fuels, such an assay is complicated by the similar response of the various fissile isotopes and the relatively large fast fission contribution from the fertile isotopes. The proposal is explored here to separate the responses, thereby promoting more accurate analysis, through design of an assay reactor which would be critical in two distinct modes having different spectra (hard and soft). The constraint is that the change in spectrum be obtained with little mechanical change in the system so as to avoid excessive reactivity renormalization. The solution examined here is a concept of a dilute fast spectrum fast reactor (zero-power) which is also critical when flooded with borated water. The response matrix is computed and the errors analyzed; problems in securing greater accuracy arise from the need to attain very low powers to measure spontaneous fission sources in the presence of fission product gammas, and the need for a better low-energy neutron filter than cadmium.