GEN IV: Evolution in Nuclear Safety

July 15, 2012

Why did the Fukushima reactor explode?

A day was made infamous

by the disaster at the Fukushima Daiichi Nuclear Power Station. Following the disaster, countries such as Germany and Belgium launched initiatives to phase-out nuclear from their energy portfolio. Though such decisions were not singular responses to Fukushima, it cannot be denied that the disaster in Japan brought to the fore fears about the dangers associated with nuclear power in the minds of both the public and policy makers.

Accidents such as Fukushima are dramatic and appear quite deadly. However, in these cases, the actual health implications have been surprisingly moderate. For example, no deaths have yet been attributed to the Fukushima disaster and members of the Health Physics Society suggest that radiation doses were too small to have much effect.

Still, the meltdown was undoubtedly catastrophic. Though perhaps not subject to acute radiation sickness, tens of thousands of families were displaced from their homes as a result of the accident and the psychological and economic impacts have been extremely severe.

As the world attempts to move towards a sustainable future, nuclear may well have a critical role to play, but the decision to invest in this technology must be weighed against the risks, chief amongst them the possibility of another Fukushima. Following the accident, interest surrounding nuclear has fizzled—the increased risk factor causing investment to appear much less attractive.

Floating house after earthquake

Fukushima did not result from unsafe reactor design and operator error, but rather unprecedentedly severe natural disasters. Most are familiar with the story: On the morning of March 11th of last year a magnitude 9 earthquake hit Japan’s Northeast coast. As designed, the operating nuclear reactors automatically deployed control rods into the reactor core, suspending the fission reaction though the core continued to emit decay heat. Though the main power supply was lost, the back-up generators kicked in to power the pumps to circulate coolant, which continued to effectively cool the core.

The reactors were designed to withstand tsunamis—up to 6 meters in height. The tsunami following the earthquake on March 11th was an incredible 15 meters. It destroyed the diesel generators leading to failure of coolant systems in reactors 1-3.  With the loss of the backup generators the coolant failed to circulate and the reactor core temperature began to rise. This eventually led to the series of explosions and the reactor meltdown.  With the failure of all safety system, the response team was forced to take a variety of steps that whilst ultimately effective, severely damaged the economic viability of the reactors.

Within 10 minutes the reactor temperature had stabilized to normal operating temperature and had shut itself down safely with no human intervention.

Today, the world’s nuclear power stations are a mix of largely generation II and some generation III reactors, mostly built around 40 years ago. The designs are largely old and outdated and the safety features do not reflect current technologies. Indeed, If Fukushima had been equipped with modern safety measures, the reactor meltdown would not have occurred.

Strangely enough, these "modern" safety features are not new. Research and development of the so-called “Generation 4” or “Gen IV” reactor design has been pursued since the late 1980s. In 2002 Argonne National Laboratory published a report detailing the success of a prototype of such a naturally safe reactor as early as 1986. The reactor in question was subject to a loss of coolant and a loss of heat removal from the primary system: the very problems experienced at Fukushima. The tests were conducted with the automatic safety systems disabled. This means that, just as at Fukushima, the reactor’s pumps were not circulating coolant and the diesel engines were not engaged. However, unlike Fukushima, though the reactor experienced an initial rise in temperature, the passive safety systems began to work spontaneously. Within 10 minutes the reactor temperature had stabilized to normal operating temperature and had shut itself down safely with no human intervention.

Argonne National Laboratory during waste removal process

Argonne National Laboratory calls the design “passively safe” because it relies not on human design, equipment, and operation, but rather on the laws of nature.

A notable passively safe design is the substitution of water coolant with sodium. Sodium has a boiling point of around 300-400, as opposed to water with a boiling point of 100. The Fukushima plants were of a Boiling Water Reactor (BWR) design. Thus, as the temperature continued to rise, the liquid coolant converted to steam but was unable to condense back into water resulting in the formation of high-pressure steam bubbles. The lack of coolant meant the reactor continued to give off decay heat. The steam reacted with the decay products leading the formation of hydrogen bubbles, causing explosions.  Sodium coolant by contrast continues to absorb heat to extreme temperatures with no worry of boiling away.

