ML20207N218

Safety Evaluation Supporting Util Response to NRC 860721 Request for Addl Info Re Reactor Coolant Pump Shaft Integrity,Per IE Info Notice 86-019

ML20207N218
Person / Time
Site:	Maine Yankee
Issue date:	01/12/1987
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20207N208	List: ML20207N206 ML20207N218
References
IEIN-86-019, IEIN-86-19, NUDOCS 8701140057
Download: ML20207N218 (3)

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/  %, UNITED STATES 3 [ g NUCLEAR REGULATORY COMMISSION

;j WASHINGTON, D. C. 20555 l

l SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION MAINE YANKEE AIUMIC F0WER COMPAtiY MAINE YANKEE ATOMIC F0WER STATION DOCKET NO. 50-309 REACTOR COOLANT PUMP INTEGRITY BACKGROUND On April 23, 1986, the staff issued a request for information under 10 CFR 50.54(f)totheMaineYankeeAtomicPowerCompany(MYAPCo)inresponseto the Reactor Coolant Pump (RCP) Shaft Failure event at Crystal River on January 1, 1986. This event was described in IE Information Notice 86-19, Reactor Coolant Pump Shaft Failure at Crystal River, which was issued on March 21, 1986.

The 50.54(f) letter requested schedules for inspecting the RCP shafts and structural components. In addition, justification for continued operation prior to RCP inspections was also requested. This request included:

1. A description of those design characteristics and operational aspects of the Maine Yankee RCPs which are different from the design and/or operation of the Crystal River Unit 3 and Davis-Besse Unit 1 RCP's.

2, The results of any analysis performed subsequent to analyses done for the FSAR which would address the consequences of a locked rotor or broken shaft event during plant operation.

3. Con;idering the higher probability than previously envisioned of a postulated RCP shaft failure, describe any actions you have imple-mented or have planned, such as' operator review and associated training concerning the specific events at Crystal River Unit 3, and Davis-Besse Unit 1, and monitoring plant parameters such as primary to secondary RCP leakage.

MYAPCo responded to thir request on May 12, 1986. The staff reviewed this response and issued a request for additional information (RAI) on July 21, 1986. MYAPCo responded to the July 21, 1986 RAI on September 4,1986. The staff evaluation of HYAPCo's responses follows:

EVALUATION The licensee, in its letter dated May 12, 1986, stated that the results reported in the original FSAR indicated that less than 7.5% of the fuel rods would experience DNB with acceptable offsite dose consequences. The seized rotor analysis was updated in 1977 and 1978 for Maine Yankee to account for an

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increase in power to 2630MW, and to account for operation with a positive moderator temperature coefficient. For each subsequent fuel cycle, the seized rotor. analysis has been updated to demonstrate that offsite doses are within 10 CFR Part 100 limits for the reloaded core.

In response to a letter from NRC to MYAPCo dated November 29, 1982, MYAFCo evaluated the consequences of a seized rotor event coincident with loss of offsite power, and with a stuck open main steam safety valve as the worst single failure. This analysis included the following conservative assumptions:

(1) RCS flow in the affected loop instantaneously goes to zero; (2) complete fuel rod failure is assumed for any rod which undergoes a DNBR of less than 1.2;(3)themaximumallowablesteamgeneratortubeleakageispresent;(4)the stuck open safety valve is on the steam generator having a maximum allowable tube leakage; and (5) the affected steam generator is assumed to boil dry, so no credit is takea for partitioning factors. The assumptions used for this analysis are compatible with the criteria set forth in SRP Section 15.3.3, and the results of the analysis were used to support cycle 8 reload at Maine Yankee.

Therefore, the staff has determined that the results of analyses indicate that the radiological consequences of the locked rotor event coincident with the loss of offsite power, and a limiting single failure are within 10 CFR Part 100 limits. The staff also believes that the sheared shaft event is bounded by the locked rotor event.

.The staff, in its July 21, 1986 RAI, expressed concern related to the cracking of the RCP drive pins and capscrews found at Crystal River Unit 3 and Davis-Besse Unit 1. In light of the above findings, the staff requested MYAPCo to provide information on plans to inspect the drive pins and capscrews, and possible corrective actions which might be required as result of those inspections.

In its September 4, 1986 response to the staff RAI, MYAPCo stated that the Maine Yankee impeller'to shaft bolting differs greetly from that at Crystal River Unit 3 and Davis-Besse Unit 1. The bolting at Maine Yankee consists of

! eight li" diameter A461 capscrews torqued to 600 ft.-lbs. In addition, there are four 1-3/4" diameter A461 drive pins used to transfer torque from the pump shaft to the impeller. Crystal River Unit 3 and Davis-Besse Unit 1 RCP designs include four 11" diameter capscrews torqued to 300 ft.-lbs. The drive pins are the same except that the Maine Yankee drive pins are on a larger bolt circle.

The licensee has concluded that the Maine Yantee design results in significantly i less cyclic stress being applied to the capscrews. The staff believes the j potential for intergranular stress corrosion cracking (ICSCC) in the capscrews is reduced in the RCP design at Maine Yankee, but not eliminated.

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In addition, MYAPCo has implemented a program of replacing a RCP motor each 1 refueling,-and. refurbishing the removed rootor during the following operating cycle. They are currently planning to expand this program to include the RCP rotating assembly beginning with the next refueling outage. The program will continue until each RCP motor and rotating assembly has been refurbished.

l During the replacement, the portions of the pump not being replaced would be visually inspected. During the following operating cycle, the removed rotating assembly will be decontaminated, inspected, and refurbished as necessary. The staff finds this inspection program acceptable.

To make the Maine Yankee licensed operators familiar with indications of a sheared shaft event, Plant Shift Superintendents discussed these indications as identified in IE Information Notice 86-19 with all licensed operators. In addition, training for licensed operator qualification and requalification was modified to include classroom and simulator training on diagnosing sheared RCP shaft and seized rotor events. The simulator training was modified to include a t : zed RCP rotor as the initiating event for a functional recovery from a subt :tical challenge.

CONCLUSION MYAPCo has responded to the concerns of the staff regarding RCP's operation at Maine Yankee. They have provided justification for continued operation by:

1. Describing their program of removal and refurbishment of the RCP rotating assemblies to begin at the next refueling i outage. This program includes a visual inspection of the entire shaft, the impeller capscrews, and the drive pins.

2. Providing the results of analyses which indicate that the radiological consequences of the locked rotor event coincident with the loss of offsite power and a limited single failure are within 10 CFR Part 100. limits. The locked rotor event bounds the sheared shaft events. '

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3. ~Mo'difying the Maine Yankee current Licensed Operator Requalifi-cation Training to include a sheared RCP impeller and a seized RCP rotor as part of the classroom and simulator training on diagnosis of these events from control board indications.

The staff finds the licensee's responses acceptable if the plar described above

, in item No.1 is iraplemented. Furthermore, the staff has determined that the license for Maine Yankee need not be modified as result of the sheared shaft event at Crystal River Unit 3.

Principal Contributor:

A. Cappucci l C. Liang R. Wright

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