The heat generation process by fission of nucleus of a neutron by absorption of U-235 is described in detail. The methods to control the fissioning rate and correspondingly the heat generation rate is a reactor by adjustment of the neutron absorption rate are reviewed. In a typical nuclear plant no person is exposed during normal conditions of operation to radiation dosage of more than 0.3 rem /year. To avoid the fission products' escaping into atmosphere necessary measures have to be taken at the time of designing a reactor. The problems of personnel safety encountered in this new technology are dealt with.