Indian Point shutdown Unit 2 due to Reactor Coolant Pump leakage from seals

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The RCP circulates -100,000 gpm through the Nuclear Steam Supply System (NSSS). It takes RCS flow from the steam generator cold side outlet and pumps it to the reactor.  At Indian Point Unit 2 Nuclear Station (IP2) there have been a significant number of reactor coolant pump (RCP) seal failures.

The seal assembly consists of three seals operating in series along the pump shaft just above the main flange.The seal return line for RCPs is designed to recover leakoff volume, at low pressure and temperature, and return it to the volume control tank or charging pump suction.

As long as cooling is provided to the RCP seals, the seals function normally and reactor coolant system (RCS) inventory losses are at the pre-event seal leak-off rate (nominally 3 gpm) which is within normal RCS makeup capability. Cooling to the RCP seals is provided by seal injection with thermal barrier cooling available as backup (thermal barrier only cools if seal injection fails).

Most seal failures are attributed to improper seal injection, debris in the system, and wear and fretting of rubbing faces in the housings and inserts.Unplanned forced outages or an extension of an outage can result if leak-off rates approach their lower or higher operating limits.

During a SBO event, this situation can lead to a significant increase in RCP seal leakage which would increase the pressure and fluid temperature in the seal return line. This over pressurization could result in a pressure boundary failure of the seal return line, further increasing the RCP seal leakage beyond that assumed in the safe shutdown analysis.

The resulting rupture would divert more of the credited boric acid storage tank (BAST) volume than was assumed in the development of the licensee’s fire safe-shutdown strategies.

Power Reactor Event Number: 47582
Region: 1 State: NY
Unit: [2] [ ] [ ]
RX Type: [2] W-4-LP,[3] W-4-LP
NRC Notified By: RUSS LONG
Notification Date: 01/10/2012
Notification Time: 04:49 [ET]
Event Date: 01/10/2012
Event Time: 04:36 [EST]
Last Update Date: 01/10/2012
Emergency Class: NON EMERGENCY
10 CFR Section:
50.72(b)(2)(iv)(B) – RPS ACTUATION – CRITICAL
Person (Organization):
Unit SCRAM Code RX CRIT Initial PWR Initial RX Mode Current PWR Current RX Mode
2 M/R Y 25 Power Operation 0 Hot Standby

“Plant shutdown from 100% due to increased leakage from 21 RCP [Reactor Coolant Pump] seal return was commenced at 0100 EST. A manual reactor trip was actuated in accordance with plant procedures at 25% power. Auxiliary feedwater pump was placed in service prior to the reactor trip, per plant procedures. Decay heat is being removed via the condenser steam dumps. All offsite power sources are available. All equipment operated as expected.”

The licensee has notified the NRC Resident Inspector and other government agencies.

The No. 1 leak-off flow alarm settings are 1.0 gpm and 5.0 gpm with reactor and pump shutdown specified in procedures at less than 0.8 gpm or greater than 6 gpm. If the high alarm is reached (and verified that flow is high) the behavior of the radial bearing and No. 1 leak-off temperature dictates the shut down response.

If either temperature is trending higher and the unit is in Mode 1 or 2, the operator is to trip the reactor and stop the affected pump. If in Mode 3 or lower, the unit is permitted to operate with one less RCP without tripping the reactor; only the affected pump is stopped. If, on the other hand, the temperatures are stable or dropping, the operator is to initiate a controlled shut down to Mode 3 and should remove the RCP from service within 8 hours.

The high temperature alarm settings for both the No. 1 seal leak-off and lower pump bearing is between 180°F and 190°F, depending on the pump model. If the setpoint is reached, the operator is directed to shut the pump down as soon as possible (initiate controlled shut down to Mode 3 and remove affected RCP from service).

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