ML20137X141

Safety Evaluation Supporting Util 850830,0910,11 & 23 Requests for Authorization to Operate at Max 8% Power,For Up to 45 Days,To Remove Moisture from RCS

ML20137X141
Person / Time
Site:	Fort Saint Vrain
Issue date:	09/30/1985
From:	NRC
To:
Shared Package
ML20137X130	List: ML20137X124 ML20137X141
References
GL-85-15, TAC-59787, NUDOCS 8510040463
Download: ML20137X141 (5)

Text

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SAFETY EVALUATION CONCERNING LIMITED LOW POWER OPERATION OF FORT ST. VRAIN DOCKET NO. 50-267

1.0 INTRODUCTION AND BACKGROUND

By Confirmatory Action Letter dated July 19, 1985, the staff authorized the restart of Fort St. Vrain (FSV) and its operation up to 15 percent of Rated Thermal Power. This allowed the licensee to operate the reactor to remove moisture, but limited operation until certain equipment qualification issues were resolved. Specifically, the licensee had to complete aging and equipment operability time studies required for equipment qualification under 10 CFR 50.49.

By letter dated August 20, 1985, the licensee reported to the staff on the progress of his equipment qualification program. New problems discovered by the licensee includad:

- Equipment items subject to submergence Unqualified taped electrical splices

- Contaminated or rusted electrical junction boxes

- Discrepancies in equipment model numbers in comparison to test.

reports

- Additional equipment potentially exposed to a harsh environment In discussions between the licensee ar,j the staff, the licensee agreed not to operate the reactor until this situation could be reviewed and approved by the staff.

By letter dated August 30, 1985, the licensee requested authorization from the staff to operate FSV at low power for a limited period to continue moisture removal. Specifically, the licensee requested NRC approval to operate at power levels not to exceed 8 percent (of full power) for a period not to exceed 45 days or extend beyond November 30, 1985. The licensee supplemented this request with additional information by letters dated September 10, 11 and 23, 1*

2.0 EVALUATION 2.1 Desinability of Continued Moisture hemoval.

The licensee has committed to and initiated multiple efforts to minimize moisture ingress into the FSV reactor. The licensee considers it prudent to continue to reduce the level of moisture in the reactor vessel.

8510040463 850930 PDR ADOCK 05000267 P PDR

The staff has reviewed the question of moisture ingress into the FSV reactor in its October 1984 Assessment Report (Reference 1). The staff has concluded in this report that reducing moisture ingress would improve the reliability of overall plant operations and potentially improve the performance of the control rod drive mechanisms. The staff required the licensee to implement modifications to reduce the frequency and severity of further moisture ingress events.

Hence, the staff concludes that further lowering of the reactor moisture level is consistent with the staff's previous findings on moisture ingress, and therefore should be continued.

2.2 Qualification Status Of Equipment For Decay Heat Removal The licensee has stated that a number of equipment r"lalification problems at FSV remain unresolved. The licensee is working to resolve these issues as rapidly as possible. The staff has found that pending resolution of these issues, equipment necessary to maintain forced Helium circulation cooling can not be considered qualified. Therefore, no credit can be taken for automatic operation of this equipment.

The licensee has stated, however, that operation at low power for a 45 day period before November 30, 1985, would rely only on manual operation of the liner cooling system for decay heat. removal following a high energy (steam) line break accident; and that operation of this equipment in the manual mode is not vulnerable to an accident environment.

The liner cooling system has two redundant loops and is in continuous operation during reactor operation. The normal supply for liner cooling water is the Reactor Plant Cooling Water System (System 46) pumps.

These pumps are located on Level 8 in the Reactor Building and east of the 4A wall. Therefore, with the relatively low Reactor Building temperature profiles at 8 percent power, it is expected that these pumps would not be affected during the steam line rupture. The remainder of System 46 is also expected to survive the accident due to the relatively low temperature-profile.

However, even if all electrical items in System 46 fail, fire water can be manually valved into the liner cooling system. This would be a once ,

through system with the water being supplied by the fire water pumps located outside of the building. In this case, no qualification is required of any electrical items. Portions of the liner cooling system exposed to a harsh environment can be manually operated.

The licensee has committed to have in effect, prior to restart, emergency procedures which reflect the dependence on the liner cooling system as the decay heat removal path. Operators would verify operation of the liner cooling system following a high energy line break accident. This verification would be by direct local observation of equipment operation and valve lineup. Operation would be periodically reverified. In the event.that the operators conclude that the liner cooling system is not

performing its design function, they would restore its operation through manual actions. The licensee is not taking credit for any automatic equipment operation in assuring operation of the liner cooling system.

