ML20245B606

Inadequate NPSH in HPSI Sys in Pwrs, Engineering Evaluation Rept

ML20245B606
Person / Time
Site:	Oconee, Turkey Point, 05000000
Issue date:	08/31/1988
From:	Israel S NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD)
To:
Shared Package
ML20245B599	List: ML20245B596 ML20245B606
References
TASK-AE, TASK-E803 AEOD-E803, NUDOCS 8904260181
Download: ML20245B606 (6)

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AEOD/E803 ENGINEERING ' EVALUATION REPORT INADEQUATE NPSH IN HIGH PRESSURE SAFETY INJECTION SYSTEMS IN PWRs August, 1988 Prepared by: Sanford Israel Office for Analysis and Evaluation of Operational Data J U.S. Nuclear Regulatory Cornmission 8904260181 esogop PDR I

.ORG NEXD PDC '

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.0 INTP.000CTION i Two recent reports highlighted a potential problem of inadequate net positive l suction head (NPSH) for high pressure safety injection pumps during the recir-l- culation phase following a loss of ccolant accident. This problem is the same l

issue reported in Ref.1 on inadequate NPSP for low pressure safety systems in PWRs. An information notice (IN 87-63), published in December 1987, addressed the low pressure pump NPSH concern. The Sequence Coding and Search System was used in this present study, but did not provide any additional events beyond those found in the inspection reports.

2.0 ~ DESCRIPTION OF EVENTS

! The following events were reported as 10 CFR 50.72, immediate notification l

reports. '

Turkey Point, May 13, 1988 As part of an ongoing design basis reconstitution, the licensee discovered a condition of potential inadequate NPSH for the containment spray and high pressure safety injection pumps during the recirculation mode following a LOCA l

(Fef. 2). Under these conditions, the low head safety injection pump is providing coolant to the containment spray pumps and the high pressure safety ir.jection pumps. A throttle valve in the P.HR discharge line is normally throttled to 30 percent. This valve was opened completely while the licensee conducted an engineering analysis of the situation. Subsequent evaluation showed the original configuration to be acceptable.

Oconee, May 19, 1988 During the review of safety analysis calculation files, the licensee discovered S a set of conditions that could defeat the recirculation function following certain small break LOCAs (Ref. 3). Under these conditions, the low pressure ir.jection system provides suctica flow to the containment spray system and the high pressure injection pumps in a piggyback mode. With inadequate procedural guidance, the flow configuration may result in inadequate NPSH for the high pressure pumps and thus fail the system. The licensee is modifying appropriate emergency procedures to correct the problem.

3.0 ANALYSIS AND EVALUATION In many plants, the high pressure safety injection (HPSI) pumps and, in some ,

cases, the containment spray system is "piggybacked" on the discharge of the  !

low head safety injection pumps during the recirculation phase following a small break LOCA. This situation occurs where the low head pumps can not '

provide core cooling directly because the reactor vessel has not depressurized sufficiently. The design discharge pressure for the low head pumps, which is on the order of 100 psi, should be sufficient to provide adequate NPSH for the high head pumps in the " piggyback" mode for reasonable installations. This apparently is not the case at all plants.

-2 A typical ECCS installation of interest is shown in the enclosed figure. A crossover line downstream of the decay heat coolers connects the discharge of the low head safety injection pumps to the containment spray pumps and the HPSI pur;ps . There may be a throttle valve located in this crossover line in some plant designs such as Turkey Point.

NPSH deficiencies may be incurred because of several different factors.

Running multiple pumps off the discharge of a single pump may increase the flow above design flow rate and consequently reduces the available pressure at the low head pump discharge because of the characteristic pump performance curve.

Another factor is the as-built cross connect piping between the low head pump and the high head pump which may have higher resistance than originally considered in the design calculations because of the length and size of the piping and any intervening partially closed throttle valves. Thus, the actual  ;

attainable pressure at the high head pump inlet during recirculation conditions {

may be less than the design estimate.

