Crews briefly shut off residual heat removal systems at units 5 and 6 to test transformers and switched core cooling to external fire trucks at units 1 and 2 to install alarm systems Monday at Japan’s severely damaged Fukushima Daiichi nuclear plant.

According to a statement issued by Japan’s Nuclear and Industrial Safety Commission Tuesday, those systems were safely put back on external power afterward as Tokyo Electric Power Co. continues its fight to control three reactors and four spent fuel tanks damaged following a March 11 tsunami and station blackout.

More details also emerged Tuesday on an air filtration system the utility plans to install in unit 1. According to the Daily Yomiuri newspaper, installation of the six filtration units began Tuesday. The system will be installed in the unit’s turbine building, which will be sealed off to keep it at a higher air pressure than the atmosphere outside and force contaminated air out of the structure through hoses.

(PackBot Robots Work Inside Unit 1 on April 29. Source: TEPCO)

Contamination from radioactive particles escaping from the plant grounds has been an ongoing concern. On Tuesday Kyodo News quoted TEPCO reports that samples taken from the sea floor near the coastal plant have shown radiation levels 100 to 1,000 times background levels. The samples, recorded Friday, were taken at a depth of 20 to 30 meters.

Radioactive contaminants also have been found at a nearby sewage treatment plant, according the Asahi Shimbun newspaper. On April 30, sludge and the remnants of incinerated sludge from the sewage treatment plant in Koriyama, Fukushima Prefecture, tested positive for cesium at levels of 334,000 becquerels. Japanese regulators plan to meet to discuss the handling of the radioactive sludge, which does not fall within any current safety guidelines.

Officials believe runoff from rain washed contaminants accumulated on the ground into sewers. Material from the incinerated sewage routinely is shipped to cement companies in Japan. Local authorities are tracking shipments of potentially radioactive material, and officials say sludge at the treatment plant is being stored safely.