Kyodo News March 23, 2011 photo of Fukushima I, from security gate of #1 and #2 reactors.
In what appears to be the first report of possible acute injuries received directly from radiation at Fukushima, three Tepco employees were reportedly exposed to “high” levels of radiation — approximately 173-180 millisieverts — while laying electrical cables in the first floor and basement of Fukushima I’s #3 reactor. Two of the three were taken to the hospital with “possible” radiation burns on their feet.
Kyodo News reported that the injuries are from beta radiation, one of four types of radiation at Fukushima (gamma, neutron, alpha and beta). Beta particles are high-energy, high-speed electrons emitted by some radioactive materials that can in some cases cause cancer and death. However, the amount they received appears to be well below the threshold for causing radiation sickness. The amounts reported are markedly above the technical minimum established potentially carcinogenic dose, in practical terms, from what’s being reported now, the workers are likely not to be at significant risk for cancer. From what I can tell, they are unlikely to even have actually received burns — meaning the trip to the hospital is probably precautionary.
Alpha and beta radiation does not penetrate material the way that gamma and neutron radiation does; alpha and beta are only really a problem when they’re ingested or absorbed through the skin. A whole lot of variables determine how serious or dangerous the exposure was, so it’s impossible to tell whether this is a serious incident. Nonetheless, it clearly doesn’t represent anything like an overall escalation of the crisis.
Kyodo News reports that the three workers had stepped in a puddle with about six inches of radioactive water. According to Tepco, they had not tested the radiation at the puddle before they stepped in it, and their protective boots may have leaked, exposing the employees’ feet to beta radiation severe enough to cause, in two of the three cases, burns severe enough to require a trip to the hospital (which may have been precautionary).
As I understand it, the potential for beta burns would be measured in grays, which is an absorbed radiation dose equal to a Sievert per kilogram of tissue (1 Sievert to 1 kg of tissue = 1 gray). That would make 173-180 mSv well below the level where any clinical effect would be seen — but there are many other variables to consider. Since beta radiation is a non-penetrating radiation, skin burns would come from the contact of radioactive material (in this case, in the puddle water) with the skin. The acute danger is dependent on how long the water was in contact with the skin, and whether any of it was in contact long enough to be absorbed. If I’m reading the US Army’s information right, it takes something like 6 grays to cause a burn, so my guess is that either the trip to the hospital was entirely precautionary, or a very small amount of skin on each worker received that 173-180 mSv. That is above the minimum established carcinogenic dose, but again, beta radiation doesn’t penetrate like gamma or neutron. While it’s also above the usual safe dose for Japanese workers (100 mSv), the level was raised by the government to 250 mSv during the crisis. In the U.S., workers have an automatic exposure limit of 250 mSv in emergency situations, and 100 mSv otherwise.
Kyodo reports the radiation at the surface of the puddle was 400 millisieverts (mSv) per hour, and 200 mSv per hour in teh air around the puddle. However, Tepco later denied there was a puddle. It’s unclear from the Kyodo report if that’s ’cause it evaporated (puddles do that sort of thing) or if the original Tepco report had been in some way erroneous.
A kind of weird headline at Kyodo News that appears to be subscribers-only proclaimed that the radiation in the water where the workers were exposed was “10,000 times normal,” which, like most reports of radiation amounts in the press, is not really a meaningful or helpful assertion but sure sounds scary.