ML20033B164
| ML20033B164 | |
| Person / Time | |
|---|---|
| Site: | Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png |
| Issue date: | 11/04/1981 |
| From: | Dircks W NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| To: | Dodd C SENATE |
| Shared Package | |
| ML20033B165 | List: |
| References | |
| NUDOCS 8111300493 | |
| Download: ML20033B164 (9) | |
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DISTRIBUTION Docket r/ incoming HSmith NRC POR w/ incoming RLilmer Local PDR w/ incoming BSnyder ORB Reading TMurley NOV 4 ggj EDO Reading RMattson Docket No. 50-213 OELD SHanauer OCA (3)
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Shapar EHughes Dircks The Honorable Christopher J. Dodd MStine Cornell United Statas Senate DEisenhut Rehm Washington, D. C.
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Dear Senator Dodd:
WPaulson DtmtUn 6h kU"NI) f Your letter dated October 8,1981 addressed to Chairman Palladino has been referred to me for response. You requested information regarding radiation induced embrittlement of the reactor vessel at the Connecticut Yankee Nuclear Power Plant at Haddam Neck, Connecticut.
The enclosure to this latter summarizes this issue. With regard to the Haddam Neck Plant, by letter dated May 22, 1981, Connecticut Yankee Atomic Power Company (CYAPCC) responded to our request of April 20,1981, re-qu'esting that each licensee provide the NRC staff with the specific actions which they propose to undertake to resolve this issue. The Pressurized Water Reactor (PWR) Owners' Group also responded to our letter. We have reviewed the PWR Owners' Group and the licensee responses to our letter.
On the basis of our review, we determined that the present fracture toughness of operating reactor vessels is such that all plants could survive a severe overcooling event for at least another year of full power operation. The CYAPC0 letter further stated that the Westinghouse Owners Group program is scheduled to be completed in December 1981 and a report will be submitted to the NRC upon completion of the program. The CYAPC0 letter also stated that they will provide additional details including a schedule for analysis and/or remedial action, if required, and their technical basis. This addi-tional information, when it is received, will receive staff review.
Based on our continuing review of this issue, we concluded that additional action should be taken to resolve the long-term problem. Accordingly, on August 21, 1981, we sent letters to eight licensees of operating plants requesting additional information regarding this issue. The selection of these plants was based on our review of vessel properties as well as other plant characteristics. The Haddam Neck Plant was not one of the plants for which additional information was requested at this time. We will review
@ghg r,esponses to this letter and assess the application to all operating planus including the Haddam Neck Plant.
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Cornell EHughes Rehm MStine Stello The Honorable Christopher J. Dodd DLisenhut Case United States Senate JHeltemes, AEOD Denton Washington, D. C.
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Dear Senatdr Dodd:
HSmith RVollmer Your letter d'ated October 8,1981 addressed to Chairman Palladino has been referred to re for, response. You requested information regarding radiation induced embrittlement of the reactor vessel at the Connecticut Yankee Nuclear Power Plant at Haddah4Jeck, Connecticut.
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The enclosure to this let er summarizes this issue. With regard to the Haddam Neck Plant, by lett dated May 22, 1981, Connecticut Yankee Atomic Power Comany (CYAPCO) respo ed to our request of April 20, 1981, re-questing that each licensee pr ide the NRC staff with the specific actions which they propose to undertake 1 resolve this issue. The Pressurized Water Reactor (PWR) Owners' Groups 1so responded to our letter. We have reviewed the PWR Owners' Group and th licensee responses to our letter.
On the basis of our independent review, f the plants where neutron irra-diation has significantly reduced the frac re toughness of the reactor pressure vessels (RPVs), all plants could s vive a severe overcooling event for at least another year of full power o ration. The CYAPC0 letter further stated that the Westinghouse Owners Group ogram is scheduled to be conpleted in December 1981 and a report will be s 91tted to the NRC i
upon conpletion of the program. The CYAPC0 letter also tated that they will provide additional detail including a schedule for an sis and/or remedial action, if required, and the basis therefore. We w 1 review this additional information when it is received.
a Based on our continuing review of this issue, we concluded that add ional action should be taken to resolve the long-term problem. Accordingly, on August 21, 1981, we sent letters to eloht 11censees of operating plants requesting additional information regarding this issue. The Haddam Neck Plant was not one of the plants for which additional information was requested at this time. - We will review the responses to this letter and assess the application to all operating plants, including the Haddam Neck Plant.
