ML20116H021
| ML20116H021 | |
| Person / Time | |
|---|---|
| Site: | Limerick  |
| Issue date: | 11/05/1992 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20116H019 | List: |
| References | |
| NUDOCS 9211120239 | |
| Download: ML20116H021 (8) | |
Text
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SAFETY EVALUATION BY THE Offl(LOF NUCLEAR REACTOR RErWLATION flELATED TO AMENDMENT NOS. 57 AND 23 10 FACILITY OPERATING LICENSE N05. NPF-39 AND NPF-85 PHILADELPHI A ELECTRIC COMPANY LIMERICK GENERATING STATION. UNITS 1 AND 2 DOCKET N01 50-352 AND 50-353 1.0 1RIf0 DUCT 10N By letter dated August 11, 1992, the Philadelphia Electric Company (PEco, the licensee) submitted a request for changes to the Limerick Generating Station, Units 1 and 2, Technical Specifications (TS). The requested changes would change the T!s to clarify the flow Surveillance Requirement (SR) for the Suppression Pool Cooling (SPC) Mode of the Residual Heat Removal (RHR) System.
2.0 BACKGROUND
The RHR system is described in Section 5.4.7 of the Updated Final Safety Analysis Report (UFSAR) for the Limerick Generating Station (LGS), Units 1 and 2.
The RHR system is comprised of four independent loops.
Each loop contains a motor-driven pump, piping, valves, instrumentation, and controls.
Each loop takes suction from the suppression pool and is capat'e cf discharging water to the reactor vessel via a separate vessel nozzle loop c bart +.o the suppression pool via a full flow test line.
In addt'.;on, h 2ps A and B have heat exchangers that are cooled by the Residual Hea. 0 ment dervice L'er 9
(RHRSW) system.
These two loops can also take suctu fica the r act-recirculation system suctior and can discharge into the reactor recir ion ystem discharge.
The RHR system has five subsystems or modes of operation:
- 1) the Residual Heat Removal or shutdown cooling mode, 2) the Low Pressure Coolant Injection
'LPCI) mode, 3) the Suppression Pool Cooling (SPC) mode, 4) the Containment Spray Cooling mode and 5) the Reactor Steam Condensing mode. The latter has been abandoned at both Limerick and Susquehanna.
The functional design basis for the LPCI mode is to pump 10,000 gpm of water per loop, using the separate loop pumps, from the sup)ression pool into the core region of the reactor vessel.
In this modo, tie RHR pump recirculates the suppression pool water directly to the reactor vessel via the RHR heat exchanger bypass line without going through the RHR heat exchanger. When the RHR system operates in the SPC mode, the suppression pool water is pumped from the pool through the shitil-side of the RHR heat ed. N ger and returned to the suppression pool.
The heat in the suppression po G water is transferred to the RHRSW which flows through the tube-side of the RHR heat exchanger.
In the SPC mode of operation, the butterfly bypass valve around the heat exchanger is closed. A simplified diagram of the SPC mode for one heat exchanger is shown in the attached Figure 1.
9211120239 921105' PDR ADOCK 05000352 V
PDR 1
The RHR heat exchangers are vertical shell and tube units with 530 type 304L stainless steel "U" bend tubes, nominally one-inch diameter and 0.049 inch wall.
The tube sheet is also stainless ; teel clad.
Tht shell is carbon 2
steel.
The effective surface area of the tubes was initially about 6281 ft,
The heat exchangers were sized on the basis of duty for the shutdown cooling mode.
All other modes, such as SPC, require less heat transfer.
During normal plant operation, the RHR system is shut down and thus the RHR heat exchangers are not in operation. They normally are filled with domineralized water (wet layup) and a biocide and/or corrosion inhibitor.
Shutdown cooiing would normally be expected to be the most common mode of operation for the RHR heat exchangers. As discussed previously, the RHR heat exchangers also provide suppression pool cooling. Any time the suppression pool temperature exceeds a pre-set limit (e.g., 90'F during normal operation) flow commences through one of the RHR heat exchangers in order to cool the suppression pool water. During the Unit I cycle four operation, the suppression pool water had to be cooled periodically due to leakir.g safety relief valves (SRVs). During June, July and August of 1991, the IA heat exchanger was used for several hours, nearly every day, to cool the suppression pool water.
Between April 1991 and March 1992, the IA RHR heat exchanger was run for over 1200 hours0.0139 days <br />0.333 hours <br />0.00198 weeks <br />4.566e-4 months <br /> to cool the suppression pool water.
