ML13317A614
| ML13317A614 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 05/13/1981 |
| From: | Crutchfield D Office of Nuclear Reactor Regulation |
| To: | Dietch R Southern California Edison Co |
| References | |
| TASK-09-04, TASK-9-4, TASK-RR LSO5-81-05-017, LSO5-81-5-17, NUDOCS 8105180191 | |
| Download: ML13317A614 (17) | |
Text
May 13, 198]
Docket No. 50-206 LS05-81-05-017 Mr. R. Dietch, Vice President Nuclear Engineering and Operations Southern California Edison Company 2244 Walnut Grove Avenue Post Office Box 800 Rosemead, California 91770
Dear Mr. Dietch:
SUBJECT:
SEP TOPIC IX-4, BORON ADDITION SYSTEM (SAN ONOFRE)
Enclosed is a copy of our draft evaluation of Topic IX-4, "Boron Addition System" for the San Onofre Plant. This assessment compares your facility as described in Amendment No. 25 to Provisional Operating License No.
DPR-13 for San Onofre Plant with the criteria currently used by the regul atory staff for licensing new facilities.
You are requested to examine the facts upon which the staff has based its evaluations and respond either by confirming that the facts are correct, or by identifying any errors. If in error, please supply corrected infor mation for the docket. We encourage you to supply for the docket any other material related to this topic that might affect the staff's evaluation.
This assessment;-hiaybe revised in the future if your facility design is changed or if the NRC criteria relating to this subject is modified before the integrated assessment is complete.
Your response within 30 days of the date you receive this letter is re quested. If no response is received within that time, we will assume that you have no comments or corrections.
Sincerely, Dennis M. Crutchfield, Chief Operating Reactors Branch No. 5 Division of Licensing 8105180191
Enclosure:
As stated OFFICE CC w/enclosu e: SEPB SEP SEPB 14 ORB#
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SREG UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 May 13, 1981 Docket No. 50-206 LSO5-81-05-017 Mr. R. Dietch, Vice President Nuclear Engineering and Operations Southern California Edison Company 2244 Walnut Grove Avenue Post Office Box 800 Rosemead, California 91770
Dear Mr. Dietch:
SUBJECT:
SEP TOPIC IX-4, BORON ADDITION SYSTEM (SAN ONOFRE)
Enclosed is a copy of our draft evaluation of Topic IX-4, "Boron Addition System" for the San Onofre Plant. This assessment compares your facility as described in Amendment No. 25 to Provisional Operating License No.
DPR-13 for San Onofre Plant with the criteria currently used by the regul atory staff for licensing new facilities.
You are requested to examine the facts upon which the staff has based its evaluations and respond either by confirming that the facts are correct, or by identifying any errors. If in error, please supply corrected infor mation for the docket. We encourage you to supply for the docket any other material related to this topic that might affect the staff's evaluation.
This assessment may be revised in the future if your facility design is changed or if the NRC criteria relating to this subject is modified before the integrated assessment is complete.
Your response within 30 days of the date you receive this letter is re quested. If no response is received within that time, we will assume that you have no comments or corrections.
