Emissions of radioactivity from the Fukushima Daiichi nuclear power plant are a tiny fraction of those during the height of the accident as Tokyo Electric Power Company moves to the next phase of its stabilisation roadmap.
Provisional analyses based on radiation dose rates at the site boundary show that emissions to air have reduced by a factor of two million compared to those at the height of the crisis, when the torus suppression chamber of unit 2 ruptured on 15 March.
Someone standing at the western border of the power plant today could expect to receive a maximum of 1.7 millisieverts per year (mSv/y) from airborne radioactivity from the three ruined reactors. This compares to the 2.4 mSv/y average that people worldwide receive from background sources, and the operational limit for nuclear power plants in Japan to limit public exposure to 1.0 mSv/y.
Dose rates from emissions drop dramatically away from the site: five kilometres away the maximum rate from newly released radiation is 0.3 mSv/y; ten kilometres away it is 0.09 mSv/y; and 20 kilometres away it is 0.03 mSv/y.
It is important to note that these figures apply only to the rate of release of radiation now, and do not include the effects of any materials already deposited on the ground, some of which will continue to emit radiation for many years. There are areas totalling about 1000 square kilometres where dose rates have been elevated beyond 20 mSv/y due to caesium-137 deposited on the ground.
However, the lowering of release rates from about two tera-becquerels per hour to around one giga-becquerel per hour means that Tokyo Electric Power Company is approaching a landmark in its control of the site.
One of the Step 1 roadmap targets, now officially completed, was 'mitigation of dispersion'. Towards that aim the company sprayed about 400,000 square metres of the site with dust control agents and removed 500 containers of contaminated debris. Work is ongoing now in preparation to install an airtight structure over unit 1, which will further control airborne emissions. Similar structures may be put in place over the other three damaged units.
Emissions to sea have been tackled separately. These too have been stopped, and water in the plant's port area filtered through zeolite blocks to reduce radioactivity from caesium. Water trapped in the basements of the power plant is being pumped to a storage and filtration centre before re-use as coolant for injection. Overall water levels in the buildings have fallen marginally as the system has come into operation, reducing the risk that any could overflow to the sea.
One other target area is in sight - cold shutdown of the reactors. A definition published yesterday by Tepco specified that cold shutdown is achieved when the temperature at the bottom of the reactor vessel is 'in general' below 100ºC. While the three crippled reactor cores have usually been above this, today the reported temperature of unit 1 was 100.0ºC, placing that goal in reach. Tepco expects emission rates from the power plant to go down further as temperatures are reduced.
Documents on the roadmap for restoration show that Tepco has met the conditions of Step 1, summarised as 'stable cooling' on schedule. It is now moving to Step 2, described as 'more stable cooling', which covers in a three to six month period:
- Ensuring the robustness of water injection and treatment facilities
- Installation of heat exchangers for the used fuel ponds of units 1 and 4
- A detailed investigation of groundwater conditions
- Expansion of water treatment and management of the sludge produced
- Installation of the cover on unit 1
- Removal of debris from the roofs of units 3 and 4
- Beginning 'full-fledged' decontamination
Researched and written
by World Nuclear News