ML20056H021
| ML20056H021 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 08/20/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20056H018 | List: |
| References | |
| NUDOCS 9309080085 | |
| Download: ML20056H021 (5) | |
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'f NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 205500001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REGARDING 10 CFR 50.44 RECOMBINER REQUIREMENTS GENERIC LETTER 84-09 NORTHEAST NUCLEAR ENERGY COMPANY MILLSTONE NUCLEAR POWER STATION. UNIT NO. 1 DOCKET NO. 50-245
1.0 BACKGROUND
On December 2,1981,10 CFR 50.44 was changed to add additional post-TMI i
requirements for combustible gas control in light water power reactors.
The revised 10 CFR 50.44 established new requirements for Mark I containments j
including requirements that (1) the containment be inerted, and (2) if the primary means of combustible gas control is a purge-repressurization system, additional recombiner capability be provided.' In response to (1), it is noted that the Millstone Unit I facility was already licensed and operating with an inerted containment.
Northeast Nuclear Energy Company (NNEC0) was attempting to incorporate an air containment atmosphere dilution system ir,to the facility licensing basis in order to comply with the version of the rule published in the Federal Reaister on October 27, 1978.
It is the recombiner requirement resulting from the December 2,1981, rule change that is the subject of this evaluation.
In response to the recombiner requirement, the BWR Owners Group (BWROG) undertook an analytical effort to demonstrate that inerted Mark I containments do not require recombiner capability to ensure that they will remain inerted under postulated post-accident conditions.
The BWR owners concluded (Ref.
NED0-22155 forwarded by letters from T.J. Dente dated June 21, 1982, and August 12, 1982) that an inerted containment alone, without recombiners or venting, precludes the formation of a combustible mixture and the staff should 4
i reconsider the recombiner requirement for Mark I plants.
In response to the BWROG request, the staff, with the assistance of its consultant (Dr. A.O. Allen), conducted independent confirmatory analyses for a wide spectrum of core damage accidents. The staff's analyses confirmed the BWROG findings.
Based on realistic radiolysis oxygen generation rates,
'Those plants for which notices of hearing on applications for construction permits were published on or after November 5,1970 are not permitted by
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10 CFR 50.44(e) to rely on purge-repressurization systems as the primary means for hydrogen control.
Therefore, those plant were not affected by the new recomDiner requirements.
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. neither purge-repressurization capability or recombiner capability would be l
required following an accident.
The staff then concluded that the BWROG methodology provides an acceptable basis for a finding that purge-repressurization need not be considered the primary means of combustible gas control.
(However, the staff continued to recognize the methodology of l
Regulatory Guide 1.7 as the appropriate methodology for use in evaluating the i
adequacy of systems which were a part of the original design basis accident-loss of coolant accident (DBA-LOCA) mitigation schemes which, due to the I
conservative radiolysis source term prescribed by Regulatory Guide 1.7, require air or nitrogen purge-repressurization systems.
This staff position l
precludes the use of initial containment inerting as the sole means of combustible gas control.)
The staff findings were presented to the Commission in SECY-83-292. The staff subsequently issued Generic letter (GL) 84-09 with the understanding that it was applicable only to inerted Mark I facilities already having a combustible gas control system capable of mitigaUng DBA-LOCA conditions is defined by Regulatory Guide 1.7.
GL 84-09 provided a means to avoid imposing the 10 CFR 50.44 recombiner requirements on thse facilities not already having them.
NNEC0 responded to GL 84-09 in a letter dated July 17, 1984.
NNECO's letter cited previous correspondence supporting a position that recombiner capability is not required for Millstone Unit 1.
The staff evaluated the NNECO response and, in a letter dated November 1, 1984 (Technical Specifications amendment which added a 4% limiting condition for operation for 0 concentration) 2 notified NNECO that the staff considered Millstone I to be in conformance with 10 CFR 50.44(c)(3).
In the November 1, 1984, letter, the staff also stated that Millstone 1 met the criteria of GL 84-09.
