TEPCO clears radioactive dust out of major car park

7 February 2012

Workers have reduced surface radiation of the 6000 m2 Main Anti-earthquake Building by more than three times, to 68 microSv/hr.

TEPCO vacuum unit collects dust and small rubble
TEPCO vacuum unit collects dust and small rubble

First, 60 workers swept up surface dirt and mopped the surface, filling 150 sandbags. Then, over the course of three weeks, a digger scraped the surface with a wire brush attached to a backhoe, and vacuumed up dust and rubble with a trailer-mounted dust collector.

Aerial radiation (measured 1m above the surface) also fell by 65%, to 82 micro Sv/hr.

Outside the site, the Japanese environment ministry has published plans for decontamination of evacuated zones. The first areas to be decontaminated would be public structures, including the Joban expressway that leads to Tokyo, according to Japan Atomic Industrial Forum's Atoms In Japan news service.

Decontamination of those areas with contamination levels of 20 mSv/year or less—where evacuation orders are planned to be lifted—will start at the high end, with work to be completed by the end of the year. The deadline for cleanup of areas with 5-10 mSv/year contamination is 31 March 2013, and for areas with 1-5 mSv/year contamination 31 March 2014. For areas with contamination above 20 mSv/year, decontamination work will carry on into next year. Pilot projects are now beginning for areas with contamination of 50 mSv/year. A full decommissioning programme is to be issued by the end of March.

In other news, reactor temperatures at Fukushima Daiichi have increased after TEPCO altered the water injection path for units 2 and 3.

The temperature reading of one of the three unit 2 upper bottom head sensors has climbed to 72°C from about 50°C two weeks ago. The other two sensors are showing readings about 45°C. The hot sensor happens to be nearly directly beneath the core spray header inlet, which might be a blind spot for the core spray header (although this is only supposition).

TEPCO has been changing the reactor injection lines to polyethylene pipes to improve injection reliability. Along with that, it decided to increase the proportion of water going to the core spray system on 1 February. Water is injected into units 1, 2 and 3 via the feedwater injection system, which enters the RPV underneath the fuel, and the core spray system, which enters the RPV above the fuel.

However, after the increase in temperature was noted in unit 2 on 2 February, engineers reversed the change and increased feedwater flow, first from 2.9 -4.9 m3/hr, and then again to 5.8 m3/hr on 5 February, and then again to 6.8 m3/hr on 6 February. Workers initially decreased core spray flow from 5.8-3.8m3/hr, and have maintained it since.

On 2 February, TEPCO engineers also changed injection at unit 3, from 5.5-6.0 m3/hr, but maintained feedwater injection at 3.0 m3/hr. Its upper bottom head measurement is cooler, at 50.1°C.

The unit 1 RPV temperature has not been affected by the change, and remains at a relatively cool 24°C with a constant injection of 4.6m3/hr through the feedwater system and 2.0 m3/hr through the core spray system.

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