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

The reactor was shut down April 15, 2013, to enter a refueling outage.

Workers removed the concrete shield plugs, the drywell head, and the reactor pressure vessel head vent pipe (not shown in diagram) and were preparing the unbolt and remove the reactor pressure vessel head.

The water inside the reactor vessel was flooded up to just below the flanges where the head is bolted on.

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

At the time, one of three shutdown cooling (SDC) pumps was operating to cool the water inside the reactor pressure vessel.

The SDC system takes water from the suction piping for reactor recirculation pump (RRP) 14 and returns cooled water to the discharge piping of RRP 14.

Boiling water reactors (BWRs) like Nine Mile Point Unit 1 use recirculation pumps to control the flow of water through the reactor core.

During reactor operation, increasing the recirculation flow rate “sweeps” bubbles from the reactor core faster, causing the power level to rise. Decreasing the recirculation flow rate has the opposite effect – reducing the reactor’s power level.

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 was sent by SDC pump 12 though tubes within a heat exchanger. Water from the Reactor Building Closed Loop Cooling (RBCLC) system flowed outside the heat exchanger’s tubes and removed heat from the reactor water. The cooled water was returned to the recirculation system and then the reactor pressure vessel.

A team was preparing to work on the electromatic relief valves that open to discharge steam to the torus to protect the reactor pressure vessel from excessive pressure.

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 the electromatic relief valves has been de-energized so their work could be performed safely.

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

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 vessel water.

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 plant’s computer.

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

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

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

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

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

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

One of the indicators available to them was the temperature of the RBCLC water steadily decreasing. Because SDC pump 12 was no longer sending water from the reactor pressure vessel through the heat exchanger, the RBCLC water was no longer carrying 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.

When the shut down of SDC 12 was noticed, the operators took steps to turn on SDC pumps 11 and 13. These pumps has been physically disconnected from their power supplies. Operators reconnected the pumps to their power supplies and started them.

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

At the time of this event, the condensate/feedwater system was supplying about 40 gallons 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 the liquid radwaste system for treatment.

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

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

The condensate/feedwater pumps and the control rod drive system’s two pumps were among the means available to the operator to supply water to the reactor pressure vessel to compensate for inventory being boiled away.

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

X

XXX

Without ac power, the pumps normally adding water to the reactor vessel are unavailable.

(Nine Mile Point Unit 1 does not have a RCIC pump.)

X X

X XXX

OWithout ac power, most of the emergency pumps providing water to the reactor vessel are unavailable. HPCI is a steam-driven pump and no steam was available during refueling.

The NRC’s preliminary greater-than-green finding for this loss of shutdown cooling event 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 of offsite power disabled all of the available makeup pumps.