ML19317H498
| ML19317H498 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 02/19/1980 |
| From: | Diianni D Office of Nuclear Reactor Regulation |
| To: | Reid R Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8006110503 | |
| Download: ML19317H498 (4) | |
Text
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1 UNITED STATES a 'f
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NUCLEAR REGULATORY COMMISSION 3.vh.e[. i WASHINGTON, D. C. 20555 t '.>4'f l h
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February 19, 1980 MEMORAtiDUM FOR: Robert W. Reid, Chief THIS DOCUMENT CONTAWS Operating Reactors Branch Division of Operating Reactors POOR QUAUTY PAGES FROM:
D. C. Dilanni, Project fianager Operating Reactors Branch M Division of Operating Reactors
SUBJECT:
UPGRADING THE RELIABILITY OF POWER OPFRATED RELIEF VALVE (PORV)
The accident at Three Mile Island on March 28, 1979, has focused our attention on the reliability of power operated relief valve (PORV) since the mal-function of this valve contributed to severity of the accident.
By memorandum dated October 4, 1979, the undersigned expressed his views and assumed the PORV will be used in the future as a means of controlling pressure in a pressured water reactor.
Such an assumption has been questioned since during the period following the accident actions have been taken and programs recommended that could have diminished the importance of this valve as a contributor to similar events in the future (i.e., hard wire reactor trip from a turbine feedwater trip, changes in valve pressure actuation setting, installation of subcool meter, flow measuring elements downstream of the PORV, etc.).
The TMI-2 task action plan and elsewhere recommends testing of the safety relief valves and PROV's as well as automating the block valve if PORV fails.
Based on discussions with the staff and the continuing evaluation of this matter, one area that does not appear in the current crocosed programs is the improvement of the reliability of the ?ORY itself.
Prior to imolementing a program for imoroving the reliability of the PORV, we should define th_e mission of the PORV in light of recent actions taken to date.
In defining such a mission, it appears tne following needs to be addressed:
1.
The fundamental safety requirements of the PORV and its block valve need to be defined.
In the TMI-2 action plan ar.d elsewhere, it is recon 7nended that the system be modified such that tne block valve automatically closes if the PORY fails to close.
Currently, there is no requirement that the block valve be~ open and in fact, many reactors operate with the block valve closed for long periods of time.
Therefore, if we don't need the block valve to be open for safety reasons, we should re:uire that the block valve and the PORY be closed at all times during ooeratien.
If, on the other hand, there are safety reasons for naving the block valve coen, these reasons should be cefined, the recuired reliability of closure (and opening) should be defined, tne recuired resocr.se of the valve snould be cefined (pressure setting to open and close, c:ening time), valve cacacity and the environmental conditions of the system s.:ald ce defined (temoerature, seismic, etc.).
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~ 2. 'The-acceptable. operational benefits of the valve should be defined.
'The purpose of the PORY was to reduce the number of reactor scrams associated with pressure increase transients, thereby reducing the number of thermal cycles on reactor components and improving reactor operating' efficiency. This is considered as an important benefit by
-many including the undersigned.
3.
Consideration of the above will determine the required closure reliability of the PORV/ block valve system.or conversely -the closure reliability will determine which alternatives are acceptable.
The closure reliability of the PORV/ block valve system if modified for auto.atic block valve is dependent on the reliability of PORV to close and the reliability of the block valve'to close upon failure of the PORV to close. As of jet, no
' definition has been made of the required reliability or the attainable reliability of the automatic block valve closure.
If the required closure reliability is high, it is likely that the required PORV closure reliability would need:to be much higher than it is now.
It should be noted that the automatic-closure feature of the block salve could likely decrease the opening reliability of the combined system.
Therefore, it is recommended that a study be performed to establish the basic requirements of the PORV/ block valve system to resolve whether or not the system is needed for safe power operation.
If it is determined that such a system is -
needed then 'it is further recormended that a program be initiatied to improve the reliability of the PORV itself.
Such a program should consider the following areas for improving the PORV/ block valve system.
A.
Voorading the Mechanical Design of POR7/ Block Valve The establishment of. requirements for upgrading the Mechanical design of PORV/ block valve cannot be completed until the staff consults with both the NSSS vendors and the valve manufacturers (i.e., item 1 and 2 recommen-dation, October 4, 1979 memorandum).
However, some design areas which affect the movement of valve internal par.ts that should be investigated are as follows:
a.
Review tolerances of the valve moving parts during operation
-b.
Review material selection to prevent galling of moving parts c.
Analy:e the energy requirements during valve operation d.
Consider insitue replacement o# valve carts subjected to long term wear for those valves welded in place e.
