Analysis of LWR Safety

As the majority of power reactors are of the BWR or PWR design, this is the type on which the most extensive safety studies have been made. A 1957 study, Theoretical Possibilities and Consequences of Major Accidents in Large Nuclear Power Plants WASH-740), was intended to provide an estimate of 

In the event of the occurrence of a major accident, resulting in severe damage to the structure of a full-scale power reactor, the principal hazard would arise from the dispersion of the very large quantities of radioactive materials which accumulate over the life of the reactor core. It should be noted that the magnitude of any explosive energy release would be at worst only a very small fraction of that involved in the deliberate explosion of a nuclear bomb, which has to be designed very specifically to achieve a rapid conversion of mass to energy. 

The principal sources of radioactivity in the reactor are the fission products, supplemented by the transuranic elements, or actinides, formed as a result of successive captures of neutrons in the uranium fuel. The fresh fuel loaded into the core is only mildly radioactive (about 300 Ci for the initial core of a typical BWR), but the activity increases steadily over the core life to a value of the order of 1.7 x 10 Ci just prior to refueling. 

The principal components of the stored activity for an 1100-MWe PWR are given in Table 12.7, which is taken from the APS study. The top line applies to the reactor immediately after shutdown following a long period of operation at full power, while the remainder of the table illustrates the rate of decay of activity, as a function of time after shutdown. The activities of the iodine and bromine isotopes, and the noble gases, whose behavior is particularly important in the event of a major dispersal event, are shown separately, and the activity induced by neutron irradiation of the reactor structure and coolant is also listed. The absorption of jS particles and y rays in the fuel and cladding leads to a considerable generation of heat in the core the last column of the table gives the total thermal power produced in this way as a function of time after shutdown. 

The release of a significant fraction of the stored radioactivity requires the breaching of multiple barriers. In the first place, most of the fission products and actinides are embedded in the fuel pellet matrix, and large-scale escape would only occur if the fuel were to melt. The fuel is enclosed in zircaloy cladding and the core is enclosed within the pressure vessel which forms part of a sealed primary circuit. Finally, the whole of the primary arcuit is enclosed within a containment structure which is specifically designed to minimize release of radioactivity to the environment. 

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