Sincerely, Dennis M. Crutchfield, Ch f
Operating Reactors Branch No. 5 Division of Licensing
Enclosure:
As stated cc w/enclosure:
See next page
Mr. R. Dietch cc Charles R. Kocher, Assistant Director, Criteria and Standards General Counsel Division Southern California Edison Company Office of Radiation Programs Post Office Box 800 (ANR-460)
Rosemead, California 91770 U. S. Environmental Protection Agency David R. Pigott Washington, D. C. 20460 Samuel B. Casey Chickering & Gregory U. S. Environmental Protection Three Embarcadero Center Agency Twenty-Third Floor Region IX Office San'Francisco, California 94111 ATTN:
EIS COORDINATOR 215 Freemont Street Jack E. Thomas San Francisco, California 94111 Harry B. Stoehr San Diego Gas & Electric Company P. 0. Box 1831 San Diego, California 92112 Resident Inspector/San Onofre NPS c/o U. S. NRC P. 0. Box 4329 San Clemente, California 92672 Mission Viejo Branch Library 24851 Chrisanta Drive Mission Viejo, California 92676 Mayor City of San Clemente San Clemente, California 92672 Chairman Board of Supervisors County of San Diego San Diego, California 92101 California Department of Health ATTN: Chief, Environmental Radiation Control Unit; Radiological Health Section 714 P Street, Room 498 Sacramento, California 95814
SYSTEMATIC EVALUATION PROGRAM BRANCH TOPIC IX-4, BORON ADDITION SYSTEM SAN ONOFRE PLANT I. INTRODUCTION Following a LOCA, boric acid solution is introduced into the reactor vessel by two modes of injection. In the initial injection mode, borated water is provided from the accumulators, from the refueling water storage tank and from the boron injection tank (Westinghouse plants only). After this initial period, which may last somewhere between 20-60 minutes, the Emergency Core Cooling System (ECCS) is realigned for the recirculation mode. In this mode borated water is recirculated from the containment sump to the reactor vessel and back to the sump through the break. A portion of the water introduced into the reactor vessel is converted into steam by the decay heat generated in the core. Since the steam contains virtually no impurities, the boric acid content in the water that was vaporized remains in the vessel.
The concentra tion of boric acid in the core region will therefore continuously increase, un less a dilution flow is provided through the core. Without the dilution flow the concentration of boric acid will eventually reach the saturation limit and any further increase in boric acid inventory will cause its precipitation.
Boric acid deposited in the core may clog flow passages and seriously comprom ise the performance of the ECCS. Topic IX-4 is intended to review the boron addition system, in particular with respect to boron precipitation during the long term cooling mode of operation following a loss of coolant accident, to assure that the ECCS is designed and operated in such a manner that a suff icient throughflow is provided before the concentration of boric acid will reach its saturation limit.
II. REVIEW CRITERIA The plant design was reviewed with regard to Appendix A, 10CFR Part 50, General Design Criteria - 35, "Emergency Core Cooling", which requires that a system to supply abundant emergency core cooling shall be provided. In addition, the plant design was reviewed with regard to 10CFR 50.46, "Acceptance Criteria for Light Water Nuclear Power Reactors", and Appendix K to 10CFR Part 50 "ECCS Eval uation Models", which set forth the requirements to maintain coolable core geometry and to provide long-term core cooling; the basis for the boron precipi tation reviews.
III.
RELATED SAFETY TOPICS Topic VI-7.A.3 reviews the ECCS actuation system with respect to the testing for operation and design performance of each component of the system.
Topic VI-7.B reviews the procedures for ESF switchover from injection to recirculation mode and recommends some automatic features to increase reliability.
2 IV.
REVIEW GUIDELINES There are no unique SRP sections that deal with this issue. The primary cri terion used for review of this system was discussed in a memo dated January 21, 1976 entitled, "Concentration of Boric Acid in Reactor Vessel During Long Term Cooling - Method for Reviewing Appendix K Submittals."
V. EVALUATION Control of the reactor system boron concentration, which is responsible for normal reactor shutdown, is provided by the Chemical and Volume Control Sys tem. This is accomplished by using either one of the two charging pumps or the test pump (Chemical and Volume Control System test pump installed in parrallel with the charging pump) to inject concentrated boric acid solution into the reactor coolant system. There are two sources of borated water a vailable for injection through three different paths as follows:
- 1.
The boric acid injection pump can deliver the boric acid tank contents to the charging pump and/or test pump.
- 2. Boric acid transfer pumps can deliver the boric acid tank contents to the charging pumps and/or test pump.
- 3.
The charging pumps and the test pump can take suction directly from the refueling water storage tank (3750 ppm boron).
