ML20198F353

Forwards Chapter 7 to Des Re Environ Impact of Postulated Accidents

ML20198F353
Person / Time
Site:	Columbia
Issue date:	09/09/1977
From:	Bunch D Office of Nuclear Reactor Regulation
To:	Regan W Office of Nuclear Reactor Regulation
References
CON-WNP-0877, CON-WNP-877 NUDOCS 8605280485
Download: ML20198F353 (9)

Text

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JnYYhk SEP 9 1977 Docket hos. 50-397 MEMORANDLE FOR: W. H. Regan, Jr., Chief. Environmental Projects Branct.

ho. 2, DSE FROM: D. F. Bunch, Chief, Accident Analysis Branch, DSE ,

SUBJECT:

REALISTIC ACCIDENT ASSESSMENT PLAhT hAME: Washington Nuclear Plant Unit No. 2 LICENSING STAGE: OL DOCKET ht#iBERS: SU-397 MILESTONE NUl:BEP.: 33-31 RESPONSIBLE BPANCH: EPB No. 2 PROJECT IW:AGER: R. Boyle REQUESTED C0t'PLETION DATE: September 9, 1977 REVIEW STATUS: AAB Input Complete The Accident Analysis Branch has enclosed Chapter 7 of the Washington Nuclar Plant Unit No. 2 Draft Environmental Statement. This section was prepared L'y P. Tan, Nuclear Engineer, Accident Analysis Brancb.

ND. F. Bunch, Chief Accident Analysis Branch Division of Site Safety and Environmental Analysis Enclosurc:

Chapter 7 of Washington Nuclear Plant Unit No. 2 cc: See next page g5280485770909 D DOCK 05000397

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ENVIRONf1 ENTAL IldPACT OF POSTULATED ACCIDENTS A high degree of protection against the occurrence of postulated accidents in the Washington Nuclear Plant Unit Nc. 2 is provided through correct design, manufacture, and operation, and the quality assurance program used to establish the necessary high integrity of the reactor system, as will be considered in the Comission's Safety Evaluation. System transients that may occur are handled by protective systems to place and hold the plant in a safe condition. Notwithstanding this, the conservative postulate is made that serious accidents might occur, even though they may be extremely unlikely; and engineered safety features will be installe'd to mitigate the consequences of those postulated events which are judged credible.

The probability of occurrence of accidents and the spectrum of their consequences to be considered from an environnental effects stancpoint have been analyzed using best estimates of prooabilities and realistic fission product release and transport assumptions. For site evaluation in our safety review, extremely conservative assumptions are used for the purpose of comparing calculated doses resulting from a hypothetical release of fission products from the fuel against the 10 CFR Part 100 siting guidelines. Realistically computed doses that would be received by the population and environment from the accidents which are postulated are significantly less than those presented in the Safety Evaluation Report.

The Commission issued guidance to applicants on September 1,1971, requiring the consideration of a spectrum of accidents with assumptions as realistic as the state of knowledge permits. The applicant's response was contained in the Environmental Report.

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. The applicant's report has been evaluated, using the standard accident assumptions and guidance issued as a proposed amendment to Appendix D of 10 CFR Part 50 by the Comission on DecemDer 1,1971. Pline classes of postulated accidents and occurrences ranging in severity from trivial to very serious were identified by the Commission. In general, accidents in the high potential consequence end of the spectrun have a icw occurrence rate and those on the low potential consequence end have a higher occurrence rate. The examples selected by the applicant for these cases are shown in Table 7.1. These examples are reasonably homogeneous in terns of probability within each class.

Our estimates of the dose which might be received by an assumec incivi-dual standing at the site boundary in the downwind di*ection, using the assumptions in the proposed Annex to Appendix D, are presented in Table 7.2. Estimates of the integrated exposure tnat night be delivered to the population within 50 miles of the site are also presented in Table 7.2. The man-rem estimate was based on the projected population within 50 miles of the site for the year 2000.

To rigorously establish a realistic annual risk, the calculated doses in Table 7.2 would have to be multiplied by estinated probabilities.

The events in Classes 1 and 2 represent occurrences which are anti-cipated during plant operations; and their consequences, which are very small, are considered within the framework of routine ef fluents from the plant. Except for a limited amount of fuel failures, the events in Classes 3 through 5 are not anticipated during plant operation; but events of this type could occur sometime during tne 40-year plant lifetime. Accicents in Classes 6 and 7 and small accidents in Class 8 are of similar or lower probability than acci-dents in Classes 3 through 5 but are still possible.

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• I The probability of occurrence of large Class 8 accidents is very small.

Therefore, when the consequences incicated in Table 7.2 are weighted ny probabilities, the environmental risk is very low.

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l The postulatt:d occurrences in Class 9 involve sequences of successive failures more severe than those required to be considered in the design bases of pro-i tection systems and engineered safety features. Their consecuences coulc be severe. However, the probability of their occurrence is judged so small that  ;

. t j their environmental risk is extremely low. Defense in depth (nultiple physical l

barriers), quality assurance for design, manufacture and operation, continued surveillance and testing, and corservative design are all applied to provide and maintain a high degree of assurance that potential accidents in this class are, and will remain, sufficiently small in probability that the environmental i risk is extremely Icw,

, L The NRC has performed a study to assess more quantitatively these risks. The initial results of these efforts were made available for comment in draft form on August 20,1974 and released in final form on October 30, 1975.

f I This study, called the Reactor Safety Study, is an effort to develop realistic data on the probabilities and consequences of accidents in water-cooled power L reactors, in order to improve tne quantification of available knowledge related j 4

" Reactor Safety Study: An Assessment of Accident Risks in U.S. Consnercial Nuclear Power Plants, Draf t," WASH-1400, August 1974.

