ML20148H627
| ML20148H627 | |
| Person / Time | |
|---|---|
| Site: | Big Rock Point File:Consumers Energy icon.png |
| Issue date: | 03/22/1988 |
| From: | CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.) |
| To: | |
| Shared Package | |
| ML20148H610 | List: |
| References | |
| NUDOCS 8803300050 | |
| Download: ML20148H627 (13) | |
Text
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CONSUMERS POWER COMPANY Docket 50-155 Request For Change to The Technical Specifications License DPR-6 For the reaeons hereinafter set forth, it is requested that the Technical Specifications contained in Facility Operating License DPR-6, Docket 50-155, issued to Consumers Power Company on May 1, 1964 for the Big Rock Point Plant be changed as described in Section I below:
I.
CHANGES A.
Delete information note prior to Section 11.3.1.5/4.1.5.
B.
Add footnote number to end of 11.4.1.5.B and add footnote (1) to bottom of page to read as fcilows:
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"(1)For the duration of Cycle 23 operation, quarterly test operation (during cold shutdown) of the depressurizing valves is not required.
In lieu of this surveillance testing requirement, full stroke exercise one depressurizing valve and partial stroke exercise the other three depressurizing valves during the refueling outages prior to and following Cycle 23 eperation. If no shutdowns occur during Cycle 23 operation, or if all four depressurizing valves are required to be partial stroke exercised for other reasons during Cycle 23 operation, continue this surveillance testing variance throughout Cycle 24 operation."
8803300050 880322 PDR ADOCK 05000155 P
DCD MIO28S-0044-NLO4
2 II.
DISCUSSION Change A deleter an informational note which was added several years ago when Big Rock Point Technical Specifications were reformatted. The note pertained to the format of this Technical Specification section only and is no longer required.
P Change B proposes a temporary change to the Rec.ctor Depressurization System (RDS) deptessurizing valve surveillance requirements. Attachment A provides a brief system description.
Existing evidence suggests the current RDS surveillance testing f requency has placed the Plant in a cycle which results in cooldown end heatups, which place additional stress on all plant systems and equipment. The cycle is initiated by a depressurizing valve pilot valve leak. The Plant unidentified leak rate increases and the Plant is shut down to repair the leaking pilot valvo.
Because the depressurizing valve must be completely disassembled to repair the leaking pilot, a post-maintenance partial stroke test is required to verify cotreet reassembly of the repaired valve.
If it has been greater than three months since the last partial stroke test on any one of the other three valves, they are required to be partial stroke tested. Generally, if a pilot valve does develop a leak, it occurs soon after star'.up.
The unidentified leak rate gradually increases, due tc. a leaking pilot valve, and the Plant is again required to shut doun and repair the leak. Frequently, the new leaking pilot valve is in one of the three valves that was tested, not the one that was repaired. Pilot valves which aren't partial stroke tested during the previous shutdown MIO288-0044-h*LO4
3 generally don't leak, but frequently the Plant is. forced to shut down-for repair of a leaking pilot valve shortly after the three-month point, and the testing / leaking cycle is repeated. This cycle has been very difficult i
to break and is a major contributor to reduced availabflity and unneeded Plant startups and shutdowns which challenge-overall Plant safety.
Approval of the proposed temporary surveillance testing frequency will allow the collection of essential data to further substantiate the observed relationship between the current surveillance testing requirements and pilot valve leakage.
Current Surveillance Testing Requirements i
i All RDS depressurizing valves are partial stroke tested and all pilot valves are full stroke tested each cold shutdown, except where it has 1
been less than three months since the last test. These requirements are contained in Section 11.4.1.5.B of the Technical Sp2cifications and are j
paralleled in our proposed Inservice Testing Program.. The depressurizing i
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valves are ASME category B, Class 1.
Because full stroke testing of the depressurizing valves is not possible 1
without removing them from the system, relie.C from this code requirement j
has also been requested in our proposed IST Prearam. At present, these valves are partial stroke tested using air trapped in the spool piece between the isolation valve and the depressurizing valve.
Some evidence is available to show that this test is a significant cot:tributor to chronic pilot valve leakage.
MIO288-0044-NLO4
4 Proposed Tet porary Surveillance Testing Requirement Remove one depressurizing valve from the system and perform a full stroke test of that valve during the refueling outages prior to and following Cycle 23 operation.
