ML19209C053
| ML19209C053 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 10/05/1979 |
| From: | Mayer L NORTHERN STATES POWER CO. |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 7910110441 | |
| Download: ML19209C053 (6) | |
Text
- -
NSF NORTHERN STATES POWER COMPANY M I N N E A PO Li e. M t N N E S OTA 5 5 401 October 5, 1979 Director of Nuclear Reactor Regulation U S Nuclear Regulatory Commission Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket No. 50-282 License No. DPR-42 50-306 DPR-60 Information Concerning Potential for Adverse E.aironmental Effects Causing Interaction Between Non-Safety Grade and Safety Grade Systems In a letter dated September 17, 1979 from Mr Harold R Denton, Director, Office of Nucim e Reactor Regulation, all light water reactor licnesees were requested to provide the Commission with information related to the effects of adverse environ-ments on non-safety grade systems and the results of these effects on safety systems. This concern arose from a recent Westinghouse Electric Corporation recommendation to owners of their plants to review four non-safety grade systems for cases where an adverse environment could lead to system operation which would impact the performance of safety systems. The four systems involved were:
Steam generator power operated relief valve control system Pressurizer power operated relief valve control system Main feedwater control system Automatic rod control system
'Ihese systems were ideatified following a consideration of all control systems included in Westinghouse functional requirements and all high energy line break locations and sizes. Possible interactions were found in only these four systems.
The impact on any specific plant would depend upon instrument and control location (inside or outside break area) and qualification of the components (whether qualified for the high energy line break environment or not).
The review recommended by Westinghouse was underway at Prairie Island when Mr Denton's September 17, 1979 letter was received. This review to date has not identified any condition which would jeopardize safe operation of the plant. A summary of our findings is provided below:
lili 257 791011o W//
P
NORTHERN STATES POWER COMPANY October 5, 1979 Page Steam Generator Power Operated Relief Valve (PORV) Control System The sequence of events reported by Westinghouse for this potential interaction is as follows:
1.
Feedwater line break in Auxiliary Building.
Single failure is loss of motor driven auxiliary feedwater pump.
2.
Feedwater spills out rupture creating adverse environment.
3.
Reactor trips on low-low steam generator level on side with break. Turbine trip occurs.
4.
Steam driven auxiliary feedwater pump starts.
5.
Adverse environment in Auxiliary Building fails unaffected steam benerator PORV open causing loss of steam to turbine driven auxiliary feedwater pump.
6.
Primary system heatup due to loss of secondary heat rink. Hot leg boiling occurs.
Investigation of this event at Prairie Isiand irdicates that it cannot occur as postulated for the following reasons:
- The auxiliary building environment will not be as severe as postulated. Feedwater line check valves at Prairie Island are inside containment.
Following turbine trip, the energy of the feedwater spilling f rom the break will f all rapidly.
Since the steam generator contents cannot blow down rapidly into the auxiliary building and the temperature of the spilled feedwater drops quickly, the environment will not be severe.
- Pressure transmitters controlling the steam generator power operated relief valves (PORV) are qualified for the high energy line break environment.
This modification was made as a result of the high energy line break study in 1973 and is described in Appendix I of the Prairie Island FSAR.
Associated cabling is also suitable for the high energy line break environment.
- At Prairie Island, the motor driven auxiliary-feedwater pump from the unaffected unit can be lined up to supply feedwater at the affected unit.
- At Prairie Island, control air to the steam generator PORV's can be isolated in the control room. This will cause the valves to close.
113J 258
NORTHERN STATES POWER COMPANY October 5, 1979 Page Main Feedwater Control System The sequence of events reported by Westinghouse for this potential interaction is as follows:
1.
Small feedwater line rupture in auxiliary building.
9 2.
Feedwater spills 'ut rupture creating adverse environment.
3.
Feedwater control values to intact steam generator f ail closed due to adverse environment.
4.
Reactor trips with both steam generators at low-low setpoint. Turbine trip.
5.
Accident will be more severe than previously calculated.
Hot leg boiling will occur.
Investigation of this event at Prairie Island indicates that it has been previously analyzed. As noted in Section 14.1.10 of the Prairie Island FSAR, analysis of the Loss of Normal Feedwater accident assumes both steam generators are at the low-low setpoint initially. The analysis shows the plant is adequately protected for this accident.
Pressurizer PORV Control System The sequence of events reported by Westinghouse for this potential interaction is as follows:
1.
Feedwater line break occurs inside containment.
2.
Feedwater spills out rupture creating adverse enriron-ment in containment.
3.
Reactor trips on low-low level in ruptured steam generator.
Turbine trip and auxiliary feedsater initiated.
