Resolution of Generic Safety Issues: Item A-17: Systems Interactions in Nuclear Power Plants (Rev. 2) ( NUREG-0933, Main Report with Supplements 1–34 )
Nuclear power plants contain many structures, systems, and components (SSCs), some of which are safety-related. Certain SSCs are designed to interact to perform their intended functions. These "systems interactions" are usually well recognized and, therefore, are accounted for in the evaluation of plant safety by designers and in plant safety assessments. However, prior to the time this issue was identified in 1978, a number of significant plant-specific events had occurred that involved unintended or unrecognized dependencies among the SSCs. Some of these events involved subtle dependencies between safety-related SCCs and other SCCs, while other events involved subtle dependencies between redundant safety-related SSCs that were believed to be independent. This issue was originally identified in NUREG-03712 and was later declared a USI in NUREG-0510.186
The purpose of this issue was to investigate the potential that unrecognized, subtle dependencies among SSCs have remained hidden and that they could lead to safety-significant events. The term used to describe these unrecognized, subtle dependencies is adverse systems interactions (ASIs). In resolving this issue, the staff did not recommend that licensees conduct further broad searches specifically to identify all ASIs because such searches had not proved to be cost-effective in the past, and there was no guarantee after such studies that all ASIs had been uncovered. Rather, in its study, the staff concluded that certain more specific actions, together with other ongoing activities, could reduce the risk from ASIs.
The staff concluded from its investigations that the following actions should be taken:
(1) Issuance of a generic letter that included: (a) the bases for
resolution of USI A-17; and (b) a summary of information relevant
to existing operating experience reviews.
(2) Recognition that the IPE Program already included the evaluation of internal flooding and the insights from USI A-17 were to be referred to in the IPE guidance documents. If licensee action regarding flooding and water intrusion was implemented as proposed, there would be no further action on Issue 77 which was integrated into the resolution of USI A-17.
(3) Recognition that the USI A-46 implementation was expected to address seismically-induced systems interactions to verify that components and systems needed to safely shut down a plant were protected, given a loss of offsite power. (New plants, not covered by USI A-46, were reviewed to existing requirements that addressed seismically-induced systems interactions.)
(4) Communication of information regarding ASIs for staff review of PRAs and for staff evaluation of electric power supplies as part of Issue 128.
(5) Identification and definition of concerns related to USI A-17 and other programs that had not been specifically addressed in this or other generic issues. The staff established the Multiple System Responses Program (MSRP),1237 the objective of which was to define the concerns with sufficient specificity to permit them to be evaluated as potential GSIs.
(6) Development of an SRP for future plants that would include guidance regarding protection from internal flooding and water intrusion events.
The staff's technical findings were published in NUREG-11741232 and the regulatory analysis associated with the resolution of this issue was published in NUREG-1229.1233 The Commission was informed of the staff's resolution in SECY-89-2301234 and Generic Letter 89-181235 was later issued to licensees. Thus, this issue was RESOLVED with no new or revised requirements for licensees.1236