۱- Introduction

۲- WL/SL properties

۳- Background: CDFs for link failure time, link property value at link failure, and time at which LOAS occurs

۴- CDFs for failure time margins

۵- CDFs for system property values at LOAS

۶- CDFs for margins based on WL and SL property values

۷- CDFs for margins involving only SL property values

۸- Summary discussion

References

Abstract

Representations for margins associated with loss of assured safety (LOAS) for weak link (WL)/strong link (SL) systems involving multiple time-dependent failure modes are developed. The following topics are described: (i) defining properties for WLs and SLs, (ii) background on cumulative distribution functions (CDFs) for link failure time, link property value at link failure, and time at which LOAS occurs, (iii) CDFs for failure time margins defined by (time at which SL system fails) − (time at which WL system fails), (iv) CDFs for SL system property values at LOAS, (v) CDFs for WL/SL property value margins defined by (property value at which SL system fails) − (property value at which WL system fails), and (vi) CDFs for SL property value margins defined by (property value of failing SL at time of SL system failure) − (property value of this SL at time of WL system failure). Included in this presentation is a demonstration of a verification strategy based on defining and approximating the indicated margin results with (i) procedures based on formal integral representations and associated quadrature approximations and (ii) procedures based on algorithms for sampling-based approximations.

Introduction

Representations for margins associated with loss of assured safety (LOAS) for weak link (WL)/strong link (SL) systems [1-6] involving multiple time-dependent failure modes are developed. As described in Ref. [7], the descriptor “loss of assured safety” for the performance under accident conditions (e.g., a fire) of a high consequence system with multiple WLs and SLs is used for the situation in which (i) a necessary set of SL failures places the system in a potentially operational condition before (ii) an appropriate set of WL failures places the system in an inoperable state. In contrast, safety is “assured” if (i) an appropriate set of WL failures places the system in an inoperable state before (ii) a necessary set of SL failures places the system in a potentially operational condition. Consistent with the preceding description, LOAS does not mean that the accident will progress to operation of the high consequence system; rather, it only signifies that certain conditions exist that could contribute to this progression.