So, what is the excuse by the Tokyo Metropolitan government for withholding this information? "It was detected inside the building. The radiation was no problem at the perimeter of the facility."
It's not even a good excuse.
From Yomiuri Shinbun (2:33PM JST 6/8/2011):
東京都大田区の下水処理施設内の空気中から、毎時約２・７マイクロ・シーベルトの放射線量が検出されていたことが、都の調査で分かった。
計画的避難区域の福島県飯舘村の放射線量と同程度で、文部科学省によると、都内でこれほどの放射線量が検出されたのは 初めて。放射性物質を含む汚泥の影響とみられるが、都は「検出場所は屋内。敷地の境界では問題なく、誤解を招く恐れがある」とし、調査結果を公表していな かった。
The survey by the Tokyo Metropolitan government revealed that the 2.7 microsieverts/hour radiation was detected in the air inside the sewage treatment facility in Ota-ku, Tokyo.
The radiation level is almost the same as in Iitate-mura in Fukushima Prefecture which is designated as "planned evacuation zone". According to the Ministry of Education and Science, it is the highest air radiation level ever detected in Tokyo. The radiation is considered to have come from the radioactive sewage sludge, but the Tokyo Metropolitan government didn't disclose the survey result, because "it was detected inside the facility. There was no problem on the facility's perimeter, and it would have caused confusion if we had disclosed the result."
都によると、この施設は都下水道局の「南部スラッジプラント」で、都内２か所の下水処理場で発生した汚泥を集めて焼却し、灰を東京湾に埋め立てる などしている。都の５月の調査では、この施設の焼却灰から１キロ・グラム当たり１万５４０ベクレルの放射性セシウムを検出していた。
According to the Tokyo Metropolitan government, the facility is "Nanbu [Southern] Sludge Plant", which collects sewage sludge from two sewage treatment centers in Tokyo and burn it, and bury the ashes in the landfill on the Tokyo Bay. In the government survey in May, 10,540 becquerels/kilogram of radioactive cesium was detected from the slag from this plant.
As this blog posted back in May, "Tobu [Eastern] Sludge Plant" in Koto-ku had 170,000 becquerels per kilogram of radioactive cesium in the slag, which had already been sold as cement material. I hate to think how high their air radiation level was inside the plant.