In early 1992, the licensee noted a decline in the heat transfer performance of the lA RHR heat exchanger and operations personnel began to use the IB heat exchanger to augment the IA unit.
timerick Unit I shutdown for the fourth refueling outage on March 20, 1992.
Because of the noted decline in the IA RHR heat exchanger performance near the end of the cycle, in May 1992 the licensee tested both the lA and IB units in accordance with Generic Letter (GL) 89-13 for heat transfer duty.
This testing (May 8, 1992) revealed that the lA heat exchanger exhibited a fouling factor in excess of design limitations.
The IB unit passed, but some degradation was noted.
To restore performance, the 1A unit was chemically cleaned with a relatively mild mixed organic acid solution primarily designed to remove suspected hardness scale.
The subsaquent testing showed little improvement in heat transfer.
The licensee decided to remove the bottom head and perform a visual examination of the tubes in the 1A RHR heet exchanger.
There are inlet and outlet isolation valves on the service water (Spray Pond water) lines into and out of each RHR heat exchanger.
The ll:ensee has experienced many problems with these 20" Anchor Darling valves, such as broken internals, sticking, leakage, etc. (See the Resident inspector's reports for November 18 to December 31, 1990, 50-352/90-27 and 50-353/90-27; September 1, to October 5, 1991, 50-352/91-18 and 50-353/91-19; January 5 to February 15, 1992, 50-352/92-03 and 50-353/92-03; et al).
Because the isolation valves were not leak-tight, the licensee had to establish and maintain freeze seals on the inlet and outlet lir.es to the 1A (and later the 18) heat exchanger to remove the bottom head. Upon removing the bottom' head of the lA heat exchanger on May 27, 1992, an examination of the lower part of the tubcs d sclosed large quantities of a black, slimy, tar-like substance that remained
,rter chemical cleaning.
The licensee contracted to hydrolyze the 530 tubes,
i 4 including the U-Bend areas. There is no estimate of the total amount of the black gunk removed, but from the writer's observation, there was a significant volume of black crud on the floor from hydrolyzing a single leg of each tube.
Following cleaning, the licensee performed a ;D0% eddy current (ECT) inspection of all 530 tubes, including the 'U" bend area where possible.
The ECT reveal-d defects (pits) in all of the tubes examined; 72 tubes (13.6%)
exhibited indications greater than 90% through wall.
The ECT indicated that over half the tubes contained defects of more than 60% through-wall.
Straight sections of two tubes were removed for examination.
As a result of various tests and analysis, the licensee plugged 37 tubes in the 1A heat exchanger.
In view of what they found in the lA heat exchanger, the licensee removed the head from the IB heat exchanger, and inspected the tubes.
The same black, slimy, tar-like substance was on the inside of the tubes.
The tubes were hydrolyzed, and inspected by eddy current testing.
Six of the U-Bends with the deepest measured pits (>89%) were evaluated by EPRI with an MRPC (Pancake Probe) to assess the characteristics of the pits.
All of the 530 tubes showed some pitting; 70% of the tubes indicated pit depths greater than 50% through-wall. A total of 35 tubes (6.60%) were plugged.
The general guideline adopted by PEco Engineering was to plug any tube in which the ECT measurements in either the inlet, outlet or U-Bend areas of the tubes indicated a maximum pit depth of 80% through wall or greater.
On June 25, 1992, representatives from PECo met with the NRC staff at the hRC Region I Head uarters to discuss the results of their testing and proposed actions to address the corrosion problems.
PECo also presented their safety assessments to confirm that there was no reduction in margin of safety with the plugged and pitted tubes.
(See the Resident inspector's reports for June 7-July 18, 1992, and July 19-August 29, 1992, 50-352/92-17 and 50-353/
92-17 and 50-352/92-23 and 50-353/92-23, respectively.)
Because of the number of plugged tubes in each RHR heat exchanger, in the later part of June 1992 and early July 1992, PEco conducted comprehensive flow and heat transfer tests on the RHR and the RHRSW systems in both Units 1 and 2.
TS SR 4.6.2.3.b currently states that the SDC mode of RHR shall be demonstrated to be operable, "By verifying that each of the required RHR pumps develops a flow of at least 10,000 gpm on recirculation flow through the RHR neat exchanger, the suppression pool, and the full flow test line when tested puraant to Specification 4.0.5."
