ML17179A985
| ML17179A985 | |
| Person / Time | |
|---|---|
| Site: | Dresden, Quad Cities  |
| Issue date: | 06/29/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML17179A984 | List: |
| References | |
| GL-84-09, GL-84-9, NUDOCS 9307080375 | |
| Download: ML17179A985 (7) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON. D.C. 20666-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REGARDING POST-ACCIDENT COMBUSTIBLE GAS CONTROL SYSTEM AT DRESDEN, UNITS 2 AND 3, AND QUAD CITIES. UNITS 1 AND 2 COMMONWEALTH EDISON COMPANY DOCKET NOS. 50-237, 50-249, 50-254, AND 50-265
1.0 INTRODUCTION
On December 2, 1981, 10 CFR 50.44 was changed to add additional post-TM!
requirements for combustible gas control in light water power reactors.
The revised 10 CFR 50.44 established new requirements for Mark I containments 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 Quad Cities and Dresden facilities were already licensed and operating with their containments inerted. It is the recombiner requirement which is the subject of this evaluation.
In response to the recombiner requirement, the Boiling Water Reactor Owners Group (BWROG) undertook an effort to demonstrate that inerted Mark I containments do not require recombiner capability to ensure that they will remain inerted under accident conditions.
The group's effort resulted in its conclusion (Ref: NED0-22155 forwarded by letters from T.J. Dente dated June 21, and August 12, 1982) that an inerted containment alone, without recombiners or venting, precludes the formation of a combustible mixture and the staff should therefore 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 degraded core events.
The staff's analyses confirmed the BWROG findings.
Based on realistic radiolysis oxygen generation rates, neither purge/repressurization capability nor recombiner capability would be required following an accident.
The staff then concluded that the BWROG methodology provides an acceptable basis for a finding that purge/repressuri-
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zation need not be considered the primary means of combustible gas control.
However, the staff continued to recognize the methodology of Regulatory Guide 1.7 as the appropriate methodology for use in evaluating the adequacy of systems which were a part of the original Design Basis Accident - Loss-of-Coolant Accident (DBA-LOCA) mitigation scheme.
(This position precludes the use of initial containment inerting as the sole means of combustible gas control.)
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The NRC staff findings were presented to the Commission in SECY-83-292 and the staff subsequently issued Generic Letter 84-09 which identified acceptance criteria for a staff finding that a Mark I containment does not rely on purge-repressurization as the primary means of combustible gas control and is thus not subject to the recombiner requirement.
2.0 DISCUSSION AND EVALUATION 2.1 Conformance to Generic Letter 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 10 CFR 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 percent 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.
Dresden and Quad Cities Technical Specification 3.7.A.5.a requires that the primary containment oxygen concentration to be maintained below 4 percent by volume.
The Technical Specifications operability and surveillance requirements thus assure that the first criterion is satisfied.
The licensee has stated (Letter to H. R. Denton dated June 25, 1984) that all pneumatic control systems use recycled containment atmosphere for operation.
This satisfies the second criterion.
The final criterion in GL 84-09 is that there be no potential sources of oxygen in containment other than that resulting from radiolysis of the reactor coolant.
However, the Dresden/Quad Cities facilities presently have Air Containment Atmosphere Dilution (ACAD) systems designed to inject air into the containment under post-accident conditions for the purpose of diluting the hydrogen concentration below combustible limits.
The staff informed the licensee of its position that Mark I facilities must, in addition to being operated with an inerted containment, have a post-accident nitrogen purge/repressurization capability, in order to meet DBA-LOCA combustible gas control requirements.
The staff also indicated its concern that the ACAD system is a possible post-accident source of oxygen which could deinert the containment.
(Letter from R. M. Bernero to D. L. Farrar dated August 11, 1986).
After subsequent correspondence and meetings with the staff, the licensee proposed modifications to the facilities addressing the staff positions.
The licensee proposed to modify the existing two-train ACAD into a two-train Nitrogen Containment Air Dilution (NCAD) system.
However, upon learning of the plans of another utility, the licensee revised its proposal in such a manner as to provide single failure proof capability in the existing nitrogen inerting/makeup system in lieu of providing a separate new NCAD system (Letters from Commonwealth Edison Company (CECo) dated March 6, 1992, and April 16, 1993).
