ML20003C749
| ML20003C749 | |
| Person / Time | |
|---|---|
| Issue date: | 02/03/1981 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML20003C742 | List: |
| References | |
| FRN-45FR65474, REF-10CFR9.7, RULE-PR-50, RULE-PRM-50-29, TASK-OS, TASK-RS-220-5, TASK-SG-029-3 NUDOCS 8103180196 | |
| Download: ML20003C749 (18) | |
Text
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ENV!RONMENTAL ASSESSMENT FOR DROPOSED RULEMAKING
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TO AwEND 10 CTR PART 50 CONCERNING ANTICIPATED TRANSIENTS WITHOUT SCRAM (ATWS) E'/ENTS 1.
Introduction The NRC staff has recently completed a review and evaluation of information that has been developed over the past 10 years on Anticipated Transients Without Scram (AWS) events and the manner in which they should be considered in the design and safety evaluation of nuclear power plants." The esult of these efforts indicate the potential extent and probability of serious consecuences from such events and the NRC is proposing to amend 10 CFR Part 50, Section 50.49 (SECY-80-409).
Since this amendment to the regulations governing the licensing of production and utili:ation facilities is substantive and ray have a signif-icant impact on the human environment, an tavironmental assessment has been prepared to determine whether an environmental impact statement should be developed for the proposed rulemaking.
2.
Need for the Amendment The significance of AWS for reactor safety is that some ATWS events could result in melting of the reactor fuel and the release of a large amount of radio-active fission products.
The NRC has concluded that the probability of ATWS events cccurring over the lifetime of nuclear power plants and the potential magnitNe of consequences arising from such events, should they occur, are sufficiently great to warnnt "Anticipateo T n nsients Without Scram for Light Water Reactors, NUREG-Ot.60, Vol. 1 througn 4 1
8103180 %
5 the imposition of aeditional requirements to recuce the procacility anc mitigate the consequences of ATWS events.
3.
The Procosed Action The NRC staff considered two alternatives in developing the proposed ATWS rule:
Alternative 1 - Maintain present regulation's - Plants as designed are safe enough and ATWS is not a safety issue.
Alternative 2 - Publish for comment a proposed rule to esttblish design requirements to reduce the likelihood of and/or mitigate the consequences of ATWS events.
Based upon analyses conducted by the staff, the NRC has concluded that the probability of an A7WS event occurring, as well as the consequences of such an event, should one occur, are sufficiently great to war-ant the imposi-tion of certain requirements. Accordingly, the NRC has chosen to pursue Alternative 2.
The proposed rule would require that each light water-cooled nuclear power plant licensee modify its design as necessary to comply with the following acceptance criteria before January 1,1964; (i) Primary system pressure. The calculated reactor coolant system (RCS) pressure and temperature resulting from postulated ATWS
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events shall be limited so that the calculated maximum primary stress anywhere in the RCS pressure bouncary does not exceed that permitted by the " Level C Service Limit" as defined in Article NS-3000 of Section III of the ASME Boiler and Pressure Vessel Code and the calculated deformation of RCS components I
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. is limited so that the operacility of componer ?.s necessary to safely bring and maintain the reactor at a cold shutcown condi-tion is not impaired.
(ii) Fuel integr'ity.
The calculated damage to tne reactor core as a consequence of postulated ATWS events, including oscillations of power and flow, shall be limited to. assure that the core geometry is not significantly distorted such as to impair core cooling or safe shutdown.
(iii) Radiation Release.
The calculated release of radioactivity from the fuel rods to the reactor coolant system dering postulated ATWS events shall not exceed one pertent of the t'adioactivity within the fuel rods of a pressurited water reactor or ten percent of the radioactivity within the fuel rods of a boiling water reactor.
(iv) Containment. The calculated containnent pressure, temperature, and
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humioity resulting from postulated ATWS events shall not exceed the design values of the containment structure and components or tne cohtained mitigating systems, equipment and coreponents.
For boil-ing water reactor pressure suppression containments, the relief or safety valve discharge line flow rates and suppression pool water temperaturos shall be limited so that steam quenching instability will not result in destructive vibrations.
(v) Long-term shutdown and cooling.
The reactor design shall permit i
the reactor to be' safely brought to and maintained at a cold shutdown. condition following postulated ATVS events without inser-(
tion of control rods.
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The tiRC staff developed fixes (called alternatives in NUREG-0460, Vol. 4 -
Anticipated Transients Without Scram for Light Water Reactors) representing different levels of safety.
