ML20028A757
| ML20028A757 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 11/23/1982 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20028A755 | List: |
| References | |
| NUDOCS 8211290003 | |
| Download: ML20028A757 (4) | |
Text
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$UPPORTING AMENDMENT N0. 76 TO FACILITY OPERATING LICENSE N0. DPR-3 YANKEE ATOMIC ELECTRIC COMPANY YANKEE NUCLEAR POWER STATION
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DOCKET NO. 50-29
1.0 INTRODUCTION
By letters da'ad October 15, 1982 and October 29, 1982 Yankee Atomic Electric Company (YAEC) (the licensee) proposed changes to Section 3/4 1 and 3/4 9 of the Technical Specifications for the Yankee Nuclear Power Station (Yankee).
The proposed changes would revise the minimum permissible flow rate for the shutdown cooling system during Mode 6 operation (refueling).
The shutdown cooling system is also known as the residual heat removal system.
2.0 BACKGROUND
ec-The shutdown cooling system is provided to remove the heat generated by radioactive decay of fission products in the reactor core during ex-tended shutdown periods.
The shutdown cooling system is placed in service af ter the main coolant temperature has been reduced to approxi-mately 3300F and the pressure to less than 300 psi gage.
The shutdown cooling system then reduces the main coolant temperature to 1400F or less and operates continuously to maintain this temperature as long as is required by maintenance or refueling operations.
The shutdown cooling system consists of a heat exchanger, circulating pump, piping, valves,. and instruments arranged in a low rressure auxiliary loop in parallel with the main coolant loops.
The shutdown cooling pump takes suction from the hot leg of the main coolant piping of loop 4 on the reactor side of the loop stop valves and recirculates radioactive main coolant water through the tube side of the shutdown cooler and back into the cold leg of loop 4 main coolant piping, also on the reactor side of the loop stop valves.
The main coolan,t is contained in a closed system and reactor decay heat load is transferred through the shutdown cooler to the component cooling system which in turn is cooled by river water.
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Complete backup of the system is provided by the low pressure surge tank pump and heat exchanger which are identical units connected in parallel.
By employing double valving in the inlet and outlet lines to the main coolings piping, required maintenance can be accomplished on the shutdown cooling system components while the reactor plant is pressurized and in full power operation.
The shutdown cooling system is designed to remove 4 X 106 Btu /hr.
with a component cooling shell side inlet temperature of 750F.
Based 0
on a shell side inigt of 70 F the heat removal capabilities are in-
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creased to 6.5 X 100 Btu /hr.
The cooling system will also reduce the main coolant temperature from 3300F to 1400F in approximately 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />.
3.0 DiSCUSSIONANDEVALUATION The' current Technical Specification requires a flow rate of 950 gpm through the shutdown cooling system to adequately remove core decay.
heat and to prevent stratification of soluble boron in the reactor coolant system during Mode 6 operation. A recent plant modification to the control value in the system has caused the flow to drop from its previous normal value of approximately 1000 gpm to exactly 950 gpm.
The licensee is investigating this change but has not yet determined its cause.
The next step in its plan of action is to secure the shutdown cooling system and open it up for inspection, but action cannot be taken until the completion of the current refueling outage.
The situation has been discussed with NRC Region I, the NRC; Resident Inspector, and NRC Headquarters staff members, and ill have concluded that no hazard exists as a result of this change in flow rates because the changes is relatively small (5%), because of the existing redun-dancy in the shutdown cooling system, because the licensee has experienced no problem in maintaining reactor coolant temperature below 1400F, and because the flow rate is still within specification.
Nonetheless, the licensee reviewed the operation of the shutdown cooling system to determine the flow rate that would actually be necessary to maintain the coolant temperature below 140 F.
The flow rate specifica-tion applies only to tjode 6 operation.
The licensee assumed a value of 2.06 Mw for decay heat generation, which corresponds to 5 days after shutdown.
This value was used because it takes 5 days to shutdown and cooldown, and make preparations for removal of the reactor vessel head, which marks the change from Mode 5 (Cold Shutdown) to Mode 6 (Refueling).
This value agrees well with the 2.04 Mw decay heat generation rate calculated by the staff using BTP ASB 9-2.
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Using thi! design maximum river water temperature of 81of, the licensee determined that a flow rate of 770 gpm would be necessary to maintain the reactor coolant temperature below 1400F.
In or'8ef to provide ad-ditional margin, t.he licensee proposed that the minimum flow rate 'be established at 850 gpm.
This is 11% below the existing specification but still 10% above the absolute minimum required flow.
The staff has reviewed these calculations and concludes that the proposed flow rate of 850 gpm is acceptable for the removal of decay heat in Mode 6.
Besides maintaining the temperature in the reactor coolant system, the shutdown cooling system circulates the coolant in such a way as to ensure that no stratification of the dissolved boron occurs.
Good ntixing of the boron is desirable to ensure that eactivity changes will be gradual during boron concentration redu:tions in the reactor coolant system. A recent boron dilution experir.ent at the LOFT facility considered the effect of varying flow rates on the mixing of boron in the reactor coolant system (EGG-LOFT-5867, Quick-Leok Report on LOFT Boron Dilution Experiment L6-6, May 1982).
The report concluded that effective mixing occurs for a wide range of flow rates, down to nearly stagnant flow conditions, and that the natural circulation currents set up by decay heat from fuel elements are in themselves an effective echanism for mixing.
Considering these results, the staff concludes that a reduction in shutdown cooling system flow specification of 11% would have no effect on boron stratification and is acceptable.
4.0 EllVIRONMENTAL CONSIDERATION We have determined that the amendment does not authorize a change in effluent types or total amounts nor an increase in power level and will not result in any significant environmental impact.
Having made this determination, we have further concluded that the amendment involves an action which is insignificant from the standpoint of environmental impact and, pursuant to 10 CFR 551.5(d)(4), that an environmental impact statement or negative declaration and environmental impact appraisal need not be prepared in connection with the issuance of this amendment.
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5.0 CONCLUSION
We have concluded, based oa the considerations discussed above, that:
(1) because the amendment does not involve a significant increase in the probability or' consequences of an accident previously evaluated, does not create the possibility of an accident or a type different from any evaluated previously, and does not involve a significant reduction in a margin of safety, the amendment does not involve A significant hazards consideration; (2) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner; and (3) such activities will be conducted in compliance with the Commission's regulations and the issuance of this amendment w.ill not be inimical to the common defense and security or to the health and safety of the public.
6.0 ACRNOWLEDGEMENT This evaluation was prepared by R. Caruso.
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November 23, 1982 DN," -
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