dispersion of fission products are assumed not to function, and all the other important safety factors may be neglected. This simplistic assumption cannot be considered justifiable in determining the minimum Distance necessary and is not what the regulatory guide requires.

Based on this viewpoint, with the considerations on the structure, characteristics and various safety measures of present light water reactor design, a case for radioactive material release within the PCV and a case for the release outside of the PCV are considered as possible accident modes where the dispersion of radioactive materials occurs, and the safety analysis results of the Accident are referred to for each of these cases. And from the viewpoint of the Distance evaluation between the reactor and the public, the Major Accident is defined in which the maximum amount of radioactive materials released is technically and reasonably assumed. Furthermore, an accident classified as a Major Accident, in which the amount of radioactive materials released exceeds that of the Major Accident when postulated hypothetically from an engineering viewpoint, is defined as the Hypothetical Accident

2. Procedure for Radiation Effect Evaluation

As discussed above, the evaluation of postulated accidents in connection with the site assessment comes back to the effects of radiation. The most important factor in the evaluation is the estimation of the source of radioactivity release (“Source Term”), in other words, estimation of how much is released from the system in which radioactive materials are normally contained.

71ie procedure for radiation safety evaluation in a postulated accident consists of only the evaluation, with enough margins, of the process in which the estimated amount of radioactive materials passes through all possible routes within the reactor system and reaches the public, resulting in radiation exposure. The source of radioactivity is defined, at present, in the “Regulatory Guide for Reviewing Safety Assessment of Light Water Nuclear Power Reactor Facilities” ^Wef4) as shown in Table 9.3.1 Details of the evaluation method are presented in Sections 7.4 and 8.4.

In discussing the hazard analysis in general for a postulated accident, it is important to focus on the assumptions made in the analysis and to pay attention to the conservatism of such assumptions. In relation to these considerations, two notable examples are discussed next

They are the Three Mile Island (TMD and the Chernobyl accidents. In the TMI accident, 70% of the accumulated noble gases and 30% of the iodine accumulated in the core were released into the PCV and the amount released to the atmosphere was estimated to be 5% and 10-5% respectively. The effect of radiation to the public was, at most, less than ImSv. On the other hand, at Chernobyl, because there was no reactor containment system, 100% of the noble gases and 20% of the iodine were estimated to be released to the atmosphereFrom these two accidents it was evident that the PCV has a significant role and the amount of the Source Term postulated for the present site assessments has sufficient conservatism in its assumptions and is reasonable in terms of prevention of radioactive material release to the environment. Because of the unexpectedly small amount of iodine released to the environment in the TMI accident, studies on the behavior of iodine during accident situations were carried out The study results indicated that iodine in the environment of the PCV that contains water and steam readily forms salts in combination with metals like cesium and is eliminated in much larger amounts than expected. When the behaviors of radioactive materials released from the damaged core are understood much more realistically, they are expected to be reflected in emergency countermeasures as well as in future siting

Table 9.3.1 Radioactive materials released to the PCV during a major accident and a hypothetical accident (source term)

In the event of loss of coolant accident (LOCA)

Major Accident: Of the amount accumulated in the core,

Noble Gas 2%, Iodine 1%

Hypothetical Accident: Of the amount accumulated in the core

Noble Gas 100%, Iodine 50%

For postulated accidents other than LOCA the main steam line break for a BWR and the steam generator tube rupture for a PWR, sources for each case are considered.

NSRA, Japan

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