ML20195K276

Submits Addl Info Re 870928 Application for Amends to Licenses DPR-42 & DPR-60 Re Turbine Valve Test Frequency Reduction.Valves Tested for Proper Operation on Monthly Basis

ML20195K276
Person / Time
Site:	Prairie Island
Issue date:	06/24/1988
From:	Musolf D NORTHERN STATES POWER CO.
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
NUDOCS 8806290383
Download: ML20195K276 (3)

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Northern States Power Company 414 Nicollet Mall Minneapohs. Minnesota 55401 Telephone (612) 330 5500 June 24, 1988 Director cf Nuclear Reactor Regulation i U.,S Nuclear Regulatory Commission

} Attn:.DocumentControl' Desk -l

• Washington, DC '20555 PRAIRIE ISIAND NUCLEAR GENERATING PIANT Docket Nos. 50-282 License Nos. DPR-42 50-306 DPR-60 Additional Information Related to License Amend. ment Request Dated September 28, 1987 Turbine Valve Test Frequency Redt.ction The purpose of this letter is to provide additional information requested during a telephone conference between NRC staff members John Craig, Charles Nichols, Richard Emsht, and Dominic DiIanni and Ron Meyer of NSP. This information supplements Prairic Island license amendment request dated September 28, 1987, "Turbine Valve Test Frequency Reduction".

Turbine Valve Testing Experience The turbine valves installed at Prairie Island plant are tested for proper operation (close on demand) on a monthly basis. In the life of the plant the turbine valves have undergone surveillance testing 112 times on Unit 1 and 104 time on Unit 2. In addition to valve testing during monthly surveillances. the valves have been cycled with subsequent verification of operation, 36 and 32 times respectively during unit cooldowns and 41 and 42 times respectively on unit trips. This gives a combined total of 367 turbine valve operations during 28 years of combined operation.

During this time, there have been two incidences of turbine valve failures.

a) During a turbine valve surveillance test on Unit 1, a turbine stop valve bypass valve (SVBV) was found in the open position. The SVBV failed to reclose during the previous surveillance due to mechanical binding. This failure did not represent a threat to a

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8806290383 880624 PDR ADOCK 05000282 P DCD

t Director of NRR

. June 24, 1988. Northem States Power Company  ;

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turbine overspeed incident since the redundant control velve l closed as designed during the subsequent test.

4 1 b) During a turbine valve surveillance test on Unit 2, a reheat l interceptor valve failed to close. Investigation found that the operating lever was 1 ,und-on the. actuator. This failure did not represent a threat'to a turbine overspeed incident since the redundant reheat stop valve closed as designed.

Performance of Turbine Overspeed Protection Systems During the life of the Prairie Island units, there have been no incidents of unplanned turbine overspeed and no single turbine valve Lfailures that could have led to a turbine overspeed. condition.

Additional Discussion of Safety Impacts of Reduced Frecuency Testing A tes't of the turbine valves at the Prairie Island plant requires that unit power he dropped to 50 to 60%. This monthly reduction in power causes a nunber of reactor. power distribution perturbations and system challenges which are undesirable:

a. Reducing reactor power is typically accomplished by the addition of boron to the Reactor Coolant System (RCS) with the control rods being used primarily for Delta I control. Upon return to power, the added boron must be removed from the RCS. This results in large amounts of water which must be processed by plant systems and the generation of radioactive vaste.

s b. The Prairie Island units were designed primarily-for base load operation. Load following and load reductions increase the possibility of equipment malfunctions which may lead to reactor trips. Lord reductions also result in additional wear and stress on components.

c. Following a load reduction, it may take several days for the reactor power distribution to stabilize. During this period, actual total reactor power can remain constant, while the indicated reactor power, as measured by the Nuclear Instrumentation System (NIS), may be changing. This is the result of xenon changes associated with the power reduction,

d. Recent fuel and reload designs have resulted in cores that are more sensitive to Delta I (power in the top of the core minus power in the bottom of the core) control. These cores can develop divergent xenon (Delta I) oscillatioas during the middle of core life. Although procedures exist to control and dampen these oscillations, they could result in reactor power reductions to remain within Technical Specification limits. Near the end of life these problems are magnified by the larger isothermal temperature

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Director of NRR June 24, 1988 Northem States Power Company

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., coefficiant, lower boron concentration, and larger differential

'xenca transients.

~ Role of WCAP-ll525 in the Licensing Process WCAP-ll525, "Probabilistic Evaluation.of Reduction in Turbine Valve Test Frequency",: serves as .the- manufacturers recotmendation for turbine valve testing for the plants represented in the Westinghousc Owners Group -

Turbine Val'e v Testing Frequency Subgroup. This analysis recommends a test interval of up to, but not exceeding one year for both Prairie Island. units. ' Recommended testing intervals are to be re-evaluated

'following any . turbine rotor modifications which are Implemented the Prairie Island units.

Based on the above additional information and the analysis and recommendations presented in WCAP-11525, we feel that reduced turbine valve testing frequencies would enhance the safety and reliability of the Prairie Island units. Reducing the frequency of power reductions for testing would place less wear and stress on critical plant systems, reduce the quantitics of contaminated reactor coolant and radioactive waste which must be processed and enhance operational safety.

Please contact us it you have any questions related to the ..: formation we

.have provided.

D ,2 D M ws _

David Musolf Manager - Nuclear Support Services fDKM/RJH/rjm c: Regional Administrator-III, NRC NRR Project Manager, NRC Resident-Inspector, NRC MPCA Attn: F W Ferman G Charnoff

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