Publisher Summary This chapter the current nuclear power technologies–the LWR and HWR, and discussed other reactor concepts proposed as more efficient or proliferation resistant follow-on to the current generation of converter reactors. A conceptual way of viewing all nuclear reactor systems is to consider them essentially as electricity-producing black boxes. The basic input to such a device is an external stock of natural uranium. In a light water reactor it is necessary to use slightly enriched uranium in order to maintain the fission chain reaction. The nuclear chain reaction results in three major products or outputs. About 75% of the initial fissile uranium-235 input is destroyed by the fission process, thus releasing large amounts of thermal energy, which is recovered in the turbine generators to produce electricity. The fission products, which are highly radioactive and decay eventually with a half-life of about thirty years, are the major waste products of the reactor operation and need to be disposed of. Two publicly perceived problem areas may hinder the growth of nuclear power production–the issues of ultimate waste management and of nuclear weapons proliferation. Eventually, a choice must be made between providing adequate power supply for the current and future generations, and living with the residual risk of nuclear weapons proliferation, which is essentially independent of the existence of nuclear power.