How to store spent nuclear fuel
In “Improving Spent-Fuel Storage at Nuclear Reactors” (Issues, Winter 2012), Robert Alvarez describes a lesson relearned at the Fukushima Daiichi plant in Japan. The reactors at Fukushima and roughly one-third of our reactors have spent-fuel pools located inside the same building surrounding the containment that houses all the emergency pumps providing reactor core cooling and makeup.
This cohabitation allows reactor accidents to cascade into spent-fuel pool accidents and for spent-fuel pool accidents to in turn trigger reactor accidents. For example, the radiation levels inside this building during a reactor accident can prevent workers from entering to restore cooling for or makeup to the spent-fuel pool. And in the converse, the water escaping from a boiling spent-fuel pool can condense and drain down to the basements, disabling all the emergency pumps by submergence—if the elevated temperature and humidity conditions have not already done so.
Alvarez does more than merely describe a safety problem. He defines its ready solution. Five years after discharge from reactor cores, spent fuel can and should be transferred to dry storage. The accelerated transfer will result in more spent fuel being in dry storage, which translates into an increased dry storage risk. But that risk increase is more than offset by the risk reduction achieved in the spent-fuel pool. As Alvarez states, the typical spent-fuel pool for this type of reactor contains 400 to 500 metric tons of irradiated fuel. A single dry cask contains only 15 to 20 metric tons. Thus, unless something causes many dry casks to nearly simultaneously fail, the radioactivity emitted from a spent-fuel pool accident is significantly greater than from a dry cask accident.
The relatively higher hazard from irradiated fuel in spent-fuel pools as compared to dry casks has been known for many years. After our 9/11 tragedy, the Nuclear Regulatory Commission (NRC) issued a series of orders requiring plant owners to upgrade security measures. The first orders went out for greater protection of the reactors, followed by orders seeking protection of spent-fuel pools, and trailed months later by orders for better security of dry storage facilities. The NRC knows the relative hazards.
Just as the Fukushima Daiichi tragedy rediscovered this spent-fuel storage problem, it also revisited its solution. There were nearly 400 irradiated fuel assemblies in dry storage at Fukushima when it encountered the one-two punch from the earthquake and tsunami. The tsunami’s water partially flooded the dry storage facility, temporarily replacing the normal cooling by air convection with water cooling. When the floodwaters receded, the normal cooling process restarted automatically. There was no need for helicopters to drop water or truck-mounted water cannons to spray water to prevent damage to irradiated fuel in dry storage.
If irradiated fuel in a spent-fuel pool causes an accident or increases one’s severity, shame on us. We know the problem and its solution. We have no excuse for failing to implement the known fix.