ML19248A222
| ML19248A222 | |
| Person / Time | |
|---|---|
| Issue date: | 07/30/1981 |
| From: | Palladino N NRC COMMISSION (OCM) |
| To: | Udall M HOUSE OF REP., INTERIOR & INSULAR AFFAIRS |
| Shared Package | |
| ML19248A223 | List: |
| References | |
| SECY-81-286, NUDOCS 8108130032 | |
| Download: ML19248A222 (3) | |
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The Honorable Morris K. Udall, Chairman f.!
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Washington, D.C.
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Dear Mr. Chai ran:
This letter is in response to your inquiry of May 1,1981 regarding the question of nuclear reactor pressure vessel integrity when subjected to thermal shock and subsequent repressurization during an overcooling transient.
Pressure vessel thermal shock has been considered for many years in the context of assuring integrity of the vessel when subjected to cold emergency core cooling water during a large loss of coolant accident (LOCA). Based on a series of thermal shock experiments (unpressurized) conducted at Oak Ridge National Laboratory (ORNL) beginning in 1976 and fracture mechanics analyses, it was concluded that a postulated flaw would not propagate through the vessel wall during a large LOCA and that the vessel integrity would therefore be maintained during subsequent repressurization.
As the result of operating experience and further analysis, it was subsequently recognized that there could be transients in pressurized water reactors (PWRs) in which the vessel could be subjected to overcooling (thermal shock) at the same time that primary system pressure remained high.
In those pressurized thermal shock transients, the vessel would be subjected to pressure stresses superimposted upon tha thermal stresses resulting from the temperature difference across the vessel wall.
In order to define what conditicas would be necessary to propagate a flaw through the entire vessel thickness under those conditions, a number of investigations were initiated by the staff beginning in early 1980 including defining the cooldown transients of interest and their respective probability, development of a computer code to perform the thermal transients 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 are sumarized in a recent
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Comission Paper (copy enclosed).
In this paper the staff indicated that there could be a problem if pressure vessels having initial material
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In order to assess the need for any imediate hetion, 8108130032 810730 T
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The Honorable Morris K. Udall the PWR industry Regulatory Response Groups (RRGs) and PWR reactor manufacturers were briefed on this ime 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 irrnediate corrective action.
On May 15, 1981, the Westinghouse, Combustion Engireering and Babcock & Wilcox Owners' Groups filed written responses supporting and reiterating their conclusion that no immediate action was required on any sperating reactor.
Based on our independent review, the Office of Nuciear Reactor Regulation (NRR) staff concluded, as discussed in the enclosed paper dated May 4, 1981, that no irraediate licensing actions are required for operating reactors because there is no evidence indicating that the radiation damage to these reactor vessels has reached an extent that requires such in a B&W reactor is estimated to be aboct 10 gy of such an event occurring actions. Please also note that the probabili per reactor per year and for Westinghouse and Combustion Engineering designed reactors, it is lower, perhaps by an order of magnitude. However, the staff has been performing a detailed review of the problem and will report its recorraenda-tions to the Commission in about five months. As a result of this review, equipment and/or procedural changes may be required.
A number of efforts are now underway by HRC to develop a full t.nderstanding for this problem. These programs may show the need for more extensive corrective action. A new project has been initiated at 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 t%t facility are scheduled for FY 1982. The development of a computer code for probabilistic analysis of reactor pressure vessel failure utilizing fracture mechanics and Monte Carlo simulation techniques is continuing.
Several potential corrective actions are possible, and will be considered.
These include:
1.
Reducing the neutron irradiation of the pressure vessel by replacing some or all of the outer row of fuei elements in the core with partially loaded or reflector elements; 2.
Annealing the reactor pressure vessel in-situ to restore a major fraction of the fracture toughness which 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 by raising the temperature of the emergency core cooling system (ECCS) injection water; and 4.
Reducing the probability of the event by control system desiges that would prevent repressurization, and/or by operator actions to prevent ronroc cari n tinn _
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The Honorable Morris K. Udall The NRC staff and its contractors have been, and will continue to be, extensively involved in the development of the technology of this issue.
The independent capability of the staff and its contractors is briefly sumarized in Enclosure 2.
The Comission regards pressurized thermal shock as a' serious problem.
Any significant possibility of reactor pressure vessel failure has to be regarded as s9rious because there are no engineered safety systems designed to deal with a large pressure vessel rupture. The Commission has directed the staff to continue close involvement with the affected licensees on this issue. The staff will be conducting meetings with the PWR Owners Groups during the week of July 27 to discuss current and future activities.
In the near future, the staff will be issuing letters to several licensees requiring the submittal of an action plan within 90 days for dealing with this problem. The plan must consider at least the corrective actions listed earlier.
The NRC staff and its contractors have the capability to evaluate and resolve this issue. Any specific regulatory guidance that is needed as a result of our ongoing evaluations wi11 be premulgated promptly so that any potential threat can be adequately addressed and the required integrity of pressure vessels in operating facilities can be assured for their remain-ing lifetime.