This staff review concentrated on confirming that the modified systems will provide purge/repressurization capability meeting the system design requirements of GDC 41, 42 and 43, and have capacity sufficient to cope with Regulatory Guide 1.7 radiolysis oxygen generation assumptions.
2.2 Analysis to Determine R.G. 1.7 Capacity Requirements In a letter dated April 30, 1993, the licensee provided a supporting analysis identifying the combustible gas control system capacity requirement based on Regulatory Guide 1.7 criteria.
The purpose of the licensee's analysis was to determine the time at which nitrogen injection would be required and the required flow rates.
The initial assumed conditions in the containment were as follows:
Temperature, ° F Relative Humidity, %
Air Volume, ft 3 Pressure, psig Drywell 150 0
158,236 15.7 Wetwell 95 100 112,800 14.7 An initial oxygen concentration of 4.0 percent by volume was assumed in the analysis to be consistent with the plant technical specifications during reactor power operation.
Reactor power is 2578 MWt (102 percent).
The licensee's analysis indicates that the flammability limit for hydrogen would be reached about 15 hours1.736111e-4 days <br />0.00417 hours <br />2.480159e-5 weeks <br />5.7075e-6 months <br /> into the design basis (Regulatory Guide 1.7 hydrogen source terms) hydrogen generation event providing that no specific actions have been taken before that time.
At 15 hours1.736111e-4 days <br />0.00417 hours <br />2.480159e-5 weeks <br />5.7075e-6 months <br /> after the LOCA, the licensee would manually initiate nitrogen dilution.
The maximum required injection flow rate is approximately 29 scfm during the initial nitrogen makeup stages, and 5.2 scfm steady state at 32 days.
The required injection flow rate is within the flow rate capability for each of the two nitrogen injection pathways of the modified nitrogen inerting system.
The 15-hour time frame considerably exceeds the time period estimated for power restoration following a station blackout.
2.3 Description of Modified Nitrogen Inerting System and Conformance to GDC 41, 42 and 43
- Description of Proposed System:
The nitrogen makeup and inerting system provides multiple pathways for containment post-accident nitrogen dilution.
One pathway utilizes the smaller "makeup" path normally used to maintain a positive pressure in the containment.
Another utilizes the high capacity
inerting" pathway normally used during startup to establish an inerted containment.
Due to single failure concerns relating to manual cutout valves that would be inaccessible under post-accident conditions, additional redundant pathways will be provided in the makeup and inerting pathways for Dresden 2/3 and in the makeup pathways for Quad Cities 1/2.
The systems will use the existing nitrogen storage tanks (two tanks at Dresden 2/3; one tank at Quad Cities 1/2) provided for inerting and makeup to the containment.
In the event of loss or depletion of the on-site storage tank(s), nitrogen can be supplied from tank truck connections.
The licensee has identified vendors capable of providing additional nitrogen within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> during emergency situations.
Nitrogen vaporization capability will be provided by steam vaporizers in the inerting pathway (one shared vaporizer at each site), redundant electric vaporizers in the makeup pathway (Quad Cities only), a shared atmospheric vaporizer in the makeup pathway (Dresden 2/3 only), and a new atmospheric vaporizer for the truck supply connection (one shared vaporizer at each site).
These combinations of vaporizers provide multiple redundancy and including capability for passive vaporization.
The vaporizers, flow-limiting orifices, and plant procedures will preclude inadvertent liquid nitrogen injection into containment.
Compliance with General Design Criterion 41:
GDC 41 requires systems to control fission products, hydrogen and oxygen to have redundant 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.
In order to meet GDC 41, the licensee analyzed the system to determine single failure vulnerabilities (Letters from CECo dated March 6, 1992, and April 16, 1993).
The single failure vulnerabilities and proposed corrective fixes are identified below.
Reopening of Primary Containment Isolation Valves {PCIVs):
The licensee identified that a failure of either of two 120VAC distribution busses would result in the inability to reinert under post-LOCA conditions since all containment inboard isolation valve solenoid pilots are powered from one bus, and all outboard pilot valves are powered from the other bus.