It subsequently came to the attention of the staff that Millstone Unit I was lacking a purge-repressurization capability which, while not required for compliance with the additional recombiner requirements of the new 10 CFR 50.44(c), is needed as the primary means of combustible gas control in the absence of recombiner capability or "another type of combustible gas control syrtem." The staff advised NNECO (Ref. NRC letter dated August 12, 1986) and licensees of several other facilities (i.e., Cooper Nuclear Station, Dresden Units 2 & 3, Quad Cities Units 1 & 2, and Oyster Creek) that their responses to GL 84-09 could not be accepted.
After a series of meetings and considerable correspondence, the staff advised the licensees that it would recognize the nitrogen inerting system (NIS) as providing a purge-repressurization capability equivalent to the purge-repressurization system required by 10 CFR 50.44 if an equivalent level of reliability could be shown.
This approach was discussed with NNECO at a meeting on April 22, 1993.
In a subsequent letter dated June 30, 1993, NNEC0 presented its proposal for modifications, stating the position that the facility was already in conformance with the rule, and that the modifications
. are being proposed to provide increased reliability for the mitigation of beyond design basis accidents. This safety evaluation addresses the I
acceptability of the Hillstone 1 purge-repressurization system.
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NNEC0's June 30, 1993, letter indicates that a hardened containment vent will be incorporated into the NIS system. This vent flow path proposal replaces that of an earlier commitment. This safety evaluation does not review the extent of conformance to GL 89-16, " Installation of a Hardened Wetwell Vent."
2.0 DISCUSSION AND EVALUATION 2.1 PURGE-REPRESSURIZATION CAPACITY AND RELIABILITY Millstone Unit 1 Technical Specifications require that the primary containment be inerted to maintain an oxygen concentration s; 4%.
This is accomplished by l
diluting the containment atmosphere with nitrogen introduced by the NIS.
The existing NIS has no accident mitigation function and was not designed to Engineered Safety Features quality standards. The principal components of the i
NIS are a liquid nitrogen storage tank, a pressure build-up coil, a steam vaporizer, containment isolation valves (CIVs) and three process valves.
All of these components except the CIVs are located in a yard outside the reactor building.
The licensee has proposed modifications to the NIS to improve its reliability under post-accident conditions. The staff reviewed the proposed modifications to determine if the modified system provides a level of post-accident purge-repressurization reliability equivalent to a safety-grade system meeting the 1
criteria of General Design Criteria (GDC) 41, GDC 42 and GDC 43 as specified I
in 10 CFR 50.44(g) and sufficient capacity for purge-repressurization under pressurized containment conditions.
2.1.1 NIS RELIABILITY 10 CFR 50.44(g) requires that a purge-repressurization system be designed to conform to the requirements of GDC 41, 42 and 43.
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Compliance with General Desian Criterion 41:
GDC 41 requires systems to have suitable redundancy in components and power sources to assure that the design basis safety function can be accomplished with loss of either on-site power or off-site power and with a single failure.
NNECO performed a single-failure vulnerability study for the to-be-modified NIS. The study assumed that isolatable threaded and flanged components outside the reactor building could be repaired or replaced in a timely manner under post-accident conditions.
For all vulnerabilities identified in the j
study, only the failure of valve AC-17 was found to be unremediable. AC-17 is a designated containment isolation valve subject to the Appendix J and ASME Section XI periodic test requirements.
It is a resiliently seated butterfly valve with a solenoid controlled air cylinder operator. NNEC0 reviewed in detail the operational history of AC-17 and the failure data of similar valves j
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. in similar service throughout the industry.
Except with respect to leaktightness (which would not affect purge-repressurization reliability), the valve was found to have an exceptional reliability history, the only significant failure mode being that resulting from failure of the Bettis air actuator controls.
NNEC0 has proposed a modification which enables the solenoid to be bypassed effectively making AC-17 equivalent in reliability to a manual valve.
The staff acknowledges that this modification will provide a very high degree of post-accident capability to operate AC-17.
