Relay Chatter Evaluation (Task 9)
Task Overview
Objective
The objective to evaluate the effect of relay chatter on plant response after a seismic event.
Purpose
This section describes a process for the treatment of low-fragility relays or relay chatter for use in an SPRA.
Guidance
Possible effects from relay or contact chatter may include:
- Could impact the availability of components required to shut down and maintain the reactor in a safe shutdown condition (for example, trip of switchgear, lockout of diesels)
- Could cause operator confusion because of unusual equipment operating configurations, as well as inconsistent and erroneous indications on control panels
- Could induce interfacing systems LOCA (ISLOCA); that is, containment bypass events
- Could lock out containment isolation
For each such relay or contact, a common approach includes the following:
- An assessment of the plant impact should chatter occur (acceptable or not)
- A determination of how the operator can diagnose the problem, if not acceptable
- The means available for the operator to correct the problem
Many older IPEEE evaluations looked only at low-ruggedness, or “bad actor” relays. It is suggested that a formal process be followed, unless it can be shown that a screening fragility assigned to all relays and contactors still would not result in a significant contribution to CDF or LERF.
A screening process for the relay evaluation may be developed as follows:
- Screen out relays and contactors that do not support key safety functions credited in the SPRA model
- Screen out relays and contactors with very high seismic capacities
- Screen out relays and contactors whose chatter will not affect the proper execution of a safety function
This screening method is performed before including other relays in the systems model. Typically, after these screening steps are performed, only a small subset of the relays and contactors may remain that could require detailed circuit analyses. Each of the methods above can work and may be performed in any order that suits the particular project. Relay chatter can be included in the SPRA using a logical OR gate to link the seismic failure mode to the equipment impacted. Generally, the relay chatter fragility could be placed under an AND gate, with a basic event representing the potential for operator action to recover from the effects of the chatter. If there are multiple impacts from the chatter, the same recovery action event typically appears with each occurrence of the seismic failure mode.
Supplemental Guidance
Related Element of ASME/PRA Standard
Part 5, Seismic Fragility-Response (SFR)
Part 5, Seismic Plant-Response (SPR)
EPRI Guidance
Seismic Probabilistic Risk Assessment Implementation Guide (3002000709) Section 5.3.4.4 provides example guidance on relay chatter evaluation.
Seismic Fragility and Seismic Margin Guidance for Seismic Probabilistic Risk Assessments (3002012994) Section 6.3 provide guidance on relay evaluations.
Other Guidance
(none noted)