In 1950 the then Reactor Safeguards Committee under the old AEC pre­pared a report (AEC, 1950) proposing the creation of a so-called exclusion radius around a nuclear reactor where residences were not permitted. It was also proposed that such a radius, measured in miles, be 1/100 of the square root of the reactor power measured in thermal kilowatts, i. e.

R (miles) = ^W P (kW); the proposal was based on the assumption that

the reactor will undergo a reactivity excursion, melting the core and ruptur­ing the coolant system with the fission products escaping freely to the environment. The application to such a rule of thumb to nuclear power plants designed in the 1960s and 1970s led to an unacceptably large exclu­sion radius.

After realizing that the proposed rule of thumb was not appropriate to the many medium-sized demonstration reactors under consideration and after accepting the principle that reactor containment was better than isola­tion, in 1959 the AEC published in the Federal Register new proposed site criteria. The new site criteria maintained the concept of exclusion radius, now called exclusion distance, which depended not only on thermal power but also on the design features, mainly the inclusion of a containment system, and the site characteristics. For the large power reactors then con­sidered the accepted exclusion radius varies from xh to % mile. It was also determined that beyond the exclusion radius population density should be small and there should be no large cities within 10 to 20 miles. In this pre­liminary document the main site parameters based on seismology, meteor­ology, geology and hydrology were also defined and considered.

After several intermediate steps, such rules were perfected and consoli­dated in a new document published in 1962 under the title 10 CFR Part 100 Reactor Site Criteria, which has been maintained up to now. The new docu­ment consolidated the concept of exclusion area and created two additional concepts: a low population zone surrounding the exclusion area containing residents ‘the total number and density of which are such that there is a reasonable probability that appropriate protection measures could be taken in their behalf in the event of a serious accident’, and a population centre distance determining the minimum acceptable distance to ‘the nearest boundary of a densely populated center containing more than about 25,000 residents’.

Methods to determine the corresponding radius and distances were also published by reference to a Technical Information Document (TID-14844) developed by DiNunno and co-workers (DiNunno et al., 1962) based on a hypothesis regarding the consequences of the maximum credible accident, a concept that was introduced in 1959 by Dr Clifford Beck, a notorious regulator within the AEC, and on limiting radiation doses to the population (Beck, 1959). These ideas and concepts are maintained today and have had a deep influence in other countries; moreover they constitute one of the pillars for the safe design of most of the currently operating reactors. A full account of these historical developments has been published by Okrent (1981).