Nuclear reactor cores brought to a non-coolable state by the earthquake

The massive magnitude 9 earthquake-virtually unparalleled in the world-left various scars on the coastal areas in the Tohoku region, and further triggered the catastrophic meltdown of reactors No. 1 to No. 3 at the Fukushima Daiichi Nuclear Power Plant, and there have been no prospects yet for resolving the accident to this day, nearly two months since the outbreak. Let us begin with the details of the accident in order to explain what is happening at that nuclear power plant.

The so-called light-water reactor plants (most nuclear plants in the world fall under this category) may be likened to a huge water boiler. A core filled with a bunch of fuel rods is located in the center of a pressure vessel (nuclear reactor), where heat is generated by nuclear (fission) reaction, and the heat brings the water inside the pressure vessel to a boil and turns it to steam to rotate the turbines and so produce electricity. There is one major difference with a water boiler, however. That is, when the gas or switch is turned off for an ordinary water boiler, the generation of heat stops immediately, but in a nuclear reactor, even if the nuclear reaction stops, the extremely strong radiation generated in the nuclear fuel becomes heat, and the heat generation does not stop immediately, but rather, it lasts for an extended period. This is called decay heat, and it is the very cause of this nuclear accident.

When the severe seismic motion hit the nuclear power plant on March 11, the control rods were immediately inserted into the cores automatically, and the nuclear reaction stopped. Transmission line towers collapsed due to the quake, and the external power supply was stopped. Because the emergency diesel generators which had been prepared in case of such events started operating and rotated the pumps to circulate water inside the pressure vessels, however, decay heat generated in the cores was removed, and the cores were successfully cooled.
If this situation had continued, the nuclear power plant would have stopped uneventfully as usual. However, a massive tsunami over 14 meters high struck an hour after the earthquake, destroyed the outdoor fuel tanks of the emergency diesel generators, and flooded the diesel generators located in the basement of the turbine buildings, precluding power generation. Therefore, the entire power source was lost. This caused the water circulation pumps to stop, and resulted in a failure to cool the cores, so that the cores became exposed due to a rise in temperature in the reactors and evaporation of water (an accident under such boil-dry conditions as this is called a loss of coolant accident). As a result, with the clad temperature of the fuel rods continuing to rise, the cladding tubes became oxidized, shattered, and melted (the melting point of zirconium alloy, the material of cladding pipes, is 1800属C). I am not sure until I can approach the building in about five years after the accident and then check on the inside after it is dismantled, but I believe the cores melted like starch syrup, including uranium oxide pellets (melting point: approximately 2800属C), and a part of them has accumulated at the bottom of the pressure vessels.

Fuel rods damaged, radiation leaked into the environment

When the reactor temperature exceeds 1000属C, the reaction of the zirconium alloy in cladding tubes with water generates large amounts of hydrogen. This hydrogen leaked and collected near the ceiling of the reactor buildings, causing immense explosions. As seen on television, the buildings ended up being a horrible scene. Outside the pressure vessels are flask-shaped containment vessels-as seen in the newspapers as well-designed to contain radiation, but these vessels were damaged in the course of the accident, so hydrogen and radioactive materials started to leak into the reactor buildings and then into the environment.

Present status of the accident

Leaving aside the detailed circumstances surrounding the accident, and instead describing the present status about two months after the accident occurred, even now the three threats-namely, heat, radiation, and hydrogen-have yet to be contained. A single event of improper handling may lead to a catastrophic situation. First of all, there is decay heat. Failure to cool the melted cores causes the temperature to rise instantly, and the melted cores may break through the bottom of the pressure vessels and protrude. In order to prevent such a situation, approximately 500 tons of water have been poured in for cooling every day, but this highly-concentrated contaminated water keeps flowing into so-called trenches-tunnels outside the buildings-and as much as seven tons of such water have already collected. This water is extremely highly radioactive, and some of it, approximately 520 tons, has been dumped into the ocean, and this amount alone exceeds the largest-ever amount of radiation to date, in Windscale, UK, in terms of marine contamination. Large volumes of iodine 131 and cesium 137 have been released into the atmosphere, causing damage such as the evacuation of residents and regulations on shipping vegetables, and further explosions may increase these radioactive materials.

Finally, as for hydrogen, with the pressure and containment vessels being filled with hydrogen, even a slight incorporation of air (hydrogen) may lead to an explosive area and cause an explosion by catching fire. Tokyo Electric Power Company announced that they would establish a system within a few months to continue cooling the cores stably and contain radiation (roadmap), but there are not yet clear solutions to the three threats.

Complete accident resolution and future nuclear power generation

Taking the Three Mile Island accident in the U.S., for instance, it took more than 10 years to completely dismantle the nuclear reactor and clear the land, and it cost one billion dollars. Despite technological advancements such as remote-controlled robots, it is predicted that it will take 10 years and cost hundreds of billions of yen to completely resolve the Fukushima accident.

Many people, including myself, have pointed out the potential hazards of nuclear power generation technologies. Seismologists have specifically been criticizing the way safety reviews are performed, especially in recent times. Both the utilities involved and the supervising central government have established a kind of collusive relationship and proceeded with nuclear power generation, however, and they would not bother listening seriously to the opinions of others. Some are criticizing the accident response of the Tokyo Electric Power Company. People have a strong distrust of the nuclear power technologies and the operators, and a divisive national debate over what to do with nuclear power generation is likely to occur. In my opinion, nuclear power plants on sites that have a high probability of earthquakes and dilapidated nuclear plants should be abolished immediately, and with the collusive relationships discontinued, including the separation of regulation and promotion, the remaining plants should be operated for the time being, and then directions should be determined based on the assessment of trends in energy-related technological development.