Each fix contains four sets of requirements, specific to each of the four reactor vendors. These fixes have evolved over the years and take into consideration changing plant designs, operating expe-i-ence, industry submittals, and the lessons learned from the TMI-2. The follow-ing table summarizes the requirements of these fixes:
SUMMARY
OF, REQUIREMENTS
(* Indicates implicit requirements.)
VENDOR Fix 2 Fix 3 Fix 4 B&W, CE SPS SPS SPS AMSAC AMSAC AMSAC Cont Isol Coat Isol Cont Isol Analysis Analysis Analysis Instr
- Instr
- Safety Valve *.
Safety' Valve" W
SPS AMSAC AMSAC AMSAC Cont Isol Cont Isol Cont Isol Analysis Analysis Analysis Instra Instr
'.SPS AMSAC AMSAC AMSAC Cont Isol Cent Isol Cont Isol l
Analysis Analysis Analysis 50 SD SD l
Logic Logic
- Logic" a
l Instr
- Instr
- SLCS-Auto" Incr. Cap SLCS-Auto
- Hi-Cap 4
h Notsnelature:
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AMSAC:
ATWS mitigation acuation circuitry.
Diverse and indepencent from the reactor protection system to actuate:
1.
PWR's - Turbine trip, auxiliary feecwater 2.
SWR's - Hign presta:e coolant injection (HPCI),
Stancby Liquid Control Systems (SLCS), Recirculation Pump Trip (RPT)
Analysis:
Analysis with acceptable evaluation models of performance following ATWS events Cont Isol:
Containment isolation initiated by early detection of fuel failures Instr:
Instrumentation necassary for shutdown that can withstand ATWS conditions Logic:
Logic of control circuits to reduce vessel isolation events and runback feecwater Safety Valves:
Additional safety valve relief capa-ity SPS:
Supplementary protection system that is diverse and indepen-dent from the reacgor trip portion of the reactoy protaction system --
B&W - BUSS, a diverse four-channel backup scram system CE SPS, a diverse, four channel supplementary protection system MSS, a modified scram system that is diverse and inde-W pendent from the reactor protection system (RPS)
GE - ARI, ATWS rod in.dection that has separate sensors and redundant scram air header exhaust valves SD:
Scram discht.ge volume for GE control rces that is less sus-captible to common mode failure SLCS-Auto:
Automatically initiated, Standby Liquid (neutron poison-)
Control System --
Incr-Cap: ' Capability to simultaneously inject with both pumps Hi-Cap:
Single failure proof system, with approximately 400 gpm capacity e
S A.
m The proposed rule provides for implementation of the requirements n stages in order to gain the greatest increase in safety in the snortest time and at the least cost. While the exact fixes to be imposec are che suoject of the ATWS rulemaking, the range of fixes include: modification to imorcve the reliability of the protection system and mitigating system actuation circuitry which involves instrumentation, control and logic circuits that are for the most.part 1c 2:ed outside containment and could be accomplished with the plant operating or during refueling outages; modifications to the piping of boiling water reactors standby liquid control systems which is primarily outsics containment and could be mace with the reactor operating or during refueling outages; and for some pressurized water reactors the installation of additional safety relief valves on the primary system which would require reac-ter shutdown for installation.
To assess the maximum potential environmental impact, the fixes that are the most costly and restrictive (from a plant operat-ing standpoint) and those with the highest occupation exposure were used.
Because ATVS is a low probability event, the NRC believes tnat-the likeli-hood of severe consequences arising from such an ATWS event in the next 2 or 3 years is acceptably small.. This judgment is based on a) the favorable experi-ence with the operating reactors, b) the limited number of operating nuclear power reactors, c) the inherent capability of some of the operatin; PWRs to partially or fully mitigate the consequences of ATWs events, d) partial ATWS mitigative capability of the recirculation pump trips feature which has been implemenEadonmostoperatingSWRsandwhichisrequiredtobeimplementedon the remaining BWRs by December 31, 1980', and e) the interim steps taken to develop procedures anc train operators to furtner reduce the risk from some ATWS events.
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Some ex4 sting nuclear power plants are considered to :e at hign risk sites owing to population density, meteorological conditions, anc c:ner fac-tors.
Icentification of these sites is a subject of another Commission-action and any additional ATWS rsquirements for these units would :e sucsecuently considered.
Concurrent with publication of the proposed rule for comment, the staff is also issuing a proposed regulatory guide for comment.
This regulatory guide provides guicance on the evaluation models, mitigating system design, and other licensing needs.
4 Incact of the procesed Action The environmental impact of the ATWS proposed rule can be categori:ed as follows:
i)
The effect on the natural environment and the people in it due to
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risk of exposure to radioactivity.
fi)
Economic effects.
