ML19312C584

Proposed Mod of Hpis.

ML19312C584
Person / Time
Site:	Oconee
Issue date:	07/14/1978
From:	DUKE POWER CO.
To:
Shared Package
ML19312C580	List: ML19312C579 ML19312C584
References
NUDOCS 7912180890
Download: ML19312C584 (4)

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OC0 NEE NUCLEAR STATION UNITS 1, 2 AND 3 PROPOSED MODIFICATION OF HIGH PRESSURE INJECTION SYSTEM July 14, 1978 l

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OCONEE NUCLEAR STATION UNITS 1, 2 AND 3 PROPOSED MODIFICATION OF HIGH PRESSURE INJECTION SYSTEM

1. Introduction In April, 1978, a problem was identified with regard to the ECCS per-formance analysis of small break LOCA's for Oconee class reactors. The previous small break ECCS performance analysis for Oconee class reactors, as documented in BAW-10052, considered the pump suction as the limiting break location for small breaks. The analysis of these breaks was per-formed assuming that only one train of the high pressure injection (HPI) system was operable and was shown to be adequate to provide the necessary core cooling. Recently, however, it has been determined that the limiting break location for small breaks is the pump discharge of the reactor coolant cold legs and not the pump suction, as was assumed in the BAW-10052 analysis. The analysis of a spectrum of small breaks at the pump discharge showed that for these breaks just one train of HPI flow is insufficient to maintain the core covered with fluid without any cladding excursion. Therefore, the re-analysis of small breaks (at the pump dis-charge), as documented in Reference 1, has been performed assuming HPI flow equivalent to 350 gpm at a reactor coolant system pressure of 600 psig to the three intact reactor coolant cold legs (70 percent of 500 gpm total HPI flow), and this flow was shown to be adequate to control the small break transients to within acceptable consequences. But, with the existing arrangement of the Oconee HPI system this amount of flow can be attained only when two HPI pumps and the two associated HPI flow paths are operable.

Each of the Oconee units has three HPI pumps normally available, and all three pumps are automatically started when the Engineered Safeguards (ES) signal is actuated. The flow discharged from these pumps is injected into the reactor coolant system through two independent injection lines, each branching into two smaller lines, and terminating into the reactor coolant cold legs between the pump discharge and the reactor vessel nozzle.

Thus, under normal design conditions HPI flow by two pumps through two injection paths, adequate to provide the necessary flow into the core during small break events, is available. There exists, however, two postulated failure modes of the HPI flow trains-- (1) failure of HPI pump "C" and (2) failure of the ES valve (HP-26 or HP-27) in the injection line-- which could render one HPI train inoperable. To assure that two HPI trains are available, as required by the recent analysis of small break LOCA's(l), the Oconee Emergency Operating Procedures were revised to require operator action outside the control room to establish flow in applicable HPI flow trains. In order to eliminate operator action outside the control room and to effectively mitigate the consequences of small break LOCA's, a modification of the HPI system as described in the following section will be implemented.

II. Description of Proposed Modification The proposed modification consists of installing a cross-connect line between the A and B HPI discharge lines downstream from the ES valves

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(HP-26 and HP-27) and another tie-line connecting this cross-connect line and the HP1 pumps B-C discharge header with isolation valves, as shown in Figure 1. The isolation valves HPI-X and HPI-Y (temporary designations) will be manually-controlled, electrically-operated valves (EHO valves) capable of being manipulated from the control room. The operators of these valves will be powered by a source of power supply independent of that supplying power to the HPI-A and HPI-B flow trains.

III. Evaluation The proposed modification wil} assure that two HPI trains (two pumps and the two associated flow paths) will be available during design conditions involving worst case single failure. The single failure analysis of the HPI system shows that for all postulate ( single failore conditions the HPI system with the proposed modification will be capable of supplying HPI flow by two HPI pumps through two injection paths.

The ECCS performance analysis of small breaks at the pump discharge, docu-mented in Reference 1, is performed assuming that HPI flow through one train is available at the time of the transient and that the HPI flow through the other train is established at 10 minutes following the ES actuation (total HPI flow of 500 gpm at 600 psig). The HPI flow through each train is 440 gpm at 600 psig for Oconee units. Therefore, the modified HPI system more than adequately satisfies the ECC flow require-ments of small break LOCA's.

The proposed modification is a passive system during normal operation of the plant, and utilization of the modified flow lines is required only in the event of a small br.eak LOCA and a simultaneous failure of one of the existing flow trains. The proposed modification would not increase the maximum flow by the HPI system but would only increase the minimum available flow. It is considered that the proposed modification would not adversely affect the performance of systems important to safety.

IV. Operator Action The only operator action required for the modified HPI system is the opening of the isolation valves HPI-X and HPI-Y fellowing an ES actuation.

The allowable time to accomplish this function is ten minutes. Since the controls for these valves will be located within the control room, the operator can accomplish this function promptly and easily.

Reference 1 - Duke Power Company Letter to NRC (from W. O. Parker, Jr. to Edson G. Case), May 15, 1978.

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