The Fukushima Workers, Part 2

This article assumes you've read the last piece on the Fukushima workers. You should also have some familiarity with the Sigmoid No Threshold (SNT) radiation harm model.

I have become fascinated by the Fukushima workers. Out of curiosity, I asked myself: what were the airborne concentrations in the control rooms, that gave rise to multi-hundred mSv internal doses in a small handful of days. I assumed all the dose was I-131 and then used the ICRP dose coefficient to concoct Table 1. This table claims that to get to the 590 and 540 mSv suffered by the two highest internal dose people, we would need a ambient dose rate of close to 250 mSv/h which would be an I-131 contamination of 10 MBq/m3.

When I first saw these results, I was sure somebody had screwed up: the ICRP, the Fukushima measurers, or me. The dose rates could not have been that high. But then I stumbled on Figure 1.

Figure 1. Fukushima dose rates next to the melted down and breached reactors.\cite{unscear-2013}[p 36]}

These numbers are based on surveys taken on March 20 and 22. We see dose rates close to the breached reactors of over 100 mSv/h. These are not peak numbers. WNA says "Soon after the accident a similar study put the highest dose rate at 300 mSv/h near rubble lying alongside Unit 3."[WNA, Fukushima Daichi Accident] Table 1 might not be crazy. (In the radiation protection business, being within a factor of two is considered spot-on.)

Units 1 and 2 had a single combined control room, as did Units 3 and 4. Each of these control rooms were sandwiched between its pair of reactors. The two guys with the highest internal dose were in the Unit 3/4 control room. Figure 2 is a photo of the Unit 3/4 control room after the explosions. These men were living in that building for at least 8 hours per day during the worst of the release. I believe the numbers.

Figure 2. Fukushima 3/4 Control Room is on the 2nd floor of the circled building.

The Fukushima crew were gutsy. They pushed very hard to get control of the mess. Whenever you were sent out on a task, you were given an APD (Automatic Personal Dosimeter) set to alarm when the dose on the job hit 20 mSv, at which point you were supposed to back off. We know the alarm was ignored on at least one occasion on March 24. Three workers got up to 150 mSv each laying cable in the Unit 3 turbine hall to pump water out of condenser.¹ They claimed they thought the APD's were faulty. I'm guessing that was a lie. They just decided this cable had to be laid, and they were going to do it.

People were inside Unit 1 containment on May 4. On the 16th of May, they were inside Unit 2. On June 9, nine guys attempted to enter Unit 3 containment. They ran into dose rates up to 100 mSv/h. After about 20 minutes, they had to abandon the job. All 9 received more than 5 mSv.\cite{jaif-109}

It's quite remarkable. We had thousands of people crawling all over and around three breached, melted down 1 GW reactors, and we can't reliably detect any harm. 5718 Fukushima workers including most of the high dose cases have been closely tracked in a government funded program. As of end of 2019, no significant health effects have been observed.\cite{kitamura-2022} The authors write: "The blood test results showed that the study participants were no different from the healthy Japanese men population." What does this mean for the rest of us?

The fortitude of the Fukushima personnel working near or in some cases inside the reactor buildings is almost matched by how rapidly the dose rates fall off as you move away from the breached reactors. The on-site command center during the response was the prosaicly named Seismic Isolated Building (SIB), Figure 3. Maybe it sounds better in Japanese. The SIB was a hardened structure with its own generator and ventilation system.

Figure 3. Seismic Isolated Building

Figure 4 shows where the SIB was located relative to the breached reactors. The SIB was up on the bluff but only about 400 meters west of Unit 1. Figure 1 indicates the dose rates near the SIB were roughly 25 times lower than next to the reactors. The SIB ventilation system was far from perfect. Workers coming and going had to don and undon their outfits in the space between the double doors so there was a lot of leakage. One woman working in the SIB received a dose of 13.50 mSv. This lady was apparently the highest dose female.

Figure 4. The SIB is the green circled building on the left edge of the photo. Red circles are approximate locations of the Unit 1/2 (left) and 3/4 (right) control rooms.

Close to the reactor, we would expect a quadratic fall off with distance from the source from plume spreading alone.² Deposition and decay would further reduce the dose rate with distance. At Fukushima, we got something between a factor of ten and a factor of 100 reduction in the first kilometer. We saw similar numbers at Chernobyl.\cite{flop3}[Sec 6.6] According to SNT, harm (cancer incidence) goes at about the 2.2 power of the dose rate. Using the 25 times less figure, the harm at the SIB would be 1115 times less than at the control rooms roughly 500 meters away.

Radiological radiation is a lot like fire. After all, they are both radiation. The photon portion of fire and radiological radiation is the same thing, just different frequency. Getting too close to fire can be very harmful; but you don't have to be very far away before fire is not a problem, and may even be desirable.

It's true that high energy photons can damage our DNA while thermal photons cannot. But Nature saw this problem coming, and has equipped up with remarkably effective repair system for this damage. This ability might be described as dumb luck. I prefer providential. Nature had to come up with this repair system to handle metabolic damage to our DNA. Fortunately, this system also works for radiation damage. That's why the Fukushima workers were able to do what they did and get away with it.

The combination of the fall off in dose rate and this remarkable repair system means you will almost certainly suffer no detectable harm if you are more than about 2 kilometers from a release. If the distance you need from a big fire is 20 meters, the distance you need from a big release is 100 times as much. Every big nuclear power plant should have a buffer zone of 2000 meters from the reactors to the nearest residence. If it does, the plant is no more dangerous to the public than a big refinery. It can and should be regulated in the same manner.

1. Cooling water injected into the reactor was filling up the condenser. They needed to make room in the condenser for more water or stop the cooling. The latter was not an option.

2. Once the top of the plume reached an inversion the spreading would be closer to linear.