ML20245B544

Inadequate NPSH in Low Pressure Safety Sys in Pwrs, Engineering Evaluation Rept

ML20245B544
Person / Time
Issue date:	10/31/1987
From:	Israel S NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD)
To:
References
TASK-AE, TASK-E710 AEOD-E710, NUDOCS 8904260152
Download: ML20245B544 (7)

Text

n%y n pc a v .y., +e n m u = aaa - +=a + x- - u. '

= =;=v

...7  : e 7' ?;.r 1

[7 .,.

+ ,

' jf '^ , s.

re y-i[

' AEOD/E710 V ENGINEERING EVALUATION REPORT INADEQUATE NPSH IN LOW PRESSURE SAFETY SYSTEMS IN PWRs.

October, 1987

~gc .

ki Prepared by: Sanford Israel-Office for Analysis and Evaluation of Operational Data l

U.S. Nuclear Regulatory Comission

,D , y7 a'- 4, 4

.f n

o. w ng , ".y" ~ '

g , g.

-e

_ , e -'.

t

_,4 ,_ _

--________..m_____.__m__.._____.___.________._.___._m__________._._____.-_.____.._.-___-_..._--___--___.m____ _ -__.______m_________.__._____ _ _ _ _

ww-._=.-=--- . +

~ =._ 7

1.0 INTRODUCTION

Three recent Licensee Event Reports at operating plants have identified a problem of excessive flow rates in the low pressure safety injection system that could occur during the recirculation phase following a loss of coolant accident. High flow rates are generally caused by lower hydraulic resistance downstream of the pumps than that normally estimated by the conservative design calculations. Ordinarily, one is concerned ebout satisfying a minimum flow limit in order to meet the ECCS requirements in 10 CFR 50.47. However, higher flow can lead to larger pressure losses in the suction piping and a decrease in the available net positive suction head (NPSH) at the inlet to the purtps.

Cavitation caused by inadequate NPSH can result in pump failure and loss of ECCS. This study summarizes some of available operating experience of the low pressure safety injection system involving NPSH.

2.0 DESCRIPTION

OF EVENTS The following events related to inadequate NPSH in low pressure safety systems were obtained from Licensee Event Reports, inspection reports, and licensee deficiency reports submitted to the NRC.

Haddam Neck, 1986 While performing engineering calculations in support of a proposed residual heat removal to high pressure safety injection cross-tie modification needed for HPSI pump recirculation, it was discovered that adequate core cooling could not be ensured for an intermediate break LOCA postulated in the core deluge system (Ref.1). Upon receipt of a safety injection signal, the core deluge system motor operated valves would open, thus allowing a significant fraction of the low pressure safety injection flow to be lost directly through the break during the injection phase of the accident. Previous small break LOCA evaluations showed that the charging and high pressure safety injection systems would provide adequate core cooling during the injection phase of the accident.

When approximately 100,000 gallons of ECCS water have been pumped from the refueling water storage tank, the ECCS would be switched to containment sump recirculation. In this case, the break size may require high-herd recircula-tion core cooling. This is accomplished by aligning the RHR pumps to feed either the high pressure safety injection or charging pumps. For the deluge line break, the residual heat removal pumps would be aligned to the high pres-sure safety injection pump suction. In this case, flow out of the break is postulated to cause residual heat removal pump and/or high pressure safety injection pump cavitation. Haddam Neck determined this cavitation condition may lead to ECCS flows being insufficient to assure adeouate core cooling.

Flow control valves were locked in throttled positions to correct this probiem.

Rojan,1986 As a followup to questions about an out-of-service valve in the residual heat removal system, Trojan discovered a discrepancy in ECCS operation during the recirculation phase of a LOCA (Ref. 2). During the cold leg injection phase of a LOCA, the residual heat removal cross-tie valves between trains would be open.

Upon switchover to cold leg recirculation, the low pressure cross-tie valves are closed unless only one resicual heat removal pump is operable in which case, the cross-tie valves would remain open. Under these conditions, the single low head

.___-____-__._-..__._.-___.____-_-____-m-__-_.--___

_ _ _ _ _ _ c a

2 pump would feed both charging pumps, both safety injection pump $, and all four cold leg injection paths. This alignment is beyond the pump capability. The NPSH requirement for a single pump was calculated to be 41 feet with an availa-ble NPSH of 31 feet. Similar inadequate NPSH was calculated for hot leg recirculation conditions. Emergency Procedures were modified to preclude operation in an unsafe system alignment.

J. M. Farley, Unit 1,1977 During preoperational testing, it was discovered that the residual heat removal pump flow rate would be significantly above the expected flow rate under post-LOCA cold leg recirculation mode of operation (Ref. 3). The highest flow rate would be 5000 gpm (4100 gpm is design) which corresponds to a 18.5 foot NPSH requirement with only 18.4 feet of NPSH available. Investigation revealed that the higher than expected flow rate was due to lower than expected RHR hydraulic resistance in the discharge piping. The actual roughness of the installed piping was less than the standard commercial steel roughness assumed in the calculations and the hydraulic resistance of installed check valves was less than estimated. Orifices were installed in the discharge piping to reduce the flow.

