The PBMR is one of several Generation IV reactor designs that are being looked at. I am involved in a couple of others, the Westinghouse IRIS design, which is a light water reactor, and the General Atomics MHTGR, which is a bit more exotic. There are some really exotic ones proposed, like the vapor core high temperature reactor, which features first-stage power extraction based on a magnetohydrodynamic generator. Talk about efficiency, that one has it in spades...