ML20059J317
| ML20059J317 | |
| Person / Time | |
|---|---|
| Site: | Pilgrim |
| Issue date: | 11/03/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20059J311 | List: |
| References | |
| NUDOCS 9311120166 | |
| Download: ML20059J317 (5) | |
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' 4,j NUCLEAR REGULATORY COMMISSION l
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UNITED STATES
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WASHINGTON, D.C. 20666-0001 i
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO.150 TO FACILITY OPERATING LICENSE NO. DPR-35 i
BOSTON EDISON COMPANY PILGRIM NUCLEAR POWER STATION l
DOCKET NO. 50-293 i
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1.0 INTRODUCTION
By letter dated May 20, 1993, the Boston Edison Company (BECO) proposed to change the Pilgrim Nuclear Power Station (PNPS) Appendix A of Operating License No. DRP-35 in accordance with 10 CFR 50.90 (Ref.1). One proposed change to the Technical Specification (TS) reduces the main steam isolation valve low turbine inlet pressure (MSIVLTIP) setpoint from equal to or greater-than 880 pounds per square inch gauge (psig) to equal or greater than 810 psig. This change is proposed to reduce potential spurious isolation caused by the existing higher setpoint.
The other TS change proposed is to reduce the minimum pressure in the j
definition of RUN mode from 880 psig to 785 psig. This change is proposed to realize fully the potential of enhanced operating flexibility provided by the i
proposed reduction in the MSIVLTIP.
2.0 BACKGROUND
The main steam isolation valve (MSIV) low-pressure isolation setpoint is part of the primary containment and reactor vessel isolation system.
It provides isolation in response to pressure regulator malfunction to ensure that.
o Radiological release to environs is less than the maximum allowed, j
o Thermal stress, due to excessive reactor vessel depressurization, is acceptable, o
Minimum Critical Power Ratio is acceptable, o
Bulk reactor water swell does not affect safety relief valves performance, l
o Low-Pressure Core Thermal Power Safety Limit is acceptable.
l The reactor mode switch is used to select the necessary scram functions for various plant conditions and to provide the necessary scram bypasses to facilitate operations. The initial setting of the low-pressure MSIV isolation was 100 psi less than the turbine inlet pressure. With this 100 psi or less l
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operating margin, unwanted isolation and scrams could occur since pressure regulators with built-in control time constants may not be able to limit the pressure drop before the isolation setpoint is reached. The new setpoint could l
reduce the number of spurious scrams and unwanted MSIV isolation, consequently improving plant availability. The reduction of spurious scrams increases safety because there is a potential reduction of stress upon operators and equipment.
i 3.0 EVALUATION The aspects of this modification and TS change that must be analyzed is as follous:
o Radiological Release o
Thermal Stress o
Minimum Critical Power Ratio o
Bulk Reactor Water Swell o
Low-Pressure Core Thermal Power Safety Limit t
Radioloaical Release While the HSIVLTIP can cause isolation of the reactor vessel and primary containment with the closure of the MSIVs, no credit is taken for this trip in the assessment of radiological releases.
For a design basis steam line break, the MSIVs are closed by a high steam line flow control logic. Small steam line breaks are detected by either high radiation in the steam tunnel or high temperature in the turbine building which also causes the MSIVs to close.
Thermal Stress Should the nuclear system pressure regulator fail open, the MSIVLTIP setpoint prevents excessive vessel depressurization which, if not terminated, could impose significant thermal stresses on the nuclear system process barrier and result in an increase in the nuclear system process barrier lifetime fatigue usage factor. According to the General Electric Company (GE) Topical Report NED0 31296 (Ref. 2), and BEC0's calculation I-N1-30, which used a low-pressure analytical limit of 750 psig, changing the MSIVLTIP setpoint from equal to or less than 880 psig to 810 psig does not significantly increase the lifetime fatigue usage factor assuming eight pressure regulator failure (open) events.