TS Section 4.0.5 invokes the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel (B&PV) Code,Section XI In-Service Testing (IST) of pumps and valves, indicating that the intene of this SR is to confirm the performance of the RHR pumps when aligned in the SPC flowpath.
Pump performance is therefore one of the parameters surveillance in order to determine operability of the SPC mode of the RHR System.
The initial flow tests showed that the RHR pumps were putting out more than the specified 10,000 gpm (e.g.,10,500 gpm) but that about 2500 gpm was going through the supposedly closed bypass valve rather than through the heat exchanger. The bypass valve is a butterfly type valve that was not intended by design to be leak tight.
By adjusting the limit switch, the
.. licensee reduced leakage through the valve to about 1000 gpm, which is still about 10% of the total flow.
There was no safety concern with the amount of flow being bypassed. As discussed previously, the 10,000 gpm requirement on the pumps was based on the LPCI mode of operation. Tha SPC mode only requires about 7500 gpm.
Nevertheless, in view of about 25% of the flow going through a supposedly closed valve, the performance of the valves in the RHRSW, the corrosion and plugging of small diameter lines in the RHRSW, the relative priority given to the " Raw Water issues Task Force" to develop a plan of action to address the problems in the service water systems, etc., the i.wident inspectors raised the question about interpretation of 4.6.2.3.b --
sp.ifically, did "through the RHR heat exchanger" mean that the specified b,000 gpm pump output was to be measured at the pump discharge, as PEco has been measuring it quarterly as part of the In-Service Test (IST) program or should it be measured at the outlet of the heat exchanger. There is no in-line instrumentation to measure flows at the latter locations.
The question was referred to NRR for interpretation. On June 24, 1992, we had a conference call with PEco's Engineering and Plant staff on the issue, with the NRC Resident inspectors, the NRC's Region 1 Project staff and NRR's Project Manager and Standard Technical Specification staff. The NRC's position was that there was no safety issue, but a literal interpretation of "through the heat exchanger" would require that the specified flow should go through the heat exchanger shell. We suggested that PEco submit a TS application to clarify the basis (intent) of SR 4.6.2.3.b.
On August 11, 1992, PECo submitted the subject application.
By [[letter::05000352/LER-1992-013, :on 920624,determined That RHR HX Failed Heat Transfer Capacity Test,Per Generic Ltr 89-13.Caused by HX Fouling & Leaking RHR HX 1B Bypass Valve.Restricting Orifice Removed to Allow Increased Flow|letter dated July 20, 1992]], PEco also submitted Licensee Event Report (LER) No. 1-92-013 discussing the surveillance tests.
PEco was able to demonstrate that there was 10,000 gpm of flow through the RHR heat exchangers in both Units 1 and 2 in the SPC mode.
To do so, they had to remove a restricting orifice in each of the LPCI test return lines at locations downstream of the globe valves.
The orifices were added under Modification Nos. 86-0024 and 5791 for Unit 1 and were part of the original design for Unit 2.
The orifices are needed so that adequate RHR pump discharge pressure can be achieved (during use of the LPCI test mode or SPC mode) withcut excessively throttling the globe valves. Without these orifices, the globe valves could be subject to significant damage as a result of cavitation.
The modifications in '990 and 1991 reduced suppression pool temperature stratification by promoting better mixing.
From the standpoint of plant safety, it is desirable to reinstall these orifices as soon as possible.
Implementation of the proposed TS change will permit reinstallation of the orifices.
On October 1,1992, PECo conducted the quarterly flow test of the Unit 2 'B' RHR pump and the 2B loop of SFC mode of the RHR system in accordance with TS SR 4.6.2.3.b.
Using special flow monitoring instrumentation, the plant staff determined that there was an increase in flow of about 300 gpm through the closed Unit 2
'b' RHR heat exchanger bypass valve, HV-C-51-2F0488, since the previous test on June 25, 1992. The surveillance test procedure was repeated several times and the actual flow through the Unit 2 RHR heat exchanger was 1
1
- between 9800 and 9950 gpm, slightly les; than *he 10,000 gpm in SR 4.6.2.3.b.
The licensee requested a temporary waiver of compliance from SR 4.6.2.3.b on October 1,1992, pending issuance of the subject amendments.
The licensee followed-up the verbal request with written requests on October 2 and 5, 1992.
The licensee's initial request was verbally approved by the NRC on October 1, 1992, and documented by letter dated October 5, 1992.
The temporary waiver of compliance expires with issuance of these amendments.
