[en] A new concept to improve the Doppler coefficient of metallic fuel liquid metal fast breeder reactor is proposed. This is accomplished by adding a moderating material to a metallic fuel assembly, so that the neutron spectrum becomes softer and the negative temperature coefficient are improved. The effects to the typical core parameters, such as Doppler coefficient, the sodium void reactivity and the reactor breeding ratio are investigated. The results show this concept improves temperature coefficients similar to those of mixed oxide core of the same size without much sacrificing the breeding ratio. (authors). 7 refs., 3 figs., 2 tabs