Resolution of Generic Safety Issues: Item B-12: Containment Cooling Requirements (Non-LOCA) (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–34 )
The rationale for normal and postaccident containment cooling will be reviewed to determine the adequacy of the design requirements imposed on the containment ventilation systems. By reviewing typical designs, the staff will develop a basic understanding of the consequences of a loss of normal containment cooling including the impact, if any, on the operability of safety systems and control systems. Specifically, this NUREG-04713 task will establish whether or not: (1) the normal ventilation system is essential to achieve a safe cold shutdown; (2) a failure in the system could cause an accident; and (3) the system is required to mitigate accidents.
A review991 of this issue in July 1986 showed the following:
(1) The normal ventilation system is not essential to achieve a safe cold shutdown for accidents within design bases. If the ventilation is interrupted, the drywell temperature would increase. However, this temperature increase would not affect the operability of any equipment needed to achieve cold shutdown. For accidents, the coolers are isolated upon receipt of an accident signal and two other systems provide temperature control (containment sprays and depressurization). Furthermore, the operators would be expected to restore cooler operation or the coolers would be automatically sequenced on from available emergency power supplies and buses. Therefore, within the context of beyond design basis accidents, the marginal benefits from improved cooler performance do not appear to warrant a change.
(2) A failure of the system should not cause an accident. For the extreme case of the complete loss of drywell cooling, the resulting drywell temperature increase could also cause a pressure increase which would result in a reactor scram (i.e., P>2.0 psi set point). The resulting scram will challenge ECCS equipment, but is unlikely to cause a more significant event. A review of all BWR LERs uncovered only a few cases of various drywell cooling failures but, in no case, did a failure cause a reactor scram.
(3) The drywell cooling system for all BWRs is not considered in any design basis LOCA analysis. As a result, the system is not required to prevent or mitigate an accident within design bases.
After studying the LERs involving BWR Fan Cooler units in operating reactors, the staff concluded991 that further study of this issue was not warranted since the potential impact of a loss of normal containment cooling on the operability of safety and control systems is minimal. Thus, this issue was RESOLVED and no new requirements were established.