Turkey Point, Unit 3, 1987 Reexamination of the containment spray system, while the plant was in mode 6, indicated that the containment spray system piping resistance was less than assumed in the original calculations because a restricting orifice in the line was not in the design documents and not installed (Ref. 4). As a result, the higher flow rate increased the pressure loss in the suction piping from the refueling water storage so that the NPSH requirement could not be met at the inlets to the containment spray pumps. During the recirculation phase following a LOCA, there is no concern because the containment spray pumps take suction from the discharge of the residual heat removal pumps, whose pressure is suffi-cient to maintain adequate NPSH at the containment spray pumps even with increased flow rates. The missing orifices were installed.

3.0 ANALYSIS AND EVALUATION Minimum flow requirements for different safety systems, based on a set of limiting design conditions, are established and confirmed by the plant safety analyses, such as fuel rod temperature response and containment loading following a LOCA. Conservative estimates of the system hydraulic resistances are used at the design stage to ensure obtaining the minimum flow requirements, which are validated during preoperational testing. Adequate net positive suction pressure (NPSH) at the pump inlet also is considered in designing the fluid system. This criterion focuses on potential maximum flow rates which are generally obtained by system configurations that minimize the effective hydraulic resistance downstream of the pumps.

The residual heat removal (RHR) pump may provide several functions following a LOCA in Westinghouse plants. Normally they serve as the low head safety injection and recirculation pumps taking water from the refueling water storage taak or the containment sump respectively. In many plants, the high pressure

' safety injection pumps are fed from the discharge of the RHR pumps during the recirculation phase following a LOCA. In a few plants, the RHR pumps also provide containment spray during the recirculation phase following a LOCA.

Thus, it ir possible that one RHR pump (assuming a failure of the second RHR 1

, - - - - . = = - -

w i pump) is discharging to four cold legs, two to four HPSI pumps, and two trains of containment spray. This arrangement significantly reduces the effective hydraulic resistance on the discharge side of the RHR pumps which increases the pump flow beyond simple RHR flow conditions or even simple low pressure safety l injection conditions .

The increased pump flow increases the pressure losses in the suction lines to ,

the RHR pumps thus reducing the available NPSH. Most of the plants were i licensed based on Regulatory Guide 1.1 which addresses NPSH for emergency core cooling and containment heat removal systems. Specifically, the guide states that the maximum expected fluid temperatures should be assumed and no credit l taken for increased containment pressure following the accident and calculating i the available NPSH during the recirculation phase following a LOCA. Three of the events cited above indicated inadequate RHR pump NPSH during the recircula-tion phase following a LOCA. The fourth event illustrates a deficiency in the l J

plant construction which omitted an orifice in the containment spray lines resulting in excessive spray pump flow rates and inadequate NPSH when drawing water from the RWST.

Inadequate NPSH causes cavitation which pits and erodes the impeller blades and ultimately results in pump failure. Since containment spray and low head recirculation functions mitigate loss of coolant accidents, their failure, because of design or installation deficiencies, constitutes a significant reduction in safety margin.

Identification of a deficiency in system design / installation / operation resulting in inadequate NPSH involves more than a simple confirmation of the original design calculations. In one instance, a deficiency was noted only for 6 narrow range of primary break sites which affected the containment sump level at the time of switchover. Two of the events illustrated implementation discrepar.cies between the design calculations and as-built system or system alignment during system operation. Deficiencies noted at three of the plants were identified several years after the plants were operating and then, only as a secondary outcome of some other activity. Thus, similar deficiencies in other plants may go undetected unless specifically searched for.

-syw.m =~-- -~ w . - - - -

=~===:-=-==-----

x

'f :

3. '

l*{ '

.. .4 -

, 7

'4' .0 l ' FINDINGS AND CONCLUSIONS " , ,

'1. . Design of-low pressure fluid' systems Monsiders off-design conditions to ensure operability of the safety system when required.

~2. Design criterion for addressing the net positive suction head at the . inlet-to low head safety injection and containment spray pumps are specifically identified in Regulatory Guide 1.1 which was applied to most of the '

operating PWRs. l

~3. Several plants have discovered that as-built or operated fluid systems do .

not comply with the original design basis for the. system, and have modified the systems accordingly to achieve adequate NPSH. . Deficiencies in the-adequacy of NPSH may go undetected unless the' issue is specifically

- searched for.

5.0 REFERENCES

1. Licensee Event Report 86-048, Docket 50-213,. Haddam Neck, January 12, 1987.

2. Licensee Event Report 86-003, Docket 50.344, Trojan Nuclear Plant, August 29, 1986.

3. Letter from A.' Barton (Alabama Power Company) to N. Moseley (NRC) dated

. July 15,.1977.

4. Licensee Event Report 87-014, Docket 50-250, Turkey Point, Unit 3, June 18, 1987.

i