It is anticipated this will be accomplished by sending the valve off site to be full stroke tested with steam, i
The reuaining three depressurizing valves would be partial stroke tested also during the refueling outages prior to and following Cycle 23 opera-j tion. Partial stroke testing would continue to be required after the performance of maintenance which could affect valve performance. These requirements would remafa in effect for the duration of the next operating cycle (Cycle 23).
i It is also requested that these proposed temporary surveillance testing requirements be extended an additional operating cycle under either of the following circumstances:
1.
No shutdowns occur during Cycle 23, or 2.
All four depressurizing valves must be post maintenance partial stroke tested during every Cycle 23 shutdown.
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5 If the above conditions occurred during Cycle 23, the needed operating performance data to properly evaluate partial stroke testing effect on pilot valve leakage would not be provided. Upon completion of the trial operating cycle, the operating performance of-the pilot valves would be evaluated and any additional Technical Specification changes would.be requested.
If partial stroke testing is shown to be a significant contributor to pilot valve leakage, the anticipated changes would be a permanent contin-uation of these proposed temporary surveillance testing requirements.
One depressurizing valve would be full stroke tested each refueling i
outage so that all four valves are full. stroke tested over four consee-l utive refueling outages. The remaining three valves would be partial.
stroke tested each refueling outage.
Basis for Proposed Temporary Testing Requiremente Depressurizing valve pilot valve leakage has been.a chronic Plant problem i
since the RDS system was first installed. Reducing pilot valve leakage 1
is imperative, since leakage can cause erosion which can lead to inad-vertent valve actuation and a primary system blowdown into the containment.
Deveral attempts
'.o correct this problem have been made by the Plant staff and Target Rock Corporation, the valve manufacturer. Attachment B lists past and current attempts.
Previous actions have not been totally successful.
Further attempts to reduce pilot valve leakage currently are MIO288-0044-NLO4 i
6 being developed. There is, however, evidence that pilot valve leakage is initiated by partial stroke surveillance testing.
Following refurbishment of a leaking pilot valve, it is bench tested for leak tightness. The valve manufacturers standard acceptance criteria is 32 bubbles per minute. To help reduce our leakage problem, standard Plant practice for several years has been to obtain a 0 bubble per minute 1
leak rate before accepting a pilot valve for service.
Because a partial j
stoke test is required to verify correct valve reassembly, approximately l
40% of the valves develop pilot valve leaks almost immediately upon returning to power.
1 l
A review of the surveillance test history and operating records shows 12 cases where depressurizing valves did not leak prior to a Plant shutdown, did not require testing during the shutdown, and were not tested during j
the shutdown. Not one of these valves developed a pilot valve leak during or soon after Plant startup.
There are also seven cases where no maintenance was performed on the depressurizing valves, but they were partial stroke tested as required prior to Plant startup. Upon return to power, in four of seven cases or about 57% of the time the pilot valves leaked. These results clearly show a relationship exists between the frequency of pilot valve leakage, and the number of test cycles the pilot valve experiences.
By reducing the required surveillance testing frequency, the probability of pilot valve leakage can be reduced.
MIO288-0044-NLO4
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7 Even though partial stroke testing. frequency would be reduced the surveillance test results indicate that out of 99 individual valve tests
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there have been no failures.
In 1986, before lift voltage and pilot travel were measured as part of the rebuild procedure, one valve did experience long stroke times during testing. This was found'to be due to inadequate pilot travel..This condition was corrected and now pilot travel and lift voltage are measured as.a part of the rebuild procedure.
The pilot valve which had the excessive stroke time was never used in I
service during power operations. Also, a. successful ~ full stroke test was performed in 1985 prior to installing a new main seat in one of the depressurizing valves. This excellent test success rate demonstrates j
that these valves will reliably open on demand. The pilot _ valve perfor-mance data noted above would continue to be monitored and recorded for the duration of the trial operating cycle.
In summary, the relaxation of the surveillance testing frequency is l
^
justified based on the observed relationship between the depressurizing valve surveillance testing frequency and pilot valve leakage, the excel-lent history of valve responses when called upon, and the built-in redundancy provided by each of the four blowdown paths being capable of passing one third of the total required flow rate.
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j III.
ANALYSIS OF NO SIGNIFICANT HAZARDS CONSIDERATION i
This proposed temporary surveillance testing frequency change suspends the requirements for a partial stroke test of the rea.'. tor depressurizing 1
MIO288-0044-NLO4 4
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8 valves when in cold shutdown following three months of operation. The suspension would be in effect during Cycle 23 operation and possibly Cycle 24 operation and is' intended to allow accumulation of necessary:
data to establish whether er not partial stroke testing causes or significantly contributes to che initiation of pilot valve leakage. With the current system configurati(n, partial stroke testing-af these valves i
is not possible with the Plant operating at power. Under the current requirements, if the Plant operated at power for an ent.re cycle, it d
would be permissible to not partial stroke test any of the depressurizing valves until the end of the cycle.
