And that's nothing, says Nuclear and Industrial Safety Agency. It not only has close to zero effect on the environment (I suppose NISA is talking only about the environment around the plant, which is already very, very bad) but the amount is nothing, the agency says (if I paraphrase..): "Compared to the much bigger release when we let TEPCO dump the "low-contamination" water into the Pacific, it's nothing! This time the amount of radioactive materials (we're talking only iodine and cesium, mind you, don't ask us about strontium and plutonium) was only 1/300th of that dump."
(Oh by the way, NISA's spokesman Nishiyama's daughter works for TEPCO, allegedly.)
Kyodo News (12:34AM JST 5/9/2011):
METI's Nuclear and Industrial Safety Agency disclosed on May 8 that opening the double door between the reactor building and the turbine building of the Reactor 1 at Fukushima I Nuclear Power Plant will release the total of 500 million becquerels of radioactive materials into the environment.
The amount of radioactive materials this time would be 1/300th of the amount released into the ocean when 10,000 tons of water with comparatively low-level contamination was intentionally released; there is no effect on the environment, according to NISA.
東京電力は、今回の放出により第１原発敷地内で浴びる放射線量は、最大０・４４マイクロシーベルトと推計。一方 で保安院は、緊急時迅速放射能影響予測ネットワークシステム（ＳＰＥＥＤＩ）を使い、海から内陸に向け秒速１メートルの東風が吹く場合、０・７７マイクロ シーベルトになるとした。いずれも一般人の年間被ばく線量限度である１ミリシーベルト（千マイクロシーベルト）の千分の１を下回る値。
TEPCO estimates the added radiation because of the operation will be 0.44 microsieverts maximum within Fukushima I Nuke Plant. NISA, using SPEEDI, estimates 0.77 microsieverts of added radiation if the wind is from the east at 1 meter/second. Both numbers are lower than 1/1000th of 1 millisievert (1,000 microsieverts) which is the annual allowable radiation limit for the general public.
保安院によると、５億ベクレルという総放出量は、放射性のヨウ素１３１とセシウム１３４、１３７の合計値。東電 が７日に測定した原子炉建屋内の放射性物質の濃度に、建屋の容積約２万５千立方メートルを掛けて総量を算出した。屋根が壊れている原子炉建屋上部の高さ約 ２９メートルの位置から、８時間かけて放出されると想定した。
According to NISA, 500 million becquerels is the total of radioactive iodine-131, cesium-134 and cesium 137. The number was calculated by multiplying the amount of radioactive materials inside the reactor building that TEPCO measured on May 7 by 25,000 cubic meters (volume of the building). NISA assumed the radioactive materials to be released from the height of 29 meters (upper part of the reactor building) for 8 hours.
Since the pressure inside the reactor building is lower than the outside by design, when TEPCO opened the double door the cooler outside air got sucked in, and the radioactive air that was inside the building was pushed up and out of the building through the roof that had collapsed by the explosion on March 12.
And they chose to open the door at night, when the outside air was even colder. So the whole point of this exercise was to release as much radioactive air as possible out of the reactor building, it seems. They could have chosen to do it during the day when the outside air was warmer, but that wouldn't have pushed out as much radioactive hot air as they would have liked.
Meanwhile, Asahi Shinbun says the radiation level even after the air-filtering system had supposedly reduced it to a safer level still measured several 10s of millisieverts/hr up to 700 millisieverts/hr, and the workers were exposed to as much as 10.56 millisieverts radiation for the work that lasted 29 minutes from 4:18AM JST on May 9.
Seven TEPCO employees and two from NISA entered the reactor building to measure the radiation levels at various spots inside the building.
That doesn't sound low to me. But Asahi reports that TEPCO is going to remove the air-filtering system they just installed, and continue to have human workers work inside the reactor building to install the water cooling system