MSRs may operate with epithermal or fast neutron spectrums, and with a variety of fuels.
Much of the interest today in reviving the MSR concept relates to using thorium (to breed fissile uranium-233),
where an initial source of fissile material such as plutonium-239 needs to be provided.
There are a number of different MSR design concepts, and a number of interesting challenges in the commercialisation of many, especially with thorium.