ML19308A448

Requests Const of Event Tree of TMI Accident by Probabilistic Assessment Group

ML19308A448
Person / Time
Site:	Crane
Issue date:	07/16/1979
From:	Johnson V PRESIDENT'S COMMISSION ON THE ACCIDENT AT THREE MILE
To:	Rehm T NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO)
References
NUDOCS 7910190204
Download: ML19308A448 (1)

Text

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President's Commission on the Accident at Three Mile Island

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~ Mr. 'Ihomas Rehm Issistant to the Executive

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Dear Mr. Rehm:

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The staff of the President's Comission at the Accident at Three

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Mile Island is gathering facts directly related to the accident itself, but also is interested in background information in the general area of f ~ ~.i.-j.

reactor safety and reactor operations. A part of the concerns that have been voiced derive from events that did not happen; namely, those sequences

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of events that would h;ve occurred had different decisions been taken at E4 various ti:nes during the course of the accident. Clearly, scrne branch iii points might have ter:rinated the accident, and some might have worsened n..

the situaticn.

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It is evident that an analysis of all paths is an impossible task and

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to proceed in an orderly rranner an event tree for this particular accident

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l is a necessity. We therefore ask that the NBC through its Probabilistic I,.

Assessment Group construct such an event tree with the major branch points pq h

noted and with a very brief description of the situation and consequences at each such point. We believe that this group of experts is the nest highly qualified in the nation in this specialized field.

l Unfortunately, the time scale for the President's Co:: mission is very short and to make use of this study we must ask for a report early in August. Some discussicns have already taken place between Mr. William Stratton cf our staff and Mr. Saul Levine of the NRC. You may consider Mr. Stratton to be our contact in this mtter.

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Sincerely,

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/,.: n %. vf&A-<'W"d Vincent L.

hnson Special Assistant to the Statf Director 2220 023

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Mr. Vincent L. Johnson Special Assistant to the Staff Director President's Commission on th'e i :

Accident at Three Mile Island 2100 M Street, N.W.

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Subject:

Event Tree for TMI Accident Enclosed is a discussion and. event tree for the TMI accident as requested by you in a letter to Mr. T. A. Rehm on July 16,1979.

REi This work was completed by the Probabilistic Analysis Staff in the l

Office of Nuclear Regulatory Research.

Sincerely,

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O'rigin::I Sigr.ed By l

Saul Lcvine Saul Levine, Director Office of Nuclear Regulatory Research k..

Enclosure:

Discussion and 4

Event Tree for TMI Accident m

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. August 9, 1979 MEMORANDUM FOR FILES FROM:

Joseph A. Murphy Probabilistic Analysis Staff Office of Nuclear Regulatory Research

SUBJECT:

LOSS OF FEEDWATER TRANSIENTS IN B&W REACTORS A brief scoping analysis of the likelihood of core melt accidents initiated by loss of feedwater transients for a typical Babcock & Wilcox designed reactor has been prepared.

It should be noted that this analysis is based on a typical B&W design as it existed before the modifications and improved operator training required by the bulletins and orders issued since the occurrence of the TMI-2 accident. The analyses performed are limited in scope and are based on point estimates of failure rates.

Error propagation techniques have not been employed and the uncertainty of the final results have not been determined in a formal manner.

To the extent possible, this analysis has relied on previous-analyses performed under ~the Methodology Applications Program sponsored by the Probabilistic Analysis Staff and the Reactor Safety Study (WASH-1400). The information from the Methodology Applications Program is preliminary and has not yet been published.

To analyze loss of feedwater transients at Babcock & Wilcox designed reactors, an event tree has been constructed (Figure 1, attached).

For ease of understanding, symbology of the Reactor Safety Stucy is used to designate failure states as noted at the top of the event tree (i.e., K represents reactor trip fails, etc.).

Each sequence in this event tree is discussed below:

a.

Sequence 1 (TM) - This is the expected loss of feedwater transient.

Last year it had an average frequency of approximately 3 per reactor year at B&W reactors, as noted in NUREG-0560.

The number of feedwater transients at an' individual reactor varies from zero up to an extreme of 10 or so per year, depending on how long the plant has been in commercial operation (highest number in first year or two). The loss of feedwater causes a reduction in the heat removal capability of the secondary system which, in turn, causes a pressure rise in the primary system.

This pressure. rise causes the pressurizer relief valve to open and resultant conditions cause thh reactor to trip. The auxiliary feedwater system is initiated upon the l

loss of main feedwater and has the capability of removing decay heat.

l Thennal balance is restored by the auxiliary feedwater action, relief valve h$

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