ML20125D412
| ML20125D412 | |
| Person / Time | |
|---|---|
| Issue date: | 10/07/1976 |
| From: | Baer R Office of Nuclear Reactor Regulation |
| To: | Novak T Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20125D401 | List: |
| References | |
| REF-GTECI-A-26, REF-GTECI-RV, TASK-A-26, TASK-OR NUDOCS 8001140285 | |
| Download: ML20125D412 (8) | |
Text
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4 UNITED ST ATES f,"
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WASHINGTON, D, C. 2C555
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V OCT 0 71976 e ' p s
MEMORANDUM FOR:
T. Novak, Chief, Reactor Systems Branch, DSS FROM:
R. Baer, Chief, Reactor Safety Branch, DDR
SUBJECT:
RSB BRANCH TECHNICAL POSITION ON REACTOR COOLANT SYSTEM OVERPRESSURE PROTECTION Your memorandum of September 7 requested comments to assist in formulating requirements for a branch technical position on systems to protect against overpressurization of the reactor vessel when water-solid. The Reactor Safety Branch response to that request is enclosed.
1 h n:c* -l Y,,/ 22. 3 Robert L. Baer, Chief Reactor Safety Branch Division of Operating Reactors
Enclosure:
As stated cc:
V. Stello D. Eisenhut 90017148 C. Berlinger F. Coffman S. Weiss G. Lanik
Contact:
G. Lanik, 00R X28050 8001140
4 PROPOSED BACK FIT ACTION FOR DOR _
3 The proposed position should be considered by R C since its implementation during the rereview of operating plants would impose definite backfitting requirements. Operating reactors characteristically have higher copper content in their weld materials and have experienced Therefore under upset greater neutron fluence on their vessels.
conditions, resulting from overpressurization of the RCS while cold, the likelihood of reactor vessel failure due to brittle fracture is more pronounced (i.e.: higher NDT transition temperatures) for operating plants than for newer plants and prompt action should be taken to provide adequate overpressure protection.
Reviews of available abnormal occurence reports and subsequent discussions with utilities and NSSS suppliers have indicated that there have been a great variety of causes of overpressure transients and that the administrative and procedural measures taken to prevent There are no obvious measures subsequent events have been ineffectual.
which can be taken to prevent the occurrence of all of these initiating events in operating reactors and, at present, few plants have adequate systems installed to mitigate the consequences of events that could lead to overpressurizations in violation of Appendix G limits. The 00R staff feels that for most operating reactors, procedural and design c,hanges wJodgrequired to solve the problem of meeting a safety limit based on Appendix G.
l To this end, DOR,by letter dated August 13, 1976 directed all pWR 1_icensees to submit proposals for interim and long term changes to their reactor systems and operating procedures that would prevent the E
violation of Appendix G limits. As detailed in the August 13, 1976 I.
90017149
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i s letters, DOR is presently requiring that all licensees of PWR's submit the following information:
(1) An analysis of the RCS response to pressure transients resulting from a single failure or operator error that can occur during startup and shutdown operations with particular emphasis on plant response under water-solid conditions.
A description of those design modifications determined to be (2) necessary to prevent the plant from exceeding Appendix G limits including equipment performance specifications and system operational sequences, and (3) A schedule for implementation for the proposed design modifications.
For those situations in which the necessary design changes identified cannot be implemented within the next few months, the licensees were requested to identify short-term measures to provide assurance that an overpressure incident would not occur in the interi_m period _ until the permanent design changes could be made.
Certain
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All licensees have nor submitted their initial responses.
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' Westinghouse utilities have formed an owners group to " examine the 8
complexity of the overpressurization events and to identify similarities
~j between Westinghouse plants'for the purposes of detennining a consistent Il solution to this problem." As an interim procedure, the plants operating personnel "have been informed of the potential for over-pressurization and have been cautioned to minimize conditions which could cause such an event." Other utilities have requested additional time to formulate an adequate response. At least one utility has 90017150
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l indicated that a 60-day response is forthcoming that will satisfy the requirements of the D0R staff letter.
Because of the design differences between operating PWR plants, due to vintage and vendor, all operating plants will be evaluated on a case-7 by-case basis to determine the k'of the present and proposed plant system design and procedures to either prevent or mitigate the consequences of overpressurization events. The basic criteria to be used in the -
evaluation of the present or proposed system designs and operating
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procedures is that they provide reasonable assurance that no single failure or operator error will pressurize the reactor coolant system General guidelines to satisfy (RCS) in violation of Appendix G limits. _
these requirements for operating plants are discussed below.
