Lessons from the Japanese nuclear disaster

Author: Peter Burns

Early on Friday, March 11, images of the destruction caused by the Japanese earthquake and subsequent tsunami began to appear. Several hours later, sporadic reports of fires and possible radiation leaks at Japanese nuclear power plants surfaced. The most serious nuclear accident in Japan occurred at the Fukushima Dai-Ichi nuclear power station, which is located next to the ocean.

This earthquake was amongst the most powerful in recorded history, registering 9.0 in Richter magnitude. The nuclear power station at Fukushima Dai-Ichi was engineered to withstand an 8.2 magnitude quake. The energy unleashed in a 9.0 quake is several times higher than an 8.2, as the Richter scale is logarithmic. There are six reactors at the power station, but only three were running at power when the earthquake struck. Automatic shutdown procedures were successfully executed, via insertion of control rods into the reactors to stop the nuclear fission reactions.

The Fukushima Dai-Ichi reactors use uranium dioxide fuel enriched to 3-4 percent uranium-235, the fissile isotope of uranium. Under normal operating conditions, the reactor maintains a controlled nuclear fission reaction, whereby uranium-235 atoms are split to give fission products (lighter atoms), energy, radiation and neutrons to sustain the chain reaction. As nuclear fuel “burns” in a reactor, fission products accumulate in the fuel. These are radioactive elements (radioisotopes) that include iodine and cesium, as well as many others. Some of the uranium is converted, by capturing one or more neutrons, to heavier radioactive elements including plutonium. Under normal reactor conditions, the considerable energy released by the fission process is carried away as heat by water to generate electricity using turbines.

The three operating reactors at Fukushima Dai-Ichi appear to have been successfully shut down, meaning that the fission reactions stopped. However, the fuel in the reactors remains highly radioactive. The radioactive decay of short-lived radioisotopes continues to add heat to the reactor core, even when the nuclear fission reactions have stopped. After shutdown of a reactor, circulation of cooling water must continue to remove this heat. Otherwise, the temperature of the fuel can reach the melting point of uranium dioxide (about 2,900 degrees Celsius), resulting in a core meltdown. Melting fuel rods release radioactive materials that may contaminate the environment.

About an hour after the earthquake a tsunami estimated at 6 to 14 m in height struck the Fukushima Dai-Ichi plant. It had been designed to withstand a tsunami of only 5.7 m. The influx of water caused the backup electrical generation systems at the plant to fail, and the cooling water stopped flowing. Diesel generators provided the backup power at the plant, and photographs of the site suggest that the fuel tanks that fed these generators were swept away by the tsunami. Although it will be several months before the details of the damage to the three reactor cores that had been operational before the earthquake is known, all probably suffered at least a partial meltdown. Explosions caused by accumulated hydrogen gas worsened the situation and made restoring reactor coolant more difficult. Each reactor core contains about 100 metric tons of uranium dioxide fuel.

Stabilizing the reactors

Under normal procedures, used nuclear fuel is removed from a reactor and placed in a cooling pond that is filled with water. This is to remove heat generated by the radioactive decay of short-lived radioisotopes. Approximately 1,600 metric tons of used fuel was contained in ponds at Fukushima Dai-Ichi when the tsunami struck. The cooling systems for at least some of these ponds failed, leading to overheating of fuel rods.

Heroic efforts of plant workers may have prevented larger releases of radioactivity. Seawater, and later fresh water, was pumped into the reactor cores to cool them, as well as into the ponds containing spent fuel. Much of the water is now contaminated by radioisotopes. At least some of it has been captured on site, but there is evidence that some radioisotopes are leaking into the ocean.

Each type of radioisotope decays, with emission of various sub-atomic particles or electromagnetic radiation, at a fixed rate known as the half-life. After one half-life has passed, half of the atoms of the radioisotope have decayed, giving either radioactive or non-radioactive elements. Those radioisotopes with short half-lives produce a lot of radiation in a short time (e.g., iodine-131 with a half life of a little more than eight days), but they also decay away relatively quickly. Those with longer half-lives remain radioactive much longer, and can have long-term impacts on the environment (e.g., plutonium-239 with a half-life exceeding 20,000 years).

As I write this commentary, the efforts continue to stabilize the Fukushima Dai-Ichi reactors and fuel storage ponds. As the significance of short-lived radioactivity (such as iodine-131) decreases due to decay, and the short-term risks of release of radioactive materials declines, the focus will shift to the task of reducing the long-term environmental impact of the Fukushima Dai-Ichi site. The uranium dioxide fuel is intensely radioactive due to the presence of fission products, 10-20 tons of plutonium (which was present initially in some fuels, and has been generated in all of them while the fuel was in a reactor), and other actinide elements. The presence of the plutonium and lesser actinides, as well as long-lived fission products such as technetium-99 and iodine-129, make these materials dangerous for thousands of years.

Previously, major commercial plant accidents occurred at Three Mile Island (TMI) and Chernobyl. At TMI, there was a partial core meltdown but little release of radioactivity to the environment. The reactor core was eventually removed from the site and transported to the Idaho National Laboratory. In Chernobyl, fires, explosions and a core meltdown resulted in release of substantial radioactive material. The damaged cores remain at the reactor site today, with the plant enclosed by a concrete sarcophagus. In my view, the fuel rods in the Fukushima Dai-Ichi plant and cooling ponds must be removed and properly recycled or placed in a robust geological repository. The long-term environmental impact of leaving the radioactive material in a near-surface location is simply too high.

Nuclear waste in the U.S.

Today in the United States there are 104 operating commercial nuclear reactors that generate about 20 percent of the nation’s electricity. About 70,000 metric tons of used nuclear fuel has accumulated at the reactor sites, where it is initially maintained in water-filled cooling ponds, and subsequently stored in dry casks. Federal policy had been to create a geologic repository for this material, and also radioactive waste created during the nuclear weapons build-up of the Cold War, in Yucca Mountain, Nevada. The Department of Energy abandoned Yucca Mountain about two years ago. A Blue Ribbon Commission, appointed by the president, is developing recommendations for handling nuclear waste in the United States.

Recent events in Japan highlight the dangers of storing used nuclear fuel at reactor sites for lengthy timeframes, and may increase the sense of urgency for a final solution in the United States. Congress decided more than two decades ago that high-level radioactive waste from commercial plants and the weapons program would be co-disposed in a single geologic repository. Most of the weapons-related waste would be immobilized in borosilicate glass, and commercial spent fuel would be emplaced in its current form. Development of two or more repositories in different geologic environments may facilitate matching of the materials with the geochemical environment, which would be a more scientifically defensible approach.

The Department of Energy-funded Energy Frontier Research Center, Materials Science of Actinides, led by the University of Notre Dame, was created in 2009 to create a foundation of knowledge concerning actinide materials to underpin an advanced nuclear energy system in the United States. The Center focuses on complex actinide materials (the fuels of nuclear energy and major components of used reactor fuel), the nano-scale control of actinide materials, and the behavior of actinide materials under extreme conditions of pressure, temperature, and radiation field. Our research seeks to create better materials for the long-term disposition of nuclear wastes, and better methods for recycling nuclear materials.

Professor of Civil Engineering and Geological Sciences Peter Burns is director of Notre Dame’s Energy Frontier Research Center, which seeks to understand and control the elements that are the basis of nuclear energy, uranium, plutonium and other actinides.