Because existing evidence suggests partial stroke testing is a significant contributor to leakage through the depressurizing valve pilot valves, it is expected fever Plant shutdowns would be required to repair pilot valves. If fewer Plant shutdowns are realized, an overall decrease in the probability of an accident or malfunction of equipment would result, as systems and 1
i equipment would not be subjected to additional cocidown and heatup i
cycles. The required partial stroke test provides verification that the depressurizing valve will function on a periodic basis.
In an ideal situation, this test would be performed once, at the end of the cycle.
The test nethod is not being changed. Therefore, an accident or malfunc-tion of a different type is not created. Reducing testing frequency could be considered as a reduction in the depressurizing valves margin of safety. However, the number and capacity of valves would not be altered.
Also, the valves have never failed an "as found" surveillance test since 1
1 i
6 MIO288-0044-NLO4
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9 installation. The proposed temporary surveillance testing frequency change has the same margin of safety-as would occur if the Plant operated continuously without any shutdowns for a full cycle;.ie, no partial stroke tests would be performed. Consequently, this proposed. temporary surveillance testing frequency change does not involve a significant hazards cons'.deration.
VI.
CONCLUSION The Big Rock Point Plant Review Committee has reviewed this Technical Specification Change Request and has determined this change does not-involve an unreviewed safety question and, therefore, it involves no significant hazards consideration. This change has also been reviewed under the cognizance of the Nuclear Safety Board. A copy of this Technical Specification Change Request has been sent to the State of Michigan official designated to receive such Amendments to the Operating License.
l r
I CONSUMERS P
.R C ANY s
By
/
e C' L HeYns, Se'nior Vice' President Energy Supply Sworn and subscribed to before me this 22st day of March
, 1988.
f tt)
Elaine E Buehrer, Notary Public Jackson County, Michigan My Commission expires October 31, 1989 MIO288-0044-NLO4
ATTACHMENT A Description of System The purpose of the RDS is to provide a means to depressurire the primory system following a small break loss of coolant accident so that the core spray system can provide core cooling. The RDS valves are not used for overpressure protec-tion and do not have a means of actuating in response to high system pressure.
The system is actuated as a result of low level signals combined with core spray system availability. The following pages show a simplified system piping diagram (Figure 1) and a cutaway view of the depressurizing valve (Figure 2).
The Reactor Depressurization System (RDS) consists of four depressurization paths. Each path has a depressurizing valve (pilot operated solenoid valve) and an isolation valve (split vedge gate valve with air operator) branching from a common header. A means of bypassing steam around the isolation valves is provided by a remotely operated bypass isolation valve.
It branches into four lines, each containing a manual bypass valve, which tie into the depressur-ization paths between the isolation valve and the depressurizing valve.
Both the depressurizing valves and the isolation valves are normally closed. The bypass valves are designed to be open during power operation. This allows the j
piping between the isolation valves and the depressurizing valves to be pressur-ized. The bypass line allows the isolation valves to be stroked each 90 days and reduces the posaibility of inadvertent opening of the depressurizing valve if the isolation valves inadvertently open (due to loss of air, etc) during power operation.
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FIGURE 1 REACTOR DEPRESSURIZATION SYSTEM Depressurizing Isolation Valves Valves A
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ATTACHMENT B Actions Taken to Reduce Pilct Leakage 1.
Increased seating force by addition of spring preload washer. (November 79) 4 2.
Improved disc seat alignment by using closer tolerance pilot discs.
(November 79) 3.
Eliminate "crud" trap for seat contaminants by chamfering the pilot valve inlet line socket weld connection. (February 85) 4 Change fron carbon steel valve top flanges to stainless steel in order to 1
reduce sources of possible corrosion products.
(March 87) l S.
Increase seating force of pilot disc by enlarging seat diameter and doubling preload spring. A larger solenoid coil was required to compensate for this modification.
(March 87) 6.
Added a sleeve between the pilot valve discs to eliminate the direct impingement of steam onto the pilot disc af ter passing thru the main pilot disc orifice.
(March 87) 7.
Provide a softer pilot valve disc seating surface by nickel plating its seat.
This change is currently being tested on one of the four depres-surizing trains and has not yet been evaluated.
(November 87)
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