The quantity of boric acid in storage from the above two sources is suffic ient to borate the reactor coolant system in order to reach cold shutdown at any time during the core life. Furthermore, if the letdown capability from the primary coolant system to the chemical and volume control system should be impaired the pressurizer void space volume is sufficient to accommodate the required injection. This free volume will accommodate sufficient concen trated boricacid solution such that the reactor coolant water can reach a concentration of about 400 ppm above the'required level to shut the plant down during normal operation.,
Concern was raised by the NRC that boron precipitation could occur and impair the ability of the boron addition systems to maintain safe core cooling follow ing a loss-of-coolant accident. In response to requests by the NRC in letters dated March 18, 1975, Southern California Edison Company (SCE), the licensee
-for San Onofre Unit 1' has evaluated the potential for boron precipitation dur ing long term core cooling following a loss-of-coolant accident (LOCA). After a cold leg break, decay heat is removed by boiloff of water in the core.
As water is evaporated, the concentration of boron can reach the saturation point causing precipitation of boron in the core.
Without a flushing flow through the core the precipitation could result in blockage of core channels. The safety objective is to assure that the potential for significant boron pre cipitation that would restrict core flow is eliminated by establishing
-3 such a flushing flow. Progress in the evaluation of this potenial and development of systems and procedural modifications to prevent its oc currence have been reported to the Commission by the licensee in letters dated November 14, 1975, March 16, 1976, and May 11, 1976.
To prevent this buildup and precipitation of boron in the core during the long term recirculation mode following a LOCA, SCE utilizes a dual recircu lation pattern (Dual Recirculation) involving both the cold legs (Recircu lation System) and the hot legs (Hot Leg Recirculation System). For long term cooling, the Recirculation System is supplied with borated water from the sump and injects into the cold legs while the Hot Leg Recirculation System provides an additional means of adding borated water to the reactor coolant system by flow from the sump, through the charging pumps and then through the pressurizer auxiliary spray to the loop "B" hot leg. Upon initiation of Dual Recirculation (19 hours2.199074e-4 days <br />0.00528 hours <br />3.141534e-5 weeks <br />7.2295e-6 months <br /> after the LOCA), the required flow split between the cold and hot legs is established by manipulating the flow control valves in the recirculation and hot leg recirculation systems.
At the present time, several single failures could prevent, or at least reduce, flow through the Hot Leg Recirculation System.
An acceptable mod ification to prevent these single failures involved the installation of redundant valves in the path to the loop B hot leg. SCE, in their letter dated August 10, 1978, as supplemented by letter dated September 15, 1978, submitted a proposal to postpone the implementation of four ECCS single failure modifications to permit evaluation of the potential impact of other plant modifications which may be considered as part of the integrated assessment of the facility in the Systematic Evaluation Program (SEP).
Postponement of the above modification was granted by the NRC in a letter dated October 16, 1978. Until these modifications can be made, a means to provide adequate hot leg recirculation in the unlikely event of a loss-of-coolant accident in one of the cold legs and the single failure of one of the valves in the hot leg recirculation path has been developed.
This alternate hot leg recirculation flow (see Reference 1) will provide borated recirculation water to loop "C" hot leg of the reactor coolant sys tem via the residual heat removal letdown line bypassing the RHR pump, rather than through the pressurizer spray to hot leg "B" (see Figure 1).
Thus, a single failure will not prevent Dual Recirculation.
The suitability of Dual Recirculation initiation time has been established by analysis.
Results of conservative calculations (assuming no depletion of boric acid in the core region due to any mechanism and with appropriate con-
-4 servatisms in the decay heat source) Indicate that the boric acid inventory in the core will approach the saturation point 27.8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> following a LOCA. The proposed initiation of Dual Recirculation at 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br /> therefore occurs well before precipitation.
The adequacy of flow rates to the core during Dual Recirculation has also been confirmed by analysis. Flow rates to the core were evaluated for Dual Recircu lation operation with various postulated large break LOCAs (as discussed in Reference 2, the large cold-leg LOCA is the limiting case from the standpoint of boron precipitation). The analysis indicated that with modifications to eliminate unacceptable single failures completed, at least 13.7 1bm/sec (102 gpm) will be available to the core compared with the maximum calculated boil-off rate of 8.8 1bm/sec. Therefore, more than adequate flow to the core is avail able during Dual Recirculation (Reference 3).
VI.