Sincerely, Original etCnel h7 yunzio J...Fa11adina Nunzio J. Palladino Chaiman DISTRIBUTION
Enclosures:
Central Files D. E1senhut
- 1. SECY-B1-286, dated May 4, 1981,
" Pressurized Thermal Shock" C PDR R-o r
- 2. Sumary of the Independent rapability of the NRC Staff and GIB Reading B. Snyder its Contractors in the Technology EDO Reading R. Mattson Rel ed to Pressurized Thermal n n cr der E. Case K. Knlel Representative Manuel Lujan (w/entif.)Co cc:
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Cleared with all Cmrs' Offices by SECY C/R.
V. Stello DCA Ref.-CR-81-027 H. Shapar OGC E. Case CHM RF H. Thompson P. Brandenberg (#10457)
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%i tu The r ?$we'.e Morris K. Udall The NRC staff and its contractors hcVe been, and will continue to be, extensively involved in the develppment of the technology of this issue.
The independent capability of the staff and its contractors is briefly sumarized in Enclosure 2.
In summary, the NRC staff and its contractors have the capability to evaluate and reso'ive this issue. Any specific fegulatory guidance that is needed as a result of our ongoing evaluatiofts will be promulgated promptly so that any potential threct can be ideountely addressed and the required integrity cf pressure vessels in o rating facilities can be assured for their remaining lifetime.
Sincer ly, N zio J. Palladino C ainnan
Enclosures:
1.
SECY-61-286, dtd May 4, 1981,
" Pressurized Thermal Shock" 2.
Sustwy of the Independent Capability of the NRC Staff nd its Contractors in the Tectynology Related to Pressurized Th
..al Shock cc: Rep. Mamel Lujan (w/e cls)
DISTRIBUTION:
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HShapar OCA HThompson 0GC CHN RF JCarter PBrandenburg (ED0-10457)
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s The Honorable Morris K. Udall -
The NRC staff and its contractors have been, and will continue to be, extenstve'.y involved in the development of the technology of this issue.
The independent capability of the staff and its contractors is briefly sumarized n Enclosure 2.
In sunnary, the NRC staff and its contractors have the capability to evaluata and resolve _this issue. Any specific regulatory guidance that is needed as a result of our ongoing evaluations will be pronulgated promptly so that any potential threat can be adequately addressed and the reautred integrity of pressure vessels in operating facilities can be assured for their remaining lifetime.
Sincerely.
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Josenh M. Hendrie Chairman
Enclosures:
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1.
SECY 81-286 dated fiay 4, 1981,
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" Pressurized Thermal Shock" N
2.
Summary of the Independent Capability of the NRC Staff and its Contractors x'
in the Technology Related to Pressurized s
Thermal Shock cc: w/ enclosures Representative Manuel Lujan DISTRIBUTION Central Files PPAS S. Cavanaugh i
TERA D. Eisenhut B. Gabriel k
NRC PDR S. Hanauer J. Mullin
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GIB Reading R. Vollmer J. Butts
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ED0 Reading B. Snyder
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W. Dircks R. Mattson
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H. Denton T. Murley E. Case F. Schroeder E. Cornell K. Kniel T. Rehm P. Norian R. Minogue N. Anderson V. Stello SECY 81-0631 (3)
H. Shacar OCA E. Case OGC H. Thompson CHM RF OELD OCA J. Carter P. Brandenburg (ED3-10457) *SEE PREVIOUS RECORD 0')6037 COPY FOR CONCURRENCES 05/ /81 05/ /81 f
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The Honorable Morris K. Udall I believe it is clear from the above discussion that the NRC staff and
-its contractors have been and will continue to be, extensively involved in_ the development of the technology of this issue. The independent capability of the staff and its contractors is briefly sumarized in.
In summary, I believe the NRC staff and its contractors have the capability for the appropriate resolution of this issue and that this will readily follow from the base already established.
I expect that specific regulatory guidance will be forthcoming so that any potential threat can be blunted and the required integrity of pressure vessels in operating facilitics can be assured for their remaining lifetime.
Sincerely,
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Joseph M. Hendrie 3
Chairman Enclosurcs:
1.
SEL/ 81-266 dated May 4, 1981,
" Pressurized Thermal Shock" \\
2.
Summary of the Independent Capability of the NRC Staff and its Contractors in the Technology Related to Pressurized Thermal Shock cc: w/ enclosures
'N Representative Manuel 1.ujan s
DISTRIBUTION Central Files PPAS S.Cavanaughp'g B. Gabriel, f TERA D. Eisenhut J. Mullinb ' c)1 NRC PDR S. Hanauer GIB Reading R. Vollmer J. Butts' h:
EDO Reading B. Snyder W. Dircks R. Mattson H. Denton T. Murley E. Case F. Schroeder E. Cornell K. Kniel T. Rehm P. Norian R. Minogue N. Anderson V. Stelio SECY 81-0631 (3)
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