Designer's view of source term changes

The quantity and mix of radioactive fission products released to the environment during a reactor accident is known as the source term. Early assessments of the source term (WASH-740, TID-14844) of light water reactor safety performed during the 1950's and 1960's assumed that a significant fraction of the radioactive fission products contained in a reactor core could be released to the environment during a severe accident. More recent experimental and theoretical work has shown that only a few percent of the most hazardous radionuclides such as cesium, iodine, and tellurium could be released to the environment during a severe accident and even much less during design basis events. Taking credit for reduced source terms for severe and design basis accidents can lead to numerous relaxations in plant design and operational requirements, such as emergency planning, allowable containment leakage and leakage testing, containment spray systems, and plant filtration systems. Another benefit of the current knowledge of source terms, when coupled with the low probability of such severe accidents, is to show that few of the many plant safety related backfits frequently proposed are justified on the basis of cost/benefit analysis.