The catastrophic disaster struck Japan just after I gave my word on resuming this current affairs column in April.It was certainly a strange coincidence.I can only assume that this is a mission bestowed upon me.There are so many things to write.Let me cover all the topics one after another.Today, I am starting with the topic of the greatest public interest, the state of reactor cores at the affected power station.
The power station has six units, Unit 1 to Unit 6, three of which were in operation.They were automatically shut down when the earthquake was detected, and were in the process of reactor cool-down to achieve the state of cold shutdown when Tsunami waves struck one hour later.Power supply facilities were submerged in water, rendering motors useless and disabling the reactor's cooling function.
A nuclear reactor continues to release decay heat even when it is shut down.Decay heat is the energy of radiation from radioactivity generated as a result of nuclear fission.This energy is transferred to and absorbed into the reactor core, where it turns into heat of sizeable intensity.
There are still thousands of kilowatts of heat remaining in the reactor core.There was about ten times more immediately after the disaster.Since the heat comes from radioactivity, the decay heat diminishes as time elapses.
This decay heat raised the temperature of fuel rods in the reactor core, whose cooling function was disabled.The heated cladding became oxidized with steam, and the heat of reaction caused the melting of the reactor core.Hydrogen, generated at the same time, later caused explosions.
The parameters showing the conditions of the melted reactor core, including pressure and water level, are stable.The reactors appear to be in respite due to the injection of water.
Everyone has different view on the extent of the melting of the core.The government says the core is molten at 20–70%, depending on the units.I suspect the reactor core is almost all molten by this stage.
In this case, if the molten core is still contained in the pressure vessel, it should be clam-shaped mass measuring 4 meters in diameter and 2 meters in height, its center boiling at the temperature of over two thousand degrees Celsius.The surface should form 20–30 centimeter-thick crust that resembles cast iron.Gaseous molten core (radioactive materials) should be constantly released through cracks of this crust.It is, indeed, a chilling image, but should be close to reality.
Radiation in the released gas is cooled down with surrounding water and steam, and transferred back to liquid, to increase the concentration of radioactivity.This is the major issue that I will examine in my column next time.Intensifying the cooling operation and successfully solidifying the molten core, would halt the release of radioactivity.However, that would involve difficult engineering work that must be performed under intense radiation.
The release of gaseous radioactive materials, which has been the subject of public concerns, is limited to noble gas and iodine with low boiling point.Most of them is believed to have already been released.This is because the steam cooling the reactor is just over 110 degrees Celsius, and radioactive materials with the boiling point lower than this temperature are cooled and become mixed in to water.This is why this case would not cause mass contamination of the atmosphere, as seen in the case of Chernobyl.
This is the factor cited by people who disagree with the government's decision to raise this incident's rating under the International Nuclear Event Scale from Level 5 to Level 7.The amount of radioactive materials released is about one-tenth of that of Chernobyl, and most radioactive nuclides that have escaped into the atmosphere are limited to Iodine 131 and harmless noble gas, unlike in Chernobyl, which involved the discharge of nuclides that could have complicated effects.This view is spreading not only in Japan but also across the international community.
To prove this point, let me point out that the radiation levels at various observation points have been gradually dropping according to the half life of Iodine 131, down to just one-tenth of what they were initially.It is time to start considering to allow evacuees to end their inconvenient living arrangement and return home.
Incidentally, it is worth noting that milk and local fish called sandlance, which showed the level of radiation above the government standard, should be processed into butter or dried into Himono, and left in storage for a short period of time to achieve radiation damping.I would buy and eat such products myself.The contamination of farmlands is not very serious, either.Wouldn't it be the government's job to ask local farmers to grow rice, and investigate the toxicity of the crop?The government needs to be more tactful at a time of emergency.
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