CONCLUSION Based on our review and using staff criteria referenced earlier, we conclude that the staff analysis of the dual recirculation documented in Amendment No.25 is judged to be acceptable.
However, as stated in the SE the licensee has postponed the implementation of ECCS single failure modifications to this sys tem pending their evaluation of the impact of other plant modifications as part of the Systematic Evaluation Program on the ECCS single failure modifica tions. This delay will have no effect on plant safety since an alternative flow path has been developed in the interim. A schedule for the implementation of these ECCS single failure modifications will be established during the in tegrated assessment.
1LAL 1~..ttic)
,44 FIGURE 1t Interim hot leg ricirculatlon fl ow path
REFERENCES
- 2.
Letter CLC-NS-309, dated April 1, 1975, from Mr. C. L. Caso of Westinghouse to Mr. T. M. Novak of USNRC.
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 May 13,1981 Docket No. 50-206 LS05-81-05-017 Mr. R. Dietch, Vice President Nuclear Engineering and Operations Southern California Edison Company O
I, 2244 Walnut Grove Avenue Post Office Box 800
/
Rosemead, California 91770
Dear Mr. Dietch:
SUBJECT:
SEP TOPIC IX-4, BORON ADDITION SYSTEM (SAN ONOFRE)
Enclosed is a copy of our draft evaluation of Topic IX-4, "Boron Addition System" for the San Onofre Plant. This assessment compares your facility as described in Amendment No. 25 to Provisional Operating License No.
DPR-13 for San Onofre Plant with the criteria currently used by the regul atory staff for licensing new facilities.
You are requested to examine the facts upon which the staff has based its evaluations and respond either by confirming that the facts are correct, or by identifying any errors. If in error, please supply corrected infor mation for the docket. We encourage you to supply for the docket any other material related to this topic that might affect the staff's evaluation.
This assessment may be revised in the future if your facility design is changed or if the NRC criteria relating to this subject is modified before the integrated assessment is complete.
Your response within 30 days of the date you receive this letter is re quested. If no response is received within that time, we will assume that you have no comments or corrections.
Sincerely, Dennis M. Crutchfield, Ch f Operating Reactors Branch No. 5 Division of Licensing
Enclosure:
As stated cc w/enclosure:
See next page.
&o51 0 I
Mr. R. Dietch cc Charles R. Kocher, Assistant Director, Criteria and Standards General Counsel Division Southern California Edison Company Office of Radiation Programs Post Office Box 800 (ANR-460)
Rosemead, California 91770 U. S. Environmental Protection Agency David R. Pigott Washington, D. C. 20460 Samuel B. Casey Chickering & Gregory U. S. Environmental Protection Three Embarcadero Center Agency Twenty-Third Floor Region IX Office San Francisco, California 94111 ATTN:
EIS COORDINATOR 215 Freemont Street Jack E. Thomas San Francisco, California 94111 Harry B. Stoehr San Diego Gas & Electric Company P. 0. Box 1831 San Diego, California 92112 Resident Inspector/San Onofre NPS c/o U. S. NRC P. 0. Box 4329 San Clemente, California 92672 Mission Viejo Branch Library 24851 Chrisanta Drive Mission Viejo, California 92676 Mayor City of San Clemente San Clemente, California 92672 Chairman Board of Supervisors County of San Diego San Diego, California 92101 California Department of Health ATTN: Chief, Environmental Radiation Control Unit,.
Radiological Health Section 714 P Street, Room 498 Sacramento, California 95814
SYSTEMATIC EVALUATION PROGRAM BRANCH TOPIC IX-4, BORON ADDITION SYSTEM SAN ONOFRE PLANT I. INTRODUCTION Following a LOCA, boric acid solution is introduced into the reactor vessel by two modes of injection. In the initial injection mode, borated water is provided from the accumulators, from the refueling water storage tank and from the boron injection tank (Westinghouse plants only). After this initial period, which may last somewhere between 20-60 minutes, the Emergency Core Cooling System (ECCS) is realigned for the recirculation mode. In this mode borated water is recirculated from the containment sump to the reactor vessel and back to the sump through the break. A portion of the water introduced into the reactor vessel is converted into steam by the decay heat generated in the core. Since the steam contains virtually no impurities, the boric acid content in the water that was vaporized remains in the vessel.