The environmental impact on the natural environment can be evaluated by considering the ralative radiological influence tnat tne nuclear power plant ha's with respect to the background radioactivity already present. The latter includes natural raciation background and man-made exposure sources other than from the nuclear power plant operation.
Occuoational Excesure:
Some occucational exposure occurs in all operating plants in the normal course of operation and maintenance. Accitional exposures can occur in operating plants during any installation of mocifications that e
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mignt De required.
Accitional occuDat onal exposure -culc De excectec in i
im lementing tne ATWS modifications recuired by this rulemaking.
Tre greatest occupational exposure cue to the proposed ATWS requirements is juegec to be that associated with the installation of new safety rJ 1ief valves on the primary system. To conservatively estimate the extant of sucn ex:osure, the staff selected an operating plant, that is estimated to neec the largest num-ber of new valves to meet the' ATWS requirements. To establis.} the plant and the number of relief valves needed, the staff censidered the nuclear steam supply system power / relieving flew ratic, utili:sc teginning of cyclu (BOC) core conditions, and. assumed a moderator temperature coafficient (MTC) value which is the most positive over CE% of the plant operating life. Sased on tnese considerations, the Calvert Cliff Unit-1 plant was selected and two or three additional relief valves are conservatively estimated to be requirec for this type of plant to meet the preposed ATVS modifications.
Technical staff from the Office of Inspection and Enforcement experienced in nuclear power plant construction practice used conservative as'sumptions and calculatiens along with plant information on piping configuration and radiation levels to estimate the occupational exposure for the welding cperation to be 2 to 3 man-rem per valve in'stallation.
Thus, far welding the three valves en the primary system, t'te occupational exposure is conservatively estimated to be less than 10 man-rem.
All other occupational exposures necessary to implement the ATVS modifi-ca*f ons required by ~the proposed rule are judged to be less relative ;; the occupational exposure from welding the valves; however, assuming they are of the same order of magnitude, the total occupational exposure is estimated to ce less than 100 man-rem per plant.
Tacle i summarizes the annual ex:csures O
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reported by the nuclear power facilities for 1973 tnrough 1978.
Based on this 4 6 year data base, the 100 man-rem exposure estimated for the worst case (from
. exposure standpoint) is much less than the average number of mar.-rem (400 ~ 500 man-rem) per reactor year of operation.
Most authorities" are in agreement that a reasonable, and probably consarva-tive estimate of the statistical relationship between low levels of radiation exposure to a large number of people and the subsequent health effects is about 100 potential premature deaths due to cancer per million man-rem and about s
220 genetic changes appearing over a span of five generations.
These statistics indicate that for the 15 operating nuclear power plants that require ATWS modifications, the total health effects would be 0.15 cases of cancer; and 0.375 cases for genetic effects over the the next five genera-tions. No early fatalities would occur for this type of *exp"osure.
Cancer and genetic impacts from the proposed ATWS modifications can be compared to cancer and genetic effects due to natural causes.
Statistical data *" show that approximately of every five deaths from all causes in the l
U.S.,.one is from cancer.
Using 10 CFR Part 20 ifmits of no more than 5 rem per year per wortsr, approximately 300 workers would be involved in the ATWS modifications. About 60 of these workers would be expected to die eventually j
Meeting on cavelopment of NRC position on health risk estimators - Michael A.
Parsont; Radiological Health Standards, October 22, 1980.
- "American Cancer Society, Facts and Figures, 1980.
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from latent cancer due to natural causes. Only 0.15 worker can be expected to prematurely die from cancer due to the total ATWS modifications for all d
15 operating nuclear power plants.
The range of genetic effects of radiation f cm ATWS modifications is very small (0.375 cases for all 15 plaats over the next five generations) in relation to current estimates of the incidence of serious' human disorders of genetic origin--roughly 10%" of liveborn offspring; which is 30 workers for the 15 plants.
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These figures underlie the staff's judgment that the occupational expo-sures from the proposed ATWS modifications are insignificant.
Peculation Excesure:
There are two types of potential impacts on the population exposure due to the ATVS rulemakir.g requirements. The first is the radiological impact due to the installation of ATWS modification on normal plant operations and the second is the impact related to ATWS events. ATWS modifi-cations do. ot affect normal operations other than improving the reliability of the scram system and thus the modification have essentially no impact on popu-lation exposure for those conditions.
Regarding the secon'd type of potential impact, the Reactor Safety Study (RSS) (WASH-1400) assessed the relative risk to health and safety of reactor accidents, including ATWS events, against other accidents encountered in the human environment.
