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Nine Mile Point Unit 1

April 16, 2013 Event

Loss of Shutdown Cooling During Refueling

Alliance for a Green Economy

Prepared by David LochbaumDirector of the Nuclear Safety Project, Union of Concerned Scientists

for

Nuclear Information &Resource Service

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The reactor was shutdown April 15, 2013, toenter a refueling outage.

Workers removed theconcrete shield plugs,the drywell head, andthe reactor pressurevessel head vent pipe

(not shown in diagram)and were preparing theunbolt and remove thereactor pressure vesselhead.

The water inside thereactor vessel wasflooded up to just belowthe flanges where thehead is bolted on.

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Closer view of the reactor pressure vessel head bolted on top of the reactorpressure vessel. After the head is removed, the reactor well (i.e., the volume abovethe reactor pressure vessel) is flooded with water. The gates to the spent fuel poolare removed to connect it to the reactor well and in turn to the reactor pressurevessel so spent fuel bundles can be offloaded to the spent fuel pool and replaced

with new fuel bundles.

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At the time, one of threeshutdown cooling (SDC)pumps was operating tocool the water inside thereactor pressure vessel.

The SDC system takeswater from the suctionpiping for reactorrecirculation pump (RRP)14 and returns cooledwater to the dischargepiping of RRP 14.

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Boiling water reactors(BWRs) like Nine MilePoint Unit 1 userecirculation pumps tocontrol the flow of water

through the reactor core.

During reactor operation,increasing therecirculation flow ratesweeps bubbles fromthe reactor core faster,causing the power level torise. Decreasing therecirculation flow rate hasthe opposite effect reducing the reactorspower level.

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SDC pump 12 was operating. SDC pumps 11 and 13 were out of service at the time.

Water from the recirculation system attached to the reactor pressure vessel wassent by SDC pump 12 though tubes within a heat exchanger. Water from theReactor Building Closed Loop Cooling (RBCLC) system flowed outside the heatexchangers tubes and removed heat from the reactor water. The cooled water wasreturned to the recirculation system and then the reactor pressure vessel.

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At 2:44pm, one of the team members opened the breaker cabinet for 125 volt dc

battery board 12. The worker was checking to make sure that the power to theelectromatic relief valves has been de-energized so their work could be performedsafely.

But the worker inadvertently checked the wrong power supply board. Opening thecabinet caused the breakers to 125 volt dc battery #12 and to the static batterychargers 171A and 171B to open. Their opening de-energized 125 volt dc batteryboard 12.

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The de-energization of 125 volt dc battery board 12 generated a false signal to trip(turn off) SDC pump 12. But because the board was powerless, the trip signal was

not sent and the pump continued operating to cool the reactor pressure vesselwater.

The control room operators did not recognize that the trip signal was present,despite a visible alarm in the control room and a printout on the plants computer.

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At 3:03pm and again at 3:05pm, the control room operators attempted to closethe 125 volt dc breaker and re-connect the battery to battery board 12. Bothattempted failed and the battery board remained de-energized.

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At 3:46pm, the control room operators successfully closed the breaker to thestatic battery chargers and re-connected a power supply battery board 12.

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Battery board 12 was re-energized for only a very short time. The breaker re-opened to once again de-energize battery board 12.

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The momentary re-energization of battery board 12 allowed the trip signal to besent that turned off SDC 12 at 3:46pm.

The water inside the reactor pressure vessel was no longer being cooled. It began toheat up from 115F.

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It took awhile for the control room operators to notice that the water inside thereactor pressure vessel was no longer being cooled and was heating up.

One of the indicators available to them was the temperature of the RBCLC watersteadily decreasing. Because SDC pump 12 was no longer sending water from thereactor pressure vessel through the heat exchanger, the RBCLC water was no longercarrying away its heat. As a result, the RBCLC water temperature began dropping.But this change was not detected by the control room operators for awhile.

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When the shut down of SDC 12 was noticed, the operators took steps to turn onSDC pumps 11 and 13. These pumps has been physically disconnected from their

power supplies. Operators reconnected the pumps to their power supplies andstarted them.

At 4:17pm, cooling of the water inside the reactor pressure vessel was restored. Thewater had heated up to about 145F in the 31 minutes that cooling had been lost, arate of about 1F per minute.

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At the time of this event, the condensate/feedwater system was supplying about 40gallons per minute to the reactor pressure vessel. This makeup flow compensated

for the amount of water being sent by the reactor water cleanup system to theliquid radwaste system for treatment.

Had the loss of cooling not been detected in time or if restoration of SDC pumps 11and 13 taken longer, the water inside the reactor pressure vessel would have begunto boil at around 5:30pm.

The reactor pressure vessel head vent pipe had been removed. Steam would haveflowed through this opening into the drywell.

The condensate/feedwater pumps and the control rod drive systems two pumpswere among the means available to the operator to supply water to the reactorpressure vessel to compensate for inventory being boiled away.

IF electrical power from the grid (i.e., ac power) was available to this equipment.

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X

XXX

Without ac power, the pumpsnormally adding water to thereactor vessel are unavailable.

(Nine Mile Point Unit 1 doesnot have a RCIC pump.)

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X X

X XXX

O

Without ac power, most of theemergency pumps providing water tothe reactor vessel are unavailable.HPCI is a steam-driven pump and no

steam was available during refueling.

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The NRCs preliminary greater -than-green finding for this loss of shutdown coolingevent is based on the relatively short time-to-boil for the reactor water (less than

two hours) coupled with the lack of safety-grade makeup pumps had a loss ofoffsite power disabled all of the available makeup pumps.