Because sodium will not boil away, it does not have to be circulated and condensed as does water and can therefore be applied as a pool rather than a loop. Because the pool design does not require circulation to cool the core it does not rely on external power. Thus, if Fukushima had been equipped with such a design the reactors would have cooled down naturally even after the loss of the backup generators. Other liquid metals such as fluoride are also viable options for coolant and offer similar benefits.

Additional passively safe designs such as the Westinghouse AP1000 still rely on water as a coolant, but have restructured the system such that the coolant sits above the core. In the case of a loss of power a heat sensitive valve automatically opens and the water flows down into the reactor and removes the decay heat—simply in accordance with the laws of nature.

Model of boiling water reactor

Gen IV designs also incorporate a passively safe fuel: uranium metal alloy. Such fuel naturally prevents a runaway chain reaction in the case of increased core temperature. The metal alloy component of the fuel expands naturally if the temperature of the core rises. This expansion slows the chain reaction by increasing the distance from one fissile nucleus to another, decreasing the chance of neutrons penetrating fissile nuclei. The control rods in current generation II and III reactors are an active safety measure providing the same service, but the fuel makes the reactor redundantly safe in case the fuel rods fail.

The redundant safety measures incorporated into Gen IV designs moves reactor safety beyond the Generation II and III principle of “mastering accidents”(accepting the possibility of accidents but taking care to ensure minimal human damage), seeking to “exclude accidents”. That is, to protect not only the human population, but also to protect the economic value of the plant and to reduce the “catastrophe factor”. Again to use the case of Fukushima, if the reactors had employed the passively safe design, the reactors in all likelihood would have shut down safely and been ready to go back on line as soon as power was restored. Moreover, the human displacement and suffering would likely have been avoided.

The Gen IV reactor design has safety measures to guard against the “maximum foreseeable risk”. And though there is always a chance that something unprecedented will occur, it is hard to imagine a scenario in which such a design would fail. Perhaps such reactor design will make investment in nuclear more attractive, and yet, perhaps not, for such redundant safety features will of course markedly increase the initial investment in the construction of a plant—already the most significant cost associated with nuclear power.


Related Media:


Further Reading:

  •  The Gen IV International Forum on Gen IV concepts. 



Baurac, Dave. "Passively Safe Reactors Rely on Nature to Keep Them Cool." Passively Safe Reactors Rely on Nature to Keep Them Cool. Argonne National Laboratory, 2002. Web. 05 Aug. 2012.

Biello, David. "Designs for Newest U.S. Nuclear Plants Aim to Balance Safety and Costs: Scientific American." Scientific American, 23 Mar. 2011. Web. 05 Aug. 2012. 

Harmon, Katherine. "Japan's Post-Fukushima Earthquake Health Woes Go Beyond Radiation Effects: Scientific American." Scientific American, 2 Mar. 2012. Web. 05 Aug. 2012.

"Japan Earthquake and Nuclear Crisis." Nature Publishing Group, n.d. Web. 05 Aug. 2012.

"Japan's Energy Crisis: A Matter of Trust." The Economist. The Economist Newspaper, 23 June 2011. Web. 05 Aug. 2012.

"Passive Safety" Reactor Designs Would Improve Response to Nuclear Events." Oregan State University, 21 Mar. 2011. Web. 5 Aug. 2012.

"Situation of the Evacuees." Fukushima on the Globe. Japan NGO Center for International Cooperation, n.d. Web. 01 July 2014. 

Wald, Matthew L. "Sizing Up Health Impacts a Year After Fukushima." Green Blog. The New York Times, 1 Mar. 2012. Web. 05 Aug. 2012.