The Daily Yomiuri, an online Japanese newspaper, reported on Saturday that workers had installed a ventilation system at Fukushima reactor no. 1, and I was interested to read about the radiation levels inside. In the U.S, we still adhere to rems, rads, and curies instead of the SI units of sieverts, grays, and becquerels.
Levels of radiation (energy) are measured in rems or sieverts, which equate to absorbed dose in human tissue. Radioactivity, the decay rate of a chunk of radioactive isotope, is measured in curies or becquerels. We’re looking at the former here.
Inside the reactor, levels were measured at 8-93 milliSieverts (mSv) per hour, which is 0.08-0.93 rems per hour. The workers installing and turning on the ventilation system (which is intended to capture iodine-131), got doses of from 0.31-3.16 mSv. The higher number equals 316 mrem, or about what the average person gets from natural background and medical doses in a year (about 200 of which is from radon). Radiation workers in the U.S. are allowed a maximum of 5 rems per year, or 5000 millirems, or more in lifesaving situations. Don’t forget that they also get specialized training and equipment.
Therefore, the job went well. They obviously used the principles of time, distance and shielding to keep their doses that low while doing the work. At 0.93 rems per hour, you could get your whole annual rad-worker dose in just over 5 hours.
It should also be noted that masks don’t stop radiation (energy), just radioactivity (particles), which could be inhaled or ingested, or get into eyes or cuts. The lead blanket they put on you at the dentist, or the lead aprons radiologists wear provide some protection against radiation, but you couldn’t make a whole suit out of that and be able to stand up in it, let alone work. The papery Tyvek or plasticky Saranex suits keep radioactive dust off you so you don’t take it home and breathe or ingest it later. Decontamination after doffing the suits is about as thorough as it gets in the haz industry, too.
Here’s the article: http://www.yomiuri.co.jp/dy/national/T110506005624.htm