Instead, the licensee is relying entirely on appropriate manual actions.

Reliance on manual operation for the liner cooling system is acceptable because the accident analysis indicates that there is adequate time (over 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />) in which to gain access to the necessary components and take action to assure that liner cooling is restored. This time limitation is. based on maintaining the reactor vessel concrete below 400 F, the level at which the concrete could be expected to lose a significant fraction of its compressive strength. The licensee has stated that failure of other equipment does not affect the independent operation of the liner cooling system and that even in the event of equipment failures ample time margin is available to enable operators to perform the manual operations necessary to realign the system or to effect any necessary repairs.

In summary, the licensee has shown that the liner cooling system can be used to remove decay heat, and has committed to implement, prior to.

restart, emergency procedures to assure that:

1) The operator will not be misled by failure of equipment, and,

2) required equipment is verified to be available and operable.

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2.3 Accident Consequences The licensee has stated that plant operation at 8 percent power level greatly reduces the requirements for decay heat removal. At this power level, the.

average core temperature would be 640 F. Decay heat calculations would yield a maximum temperature of 1337 F 100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 months <br /> after reactor shutdown, without any liner cooling. With one loop of liner cooling, fuel temperatures would reach 1281*F about 100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 months <br /> after reactor shutdown. These temperatures are well below the 2400 F ful temperature reached in normal operation, and the 2900 F fuel temperature at which fission product release begins to occur. Under these conditions, there would be no significant fuel failure or fission product activity released. Operation of the liner cooling system would be assured by the licensee's procedures following a reactor shutdown.

The staff performed independent calculations concerning decay heat removal for Fort St. Vrain when operated at the 8 percent level. The staff's calculations find that the maximum fuel temperature reached is about 1350*F after 3 days, with only the liner cooling system in operation (Reference 2). The staff calculated peak temperature with the liner cooling system in operation is higher than the temperature calculated by the licensee assuming no liner cooling because of different assumptions about coolant flows and core temperatures in low power operation. The staff's calculations are more conservative because the minimum allowable flow and higher core temperatures were assumed as initial conditions. It should also be noted that both the core heatup and the cooldown produced by the liner cooling system take place very gradually. Hence, temporary interruptions of the liner cooling system operation do not greatly affect fuel temperatures.

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The staff concludes that the liner cooling system provides adequate decay heat removal for FSV at the 8 percent power level and that operation in this mode has no unacceptable accident consequences.

2.4 Return To Power Operation The licensee has stated that the need to remove moisture from the FSV reactor will potentially delay eventual return to power operation. FSV technical specifications (Sections LCO 4.2.10 and 4.2.11) limit the allowable moisture levels in reactor primary coolant. The licensee states it must continue moisture removal operation or delay the ret"rn of FSV to power operation.

Eventual startup of the plant will aise require the licensee to resolve problems with equipment qualification. hence, if the reactor moisture level is reduced now, and maintained in that condition, reactor power operation can begin promptly when the other problems are resolved.

Additionally, maintaining the reactor at low moisture levels during this period minimizes the potential for the moisture to adversely affect the exposed reactor systems, thus delaying the plant's return to power operation.

The staff concludes that return to power operation of FSV is in the public interest and that lower power operation to allow moisture removal should be permitted.

3.0 CONCLUSION

The staff finds the continued low power operation of FSV to remove moisture is desirable. This is part of the licensee's overall effort to reduce the undesirable effects of moisture ingress on the reactor. The staff also finds that limited operation to remove moisture now would potentially allow Fort St. Vrain to return to power operation at an earlier date after current equipment qualification problems are resolved. In low power operation, the staff finds that decay heat can be safely removed by the liner cooling system. The licensee has committed to put in place prior to restart appropriate procedures and training to assure that the liner cooling system can be placed in operation manually and to assure that its operation will not be affected by unresolved problems with equipment qualification. The NRC will verify acceptable implementation of these procedures by inspection prior to restart. Thus, the staff concludes that Fort St. Vrain can be safely operated for a period not to exceed 45 days and at a power level not to exceed 8 percent of full power. However, operation at low power levels is only acceptable for a period of 45 days before November 30, 1985. After this period, plant operation must be in accordance with Commission policy as stated in GL 85-15.

Date: September 27, 1985 Principal Contributors:

K. Heitner, DL T. King, DL A. Masciantonio, DE

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References:

1. Preliminary Report Related to the Restart and Continued Operation of Fort St. Vrain Nuclear Generating Station, October 1984.

2. Letter to T. L. King, NRC, from S. J. Ball, ORNL, on ORECA Analysis of FSV 8% Power Severe Accident Sequences dated September 9, 1985 m

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