ML20126C192

SER Accepting Proposed Pumpback Sys for Plant

ML20126C192
Person / Time
Site:	Monticello
Issue date:	03/08/1977
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20126C191	List: ML20126C187 ML20126C192
References
NUDOCS 9212220412
Download: ML20126C192 (3)

Text

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> l SAFETY EVALUATION BY-WE OFFICE OF NUCLEAR REACIOR REGUIATION ON WE PROP 05tu PUMPBACK SYdiud FOT K NFICELLO NUCLEAR P0hER STATION D0atil NO. 50-263 -

Ba'kground c

As a result of recent stmetural analyses perfomed in conjunction.

with a generic review of pool dynamic loads for Mark I pressure - suppression containments, it was determined that the consideration of pool dynamic loads resulting from a postulated loss-of-coolant accident had reduced the margin of safety in the containment design for.the Manticello Nuclear Power Station. Subsequently, the licensee agreed to institute a " differential pressure control system".to mitigate the pool dynamic loads and thereby restore the margin of safety in the containment design. The differential pressure control system establishes a positive pressure between the drywell and torus regions of the containment which reduces :the height of the water leg in the downcomers and subsequently reduces the hydrodynamic loads.

To control combustible gases following a postulated loss-of-coolant accident,=the containment atmosphere is inerted with nitrogen during normal operation. The inclusion of a positive differential pressure between the-drywell and torus results in a loss of nitrogen from the drywell to the .

j torus airspace from leakage through the vacuum breakers on the vent headers. To minimize the loss of_ nitrogen from the system, the licensee has proposed a "pumpback" system which would collect the over-inerted atmosphere in the torus and return it _to .the drywell.

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9212220412 770308 PDR ADOCK 05000263 p PDR

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System Description

1he pumpback system consists of a two inch line which is welded into the existing toms vent bypass line and the drywell atmosphere purge line. The pumpback line takes suction from the torus vent bypass line and branches into two parallel flow paths, each containing an upstream shutoff valve, a 1001, capacity blower, a check valve, and a downstream shutoff valve in series. The parallel lines rejoin and discharge into the drywell atmosphere purge line. The existing isolation valves on both the toms vent bypass and the drywell atmosphere purge line have been changed from a nomally closed position to a normally open position to provide the flow path from the torus airspace to the drywell.

Safety Evaluation The installation of the proposed pumpback system has required the addition of a control valve on the torus vent bypass line to provide the proper containment isolation capability. The new isolation valve, designated CV-7440, is installed in series with an existing control valve, designated CV-2384, to provide redundant containment isolation capability. The outboard isolation valve, CV-7440, is an air operated valve which is open when the valve solenoid is energized and will close' upon the receipt of a Group 2 isolation signal, the loss of the instrument bus associated with the outboard isolation valves, or the loss of instrument air. The inboard isolation valve, CV-2384, receives power from a separate instrument bus, and similarly will close uyn loss of its instrument bus, on loss of instrument air or upon receipt of a Group 2 isolation signal. The position of both valves will be indicated in the control room, i

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4 and the valves will be leak tested in accordance with Appendix J to -

l 10 CFR 50.

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• We have also_ reviewed other potential effects the inclusion of the h

l pumpback systs could have on the consequences of a postulated loss-of-4-

coolant accident. Potential steam bypass of the suppression pool could i

j result- from the direct comunication of the drywell and the torus air

space. However, the low mass flow rate associated with the 2 inch size j -line in conjunction with the redundant capability to isolate both the l torus vent bypass line-and the drywell atmosphere purge line will result

i. in a negligible amount of steam bypass. In addition, a swing-disk check

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! valve, located down stream of the blower, which would prevent reverse a

j flow rom f the drywell and further mitigate the effects of steam bypass.

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' Conclusion

. We have reviewed the proposed pumpback syst m for the Monticello Nuclear Power Station with regard to both containment isolation capability h and potential adverse effects on a postulated loss-of-coolant accident.

The licensee has provided both redundant and diverse isolation capability in conjunction with the existing isolation valves and the added isolation--

valve, CV-7440. In addition, the installation of the punback system ,

will neither increase the probability of an occurrence, nor will_ it increase t

the consequences of a postulated accident. Therefore, we conclude that pumpback system,.as proposed by the licensee, is acceptable.

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