Economic development and quality of life are intrinsically tied to widely available energy supplies. An energy crisis is engulfing the world as oil prices soar, fossil fuels are depleted, and greenhouse gases build in the atmosphere. With 440 nuclear reactors existing today for electricity generation, and more than 60 under construction in energy-hungry countries like China with developing economies, we can look forward to a future with many more nuclear reactors in the world. Despite high standards of safety, accidents like Fukushima are inevitable over the long-term. Reactors cannot be designed to withstand the entire gauntlet of very unlikely events that might happen. The first reactor meltdown occurred in the 1950s, and the number now has passed 20 worldwide. Instead of assuming that we can prevent another meltdown, scientists need to lower the risk to humanity by understanding what happens when a reactor meltdown happens and how radioactive material is released.
Burning fuel in a nuclear reactor is unlike any other type of fuel we burn for energy. When fossil fuels are burned, organic chemicals react with oxygen in the air, and heat is liberated. Burning a nuclear fuel is totally different -- atoms of uranium are actually broken apart, or fissioned, producing lighter atoms and a huge amount of energy.
Fuel that comes out of a reactor is nothing like what went in. Around four percent of the uranium atoms have split, and each one that splits creates new atoms of lighter elements. Many of these newborn atoms are radioactive. Soon they will break apart, releasing particles we call radioactivity, as well as a lot of heat energy.
A nuclear reactor generates heat energy in two ways. The first is when uranium atoms are split. Pushing control rods made of carbon into the reactor can stop this process. The second is when radioactive atoms created in the reactor decay or breakdown, and this process is impossible to stop. Used fuel is one million times more radioactive than when it was fresh. The radioactive decay alone produces two megawatts of energy for each ton of fuel. Nuclear reactors have built-in cooling systems to keep them from overheating and melting down.
In Fukushima, three nuclear reactors were operating when the earthquake struck on March 11, 2011. Reactor safety systems worked as intended and control rods stopped the splitting of uranium atoms. Forty minutes later a nearly 50 foot tsunami came ashore. The earthquake wrecked the electricity grid, and the tsunami flooded backup diesel generators and washed away their fuel tanks. Reserve batteries went dead within a few hours.
Nuclear reactors are used to generate electricity, but they also need to consume electricity to avoid meltdown due to overheating. Reactors are cooled by pumping cold water through a circuit of pipes. At Fukushima the pumps that circulate water stopped because there was no electricity to run them. Radioactive atoms continued to self-destruct, quickly releasing heat that was too much for the reactors to withstand.
Nuclear reactors are built with more than one layer of containment to hold in the radioactive elements they create. When reactors melt and hydrogen gas explodes, the layers can be destroyed. This happened at Fukushima in 2011 and at Chernobyl in 1986, but importantly it did not happen at Three Mile Island in 1979.
Units 1, 2 and 3 at Fukushima overheated and uranium fuel and its zirconium containers melted. Steam reacted with the metal and released hydrogen. Because hydrogen gas is so light, it rose through the surrounding air and pooled at the top of the reactor containment buildings. Footage of the Hindenburg airship disaster tells us what happened next. Hydrogen explosions in Fukushima units 1 through 4 severely damaged the containment systems. Although unit 4 was not running at the time of the earthquake, hydrogen gas appears to have accumulated there after it travelled through pipes from unit 3.
When containment is lost, radioactive material is released from the reactor. This material itself is not radioactivity. Radioactivity is the particles and energy released by a radioactive material when its atoms break apart. To be impacted by radioactivity, a person needs to be close to the source because the dose rate decays rapidly with distance. Radioactive material contained within a reactor presents no health risks. It is radioactive material that is released into the air and water that contaminates homes and food, and the radioactivity associated with this material that causes increased risk of cancer in the population.
Radioactive material released during a reactor accident mostly comes from the fuel itself. When fuel melts, gases are released to the air. If there is no containment, the gases drift out of the reactor. But much of the radioactive material in the fuel is not released as a gas, even at meltdown temperatures. This solid material generally stays put, unless there is a transport vehicle.
At Fukushima, as meltdowns were happening, operators wisely chose to pump huge amounts of seawater into the reactors to cool them. But, this water became a transport vehicle that carried radioactive materials out of the reactor into the ocean. But how much radioactivity was released from the reactors?
In an article published last Friday in Science, my co-authors and I examine what is known about nuclear fuel meltdowns and the release of radioactive material during and after an accident. While it may come as a surprise, there is a lot of uncertainty about what happens when damaged nuclear fuel interacts with water and air. We argue that difficult and expensive research is urgently needed before another reactor meltdown happens. Better armed with an understanding of the complex chemical processes that happen during and after a meltdown, future emergency responders will be better able to protect society.