Single-failure proof electrical power is provided by the Instrument AC System which may be powered by a diesel generator, gas turbine generator or the 1
Millstone Unit 2 cross-tie.
Redundant pneumatic supplies to valve operators are provided by multiple air compressors, a Unit 2 cross-tie and portable air I
bottles.
Multiple steam supply sources are available for vaporizer heating.
These include two boilers powered by Unit 1, another boiler powered by Unit 2, and the Unit 2 reboiler.
Capability also exists for connection of a portable boiler.
I An alternate nitrogen gas supply connection will be provided on a new branch line off the existing nitrogen supply line in the yard.
(Note: the new branch line will also be common to the Hardened Vent System).
Based on the considerable redundancy described above, and the alternate measures available in the event of equipment failure, the staff considers GDC 41 to be met.
Compliance with General Desian Criterion 42:
GDC 42 requires that applicable systems be designed to permit periodic inspection of important components.
The important components include CIVs, pressure and flow control valves, vaporizers, truck connections, instrumentation and controls and the nitrogen j
storage tank.
The NIS is located in two general areas:
(1) the reactor building and (2) the
" yard." Both areas are fully accessible under normal operating conditions l
permitting periodic inspections and system walkdowns.
In addition, the yard l
is accessible under post-accident conditions. The staff thus considers GDC 42 to be met.
Compliance with General Desion Criterion 43:
GDC 43 requires that systems be designed to permit periodic testing for operability and leaktight integrity.
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The NIS is used frequently during normal plant operation for control of containment oxygen concentration and pressure control. This provides a high level of assurance that the system is functionally operable.
In addition, the isolation valves are subject to 10 CFR 50, Appendix J local leak rate testing requirements and 10 CFR 50.55a Inservice Testing requirements. The staff thus considers the GDC 43 requirement to be met.
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. 2.1.2 NIS CAPACITY Calculations by NNECO indicate that the NIS is capable of injecting nitrogen into containment at rates in excess of 1,100 scfm with low containment pressure, and 500 scfm with the containment pressurized to 50% of design pressure (i.e., 31 psig).
Based on bounding studies performed for other Mark I facilities having a higher reactor power / containment volume ratio, (e.g.,
see Dresden/ Quad Cities safety evaluation issued June 29, 1993) these rates are considerably higher than needed to dilute radiolytic oxygen and maintain the Millstone Unit I containment in an inerted condition under post-DBA-LOCA conditions. The NIS would not be needed for containment dilution for at least 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> post-accident.
The nitrogen liquid storage tank has a total capacity of 950,000 scf and is replenished when depleted to the 400,000 scf level.
This storage capacity and the administrative controls over replenishment ensure sufficient on-site nitrogen availability to meet post-DBA-LOCA hydrogen control requirements.
The staff therefore concludes that the capacity of the NIS is adequate.
2.2 CONFORMANCE TO GL 84-09 Generic letter 84-09 states that the Commission has determined that a Mark I BWR plant will be found to not rely upon purge-repressurization systems as the primary means of hydrogen control, (and thus is not subject to the $50.44 recombiner capability requirement), if certain technical criteria were satisfied.
These criteria are:
(1)
The facility has technical specifications requiring that the containment be inerted to less than 4% oxygen, (2)
The facility has only nitrogen or recycled containment atmosphere for use in all pneumatic control systems within containment, and (3)
There are no potential post-accident containment oxygen sources other than radiolysis.
In Amendment No.101, issued November 1,1984, the Millstone Unit 1 Technical Specifications were changed to limit oxygen concentration to s 4%.
In the safety evaluation for that amendment, the staff found that the above three criteria were satisfied.
3.0 CONCLUSION
The NRC staff has concluded that the licensee's proposed resolution of the hydrogen recombiner requirement of 10 CFR 50.44 is acceptable.
This acceptance is based upon the licensee's commitments to upgrade the NIS and the staff findings that (1) the upgraded NIS will provide a reliable purge-repressurization capability and (2) the criteria of GL 84-09 are met.
Principal Contributor: W. Long Date: August 20, 1993