Since risk is the product of the probability of an event
" Meeting on oevelopment of NRC position on health risk estimators - Michael A.
Parsont, Radiological Health Standarcs, October 22, 1980.
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times its consequences, a low precability event with large consecuences (like some ATWS events) may be comparable to a high probacility event with small con-sequences (like normal operational releases).
The conclusion of the RSS stucy was that overall the environmental risk of ATWS accidents is comparable to the risk from normal operational releases; and the individual cosas will still be very small compared to natural background doses (* 100 millirems per year) or the dose limits specified in 10 CFR Part 20.
Since ATWS modifications reduce the already small overall risk from ATWS events, compared to normal operation, the modifications are insignificant relative to further impacting population exposure.
Replacement Power:
Replacement power may be an impact where delay in startup or shutdown of operation is required to make ATWS modifications.
The staff's judgment is that ATVS modifications could likely be made with little or no additional construction time or shutdown time, since immediate retrofit is not proposed.
For operating plants or near term operating plants, the staff
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believes that most of the ATWS modifications that require plant shutdown could be installed at the time of scheduled plant outages.
This judgement is based on the following historical data.
During 1978, the 25 operating BWRs experienced an' average of 12.89 weeks of outage time and. the a0 operating PWRs an average of 13.01 weeks of outage of which more than 8 weeks were scheduled.
Refueling was the primary cause of schedule'd outages at both BWRs and PWRs and t5e primary cause of forced oucages at both SWRs and PWRs was equipment failure. Maint-enance or testing also accounted for a large percentage of the scheduled outage time at toth types of plants. Tables 2,'3, and 4 summarize the proximate cause of outages during 1976,1977, and 1978, respectively.
The NRC staff estimates that the ATWS modifications that recuire plant shutdown can be made in four to six weeks.
A utility has estimated from four to eignt weeks decending upon wnat
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kinds of ucexpected preolems night arise.
For either estimate, the historical I
record as summarized in the tables, indicates that th.e scheculed refueling out-ages (During 1978: 5.81 weeks / refueling for SWRs and 7.8 weeks / refueling for PWRs) provide ample time to implement these codifications and very little (if any) additional shutdown time would be requirec. Thus, the imgact of needing alternate power sources because of shutdown due to ATWS modifications is an insignificant environmental consideration.
Economic Impacts:
The economic impacts consist of direct and indirect cos ts.
The direct costs are the cost of the equipment and its installation and for replacement power if needed. The indirect costs include items such as licensing costs, operating and maf atenance, financing and escalation, taxes and insurance, continge. ley funds, and equivalents of radiation exposure (i.e.,
51,000/ man-rem)* that miri t occur during installation of hardware modifications h
in opersting plants.
fince rep 1,acement power is judged not to be needed, the indirect costs are estimated to be about equal to the direct costs.
Table 5 succarizes the staff's estima:e of the direct cost impacts, not including replacement power cost, of ATWS requirements in terms of 1S80 dollars for various vendors. The highest impact is estimated to be less than 3% of l
the capital cost of the nuclear power plant ed on S880/kW in 1980 dollars l
j and a 1,000 MWe plant.
This result Iapports t.he staff's judgment that such costs are a small fraction of the total plant investment.and are an insignifi-cant economic environmental impact.
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l "Appencix 1, 10 CFR Part 50.
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5.
S u=a rv The proposed regulatory action has the effect of furtner recucing react:r risk which will improve the human nealtn and safety environment relative to ATds events. The initial economic impact, while not trivial, is also not significant when consicered in the overall centext of tne human economic environment and could in the long run te an ec:ncaic benefit by reducing power outages and ATdS accicent consequences.
Based on :nis assessment, the pr:cesed ATdS regul* tory action coes not recuire an environmental imcact statement sinct its impact en the huean environment is judged y be insignificant and nonsutstantive.
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s Table 5 - Direct Cost Impact of ATWS Requirements 1980 Dollars in Millions Per Plant Lifetime Operating Plants Plants Under Construction future Plants Plant Alternative 2bF (Construction Permit issuerQ (Construction Permit Design Impacts Fuel Load Data fuel Load Data Hot Issued) before 1/1/84 after 1/1/84 Alternatine 2d*
Alternative 2h*
Alternative 2d*
Impacts impacts Impacts Babcock & Wilcox
- 2. 8
- 2. 0 2.9 2.5 Combustion Engineering 2.6 1.8 2.9 2.5 Westingnouse 1.7 1.2 1.8 2.0 General Electric 3.5 3.2 10.8 6.7
- Alternatives 2b and 2d are defined in the conunission Paper (SECY-80-409).
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