ML20090L569
| ML20090L569 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 04/01/1975 |
| From: | Mayer L NORTHERN STATES POWER CO. |
| To: | Anthony Giambusso Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 9102120426 | |
| Download: ML20090L569 (3) | |
Text
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FROM: Northern states Power DATE OF DOC DATE REC'D LTR TWX RPT OTHER i
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$0-263 DESCRIPTION:
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Dear Mr. Giambusso:
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Monticello Nuclear Generating Plant Docket No. 50-263 License No. DPR-22 l
Anticipated Transients Without Scram (ATVS)
Your October 19, 1973, letter instructed us to respond to the staff position on ATWS stated in WASH-1270, " Technical Report on Anticipated Transients Without Scram for Wster-Cooled Power Reactors." September, 1973. Monticello falls under Section II.C of the staff position which requires a three-stage evaluation.
1.
Analysis of ATWS Consequences - Northern States Power Company has contracted with General Electric Company to perform this analysis.
The results are presented in the attached report entitled NED0-20B46,
" Anticipated Transients Without Scram Study for the Monticello Nuclear Generating Plant".
4 2.
Review of Reacto. ;hutdown System Design - The results of this portion of the evaluation were submitted on October 1,1974, in a document entitled NEDO-20635, " Evaluation Report - Common Mode Failure Vulnerability of Reactor Protection System Instrumentation for the Monticello Nuclear Generating Station".
3.
Program for impicmentation - Having made the above submittals, WASH-1270 stipulates that the staff will evaluate the need for plant changes on an individual case basis.
We agree with the General Electric position as stated in the introduction of the ottached report.
In particular, we believe that the guidelines of WASH-1270 are not appropriate for such a low probability event as an ATWS.
Rather, we believe the guidelines suggested in NEDO-10349, " Analysis of Anticipated Transients Without Scram," are more appropriate.
We also wish to emphasize our concurrence with the conclusion that ".
.there is no undue risk to public health and safety due to the continued operation of the present population of reactors in their current configuration."
the present time there is a very low probability of an ATWS event having serious At consequences. A concern has been expressed that the probability may increase with the increasing number of operating reactors. We believe that other factors exist which will counte'.nalance the effect of additional operating plants making retrofit measures 3303
Mr. A Giambusso pa ge 2 April 1, 1975 f or Monticello unwa rranted. WASil-12 70 shows t ha t the reactor protection system unreliability (and therefore the ATWS event probability) is inversely proportional to the accumulated years of reactor operating experience.
Cnwreason for the apparent relatively high probability of the ATWS event is the absence of a nassive data bank of operating experience to quantify low probability events within a 95% confidence interval.
For exampic, WAsil-1270 reports two failures in 1627 acepmulated reactor years of operation up to March of 1973 for an unreliability of 1.6 x 10 **
based on a monthly test interval.
For the same situation but with no failures, the unreliability would have only been a factor of two better.
Since the llanford N reactor is atypical of present reactor protection systen designs, reasons are discussed (WASit-1270) for not including one of the two known potential failures to scram. Assume, for the moment, that the ratio of one failure per 1620 accumulated reactor years of operation exists in the future. When the data base expands by a factor of 14, at a constant failure rat e, the reactor protection system un-reliability will improve by a factor of three simply due to the increased data available to demonst rate a 957. confidence interval.
Iloweve r, there are reasons to believe that the f ailure rate will be reduced in the future as 1) new plants beeme operabic with improved reactor prot ection systems as required by Section 11. A of the WAsil-1270 staf f position, 2) plants identified by Sections 11.B and II.C complete their startup testing program and achieve stable, equilibrium operation, and 3) knowledge and experience are opti.mized through standardized systems and components.
A signif. cant reduction in the failure rate could also improve the reactor protection system unreliability by a factor of three.
1he combined ef fect of an tmproved failure rate over i larger data base could easily improve the unreliability by a decade.
In response to contention 11-33 of our current FTOL hearing the staf f testimon;* has determi ned that a realistig value of the unreliability of presently designed reactor protect.on systens is3x10'.
The staff concludes that the probability of an A1VS event v.th serious consequences is low enough to satisfy their safety objective today and for the next few years.
On this basis and in light of the potential reduction in unre tiebility as the operating data base is expanded, we do not believe backfitting of Monticello is presently varranted.
Yours "c ry t ruly,
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L 0 Mayer, PE Manager of Nuclear Support Services Lm /Miiv/yb cc:
J G Keppler G Cha rno f f MPCA Attn.
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