Nuclear safety stretched by Japan superquake

Different news media are giving out conflicting information on the nuclear power plants at Fukushima. It seems that the safety of one of the plants has already been compromised. The information is conflicting and confusing. The Incident and Emergency Centre (IEC) of the International Atomic Energy Agency (IAEA) is in constant touch with Japan's Nuclear and Industrial Safety Agency (NISA). The latter collects information from the operator, Tokyo Electric Power Company (TEPCO) and their information is supposed to be authentic.

From the bits and pieces of information gathered so far, it looks like a case of a Design Basis Accident, though claimed to be not as severe as the Chernobyl incident or the Three Mile Island accident in the USA. Hopefully, we'll get a clearer picture within a few days.

At around 0530 GMT, 12 March 2011, the authorities announced preparation for the venting of the containment of the Unit 1 reactor at the Fukushima Daiichi plant through a controlled release of vapour. This operation was intended to lower pressure inside the reactor containment. At 1240 GMT, NISA informed that there has been an explosion at the Unit 1 reactor, and that they were assessing the condition of the reactor core.

The Japanese authorities extended the evacuation zone around the Fukushima Daiichi plant to a 20-kilometre radius from the previous 10 kilometres. The authorities were reported to be making preparations to distribute iodine to residents within the power plant area. Iodine-131 is the radioactive gas that becomes airborne potassium iodide, an inexpensive drug that, when given within around 24 hours of exposure, prevents the thyroid from uptake of the radiation, and ultimately, from the increased dangers of thyroid disease and thyroid cancer due to iodine-131 exposure. Alarm was in the air and the delays in announcing the accident in its turn only created rumours.

Earlier on 11 March 2011, Japanese authorities had informed the IAEA that officials were working to restore power to the cooling systems of the Unit 2 reactor and that mobile electricity supplies were made available at the site and that pressure was increasing inside the Unit 1 reactor's containment, and the officials had decided to vent the containment to lower the pressure. The controlled release will be filtered to retain radiation within the containment, it was reported.

1245 CET, 11 Mar 2011: The IAEA's Incident and Emergency Centre receives information from Japan's Nuclear and Industrial Safety Agency (NISA) that a heightened state of alert has been declared at Fukushima Daiichi nuclear power plant. NISA says the plant has been shut down and no release of radiation has been detected. The World Meteorological Organisation has informed the IAEA that prevailing winds are blowing eastwards, away from the Japanese coast. Japanese authorities reported that the four nuclear power plants closest to the quake have been safely shut down. The Agency has sent an offer of Good Offices to Japan, should the country request support.

The reactor involved in the accident, Fukushima Daiichi (unit 1) is Boiling Water Type, having a rate capacity of 460 MW and has been in operation since March 26 1971. It is one of the six reactors of Boiling Water type (BWR) with combined capacity of about 5000 MW. Another site at Daini in the same prefecture has 5 reactors of the same type — 1100 MW each. All other units are reported to be safe. Even this particular reactor could be shut down.

Then why is this being reported as alarming? Even after shut down, meaning termination of chain reaction, the residual heat inside the reactor core continues to be generated and remains for days (roughly 7 percent of the nominal). This reduces with time. In the present case, this is roughly 100 MW of heat. Heat has to be removed continuously by circulating coolant (water in this case) through the core. Electricity driven pumps are intended for this purpose. But as the report suggests, when the reactor stopped producing electricity, the external supply of power was terminated due to the earthquake. Back-up pumps of 3x 100 percent capacity are provided to support the circulation. As reported by the authority, diesel generators of the plant also broke down due to the earthquake. Later on, external diesel generators were wheeled in. By that time the residual heat kept on accumulating. This was the case of a bare core. The temperature of the fuel elements had possibly exceeded the maximum allowable, at least in the parts of the core.

Luckily, any reactor design uses a concept called 'defence in depth', i.e. providing multiple barriers between the source of radiation and the environment. Most of the radiation (90 percent +) is contained in fuel element. The fuel rods are Uranium Oxide pellets shrouded in Zircaloy cladding with a gap in between. Uranium temperature must exceed about 1200 C if the cladding had to fail. Normally, the temperature of the outer wall is less than 400 C by circulating water. If even the integrity of fuel rods is compromised, radiation is leaked into the pressure vessel, a rugged steel structure, and designed appropriately not to break.

The reactor cooling system is such that heat is not transferred directly. Rather, it is transferred through a barrier. Steam thus produces flows through turbine for producing electricity.

The news that there were explosions inside is worth serious consideration. An explanation could be that super saturated steam in the core initiated interaction with Zircaloy (cladding) to produce free hydrogen and this caused hydrogen-oxygen explosion(s). Also, the excess steam increased pressure and internal structures were stressed. May be it was apprehended that the barriers to release of radioactivity could collapse. The question of venting possibly was the solution in order to release pressure. The same strategy was taken during the nuclear accident at Three Mile Island Plant of Pennsylvania, USA in the 1970s. Similar explosions were reported during the accident at Chernobyl. While venting, the steam also contained radioactivity and hence the question of distribution of iodine tablets was conceived.

Radiation embedded in parcel of air released usually travel along the prevailing direction of air before gradually becoming weaker in concentration. The distance travelled will depend on weather pattern. The weather report says the prevailing wind direction is towards the sea. Dispersion dynamics is such that any kind of precipitation will settle radiation on ground, sea, contaminating soil, water, etc. Radiation can reach living organisms through direct inhalation or through milk (from contaminated grass consumed by cows, goats, etc.), water, soil, etc.

The above are only what could be found from incomplete information made available through various open sources. The real picture will emerge as the authorities in Japan analyse and try to come up with an answer. Right now everyone in Japan must be busy managing the situation so that the damage can be minimised. All that has so far happened is already known to the industry, and the systems were designed to mitigate the consequences.

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Dr C S Karim, a former chairman of Bangladesh Atomic Energy Commission, is a nuclear safety expert. He served as a member of the 2007-08 caretaker cabinet.