Sincerely, NRR: DIR ED0 HDenton WJDircks William J. Dircks Executive Director for Oppons
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Docket w/ incoming NRC PDR W/ INCOMING Local PDR w/i coming ED0 Reading BSnyder Docket No. 50-213 OPB Reading TMurley OELD RMattson OCA (3)
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EHughes PPAS Minogue MStine Shapar DEisenhut Dircks The Honorable Christopher J. Dodd JHeltehes, AE0D Cornell United States Senate DCRutchfield Rehm 1
Washington, D. C.
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Dear Senator Dodo:
RV liner Denton 1
Your letter dated October 8,1981 addressed to Q iairman Palladino has been referred to ne for response. You requested in 6rnation regarding radiation induced enbrittlement of the reactor vessel a the Connecticut Yankee Huclear Power plant at Haddam Heck, Connecti t.
The enclosure to this letter sumarizes the tatus of this issue. Specifically with regard to the Haddan Neck plant, the f,C staff has concluded that no inmediate licensing action is required becpuse there is no evidence that indicates that the radiation damage to th s reactor vessel has reached an extent that requires such action.
"i By letter dated Itay 22, 1981, Connectic, t Yankee Atomic Pcwer Company (CYAPCO) responded to our request of April 20, /981 requesting that each licensee provide the HRC staff with the speciffc actions which they propose to under-take to resolve this issue. The CYA 0 letter stated that the Westinghouse N Owners Group program is scheduled t be completed in December 1981 and a report will be submitted to the HRC upon
. pletion of the progran. The CYAPC0 letter also stated that they afil provide additional detail including a schedule for analysis and/or remedial action, ff required, and the basis for the same. We will review this additional info,.ation when it is received.
Sincerely, 1
William J. Dircks, Executive Director of Operations ED3 WJDircks i
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Enclosure NUCLEAR REACTOR PRESSURE VISSEL INTEGRITY Wl:EN SUBJECTED TO THERitAL SHOCK AND SUBSEQUENT REPRESSURIZATION DURING
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AN OVERC00 LING TRANSIENT (PRESSURIZEDTHERMALSH0CK)
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Pressure vessel thermal. shock has been considered for many years in the context of assuring integrity of. the vessel when subjected to colf _
i emergency core cooling water during a large loss of coolant accident (LOCA).
Based on a ser.ies of thermal shock experiments (unpressurized) conducted at Oak Ridge National Laboratory (ORNL) beginning in 1976 and based on fracture mechanics analyses verified by the experim6nts, it.was concluded that a postulated flaw would not propagate through-the vessel wall during a large L'0CA. Therefore, the vessel integrity would be
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mairYtaine'd during subsequent reflooding which would occur at relatively 1
1ow pressure due to presence of the large break.
As the result of operating experience, it was subsequently recognized that there could be transients in pressurized water reactors (PWRs) in which the vessel could be subjected to severe ' overcooling (thermal e
shock) followed by repres.surization.
In these pressurized thermal shock transients, vessels would be subjected to pressure stresses superimposed upon the thermal stresses resulting from the temperature difference across the vessel wall.
The Rancho Seco event of March 20, 1978 is believed, to represent the most severe (and prolonged) overcooling transient experienced to date.
In that event, a lightbulb being replaced in the non-nuclear instrumentation / integrated control system (NNI/ICS) panel was dropped and caused a short to occur while the plant was at approximately 70% power. About 2/3 of the pressure, temperature and level indication was lost. The reactor tripped, feedwater was lost and the once through steam generators (OTSGs) dried out.
Subssauent refilling by the main.fegdwater (MFW) system caused a primary system overcooling
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and an actuation of high pressure injection (HPI) and emergency feedwater.
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(EFW). ActuationofHPIandEFWcausedsevereovercoolingrates(approx-0 imately 300 F/hr) un'til-the' pumps were partly secured: by p'lant operators'.-
Actuation of HPI also caused repressurization of the primary system.
Operators did not recognize until approximately one hour later that 0
primary system temperature had been reduced to about 285 F'(because of preoccupation with.re,storation of fiNI/ICS equipment).
If an overcooling event such as that at Rancho Seco in 1978 were.to occur l
,,even for the vessel with the, worst ma,terial properties in the current population of reactor vessels,'the staff would not expect a failure.