Power can be restored to the required valve solenoids by lifting two wires that connect the valve group to the failed supply and then connecting a separate 120VAC source from the other unit.
These actions would be performed at the main control panels.
In addition to providing power to the 120VAC bus, the containment isolation signal must also be overridden to open the required valves.
To override the signal, two jumpers, one for each inboard and outboard valve, must be placed in the same panels discussed above.
These jumpers will provide
.. the operator with normal control at the control switches.
Prestaged jumper kits will be located in the control room.
It is estimated that these actions can be completed within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.
Override switches and valve position indication for PCIVs are provided in the control room.
The motive power for the PCIVs that must be reopened to conduct reinerting is supplied by the Instrument Air System.
The Instrument Air System is not safety grade.
However, it has a very high degree of flexibility and redundancy.
The Instrument Air System is backed-up by the Service Air System in the same unit and the Instrument and Service Air Systems of each unit are cross-connected to the other unit. Also, the licensee estimates that the air compressors can be backfed with electrical power from the emergency diesel generators within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and the system repressurized within the following 45 minutes.
Other Valves:
All other valves required for system operation are manual valves with a history of reliable operation and subject to frequent operation and periodic inspection.
Nitrogen Storage:
Nitrogen supply will be available from on-site and off-site sources.
The primary source will be an existing nitrogen tank which is required under administrative procedures to contain a 2-day supply.
Instrumentation and Control:
The makeup and inerting injection pathways share a common instrument loop for pressure control.
In the current design~ a control signal is sent concurrently to both pressure control valves whether that flow path is in use or not.
In the event of a failure of the pressure transmitter or pressure controller, the pressure control valves can be remotely manually operated from the control room by lifting two wires from the pressure controller output and substituting a signal simulation.
The pressure control valves can then be regulated by adjusting the simulated signal.
The licensee has said that this alteration could be accomplished within the eight-hour time period stated above.
However, the provision of the new flowpaths provided to bypass the inaccessible manual isolation valves eliminates the need for such alteration.
Instrumentation for containment pressure monitoring and flammability monitoring is redundant and safety grade.
New pressure, flow and temperature instrumentation will be provided at the new atmospheric vaporizers and flow limiting orifices will be provided in the new piping.
This instrumentation provides redundancy with respect to the existing flow control stations.
As described above, the licensee has identified the single failure vulnerabilities and has proposed acceptable resolutions.
Based on the above, the NRC staff finds that the requirement of GDC 41 has been satisfied.
Compliance with General Design Criterion 42:
GDC requires that applicable systems be designed to permit periodic inspection of important components.
The important components include primary containment isolation valves (PCIVs),
pressure control valves, vaporizers, and truck connections, and the on-site nitrogen storage facility.
The facility technical specifications and ASME Section XI Inservice Inspection requirements ensure periodic testing of safety grade components and establish minimum out-of-service time limits for inoperable components.
For non-safety grade components, the licensee has indicated that the components will be subjected to periodic inspection consistent with manufacturer's guidance.
Based on the above, the staff finds that the licensee's proposal complies with GDC-42.
Compliance with General Design Criterion 43:
GDC 43 requires that systems be designed to permit periodic testing for operability and leaktight integrity.
The normal operation of the inerting system provides a high degree of confidence the system is leaktight and operable.
The makeup flow path is in continuous operation during normal plant operation.
The other inerting flow path is used during each plant startup after any outage which required the containment to be deinerted.
In addition, PCIVs 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.
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 remove the ACAD system as a potential post-accident oxygen source, and provide equivalent redundant nitrogen dilution capability.
Implementation of these commitments satisfies the criteria of GL 84-09.
Principal Contributors:
K. Bristow W. Long Date:
June 29, 1993
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Mr. D.. based on.the safety significance of other modifications to be completed prior to these modifications t~ me~t the requirements of GL 84-09.
Please provide this information within 60 days following. receipt of this letter.
This c6mpletes the NRC technical resolution of GL84-09 for Dresden and Quad Cities iction on TAC Nos. M55148.M55149, M56579 and.M5658b.
Sincerely, Original Signed By:
John F. *stang, *Project Manager Project Directorate 111-2
- Division of Rea~tof Prcijects 111/IV/V
- Office of Nuclear Reactor Regulatto~ *
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