Why Nuclear Scientists Have Missed the Danger of Spent Fuel Pools


Dear friends: 

Since the Fukushima nuclear power plants accident in March 2011, there have been two types of contributions by nuclear scientists. One type, who represents the voice of TEPCO, has influenced the decision makers of the solutions to the accident, the government strategy of the evacuation zone, the timing of announcement of the core meltdown and the constant public campaign to give the impression that the situation is improving rapidly. The other type, who has been warning the government of the worsening situation, finds their influence over the decision makers and the media circle limited. However, I find it puzzling that there has been so little warning from the nuclear community about the potential for catastrophic accidents or terrorist attack involving the hundreds of spent fuel pools worldwide.  I received a clear explanation from Dr. Gordon Edwards, one of Canada’s best known independent experts on nuclear technology, uranium, and weapons proliferation. 

I hope you will better appreciate the serious issues of the spent fuel pools from these comments.

Akio

 

This article is now available in Japanese.

Dear Akio,

You asked me why there has been so little warning from the “nuclear establishment” (TEPCO and the regulatory agency) about the potential for catastrophic accidents involving the spent fuel pool in reactor number 4.

In the field of nuclear safety, the focus of attention has always been on analyzing and preventing catastrophic accidents involving the core of the reactor.  In comparison, little attention has been paid over the years to catastrophic accident scenarios involving the spent fuel pool.

Since the very first US Reactor Safety Study, the “Brookhaven Report” in 1957, to the major 12-volume US NRC Reactor Safety Study (the Rasmussen Report) of 1974, and continuing right down to the present day, virtually all of the attention has been directed to extreme conditions that might develop in the core of the reactor — unterminated power excursions, loss of coolant accidents, breach of the reactor vessel, core meltdowns, and so on.

Most nuclear engineers and nuclear regulators have developed a “blind spot” about the catastrophe potential associated with the spent fuel bay because of years of neglect. Such considerations have never played a significant role in their training as nuclear engineers or in their many subsequent years of experience in the field of nuclear safety analysis.

As a result we have backup pumps, backup electrical supply systems, and backup cooling systems for the core of the reactor, but no backup pumps or electrical supply or cooling system for the spent fuel bay. We have extravagant containment systems for the core of the reactor, but no comparable containment systems for the spent fuel pool.

This absence of backup systems for the spent fuel pool is testimony to the lack of effort and lack of forethought that has been devoted to the spent fuel bay. Nevertheless, the radioactive inventory in the spent fuel pool is often much greater  than that in the core of the reactor, and a prolonged loss of coolant — or even loss of circulation of coolant — will lead to overheating of the fuel and extensive fuel damage.  This will result in significant releases of radioactive fission products into the atmosphere due to the inadequate or even non-existent containment provided for the spent fuel pool.

Moreover, a loss of coolant in the spent fuel pool — whether by leakage, spillage, or  boiling off of the cooling water — will lead to intense gamma radiation that would prevent human access for hundreds of metres in all directions around the spent fuel pool, making it very difficult to take corrective actions.

Under adverse circumstances there can even be a fuel meltdown in the spent fuel pool, if the temperature climbs to about 2800 degrees C, which would vastly increase the radioactive releases and spread those releases over a much wider area.

The overheating of the spent fuel in the pool can be exacerbated by the intense exothermic reaction between the zirconium cladding and the steam produced from the overheated water, and can even result (at around 1000 degrees C) in a very intense zirconium fire which can result in tiny particles of intensely radioactive spent fuel being liberated into the atmosphere.

Depending on the diameter of these “hot particles” (sometimes referred to as “nuclear fleas”) they can be transported greater or lesser distances by the wind, possibly affecting populations hundreds of kilometers from the spent fuel pool.  Once dispersed into the environment, these hot particles will constitute a source of radiation exposure and environmental contamination for centuries to come.

In addition to the possibility of zirconium fires (which have for a long time been almost completely overlooked by nuclear engineers and regulators) there is another, even more dangerous possibility.  An alteration in the geometry of the spent fuel in the pool, by which the separation between the spent fuel rods is slightly but significantly reduced, can lead to re-initiation of the chain reaction in the pool.

This “accidental criticality” will not only drive the temperature up rapidly, but will also replenish the supply of short-lived heat-producing fission products, accelerating the damage to the fuel, magnifying the heat loading, increasing the probability of a fuel pool meltdown, and vastly increasing the atmospheric releases of radioactivity.

It has been a standard practice in the nuclear industry to avoid consideration of all of these possibilities, based on the assumption that there will be “lots of time” to react to any emergency involving the spent fuel pool, as it will normally take days for the spent fuel to reach the melting point and it will be a “simple matter” to refill the pools with water if necessary.

This ignores the fact that major structural damage may make it impossible to approach the spent fuel pool due to the lethal levels of gamma radiation emanating from the spent fuel once the protective shielding of the water is gone.

 

Gordon Edwards is the president of the Canadian Coalition for Nuclear Responsibility and is the recipient of the 2006 Nuclear-Free Future Award . For a full biography, click here.