4.
Pressurized PORV fails open due to adverse environment.
5.
Primary pressure falls and hot leg boiling commences.
Severity of accident dependent upon time for operator to close PORV block valve.
Investigation of this event at Prairie Island indicated that it is not an unreviewed safety question. The consequences of this event are bounded by analyses contained in WCAP-9600, " Report on Small Break Accidents for Westinghouse NSS Systems".
We will alert our operators to the possibility of the pressurizer PORV's failing in the open position following a high energy line rupture inside containment. Block l l 5
9CQ l iJJ LJi
NORTHERN STATES POV'ER COMPANY October 5, 1979 Page valves in the PORV discharge lines will be closed to ensure that a secondary high energy line rupture will not cause a breach of th. primary system pressure boundary.
As a long term solution to this concern, we till investigate modifications to the pressurizer PORV control system and valves to make them capable of operating normally in an adverse environment.
Rod Control System The sequence of events reported by Westinghouse for this potential interaction is as follows:
1.
Intermediate steam line break inside containment.
Break not large enough to give immediate protective action.
Rod control system is in automatic.
2.
Steam causes adverse environment in containment.
Excore detectors and associated cabling fail causing rods to step out.
3.
Reactor trips on overpower delta T.
DNBR f alls below 1.30 prior to trip.
Accident more severe than FSAR calcualtions.
Investigation of this event at Prairie Island has shown that at steady state power levels greater than 15%, the operating procedures require Bank D rods be positioned at >215 steps. During load follow operations, the rods may be inserted further into the core, but the rods must be above the insertion limits (in accordance with Technical Specification limits). At 215 steps the differential rod worth is very low.
The rod worth is less than 2 pcm/ step, considerably lower than dif ferential rod worths used in the West oghouse analysis of this event. Because of this, the postulated transient at Prairie Island is not a significant concern.
If the plant is at steady state pover, operators would be alerted to this condition by decreasing power, shown on the control room recorder, and increasing containment temperature and pressure. Delta T also provides indication, channel alert alarm, and protection functions.
If the plant was undergoing a load change when the event occurred, the operators would be immediately aware of it because of the attention paid to indicated power.
We will alert our operators to the possibility of this event. We will also investigate the need for a plant specific analysis to determine the actual safety margins available at Prairie Island for this event.
Summary As noted above our investigation has not identified any condition which would warrant the modificatics, suspension, or revokiig of our operating licenses. We are continuing to review this matter in conjunction with the Westinghouse Electric Corporation. This review will be expanded into the investigation of as yet i13J 260
NORTHERN STATES POWER COMPANY October 5, 1979 Page unidentified instances of non-safety system and safety system interaction if any instances are identified.
Since this issue is included in the scope of future work by the NRC TMI-2 Lessons Learned Task Force, we propose to fully resolve any remaining questions through compliance with licensing requirements established by that group.
Please contact us if you have any questions related to the information we have provided.
Yo L 0 Mayer, PE Manager of Nuclear Support Services LOM/DMM/jh cc J G keppler G Charnoff 113] 261
a O
UNITED STATES NUCLEAR REGULATORY COMMISSION NORTHERN STATES POWER COMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket No. 50-282 50-306 License No. DPR-42 DPR-60 LETTER DATED OCTOBER 5, 1979 RESPONDING TO NRC REQUEST FOR INFORMATION ON ADVERSE ENVIRONMENTAL EFFECTS ON NON-SAFETY GRADE INSTRUMENTS AND CONTROLS Northern States Power Company, a Minnesota corporation, by this letter dated October 5,1979, hereby submits a response to tha NRC request dated September 17, 1979 for information on the potential for adverse environmental effects caus-ing interaction between non-safety grade and safecy grade systems.
This request contains no restricted or other defense information.
NORTHERN STATES POUER COMPANY 0 Y JbD)
By 1 J Wachter Vice President, Power Production
& System Operation On this 5th day of October,1979, before me a notary public in and for said County, personally appeared L J Uachter, Vice President, Power Production and System Operation, and bepg first duly sworn acknowledged that he is authorized to execute this document on behalf of Northern States Power Company, that he knows the contents thereof and that to the best of his knowledge, information and belief, the statements made in it are true and that it is not interposed for delay.
h p Jeanne M Hacker Notary Public - Minnesota Hennepin County p,,Gpr9s4Ahpice&<May*4 $986 1
JEANNE M. HACKER l;
{ g2-.%'
not Aav pueuc - MWNESoTA
]
4 1 Jv ll'-
HENNEPIN COUNTY u co-se E e"-5 'N ^ '" U 1l v
P__
- - ::: - :::. ::::n***n**~**n*/