Determine what measures were taken including certification testing in determining valve'lengevity f.
Evaluate the exceptions (if an.e) taken by the valve manufacturer :o to the engineering specificati:n issued by the NSSS vendor i
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B. -PORY Valve Testing It/is unreasonable to assume that the PORY will last throughout the-life of the plantf(usually 40 years) without performing periodic maintenance on the valve. Usually the valve plug and seat ring wear with valve use resulting in an increase'in the leak rate. The objective of the.PORV testing program should determine the performance limit of the valve.
In
.the case of the TMI, the temperature downstream of PORY was reading abcve 200*F for'some time prior to tre event indicating the FORY had been leak-ing. As we know, this contributed to the confusion on whether the valve was actually closed during the. accident.
-The PORV.of each valve manufacturer, fabricated to give the least-conservative performance, but still within design limits, should be tested under the worse case pl. ant conditions.
The testing program should demonstrate 'the number of cycles the valve could undergo without sticking in either the open or closed positions and determine wear as a function of leak rate. The demonstrated number of PORV cycles without sticking could be used to determine a lower safe number of cycles to be used by incastry.
It is conceivable that depending upon valve use that after several fuel cycles the safe t
number of cycles would be reached or the leak rate is high enough to warrant the replacement of valve internals.
In arriving at an acceptable leak rate, one should censider the makeup and letdown capacities of the primary system of the particular plant and still maintain adequate water chemistry.
The reactor operator.should be given soecific instructions in the use of the temperature readout downstrear of the PORV to indicate an excessive leak rate. As part of the testing program, one may consider the feasibility
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of establishing specific temperature; range cf the temperature sensor downstrear of the PORV as a means Of monitoring leak rate conditions of the.PORV.
Such information from tne test program would standardize the location of the temperature sensor-with respect to the PORV (i.e., such information would'be passed on to the Designers) and form the basis for specific instructions for the reactor operator.
Furthermore, it may be o
advisable to install totalizers on tne PORV's in service in order that an accurate valve operating history is maintained.
C.
Automatic Actuation of Block Valve to Assist PORY Automating the actuation of the bi:ck valve in the event of failure of PORY is another defense in deptn esasure that could improve the reliability of the 40RV.
It was brought to cu attention that such a featureLnow exists in a PWR in Germany. We should cb sin complete detail information regarding the au::matic scheme.that ~was deve:::ed in Germany. The automatic actuation cf.the block valves serving as a :acku: valve can be achieved by the action
- f One ?ORV, system pressure and ::ssibly tne temperature sensir.g device ese carameters can be utilized in the
.lecated :ownstream of the PORV.
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Prior to the event at TMI, the PORY would coen at a set pressure of
~2355 psig and close when the :ressure falls below ~2305 psig.
2.
A pressure signal. set point, set between the PORV's reset pressure
(-2305 psig) and.the normal operating pressure coupled with a signal from the PORV's stem position (i.e., valve stem not in closed position) would actuate the closing of the. block valve. The PORV stem position signal should orginate from a linear transducer or a similar device that monitors valve stem motion directly.
3.
The temperature sensor downstream of the FORV (upon high temperature) could also be used to actuate the closing of the block valve. However, arriving at an acceptable temperature set point for closing the block valve may be difficult in that the rate of temoerature rise is a function of the leak rate.and themoccuple location with respect to the PORV - for operating plants.
4.
The same signal used to open the FORY (i.e., system pressure ~2355 psig) could be used to open the block valve during follow on transients for pressure control. Operation in tnis mode in which pressure control is by the block valve should be c:nsidered a temporary operating condition until next reactor shutdown at which time the PORV is repaired.
5.
The operator in the control room should have the capability to i
manually everride the automatic ac:uation of the block valve.
Annunication in the control roca should also be available to alert the o:erator that the PORV has failed and the operation is continuing
-by the use of the block valve.
-u-:herore, an SI signal should De used to close both the block valve and the PORV.
In conclusion, a need exists to determine :ne rission of PORV with respect
- nor al clant o:erations, the relationshio o# tne PORY with the block valve and ne function cf the PORV during accider: c:ncitions. The reliability of the PORV/ block valve can be improved by utilizing sound engineering principles and ne degree reliability can te demonstrated oy an adequate testing program.
Furthe-more, these studies that would incluce in:ut from NSSS vendor and the valve manufacturer should be coordinated with One valve testing program.
Finally, the technical aspects of this task wo.ic be considered complete with
- ne issuance of ar. a:ceptable NRC staff cosi-icn regarding upgrading the eliability and defining the mission of the 00:V/biock valve system.
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