The effects of increased core flow combined with this fatigue analysis were evaluated and determined to be insignificant.
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y 1 Minimum Critical Power Ratio i
NEDO 31296 analysis indicates that if. the nuclear system pressure regulatorc l
fails open, assuming turbine control valves (TCVs) opening l instantaneously upon failure of the pressure regulator, the lower MSIVLTIP setpoint will cause 4 delay'in MSIVs closure but there will be no impact on the minimum critical power ratio (MCPR) because that setpoint is not near the safety limit. The voiding resulting by the opening of the turbine valves will cause a reduction in power i
level and the swell will cause a high-level turbine trip.
Bulk Reactor Water Swell Reducing the MSIVLTIP setpoint would result in extending the depressurization i
time prior to MSIV isolation and cause greater bulk water swell. GE's analysis
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MDE-70-0586 (Ref. 3) of this event (pressure regulator failure [open] transient) using a bounding set of initial conditions chosen to maximize the severity of t
the water swell (2% power with 30% flow), indicates that high reactor water level initiates a turbine trip and feedwater pump trip.(no credit is taken for a feedwater pump trip because this trip is not qualified as safety grade).
Steam continues to be bypassed to the condenser until the MSIVLTIP setpoint'is reached which causes the MSIV to close and cause a reactor trip. During the transient, the water level does not reach the bottom of the steam line nozzle elevation and hence no liquid will be trapped in the steam lines as a result of MSIV closure.
i Therefore, the safety relief valves would not be required to discharge high i
pressure liquid or two phase flow.
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Low-Pressure Core Thermal Power-Safety Limit Although the MSIVLTIP.setpoint is' lowered to 810 psig, the automatic protection i
of the low-pressure core thermal power safety limit, which is equal to or less 1
than 25% of rated thermal power, will remain functional.
j Reactor Mode Switch Reactor modes are determined by the position of the mode. switch. The safety function of the mode switch is to select the necessary scram functions' for i
various plant conditions. The mode switch also provides necessary bypasses to j
facilitate operation. The licensee proposes to reduce the minimum pressure in the definition of RUN mode from 830 psig to 785 N,9 This change in the minimum pressure for the RUN mode does not affect any of the scram functions.
l Specifically, the scram on high flux at less than 25% power (mode switch in i
STARTUP) will not be bypassed before 785 psig is exceeded. This pressure is the minimum pressure for using the minimum critical power ratio (MCPR) as the basis-for fuel cladding protection.
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4.0 CONCLUSION
Based on the results of the various analyses, the staff finds the licensee's proposed changes to the technical specifications acceptable.
5.0 EIFEREEf1 I
1.0 Letter from E.T. Boulette, Senior Vice President - Nuclear, BECO to NRC,
" Proposed Change to Technical Specification: Main Steam Isolation Valve L
Turbine Inlet Low-Pressure Setpoint" dated May 20, 1993.
2.0 General Electric Report NEDO-31296, " Safety Evaluation of MSIV Low Turbine Inlet Pressure Isolation Setpoint Change for Pilgrim Nuclear Power Station" dated May 1986.
L 3.0 General Electric Report MDE-70-0586, " Evaluation of the Effect on Plant 1
Operation of MSIV Low Turbine Inlet Pressure Isolation Setpoint Change at Pilgrim Nuclear Power Station" dated July 1986.
6.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Massachusetts State official was notified of the proposed issuance of the amendment.
The State official had no comments.
7.0 ENVIRONMENTAL CONSIDERATION
The amendment changes a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and changes surveillance requirements. The NRC staff has determined that the amendment involves 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 amendment involves no significant hazards consideration, and there has been no public comment on such finding (58 FR 36425). Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51:22(c)(9).
Pursuant to 10 CFR 51.22(b) no anvironmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendment.
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8.0 CONCLUSION
The Comission has concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Comission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor:
F. Paulitz Date:
November 3, 1993 t
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