" Reactor Safety Study: An Assessment of Accidents Risks in U.S. Corinercial Nuclear Power Plants," WASH-1400 (NUREG 75/014), October 1975.

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. i to nuclear reactor accident probabilities. The Conmission organized a special group of about 50 specialists under the direction of Professor Norman Rasmussen of MIT to conduct the study. The scope of the study has been discussed with EPA and described in correspondence with EPA which has been placed in the NRC Public Document Room (letter, Doub to Dominick, dated June 5,1973).

As with all new information developed which might have an effect on the health and safety of the public, the results of these studies will be assessed within the Regulatory process on generic or specific bases as may be warranted.

l Table 7.2 indicates that the realistically estimated radiological consequences of the postulated accidents would result in exposures of an assumed individual at the site boundary which are less than those which would result from a year's exposure tc the Maximum Permissible Concentrations (MPC) of 10 CFR Part 20.

The table also shows the estimated integrated exposure of the population within 50 miles of the plant from each postulated accident. Any of these integrated exposures would be much smaller than that from naturally occurring radioactivity.

When considered with the probability of occurrence, the annual potential radia-tion exposure of the population from the postulated accidents is an even 91 aller fraction of the exposure from natural background radiation and, in fact, is well within naturally occurring variations in the natural background. It is con-cluded from the results of the realistic analysis that the environmental risks due to postulated radiological accidents are exceedingly small and need not be considered further.

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, Table 7.2. Classification of Postulated Accidents and Occurrences Class NRC Description Applicant's Examples

1. Trivial incidents Included under routine releases

2. Small releases outside Included under routine releases containment

3. Radioactive waste systems Offgas system leakage failure

4. Fission products to Fuel Cladding defects and fuel primary system (BWR) failures induced by off design transients

5. Fission products to Not Applicable primary and secondary systems (PWR)

6. Refueling accident Fuel bundle drop

7. Spent fuel handling Fuel assembly drop on fuel storage accident pool and spent fuel shipping cash drop

8. Accident initiation Loss of coolant accident, rod drop events considered in accident, steamline break, instrument design-basis evaluation line break in the Safety Analysis Report

9. Hypothetical sequence of Not considered failures more severe than Class 8

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-  ! TABLE 7.2

! 50!4%D.Y GF RADIOLOGICAL CONSEOUENCES 0F POSTULATED ACCIDENTS i I- Estimated Fraction Estimated Dose j of 10 CFR Part 20 to Population in I' limit at site 50 mile radius, [

Class Eye 3 boundary man-rem f

10 Trival Incidents 3/ 3/

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j 2.0 Smeli releases outside 3/ 3/

L containment

, 3.0 Radwaste System failures 3.1 Equicment Leakage or malfunction 0.0034 0.32 3.2 Release of waste gas storage i tank contents 0.014 1.3  ;

4 3.3 Release c' licuid w6ste storage j contents < 0.001 0.004 ,

j 40 Fission grobucts to primary system  ;

(BWR) < 0.001 0.13 i 4.1 Fuel cl&oding defects 3/ 3/ ,

i 4,2 Of*-dasign transients that i i induce fuel fcilures above ~

j those saxpected < 0.001 0.13 l

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The doses calculsted as tortsectences of the postulated accidents are j based on airborne transport of radioactive materials resulting in both

a direct r,d an innaTation dose. Our evaluation of the accident doses

! assumes that the cpplicant's environmental monitoring program and l

appropriate adattianal monitcring (wnich could be initiated subsequent j

to a liquid release incident detected by in-plant monitoring) would a detect the presence of radioactivity in the environment in a timely manner such that remedial action could be taken if necessary to limit exposure from other potnetfal pathways to man.

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Represents the calculated f action of a whole body dose of 500 mrem, or the equivalent dose to an organ.

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~These radionuclide releases are considered in ceveloping the gaseous and

liquid source terms preser.ted in Section 3 and are included in the doses 4 in Section 5.

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. p TABLE 7.2 - Continued Estimated' Fraction Estimated Dose of 10 CFR Part 20 to population limit at site in 50 mile radius, Class Event- boundary man-rem 5.0 Fission products to primary N.A N.A and" secondary systems (PWR) 6.0 Refueling accidents 6.1 Fuel bundle drop. < 0.001 0.0069 6.2 Heavy object drop onto fuel in core- N/A N/A 7.0 Spend fuel handling accident 7 .1 Fuel assembly drop in fuel rack <0.001 0.016 7.2 Heavy object drop onto fuel rack N/A N/A 7.3 Fuel cask drop 0.008 0.75 8.0 Accident initiation events consider'ed .in design basis evaluation in the SAR 8.1 Loss-of-Coolant Accidents Small Break <0.001 0.002 Large Break 0.005 3.8 8.l(a) Break in instrument line from primary system that penetrates the containment <0.001 <0.001 8.2(a) Rod ejection accident (PWR) N/A N/A 8.2(b) Rod drop accident (BWR) <0.001 0.16 8.3(a) Steamline breaks (PWR's outside containment) N.A. N.A.

8.3(b) Steamline break (BWR)

Small Break <0.001 0.042 Large Break 0.002 0.22