TEPCO has sealed stairwells, hatches and other penetrations leading to the basements of the Fukushima Daiichi units 1-4 turbine halls and other buildings to reduce the spread of radioactive dust.
The dust is carried by draughts created by the movement of water from the basements as they are pumped dry. The work, carried out in October, involved fitting non-combustible plastic sheets over the openings and sealing with tape or urethane foam. Some entryways were blocked with boards to prevent access through the barrier. In the units 2,3 and 4 turbine buildings, the large equipment hatch was sealed with balloons.
TEPCO has announced that the presence of the short-half life fission product Xe-135 detected last week did not indicate a dangerous situation, because it was detected in small quantities, because the detected quantity did not change after injection with boric acid, and because no other reactor parameters changed. It said that the fission product reflected a continuous low-level fission reaction, and was only detected because of a new, sensitive measuring method. In a separate statement, the Japanese nuclear regulator NISA concurred with these conclusions.
In other news, Japanese national broadcaster NHK reported that the Japanese environmental registry has begun a detailed survey of radiation levels in air within the 20km exclusion zone around the plant, in order to determine decontamination priorities when work starts in January. It will use unmanned helicopters and cars to carry out the survey at 100m intervals. The NHK report was published by Japan Atomic Industrial Forum's Atoms in Japan information service.
JAIF reports that Japan's advisory committee on the Fukushima cleanup has released a provisional estimate that cleanup of the station will take 30 years or more. Work to remove fuel from spent fuel ponds could begin as early as 2014. After damaged primary containment vessels are repaired and filled with water, melted fuel could begin to be removed from them in about 2021. The committee chairman, Kyoto professor Hajimu Yamana, said that definitive estimates require investigation of the interior of the reactors.