TEPCO injects poison after fission products detected in new unit 2 gas treatment line

2 November 2011

A new gas control system installed to reduce radioactive nuclides from the unit 2 primary containment vessel has possibly detected amounts of radioactive xenon with short half-lives, TEPCO said.

Unit 2 PCV filter unit
Unit 2 PCV filter unit


The presence of these nuclides is 'undeniable' evidence that a fission reaction has occurred in the reactor recently, TEPCO said, though the nature or causes of the reaction remain unclear. However, the detected amounts of xenon are relatively small—the largest is Xe-131m at 6.9x10^-4 Bq/cm3—and a maximum of a multiple of five above the detection limits. The three xenon isotopes found, the one above and Xe-133, Xe-135, have half-lives of 12 days, 5 days and 9 hours, respectively. The samples are currently under review. TEPCO said that there were no significant variations in measured temperature or pressure readings. However, it injected the neutron poison boric acid into the reactor for an hour on 2 November as a precautionary measure. It has also slightly increased the amount of water injection, by a total of 0.5 m3/hr.

Operation of the gas control system, installed to filter out Caesium 134 & Caesium 137, began on 28 October. The units consist of high-efficiency particulate air (HEPA) and activated charcoal filters, which TEPCO claims absorb 99% of radioactive materials. The system also monitors H2 concentrations, and after noticing that H2 concentrations in the system had risen to 2.7 vol% on 30 October, increased the injection rate of nitrogen gas from 16 to 21 N m3/hr to maintain a non-combustible concentration of hydrogen (the limit is 4 vol%).

The system extracts gas from the PCV via the combustible gas density control line, from which new piping carries the gas out of the reactor building and into the turbine building. There, it is cooled by radiators and sent to one of two filters mounted in series, behind which is mounted an extractor fan that creates the vacuum pull. The air is monitored, and some air is recirculated and reheated to pass through the filters again. Treated, decontaminated air is vented outside. TEPCO aims to install similar systems at unit 1 and 3 by the end of the year.

In other news, TEPCO has announced that the unit 1 reactor building cover has been completed, after five months of work. The building includes a filtration system. The building will help reduce the amount of rainwater contaminated by radionuclides.

Research and surveying work has begun on a shielding wall to isolate the Fukushima Daiichi 1-4 groundwater, which is likely to be contaminated, from the sea. At the site, water generally flows downhill toward the sea. The wall would consist of deep sheet piles that wall off the site from the sea bed, a low-permeable layer beneath the sea-bed, and the permeable layer beneath that. Workers will also install a groundwater drain to collect contaminated groundwater. Work will begin immediately.

However, TEPCO also reports that it has decided not to isolate site groundwater from land-side groundwater by piling on the uphill side, for several reasons. It found that such a wall would likely lower groundwater by 1-2 metres, enhancing the risk that contaminated water pooling in site buildings might leak into the groundwater. Second, land-side piling work would interfere with other site recovery issues.




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