Japanese electrical utility TEPCO has published its reasoning, and plans, for flooding the primary containment vessel of unit 1 at the damaged Fukushima Daiichi plant.

TEPCO's unit 1 reactor cooling plan would use existing penetrations to cool water in two loops, connected by a heat exchanger. The secondary loop includes an external cooling tower.

Flooding the reactor is the first step in setting up a two-loop cooling circuit to achieve cold shutdown at the unit. Following a test increase in the water injection rate in late April, TEPCO began flooding the unit 1 containment on 6 May by increasing the injection rate from 6 m3/hr to 8 m3/hr.

Japanese nuclear regulator NISA reviewed the containment flooding proposal, and concluded that under the circumstances, "NISA has evaluated the measures as appropriate since there was no other effective option," according to a translation of the document provided by TEPCO in early May.

TEPCO has also published a draft schedule of work through the end of May.

1. The reasoning

TEPCO said that the flow rate of injected water is slightly more than necessary to remove decay heat. There is a relatively short time until the temperature of the spent fuel starts to increase, if water injection stops. However, flooding primary containment (the drywell) would allow more time until the temperature of the spent fuel increases, even in a contingency when the injection stops.

TEPCO said it expected that submerging the exposed portion of fuel by the flooding of the primary containment will decrease the leakage of radioactive substances.

TEPCO said it evaluated the impact of the flooding on the damaged reactor building's ability to resist a designed seismic force, and concluded that there would be no significant shear rigidity impact on the primary containment vessel, and that the building would be able to bear the weight of the water. It also concluded that the increase in pressure from flooding would not reach the point at which ventilation would be required.

Water that is injected into unit 1 is unlikely to cause the outdoor pipe trench near unit 2 to overflow, TEPCO says, because of its work to pump water out of the trench and the unit 2 turbine building. Moreover, it is planning to cover over the trench by the end of May.

TEPCO says it is impossible that much hydrogen would be generated by the flooding operation.

The water level would be monitored in several different ways, including water level indicators and pressure indicators. Water level indicators in the building will be adjusted by workers, once the radiation dose in the unit 1 reactor building has been reduced.

2. The plan

TEPCO estimates that workers could enter the unit 1 reactor building once the amount of dust in the building is reduced, and once the radiation dose in the plant is reduced. It estimates that a planned forced-air filtration system (installation of which began on 5 May) would reduce the density of iodine-131 to 0.026 Bq/cm3 in 30 hours. However, that is still more than two times acceptable levels for workers with full face masks (0.01 Bq/cm3 Iodine-131; 0.03 Bq/cm3 Caesium-134; 0.03 Bq/cm3 Caesium-137). TEPCO is planning work including decontamination, rubble removal and shielding to reduce the average dose level of 10 mSv/hr by a factor of 10.

Once workers can enter, they will be able to calibrate and install water level indicators to flood the primary containment vessel, install a new closed-circuit reactor cooling system to attain cold shutdown of the reactor.

The air supply line that is currently being used to inject nitrogen into the containment, to prevent the explosion of hydrogen gas, will be used to feed reactor coolant into the reactor. A new line will need to be laid for nitrogen injection. TEPCO's proposed line passes through the reactor building airlock.

To allow the workers to perform the work, TEPCO will have to open the reactor airlock. It has estimated the radiation dose through inhalation from this action to be 0.004 mSv iodine-131, 0.0016 mSv caesium-134,137. It concludes that the effective dose is 1000 times lower than 1 mSv, the annual dose limit for the general public.

TEPCO has published details of its proposed plan to build a new two-loop reactor cooling system. The system combines different existing systems. In the new primary loop, a pump will push water through two pipes to the containment vessel; water from the containment vessel will flow through one pipe to a new heat exchanger unit. In the new secondary loop, a pump will push water through the heat exchanger, to a new external air cooling tower, and back again. Makeup water will be supplied to the cooling tower. The secondary loop will also have a surge tank.

Because of the space required, expected airborne radiation and the length of pipes required, TEPCO plans to put the heat exchangers in the unit 1 large equipment hatch. Secondary loop equipment including the temporary cooling tower will be placed outside.

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Date: Friday, 06 May 2011
Original article: neimagazine.com/news/newstepco-begins-flooding-unit-1