Release of Radioactive Isotopes from Nuclear Power Plants

Release of Radioactive Isotopes from Nuclear Power Plants 

As a result of calculations based on a knowledge of the factors above, it is possible to set limits on the discharges of specific radionuclides from the nuclear plant. These limits are referred to as derived limits. They may specify simply the maximum allowable rate of release or the maximum permissible concentrations in air, water, or particular members of the relevant food chains. 

The principal source of radioactivity in a nuclear reactor arises, of course, from the fission products in the fuel elements. The amount of activity present will depend on the power level of the reactor and the time for which it has been operating. A 1000-MWe PWR at the end of the core life will 

Despite the large fission product and actinide inventory of the fuel, the leakage rate from the fuel pins may be kept to a very small value by adequate design and careful quality control in manufacture. The coolant circuits of reactors which are refueled on-load are particularly easy to maintain at a low level of activity, on account of the ease with which defective elements can be removed. In addition to any small escape of fission products from cracks or pinholes in the cladding, some activity will generally arise from slight contamination of the outsides of the fuel pins with fuel while they are being loaded. 

In addition to the activity stored in the fuel elements, further radioactive isotopes may be formed by activation of the moderator, coolant, and structural materials by neutron capture. Particulate material arising from corrosion or erosion of the structure by the coolant will be circulated through the core and thus become activated. This can lead to accumulation in cooler parts of the circuit, such as the heat exchangers, or, in a direct cycle plant like the BWR, in the turbine itself. 

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