Even though the increase in bypass flow was not consequencial, it was another indication that flow through the butterfly bypass valves can change, even if the valve has not been operated.
in their letter of October 2,1992, the licensee's explanation for the slight increase in bypass flow was that the " data showed a slight increase in the degradation of the valve." We understand that the licensee is evaluating the design and operation of the bypass valves to determine if leakage through the valves can be controlled within a predictable range.
As a result of the tube plugging and fouling in the 1A and IB RHR heat exchangers, the licensee reanalyzed the minimum required flows of service water and suppression pool water through the heat exchangers to perform the design functions.
In their letter of October 5,1992, the licensee reported that the revised calculations showed that an RHR system flow rate of approximately 7500 gpm is needed to pass through the RHR heat exchangers to remove the design heat load, assuming the design maximum value for the heat exchanger fouling factors, a nominal number of plugged tubes, and the maximum RHRSW inlet temperature.
In their response of January 29, 1990, to NRC Generic Letter 89-13 " Service Water System Problems Affecting Safety-Related Equipment," the licensee committed to periodically test the heat transfer performance of the RHR and other heat exchangers.
This commitment was reiterated in the licensee's letters of August 11, 1992 and October 2, 1992.
This is the important consideration, more so t,,an flows through the heat exchanger, although there is a close relationship.
3.0 flAlVATION TS SR 4.6.2.3.b currently states that the suppression pool cooling (SPC) mode of RHR operation shall be demonstrated to be operable, "By verifying that each of the required RHR pumps develops a flow of at least 10,000 gpm on recirculation flow through the RHR heat exchanger, the suppression pool, and the full flow test line when tested pursuant to Specification 4.0.5."
Since the TS Bases for this surveillance requirement did not address the LGS design which included an RHR heat exchanger bypass valve, this proposed change provides clarification of this TS SR.
PEco's position is that the purpose of this TS SR is to confirm the RHR pump performance while operating in the SPC mode, pursuant to the IST requirement of TS Section 4.0.5.
Specifically, the purpose of this TS SR is to confirm that each RHR pump develops a flow rate of 10,000 gpm through the most restrictive flow path.
This includes the RHR heat exchanger and its associated closed bypass valve, the suppression pool, and the full flow test line.
This TS SR is not intended to confirm the heat transfer capability of the RHR heat exchanger since there is no equivalent TS SR for the flow of RHRSW through the RHR heat exchanger.
Periodic heat
.. transfer testing of the RHR heat exchanger is required by 10 CFR 50, Appendix A, GDC 40, and implemented by administrative controls as committed to in PECo's response to NRC Generic Letter 89-13. Accordingly, PEco proposes to change TS SR 4.6.2.3.b to clarify its purpose as follows:
"b. By verifying that each of the required RHR pumps develops a flow of at least 10,000 gpm on recirculation flow through the flow path including the RHR heat exchanger and its associated closed bypass valve, the suppression pool and the full flow test line when tested purruant to Specification 4.0.5."
This propcsed clarification does not change the operation of the RHR system in the SPC mode, the heat transfer capability of the system, or the existing heat transfer testing requirements.
The proposed TS changes do not involve any physical changes to the RHR system components. These proposed TS changes only clarify the fact that the purpose of TS SR 4.6.2.3.b is to confirm the RHR pump performance while operating in the SPC mode, i.e., flow through the most restrictive conditions of the flow path.
The RHR heat exchanger performance will continue to be verified by periodic testing as described above.
Therefore, the pressure suppression function of the suppression pool is unaffected by these TS changes.
We agree with PECo's interpretation.
The proposed changes to the SR and Bases will clarify the interpretation as we suggested in our June 24, 1992 telecon with the PEco staff.
The proposed changes are acceptable.
4.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Pennsylvania State official was notified of the p'oposed issuance of the amendments. The State official had no comments.
5.0 ENVIRONMENTAL CONSIDERATION
The amendments change the surveillance requirements. The NRC staff has determined that the amendments involve no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure.
The Commission has previously issued a proposed finding that the amendments involve no significant hazards consideration, and there has been no public comment on such finding (57 FR 40218). Accordingly, the amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need bo i
l prepared in connection with the issuance of the amendments.
6.0 CONCLUSION
The Commission has concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the public will not ae endangered by operation in the proposed manner, (2) such activities will te conducted in compliance with the ommission's regulations, and (3) the issuance of the amendments will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor:
R. Clark Date: November 5, 1992
Attachment:
Figure 1
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