Seismic and Quality Group The equipment utilized to protect against overpressurization of the reactor vessel Mile in a cold condition should be of high quality to assure operability when required and should provide protection against 6'
potential safety problems during normal operation when it is not required, y
For operating plants, redundancy and diversity of appropriate high quality systems could be used as an alternative _t_o N class valve (s) and J /
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IEEE 279 electrical standards as long as the system is tested periodically k6'?
,a and meets single failure criteria. However, the seismic and quality group Y /
classification of all overpressurization protection equipment should be f> f
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equivalent to other safety systems in the plant.
t 90017151 l
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_ Design Criteria l
The an_alyses_ to be performed by all licensees shou
-These should include RCS themal pressure sources while water solid.
l t pumps expansion from pressurizer heaters, decay heat, reactor coo such and secondary side steam generator feedback, and fluid source Low pressure as charging pumps, safety injection pumps and accumulators.
i d primary system relief valves should provide at least 100% of the requ re t limiting relief capacjty, as detemined by analysis assuming the mos single failure or operator error as the initiating event.
A defense in depth approach should be utilized to supplem gdesion improvements to minimize the proba i i trative an overpressurization event by upgrading procedures and adm
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Methods other than providing primary system relief c controls.
i from may be considered to acconnodate the mass addition result
[d inadvertent actuation of safety injection pumps and accumu e
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Disabling of safety injection pumps and/or valves during wat operation might be an acceptable _ alternative.
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The impact of all procedural changes on overall plan For example, on present Technical Specifications should be evaluated.
idual the disabling of safety injection pumps or the opening of th heati removal (RHR', system isolation valves (to utilize R bubble in relief capacity) prior to collapse of the pressurizer steam i
some cases would be in violation of existing Technical 90017I52 i
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, Operator Action,' Controls and Instrumentation During nonnal plant operation, as well as during startup or shutdown In emergency procedures, operator action is permissible and encouraged.
f situations, credit for operator action is not permitted for a minimum p
i Therefore, operator actions would be pemitted f
of ten (10) minutes.
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he d sa 1 ng he sa et in ion sy t y
In such cases when operator maneuvering power to valves and/or pumps.
actions are specified in procedures, adequate safeguards would be f
required, ie: warnings in the procedures and automatic alarms., to provide assurance that the appropriate action has been taken.
In the event that an overpressure transient were to occur, credit 4
for operator action to teminate the transient prior to exceeding h,
Appendix G limits would also be pemitted if the licensee could demonstrate The by analyses that sufficient time and information were available.
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inclusion of alanns, annuciators and instrument readouts in the control room would be required to assure that the necessary action would be completed in a timely fashion.
For example, if the pressurizer relief valves are utilized for low pressure RCS relief, an annuciator should be provided in the control room In l
to indicate if the low pressure set point has been exceeded.
l addition, indications of relief valve operability and a status light panel showing the status of all components which could contribute to l
an overpressure transient, should be provided to enable the operator to
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terminate the transient.
90017153
. Ig, to provide maximum flexibility in plant operation, b
actuation of overpressure relief valves would be automatic with the set
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point programed in coordination with the pressure-temperature limit E
of Appendix G.
However, due to practical limitations involved in obtaining accurate temperature readings at the vessel wall, this P-T programed option would be difficult to implement, especially on operating plants. Therefore an acceptable alternative would include a system with automatic relief capability at a single low pressure
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set point, detennined by the low temperature Apoendix G limit.
This could be achieved with either pilot-operated or spring loaded relief valves, which were isolated during normal plant operation and enabled during low pressure operation as part of the normal shutdown procedure prior to initiation of water-solid conditions.
Test Requirements
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The Technical Specifications should provide for testing of the
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overpressure protection system adch)old shutdown, and, I an extended shutdown,should require die testing.
Suninary The impact of these requirements on operating plants will be evaluated on a case-by-case basis and will depend on the presently available pressure relief systems. Some plants have pressurizer relief s
valves and high pressure sensors that could be modified relatively easily for low pressure relief. Some plants, on the initiative of the utility's design review group, were originally constructed with this type 90017154
. of system protection. Other plants have relief capacity on the RHR system that could be used as a backup if the RHR system were not isolated when the relief capacity was required. Regardless of the hardware changes and additional procedural and administrative controls proposed by the licensees, they should be analyzed to assure that other aspects of plant safety are not sompromised.
90017155 i
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