As nuclear power generation has become established since the 1950s, the size of reactor units has grown from 60 MWe to more than 1600 MWe, with corresponding economies of scale in operation. At the same time there have been many hundreds of smaller power reactors built for naval use (up to 190 MW thermal) and as neutron sourcesa, yielding enormous expertise in the engineering of small power units and accumulating over 12,000 reactor years of experience. There is revival of interest in small and simpler units for generating electricity from nuclear power, and for process heat.