The concentra tion of boric acid in the core region will therefore continuously increase, un less a dilution flow is provided through the core. Without the dilution flow the concentration of boric acid will eventually reach the saturation limit and any further increase in boric acid inventory will cause its precipitation.
Boric acid deposited in the core may clog flow passages and seriously comprom ise the performance of the ECCS. Topic IX-4 is intended to review the boron addition system, in particular with respect to boron precipitation during the long term cooling mode of operation following a loss of coolant accident, to assure that the ECCS is designed and operated in such a manner that a suff icient throughflow is provided before the concentration of boric acid will reach its saturation limit.
II. REVIEW CRITERIA The plant design was reviewed with regard to Appendix A, 10CFR Part 50, General Design Criteria - 35, "Emergency Core Cooling", which requires that a system to supply abundant emergency core cooling shall be provided.
In addition, the plant design was reviewed with regard to 10CFR 50.46, "Acceptance Criteria for Light Water Nuclear Power Reactors", and Appendix K to 10CFR Part 50 "ECCS Eval uation Models", which set forth the requirements to maintain coolable core geometry and to provide long-term core cooling; the basis for the boron precipi tation reviews.
III.
RELATED SAFETY TOPICS Topic VI-7.A.3 reviews the ECCS actuation system with respect to the testing for operation and design performance of each component of the system. Topic VI-7.B reviews the procedures for ESF switchover from injection to recirculation mode and recommends some automatic features to increase reliability.
1',
-?
2 IV.
REVIEW GUIDELINES There are no unique SRP sections that deal with this issue. The primary cri terion used for review of this system was discussed in a memo dated January 21, 1976 entitled, "Concentration of Boric Acid in Reactor Vessel During Long Term Cooling.- Method for Reviewing Appendix K Submittals.
V. EVALUATION Control of the reactor system boron concentration, which is responsible for normal reactor shutdown, is provided by the Chemical and Volume Control Sys tem. This is accomplished by using either one of the two charging pumps or the test pump (Chemical and Volume Control System test pump installed in parrallel with the charging pump) to inject concentrated boric acid solution into the reactor coolant system. There are two sources of borated water a vailable for injection through three different paths as follows:
- 1. The boric acid injection pump can deliver the boric acid tank contents to the charging pump and/or test pump.
- 2. Boric acid transfer pumps can deliver the boric acid tank contents to the charging pumps and/or test pump.
- 3. The charging pumps and the test pump can take suction directly from the refueling water storage tank (3750 ppm boron).
The quantity of boric acid in storage from the above two sources is suffic ient to borate the reactor coolant system in order to reach cold shutdown at any time during the core life. Furthermore, if the letdown capability from the primary coolant system to the chemical and volume control system should be impaired the pressurizer void space volume is sufficient to accommodate the required injection.
This free volume will accommodate sufficient concen trated boric acid solution such that the reactor coolant water can reach a concentration of about 400 ppm above the required level to shut the plant down during normal operation..'
Concern was raised by the NRC that boron precipitation could occur and impair the ability of the boron addition systems to maintainsafe core cooling follow ing a loss-of-coolant accident. In response to requests by the NRC in letters dated March 18, 1975, Southern California Edison Company (SCE),
the licensee for San Onofre Unit, 1has evaluated the potential for boron *precipitation dur ing long term core cooling following a loss-of-coolant accident (LOCA).
After a cold leg break, decay heat is removed by boiloff of water in the core.
As water is evaporated, the concentration of boron can reach the saturation point causing precipitation of boron in the core. Without a flushing.flow through the core, the precipitation could result in blockage of core channels.
The safety objective is to assure that the potential for significant boron pre cipitatiori that would restrict core flow is eliminated by establishing
-3 such a flushing flow.
