Some stabilisation targets have been met at the Fukushima Daiichi nuclear power plant, but levels of contaminated water in the basements remain Tepco's biggest challenge.
Pumping water into the three ruined reactors will be essential to their cooling for some time to come, and temperatures at units 2 and 3 are slowly approaching the landmark 100ºC, while unit 1 has been below this for almost a month. However, the water becomes highly radioactive after passing over the melted cores and it goes on to accumulate in the basements of the buildings.
To achieve sub-100ºC temperatures Tepco needs to increase the rate of injection, but this is impossible with the current volumes of accumulated water. Progress has to be made in the performance of the treatment facilities that clean radionuclides from the water ready for re-injection.
Performance targets for 80% availability have been approached for spells of a few days, although at times performance has been below 60%. During times of good operation water levels in the basements have come down by about 1.5 centimetres per day, only to go back up again slightly faster when equipment breaks down.
Overall, water depths have crept down from a high of about 3.80 metres to about 3.55 metres over about 90 days. Nevertheless, Tepco has said this marginal change has been enough to put the water surface 'sufficiently' below the top of the basement pit, at which point overspillage would be a major concern.
Tomorrow the company will begin the use of the SARRY caesium-absorption facility which will improve the process and on 20 August a second new evaporative concentration line will start.
Tepco has hit one target with the installation of external cooling for all four affected used fuel ponds. Ponds at units 1, 2 and 3 have stable temperatures between 34-37ºC, with water injection systems in place should cooling fail for a long period.
At unit 4 the external cooling system was brought online at the end of July and its temperature has stabilised at around 43ºC. A separate effort is underway to desalinate the water of pond 4, much of which was sourced from the sea.
Monitoring of newly released radiation in the air has been stepped up by about a factor of ten after levels fell below detection limits. The dose rate at the site boundary from newly released radiation now is just 0.4 millisieverts per year - within normal operating limits.
With the site now substantially stabilised and radiation release all but stopped to both air and sea, the issue of disposition of caesium-137 in the wider area is becoming the main focus of attention. The Ministry of Economy, Trade and Industry is now beginning analysis to support the return of residents to the zone 20-30 kilometres from the plant. It will need to assess the extent of contamination in each individual area as well as potential remediation measures.
Researched and written
by World Nuclear News