ML19329A289
| ML19329A289 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 11/26/1971 |
| From: | US ATOMIC ENERGY COMMISSION (AEC) |
| To: | |
| Shared Package | |
| ML19329A288 | List: |
| References | |
| NUDOCS 8001020864 | |
| Download: ML19329A289 (5) | |
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ENVIRONMENTAL IMPACT OF' ACCIDENTS degree A high/of protection against the occurrence of postulated accidents in
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the Oconee reactors is provided through correct design, construction, testing, and operation, and through the quality assurance program for.
- establishing a high integrity of reactor systems, as considered in the Commission's Safety Evaluation dated December 29, 1970.
Engineered safety _ features are nevertheless provided to mitigate the consequences oi postulated accidents and occurrences.
The highly conservative assumptions and' calculations used in the Safety Evaluation are not suitable for environmental risk evaluations because the probability of occurrence for the unfavorable combinations of circumstances used in establishing tpe adequacy of reactor design is remote.
In recognition of the need for a realistic approach to environmental risk, 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
" Supplement to the Environmental Quality Features of Keowee-Toxaway Project" dated October 1971.
Nine classes of postulated accidents and occurrences ranging in severity from trivial to very serious have been identified by the Commission.
In general,'accidentsinthehighconsequenceendofthespectrumhavea low occurrence rate.and those on the low consequence end have a higher occurrence rate. The examples selected.by the applicant-for these classes are shown in Table 1.
The examples given are reasonably homogeneous in terms of in probability with/ each class, although we consider the release of the 8001020 [
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waste gas decay tank contents as more appropriately in Class 3.
The radiological consequences calculated by the applicant for Classes 2 through 8 are. summarized in Table 2.
The radiological consequences of Class 1 accidents are within those for routine effluents.
The applicant considers the consequences of Class 3 accidents to be limited by their radiation monitoring system, and even if this system were not to function properly the consequences would not exceed those of Classes 2 and 5.
The applicant calculates the Annual Average fraction of 10 CFR Part 20 MPC received by an individual at the exclusion area boundary, and the Annual Average dose received by any individual within a 50 mile radius of the pl, ant.
(The' average dose computed by this method takes wind variability as well as average meteorology into account).
The doses in Table 2 were a
calculated by the staff from the applicant's dose estimates.
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We consider the results in Table 2 to represent reasonable radiological consequences, given the occurrences of the postulated events.
Certain l
assumptions made by the applicant, such:as the assumption of no prior steam generator tube leaks in the evalua' tion of the steam generator tube rupture and the omission of the primary coolant source in evaluating secondary system incidents are' questionable, but the use of alternative assumptions does-not'significantly affect overall environmental risks.
To
establish a realistic annual risk, the calculated doses in Table 2 must i
be multiplied by estimated probabilities. 'In general, we consider the events in Classes 2 through 5 as improbable but not unlikely during the 40-year life of the plant. Accidents in Classes 6 through 7 are relatively less probable but are still conceivable.
The probability of occurrence of Class 8 accidents is very remote.
The occurrences in Class 9 involve sequences of postulated successive failures more severe than those postulated for the design basis of protection systems and engineered safety features.
Their consequences could be severe.
However, the probability of their occurrence is so small that their environmental risk is extremely low.
Defense in depth (multiple physical barriers), quality assurance for design, manufacture, and operation, continued surveillance and testing, and conservative design are all applied to provide and maintain the required high degree of assurance that potential accidents in this class are, ara will remain, sufficiently remote in probability that the environmental risk is extremely low.
-Table 2 indicates that the realistic radiological consequences of the postulated accidents for an assumed average individual at the site boundary are within the limits of 10 CFR Part 20.
It also shows that the man-rem contribution for each postulated accident is orders of magnitude smaller i
than that from the natural background radiation of 117,000 man-rem /yr.
When multiplied by the probability of occurrence, the annual potential o
radiation exposure of the population within 50 miles from all the postulated accidents identified is an even smaller fraction of that from natural back-ground radiation and well within naturally occurring variations.
Therefore, it is concluded from the-results of this analysis that the realistic environmental risks due to postulated accidents are exceedingly small.
Z IABLE I CLASSIFICATION OF POSTULATED ACCIDENTS AND OCCURRENCES NO. ' 0F AEC-APPLICANT'S
- CLASS DESCRIPTIONS EKAMPLE(S) 1 Trivial Incidents Not Considered 2
Misc. Small Releases Outside Frequent small spills Containment and leaks.
Infrequent larger pump seal or valve leaks Releases Due to Piping Failures 3
Radwaste System Failures Inadvertent Discharge of the Contents of a Reactor Coolant Waste Receiver Tank or Waste Gas Decay Tank 4
Events That Release Radioactivity Not Applicable into the Primary System 5
Events That Release Radioactivity Into
- ormal Operation with Fuel Secondary System a
Failures and Staam Generatcr Leaks Transient Operation with-Fuel Failures and Steam Generator Leaks Steam Generator tube rupture 6
Refueling Accidents Inside Containment Dropped Fuel Assembly 7
Accidents to Spent Fuel Outside Containment 8
Accident Initiation Events Considered in Steam Line Break Accident Design-Basis Evaluation in the Safety Analysis Report Rupture of Waste Gas Decay Tank Loss of Coolant Accident 9
Hypothetical Sequences of Failures More Not Considered fSevere than Class 8 p'
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- TABLE II
SUMMARY
OF RADIOLOGICAL CONSEQUENCES OF POSTULATED ACCIDEh"rS
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CLASS INCIDENT EXCLUSION AREA 50 MILE BOUNDAPY AVERAGED ANNUAL AVERAGE AND DIVIDED MAN-REM
- WHOLE BODY DOSE MREM 2
Misc Small Leaks 5
19 5
Loss _of Load 1.5 x 10-3 6 x 10
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Steam Generator Tube Leak 5
19 Steam Generator Tube Failure 0.3 1.3 667 Spent Fuel Accidents 1
4.1 8
Steam Line Failure 3 x 10-2 0.12 Waste Gas Decay Tank Rupture 0.6 2.3 Design Basis Lors of Coolant 3
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- Based on 900 000 people in 50 miles-E
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