More on Spent Fuel Pools at Fukushima

March 21, 2011 | 4:32 pm
David Wright
Former contributor

Masa Takubo this morning sent information he had collected from briefings and press reports in Japan, and the TEPCO website, about the spent fuel pools. Here is an updated table:

The pools at Units 2, 3, 4, and 5 all have a volume of 1,425 cubic meters, with dimensions of 12.2m x 9.9m x 11.8m deep.

The Unit 1 pool is somewhat smaller (1,020 cubic meters) and the Unit 6 pool slightly larger (1,497 cubic meters).

Each fuel assembly consists of roughly 60 fuel rods and has a mass of about 170 kg.

The fuel assemblies are about 4 meters long. They sit on racks in the pool, slightly off the pool’s floor. The water level is typically kept 7-8 meters above the top of the assemblies.

The sixth column of this table gives the heat generated by the fuel in the spent fuel pool. According to these numbers, the Unit 4 pool is the biggest concern for overheating.

We know that 548 fuel assemblies in the Unit 4 pool were removed from the reactor and placed in the pool three months ago when the reactor was down for maintenance. They joined 783 assemblies that were already in the pool. The value given for the amount of heat being generated in the pool is roughly equal to an estimate of the heat from the 548 fuel assemblies transferred from the core of the reactor 3 months ago, so the heat released by the remaining 783 assemblies in the pool must be relatively low.

Adding Water to the Pools

People are concerned about the level of water in the spent fuel pools because if the water level drops and the fuel is exposed to air for long enough, the temperature of the fuel rods can increase to the point they suffer damage and release potentially large amounts of radioactive gases. Cooling has been restored to the pools in Units 5 and 6, and efforts to restore cooling at Units 1 and 2 are underway.

The biggest concern is at Units 3 and 4, and in both cases explosions have damaged the reactor buildings that surround the spent fuel pools, so that gases released from the spent fuel pools would go directly into the atmosphere.

Japanese press over the past few days reported attempts to add water to the pools by helicopters and fire hoses. The first attempts were relatively small, either dropping or spraying 50-60 tons of water, with little idea of how much of that may have gotten into the pool. (A metric ton of water equals about 275 gallons.) More recently, hoses have been used at Unit 3 for many hours at a stretch.

The Japanese Nuclear and Industrial Safety Agency (NISA) updates on March 20 and 21 said that in the past day workers attempted to add 40 tons of seawater to the Unit 2 pool, 1137 tons of sea water to the Unit 3 pool, and 90 tons of fresh water to the Unit 4 pool. There has been no report of how much of this water has actually gotten into the pools.

To put that amount into perspective, the total amount of water in each of the Unit 2-5 pools is 1,300-1,400 tons. The amount of water needed to fill a pool up to the top of the assemblies—to keep them just covered—is about 500 tons.

So the amount of water workers have attempted to add to the Unit 3 pool yesterday and the day before would have more than filled the pool if a significant fraction of what was being shot at it actually entered the pool Since the heat load in this pool is low, it is not immediately clear why officials are focusing so strongly on this pool.

Because the fuel in the Unit 4 pool is hot, the water in that pool will evaporate water rapidly. If the water in the pool had been heated to boiling temperature, then heat from these assemblies would be enough to boil off about 3 tons of water every hour, or about 70 tons per day.

This amount of water would have to be replaced each day in Unit 4 to make up for that boiled away, and leaks in the pool—which have been reported—would increase the amount required. If the fuel is already partially uncovered, significantly more water would need to be added to cover it again.

The Common (Shared) Storage Pool

In addition to these individual pools, there is a larger common spent fuel pool that is used to store spent fuel from all 6 reactors once it has been out of the reactor for 19 months and has cooled down. It has a volume of 3,828 cubic meters (29m x 12m x11m deep) and currently has 6,375 spent fuel assemblies in it. It is located 50 meters west of Unit 4. Reports also say that this pool continues to have water supplies but its cooling system is not functional.

Because this pool has water, and because the fuel is much less radioactive, it has not been a concern in the current crisis.

Change of Decay Heat with Time

The plot below shows how the decay heat from spent fuel decreases with time. The hottest spent fuel in the Unit 4 pool is about 3 months, or 0.25 years, old.

Decay heat as a function of time from 0.01 years (about 4 days) to 100 years for spent-fuel burnups of 33, 43, 53 and 63 MWd/kgU. The lowest burnup was typical for the 1970s. Current burnups are around 50 MWd/kgU