Collapse of spent fuel storage pool at Fukushima Daiichi could be worse than initial accident, says new report

Publish date: July 3, 2012

Written by: Charles Digges

A collapse of the already tilting reactor No 4 building at the stricken Fukushima Daiichi plant, ¬atop which sits a spent nuclear fuel storage pool containing 1,535 fuel assemblies – including 204 unused ones – would lead to a “significant global impact,”– by far topping last year’s triple meltdown at the plant, a new report says.

According to the report (available for download in PDF at lower right in blue box) released by Holophi CH, a Swiss-based industrial analytics think-tank, even a 10 percent release of the storage pool’s inventory of radioactive cesium and strontium would “represent 3 to 10 times the March 11, 2011 release amounts, substantially increasing risk levels in Japan and marine life.”

“This is an acute example that we will have to live with the threats emanating from Fukushima for years to come,” said Nils Bøhmer, Bellona’s nuclear physicist and general manager.

The spent fuel pool was singled out by Bellona early in the Fukushima crisis as a possible source of catastrophic radiation releases due to water loss, which took a back seat to the chaos of trying to restore cooling water to reactor Nos 1, 2 and 3 with fire trucks, water cannons and seawater dropped from helicopters.

A tsunami following a 9.0 magnitude earthquake devastated primary and back up cooling to Fukushima Daiichi, causing three reactor meltdowns within three days of the March 11, 2011 disaster. All of Fukushima Daiichi’s reactors are currently in cold shutdown.

But water loss or collapse of the structure housing the spent fuel storage pool continues to pose a cataclysmic threat.

If cooling water for the pool is lost, said the report, “a major release of radioactive material could result,” adding that, “Given the large amounts of heat generated by the fuel rods, the temperature would rise quickly. These rods are surrounded by zirconium cladding and at high temperatures, this cladding catalyzes hydrogen production, can generate additional heat and even explode and burn.”

The report states there are many ways such a catastrophe could occur, from loss of water in the pool through cracks or evaporation due to the heat of the spent fuel elements, leading to a radiological fire, fuel melts, to a complete collapse of the pool, which would shed spent fuel assemblies and irradiated water in total disarray. Each fuel assembly contains approximately 50 to 70 rods of uranium fuel.

Removal of the waste in the spent fuel storage pool is hindered by debris, high radioactivity and the fact that the equipment for removing the assemblies was destroyed in the disaster.

Media reports state that removal of the spent fuel is scheduled for removal 2013. But the Holophi CH report indicates that should continue safely only in the absence of no further damage to the pool or its contents.

But Bøhmer pointed out that the spent nuclear fuel problem was not constricted to the spent nuclear fuel pool.

“All of the reactors, including those that were devastated by the tsunami, haves spent fuel in them that must be put into safe storage and that will take decades,” he said.

TEPCOs troubled efforts to cool the pool

The waste assemblies themselves produce 1 megawatt of thermal waste energy, even immersed in water.  This is down from 2 megawatts of waste energy last year.

But cooling apparatus for the spent fuel pool continues to glitch.

It was only Sunday that improvised cooling systems were brought back online after they failed for unknown reasons on Saturday, the Japan Times reported. 

The fuel assemblies must be kept at 33.3 degrees Celsius, and the weekend outage brought them to 42 degrees. A similar outage occurred on June 4, according to the paper.

The original cooling system for the pool was destroyed by a hydrogen explosion that seriously damaged reactor No 4’s building and integrity, and plant owner Tokyo Electric Co (TEPCO) has been cooling it with hoses.

Fuel pools open to the environment atop tilting building

Unlike reactor cores, spent fuel ponds are not protected from the atmosphere, meaning there is no second line of defense in the t of a leak, collapse or evaporation of the water holding the fuel elements.

The pool is perched 30 meters above the ground, and the No 4 building is reported to be leaning.

TEPCO confirmed this in a June report to regulators, saying that at least two of the walls of the No. 4 reactor building are bulging outward at various points and that the building is tilting.

TEPCO has installed steel pillars to help support the pool.

“If the pool collapses or develops serious cracks allowing the cooling water to drain, the fuel rods will be exposed to the environment,” warned the Holphi report.

Many experts say that the building would not withstand another quake – and further quakes of at least a 7.0 magnitudes are predicted in the Holophi CH report.

“Because of the great damage inflicted by the tsunami and the earthquake, there is doubt about the integrity of the other structures at Fukushima Daiichi as well,” said Bellona’s Bøhmer. “The waste inside these structures and the damaged fuel within the could trigger even larger catastrophes as time passes.”

Scenarios for disaster

The Holophi CH report analyzed contamination scenarios that may result from further damage to the pool and laid out radiation emissions that compare radioactivity released in March 2011 to what could be released from the spent fuel storage pool.

If collapse or breakage leads to only 10 percent of the pools radioactive isotopes being releases, said the report, an even distribution on land of cesium would equal some 100,000 Becquerel per square meter, compared to the 12,000-14,000 Becquerel per square meter that were measured in the Fukushima prefecture in March 2011 and the 1,500-5,000 Becquerel per square meter found in neighboring prefecures.

Strontium contamination on land is likewise projects to be far high: some 30,000 Becquerel per square meter evenly distributed on land compared the measured 3,600 to 12,000 measured in Fukushima prefecture in March 2011, and 450 to 1,500 in neighboring prefectures.

Sea contamination at only 10 percent contamination would also increase. According to the modeling structures used by Holophi, some 640,000 Becquerel per square meters would be deposited on the surface, with a volume concentration to 100 meters of 6,400 Becquerel per cubic meter.

This compared to March 2011 cesium measurements near shore of 210 to 8,000 Becquerel per cubic meter, and 6 to 1,400 Becquerel per cubic meter off shore.

Worst case scenario

According to the report, radiological fires would lead to some 30 to 100 percent of the cesium and strontium being released from the pool.

“If a radiological fire occurs and further cooling is impossible, more than 10 percent of the radioactive material of the [spent fuel pond] cold be released,” read the report. “Release of 100 percent of the cesium, strontium and other isotopes would scale the base case results by a factor of 10 and would clearly present a dire situation for areas of Japan, marine life in the Pacific and the global environment.”

The report acknowledges that it is not fully comprehensive, saying that many modeling structures about what could happen should the spent fuel pool fail “have been developed by government, but appear to be classified.”


Therefore, it says that, “the sequence of events that might follow a failure of [the spent fuel pond] is difficult to determine.”

It encouraged governments and other organizations with advanced modeling capabilities to draw their own conclusions and publicize them.

It also recommended an acceleration of the fuel removal process if there is still a significant possibility of a radiological fire.

Industries in Japan that may be affected by a failure of the pool and following radiation should draw up contingency plans for further events like earthquakes and other eventualities that might impact the pool.

The Holophi CH report also said that, “Members of the public, particularly in Japan, and the US, should pressure authorities to perform accurate and honest assessments of risks, to prepare plans to handle further earthquake or other damage to the [spent fuel pool] and the rest of the plant, and to accelerate mitigation activities.”