ML19347C198
| ML19347C198 | |
| Person / Time | |
|---|---|
| Issue date: | 09/29/1980 |
| From: | Harold Denton Office of Nuclear Reactor Regulation |
| To: | Plesset M Advisory Committee on Reactor Safeguards |
| Shared Package | |
| ML19347C199 | List: |
| References | |
| ACRS-GENERAL, NUDOCS 8010170015 | |
| Download: ML19347C198 (3) | |
Text
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SEP 2 91980 "FMORANDUM FOR: Milton S. Plesset, Chairman, Advisory Comittee on Reactor Safeguards FROM:
Harold R. Denton, Director, Office of Nuclear Reactor Regulation SUNECT:
CASCADIMG FAILURES IN NUCLEAR PLANTS Ycur letter to '/illiam J. Dircks on this subject dated August 12, 1980, indicated that the Cemittee believes more attention is needed regarding i
l the safety evaluation of cascading failures, particularly the later failures in a chain of events following an initiating event.
'le ret with the Subcomittee on Safety Philosophy, Technology and Criteria on September 3,1980, and cascading failures were discussed within the context of systems interactions, including the five examples of cascading failures cited in your letter. F!e agree that the subject needs more attention and it is expected that the Systems Interaction Branch within l
the Division of Systems Integration of NRR will take the lead role in c'evelcoing the criteria and methods tnat can be used by industry and the staff in subsequent system interaction reviews. This branch will serve as the poir.t of contact within MRR regarding cascading failures.
The transcript of the Subcomittee meeting documents proposed systems interaction goals and the program for the next several years, current activities in this area, and each of the five examples of cascading failures.
l An important aspect of the discussions on cascading failures was whether l
the examples would be iden*ified as a systems interaction if the criteria and methods now under development were used.
' hile a broad definition of i
l systems interaction included postulated failures initiated in safety i
systems, we believe that such failures are less likely systems interaction i
candidates than events that can involve failures initiating in non-safety systems which, in turn, result in a potential violation of a safety function.
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Me believe that most postulated s:'ety system failures have already been i
considered as part of the normal licensing review. Nevertheless oversi l
reviews may still pick up examples of potential safety system failures;ght j
in which case, the evaluation of these examples will be directed to the l
responsible lead technical review branch.
The staff's proposed aoproach was illustrated through the discussions held with the Subccmittee regarding the classification of the five examples of cascading failures given in your letter.
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Milton S. Plesset.
The first example postulated a seismic event causing plant shutdown concurrent with failure of the auxiliary feedwater systems.
The loss of normal decay heat removal systems leads to the use of bleed-feed mode for decay heat removal.
A further stipulation made was that the containment temperature and pressure exceeded the qualifications of the power and control circuitry on the PORVs and block valves. This example constitutes a systems interaction that wculd be acted upon by the Syste:cs Interaction Branch. The root cause originates in a non-safety system and the consequence violates a current systems inter-action criterion: an uncontrolled loss of decay heat removal.
The second example postulated a stuck-open relief valve coincident with a failure o' the discharge line as a result of sustained resonant ficw. This exampla letds to violation of the systems interaction criterion " uncontrolled release of redctor coolant." However, this scenario would not be acted upon by the Systems Interaction Branch because its root-cause originates within a safety system and it is acted upon by the Mechanical Engineering Branch witnin the present review process.
The third example pcstulates failure of an instrument line within a control system that leads te both a small break LCCA and a loss of accurate parameter sensing. This example constitutes a systems interaction that would be acted upon by the Systcms Interaction Branch. The rcot cause originates in a non-safety system and could violate a systens interacticn criterien depending upon which instrument line failed.
The fourth exampie postulates a failure cascade resulting from a short-circuit in heavy electrical equipment inside containment. The further stipulation was that the protective overpower circuit-breaker relays failed to function.
This example would not be acted upon by the Systems Interaction Branch because the entire area surrounding this concern is already covered by the Power Systems Branch in the review precess as guided by Pegulatory Guide 1.63.
The fifth example presumed a break in a BWR HPCI steam line cutboard of the isolation valves.
It was further stipulated that the consecuences of this failure wculd be beyond the demonstrated capabilities cf the isolation valves and their controls. This example fits the bread definition of systems interaction only. Although this type of event ircludes a serious consequence of a single failure, the Systems Interaction Branch would not be the branch taking corrective action. The safety concerns in this type of event are presently evaluated by the Mechanical Engineering Branch with respect to the postulated break and the piping, and ti.e Ecuipment Cualificaticn Branch with respect to valve operability.
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o Milton S. Plesset,
During'the meeting, we agreed with the Subcomitte' that the issues involved e
are complicated. Additionally, the Subcomittee acknowledged the need for i
further guidance.to the Staff regarding approaches to cascading failures.
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/H.R'.Denton, Director Dffice of Nuclear Reactor Regulation I
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