Progress in the evaluation of this potenial and development of systems and procedural modifications to prevent its oc currence have been reported to the Commission by the licensee in letters dated November 14, 1975, March 16, 1976, and May 11, 1976.
To prevent this buildup and precipitation of boron in the core during the long term recirculation mode following a LOCA, SCE utilizes a dual recircu lation pattern (Dual Recirculation) involving both the cold legs (Recircu lation System) and the hot legs (Hot Leg Recirculation System). For long term cooling, the Recirculation System is supplied with borated water from the sump and injects into the cold legs while the Hot Leg Recirculation System provides an additional means of adding borated water to the reactor coolant system by flow from the sump, through the charging pumps and then through the pressurizer auxiliary spray to the loop "B" hot leg. Upon initiation of Dual Recirculation (19 hours2.199074e-4 days <br />0.00528 hours <br />3.141534e-5 weeks <br />7.2295e-6 months <br /> after the LOCA), the required flow split between the cold and hot legs is established by manipulating the flow control valves in the recirculation and hot leg recirculation systems.
At the present time, several single failures could prevent, or at least reduce, flow through the Hot Leg Recirculation System. An acceptable mod ification to prevent these single failures involved the installation of redundant valves in the path to the loop B hot leg. SCE, in their letter dated August 10, 1978, as supplemented by letter dated September 15, 1978, submitted a proposal to postpone the implementation of four ECCS single failure modifications to permit evaluation of the potential impact of other plant modifications which may be considered as part of the integrated assessment of the facility in the Systematic Evaluation Program (SEP).
Postponement of the above modification was granted by the NRC in a letter dated October 16, 1978. Until these modifications can be made, a means to provide adequate hot leg recirculation in the unlikely event of a loss-of-coolant accident in one of the cold legs and the single failure of one of the valves in the hot leg recirculation path has been developed.
This alternate hot leg recirculation flow (see Reference 1) will provide borated recirculation water to loop "C" hot leg of the reactor coolant sys tem via the residual heat removal letdown line bypassing the RHR pump, rather than through the pressurizer spray to hot leg "B" (see Figure 1)
Thus, a single failure will not prevent Dual Recirculation.
The suitability of Dual Recirculation initiation time has been established by analysis. Results of conservative calculations (assuming no depletion of boric acid in the core region due to any mechanism and with appropriate con-
-4 servatisms in the decay heat source) indicate that the boric acid inventory in the core will approach the saturation point 27.8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> following a LOCA. The proposed initiation of Dual Recirculation at 19 hours2.199074e-4 days <br />0.00528 hours <br />3.141534e-5 weeks <br />7.2295e-6 months <br /> therefore occurs well before precipitation.
The adequacy of flow rates to the core during Dual Recirculation has also been confirmed by analysis. Flow rates to the core were evaluated for Dual Recircu lation operation with various postulated large break LOCAs (as' discussed in Reference 2, the large cold-leg LOCA is the limiting case from the standpoint of boron precipitation). The analysis indicated that with modifications to eliminate unacceptable single failures completed, at least 13.7 1bm/sec (102 gpm) will be available to the core compared with the maximum calculated boil-off rate of 8.8 1bm/sec. Therefore, more than adequate flow to the core is avail able during Dual Recirculation (Reference 3).
A VI. CONCLUSION Based on our review and using staff criteria referenced earlier, we conclude that the staff analysis of the dual recirculation documented in Amendment No.25 is judged to be acceptable. However, as stated in the SE the licensee has postponed the implementation of ECCS single failure modifications to this sys tem pending their evaluation of the impact of other plant modifications as part of the Systematic Evaluation Program on the ECCS single failure modifica tions. This delay will have no effect on plant safety since an alternative flow path has been developed in the interim. A schedule for the implementation of these ECCS single failure modifications will be established during the in tegrated assessment.
VC11-YRUIA hTS
- k.
MO.
Eux MiE IL FIGURE 11.
-Interim hot leg recirculatlon flow path
REFERENCES
- 2. Letter CLC-NS-309, dated April 1, 1975, from Mr. C. L. Caso of Westinghouse to Mr. T. M. Novak of USNRC.