Spent fuel removed from a nuclear reactor is highly radioactive and generates intense heat.
This irradiated fuel needs to be actively cooled for one to three years in pools that cool the fuel, shield the radioactivity, and keep the fuel in the proper position to avoid fission reactions.
If the cooling is lost, the water can boil and fuel rods can be exposed to the air, possibly leading to severe damage and a large release of radiation.
Nuclear power plants must replace fuel every one to two years, and the Fukushima Daiichi reactors typically remove about 25 percent of the reactor’s fuel – to be replaced with fresh, or unirradiated, fuel – during each refuelling outage.
The spent fuel, which is hottest immediately after it is removed from the reactor, is placed in the spent fuel pool until it is cool enough to be moved to longer-term storage.
The concern about the spent fuel pools at Fukushima Daiichi is that the capability to cool the pools has been compromised. See diagram below for location of the pool in each reactor building.
Elevated radiation measurements at the site may be partially of the result of uncovered or overheated spent fuel.
Number of Fuel Assemblies in Cooling Pools at Fukushima Daiichi
Here is a summary of spent fuel conditions at Fukushima Daiichi nuclear power plant, based on documents and confirmed by Japanese officials:
Unit 1 experienced an explosion on 12 March that destroyed the outer shell of the building’s upper floors. No precise information has been available on the status of the spent fuel pool.
Precise information on the status of the spent fuel pool was unavailable in the days following the earthquake, but Japan’s Nuclear and Industrial Safety Agency began to release temperature data on 20 March:
20 March, 23:00 UTC: 49 °C
21 March, 05:25 UTC: 50 °C
21 March, 21:20 UTC: 51 °C
22 March, 02:20 UTC: 53 °C
22 March, 06:30 UTC: 50 °C
Workers conducted an operation to spray 40 tonnes of seawater to the spent fuel pool on 20 March.
Unit 3 experienced an explosion on 14 March that destroyed the outer shell of the building’s upper floors. The blast may have damaged the primary containment vessel and the spent fuel pool. Concerned by possible loss of water in the pool, authorities began spraying water into the building in an effort to replenish water levels. First, helicopters dropped seawater on 17 March, and every day since then, including 21 March, emergency workers have sprayed water from fire trucks and other vehicles, so far spraying at least 3,742 tonnes.
This reactor was shut down 30 November 2010 for routine maintenance, and all the fuel assemblies were transferred from the reactor to the spent fuel pool, before the 11 March earthquake. The heat load in this pool is therefore larger than the others.
On 14 March, the building’s upper floors were severely damaged, possibly causing a reduction of cooling capability in the spent fuel pool. Emergency workers began spraying water into the building on 20 March, and have continued daily since then, so far spraying at least 255 tonnes.
Units 5 and 6
Instrumentation at these reactors began to indicate rising temperatures at their spent fuel pools starting on 14 March. Three days later, Japanese technicians successfully started an emergency diesel generator at Unit 6, which they used to provide power to basic cooling and fresh-water replenishment systems. Workers created holes in the rooftops of both buildings to prevent any hydrogen accumulation, which is suspected of causing earlier explosions at Units 1 and 3.
A second generator came online on 18 March, and the next day, the higher-capability Residual Heat Removal system recovered full function. Temperatures in the spent fuel pools of Units 5 and 6 have gradually returned to significantly lower temperatures .
Common Use Spent Fuel Pool
In addition to pools in each of the plant’s reactor buildings, there is another facility – the Common Use Spent Fuel Pool – where spent fuel is stored after cooling at least 18 months in the reactor buildings. This fuel is much cooler than the assemblies stored in the reactor buildings. Japanese authorities confirmed as of 18 March that fuel assemblies there were fully covered by water, and the temperature was 57 °C as of 20 March, 00:00 UTC. Workers sprayed water over the pool on 21 March for nearly five hours, and the Nuclear and Industrial Safety Agency reported that the pool temperature had risen to 61 °C as of 21 March, 07:30 UTC.