Another aspect of this problem is that preoperational flow tests were not performed with the ECCS aligned in the recirculation mode. According to the updated FSARs, the recirculation tests are performed with the pumps blocked to evaluate system realignment, not functional capability. It would be difficult to run a full blown flow test that included the containment spray pumps as well.

Post-operational surveillance tests suffer from the same limitations. The extent to which the results of partial flow tests have been used to confirm the design basis conditions is unknown. Retrieval of archival documents related to these ,

calculations is fragmentary, so that their robustness may not be easily ascertained.

The discovery of the potential deficiency in NPSH at Turkey Point and Oconee was the cutcome of post-operational programs to reconstitute design calculations.

In one instance, the effort was prompted by comments in an NRC inspection report on a totally unrelated fluid system. The utility took the initiative to explore similar deficiencies in other systems. It is doubtful that these deficiencies would have been uncovered without these specific investigations by the utilities.

Inadecuate NPSH causes cavitation which ultimately results in pump failure. The high pressure safety injection system is essential for mitigating small break LOCAs and at some plants, it is used for hot leg recirculation even for large LOCAs. Thus, loss of this system in the recirculation mode would represent a significant reduction in safety margin.

i 4.0 FINDINGS AND CONCLUSIONS l l

1. Two utilities have discovered that their high pressure safety injection )

system may have inadequate NPSH during the recirculation phase following ]

a LOCA. j l

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2. Plant specific piping arrangements and the presence of throttling valves between the low head pumps and the high head pumps contribute to the deficiencies in NPSH.

3. Potential deficiencies in NPSH for high head pumps will go undetected because the surveillance tests, including preoperational tests, do not test the complete ECCS in the recirculation mode. i

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5.0 REFERENCES

1. Memorandum from T. Novak (AE00) to C.E. Rossi (NRR), " Inadequate NPSH in L Low Pressure Safety systems in FWRs", AE00/710, dated October 7, 1988.

2. Immediate Notification 50.72 Report No.12278, Turkey Point plants, dated May 13, 1988.

3. Immediate Notification 50.72 Report No. 12326, Oconee plants, dated May 19, 1988.

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UNITED STATES j NUCLEAR REGULATORY COMMISSION 0FFICE OF NUCLEAR REACTOR REGULATION.

WASHINGTON, O. C. 220555 August , 1988 INFORMATION NOTICE NO. 87-63, SUPPLEMENT 1: INADEQUATE NET POSITIVE SUCTION j HEAD IN LOW PRESSURE SAFETY SYSTEMS 1

)

' Addressees.

All pressurized water reactors holding an operating license or a construction j pennit.  ;

Purpose:

Ir. formation Notice 87-63 provided information to the addressees about inadequate NPSH in low pressure safety systems because of. deficiencies in design, installa-tion, or operation of these systems. This supplement extends this concern to high pressure safety injection systems in certain PWRs which " piggyback" the HFSI pumps off the low head pumps in the recirculation phase following a LOCA.

Fecipients are expected to review the information provided for applicability to their facilities and consider actions, if appropriate, to preclude similar occurrences at their plants. However, suggestions contained in this information netice do not constitute NPC requirements; therefore, no specific action or written response is required.

Discussion: -

Recently, two different utilities reported potential deficiencies in the NPSH fer HPSI pumps when operating in the recirculation mode following a LOCA. In bcth instances, the HPSI would be "piggybacked" off the low pressure pumps dLring the recirculation phase. Although the low head pumps provide relatively high discharge pressure, the intervening cross-connect piping between the HPSI ar.d the low head pumps and partially closed throttle valves provide sufficient hydraulic resistance to cause potential problems at the HPSI pump inlets.

Corrections include opening partially closed throttle valves and procedure modifications to improve the hydraulic characteristics of the ECCS in the recirculation mode. This is a significant safety concern because of the reliance on HPSI for mitigating small LOCAs and in some cases, they are used fer hot leg recirculation even for large LOCAs. No specific action or written response is required by this information notice. If you have any questions about this matter, please contact the technical contact listed below or the Regional Administrator of the appropriate regional office.

Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical

Contact:

S. Israel, AE0D 301 492-4437