The staff conclusion is supported by an analysis of 1;he. Rancho Seco event performed by the Oak Ridge National Laboratory which indicat'ed-that it would be several years before any B&W-designed facility reached l
l the threshold irradiation level for crack initiation (that is, small' cracks growing to larger ones assuming conservative initial material properties for pressurized overcooling events equal in severity to the Rancho Seco event).
Some reactor vessels in Combustion Engineering (CE) and Westinghouse (W) facilities have somewhat higher irradiation histories; however, other mitigating factors provide a significant. margin to failure should a pressurized overcooling event similar to that at
- Rancho Seco occur.
In order to define what transient conditions more severe than the Rancho Seco event would be necessary to propagate a flaw through the entire vessel thickness, a number of investigations were initiated by the staff beginning in early 1980.
These investigations included defining the cooldown transients and accidents of interest and their respective probability, development of a computer code to perform the thermal transient and fracture mechanics analyses, and planning for pressurized thermal shock tests in the Heavy-Section Steel Technology Program at ORNL.
The, staff evaluations of this analytical work indicated that there could be a' problem if pressure vessels having initial material properties (fracture, toughness) less favorable than those fabricated more recently l..
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9 were subjected to severe pressurized cooldown transients after many years of neutron irradiation.
In order to assess the need for any immediate action, the PWR' industry Regulatory Response Groups'(RRGs) and PWR reactor manufacturers were briefed on this issue by the staff on March 31,.1981.
In a progress briefing on April 29, 1981, the PWR Owners' Group asserted that ~there was no need for immediate corrective action.
On May 15;-1981, the Westinghouse, Combustion. Engineering and Babcock & Wilcox Owners' Groups ' filed written responses supporting and -
i reiterating their conclusion that no immediate action was required on any operating reactor.
The ' staff has determined' tihat'no irmiediat,e licensing actions are required
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for plants under construction, plants under review for operating licenses, l
or operating faciliti.es; however, the staff has taken the following
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actions,:
1.
Meetings have been held on many occasions with industry representatives r
for detailed discuss' ions and exchanges of information.
2.
Evaluations are continuing for refinement of the staff.'s understanding of this safety concern and better definition of wha't actions the' industry and staff.must take to resolve this issue.
A number of efforts are now underway by the NRC staff to develop a better technical basis for a final resolution for this problem.
These programs may show the need for more extensive corrective action before vessels approach their end of design life state. A new project has been initiated at Oak Ridge National Laboratory (ORNL) to bring together a comprehensive evaluation of the many aspects of this problem in order to define the best course of regulatory action toward its understanding and resolution.
The Heavy-Section Steel Technology Program at ORNL is continuing, and first tests using a new pressurized thermal shock test
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facility are scheduled for FY1982. Tiie development of a computer code for probabilistic analysis of reactor pressure vessel failure utilizing
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fracture mechanics and Monte Carlo simulation techniques is continuing.
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Several potential co'rrective actions are possible, and will be considered.
These include:
1.
Reducing the neutron irradiation of the pressure vessel by replacing some or all, pf the outer row of fuel elements in the core with
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partially loaded or reflector elements; t
2.
Annealing the reactor pressure vesstl ';n-situ to restore a major _
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fraction of the fcacture toughne'ss whicn was lost due to. neutron irradiation.
Annealing is feasible from a metallurgical standpoint, but practical application.is difficult and potentially expensive; 3._
Reducing the thermal shock during some transients.by raising the temperature of the emergency core cooling system (ECCS) injection water; and 4.
Reducing the probability of the event by control system designs that would prevent repressurization, and/or by operator actions to prevent repressurization.
The NRC staff and its contractors have been, and will continue to be, extensively involved in the development of the technology of this issue.
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81-2174 Logg ng Date No.
NRC SECRETARI AT-I s
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TO: O commissioner X XU E e.c. Der /Oper.-
O cen.coun i O cene ti on O soricitor O secretary O rubi.c Affairs O in.oector a Auditor a
O e iieveveiverion Sen Christooher J. Dodd incoming:
From:
Pall adi nn Date 10/R/R1 To:
subieccnneprned ahnut recent rots that radiation.may be causina embrittlement in reactbr vessel at Haddam Neck uraes NRC to comDiete evaluation of North East Utilities mission providing info.on this situation Prep.r. reply for sign.tur. of.
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