Thursday, March 17, 2011

Doses and Dose Rates

The reporting on doses and dose rates has been particularly bad and hard to understand, but I think it may be possible to pull out some information from it to help understand how bad the incident is. In this post, I want to provide a framework in which to think about what’s going on at Fukushima. I’ll work on the numbers in another post.

Reporters on the Nuclear Energy Institute call-in a few days ago seemed to want a single number to summarize whether reactors were safe or not. The NEI speakers weren’t willing to give that kind of answer. There’s an argument going on that I’ve been ignoring as to whether the situation at Fukushima qualifies as a 4 or a 6 or a 7 on a seven-point scale for rating nuclear accidents. Given the uncertain state of the knowledge that is input for that rating, this argument at this time seems rather silly to me. I suppose the benefit of this argument would be a big headline, “Fukushima at top of accident scale!”

The situation at Fukushima is constantly changing. We get snapshots from the Tepco press releases or aerial photographs. If we want to understand what is happening and what is likely to happen, though, we have to put those snapshots into time sequence.

Thinking about time sequence means thinking about rates. The rate most of us work with most frequently is the speed we drive our cars: miles per hour (or miles/hour, or mph). If we’re driving 50 mph, we expect to cover 100 miles in 2 hours. That’s multiplying 2 hours by 50 miles/hour. I’m going through it step by step because it is analogous to dose calculations. The slash in miles/hour indicates division. So when you multiply hours by miles/hour, the hour unit cancels out to give miles as the result.

Now on to radiation doses.

What Kind of Exposure Is It?
I discussed this a few days ago, but I’ll go into more detail here.

An operating reactor, or the fuel in spent pools, spews out neutrons and alpha, beta, and gamma radiation. Under normal conditions, the reactor or pool shielding protects the operators from that radiation. But in reactor accidents, there have been times when operators have to go inside the shielding and are irradiated (or “exposed to radiation,” as the media often put it).

The kind of radiation I’m talking about is emitted by unstable atoms, so there is always matter associated with the radiation, radioactive matter. The matter doesn’t escape from an intact reactor or pool, so the irradiated person doesn’t become radioactive* and the effects of his exposure don’t transfer to people in contact with him. This is a concern for the people at the reactor only.

What is of concern for the people in the areas around Fukushima is radioactive matter that comes out of the reactors or spent fuel pools. When steam is released from the reactor, it may carry gaseous fission products like krypton and iodine. The krypton remains gaseous and dilutes itself in the atmosphere, but the iodine cools and solidifies into particles or attaches to dust particles already in the atmosphere. If material is on fire, as in some reports about the spent fuel pool at reactor #4, solid particles containing fission products and uranium and plutonium** will be lofted into the air with heated combustion gases. The particles will eventually fall out of the atmosphere onto people, houses, plants, animals, and the ground. This kind of contamination can be spread by those who carry it, but, because it consists of solid particles, as it spreads, it becomes more dilute and less harmful.

What’s the Dose?
This can get complicated fast, but I’m going to leave most of the complications out to emphasize the general concepts. I’ll be happy to handle questions if you put them in the comments or tweet them to me (@cherylrofer).

For the operator inside the reactor shielding, there is a radiation flux that can be translated into units of sieverts, which indicate its effect on his body. Here’s an explanation of radiation units from the CDC. The dose rate, which is the number that usually shows up on instruments, is sieverts/unit time, or fractions of sieverts/unit time. Those fractions are usually microsieverts (μSv) or millisieverts (mSv). Most instruments use hour as the unit of time, but that is not always the case. Popular listings of everyday exposures, as in x-rays or in airline flights, are usually per incident.

The rate at which a person is irradiated makes a difference; small doses over years may have no effect at all or may cause cancer; the same total all at once could produce radiation sickness. It’s the total dose that is important, so we need the dose rate and duration of exposure, in the same units, to be multiplied together to give the dose, just as we multiply speed times time to get distance.

Many news reports provide no time units at all, and some have confused milli- and micro-. Ezra Klein posts a color-coded chart that lists doses, dose rates, and doses per incident. From orange down, they’re for immediate exposure, not exposure over time. Apples and oranges. He’s evidently received some critical comments (including an e-mail from me), but he doesn’t try to clarify. I doubt that a variety of comments from anonymous posters are helpful. The numbers seem to be all right, although confusing in the way Klein has presented them.

More reliably, the American Nuclear Society provides an interactive dose estimation chart that you can use to estimate your yearly dose from common sources of radiation. Unfortunately, their doses are given in rem. One sievert (Sv) equals 100 rem.

I’m off to collect some numbers and try to do some comparisons for the next post.

*This statement is not quite true, but, unless the dose is enormous, the amount of material in the person’s body that is activated is small and of no danger to other people.

**Plutonium is always present in spent reactor fuel. It is produced when uranium-238 absorbs neutrons.


MT said...

Is one sievert really the same as another? as in Gamma rays vs neutrons vs alpha and (the notoriously wimpy) beta particles? Who's liable to be exposed to which kind, or do we learn all we need to about the danger of exposure from the sieverts dosage?

MT said...

I think there might be biological units for radiation, based on the rate of breakage or crossover of the chromosomes in yeast or whatever. Or maybe I'm confusing this with Morgans...

MT said...

Ah. Per your link the sievert (like ye olde "rem") is itself the biologically defined metric. So any 17th sievert at least ought to be like the 17th any other individual receives.

Cheryl Rofer said...

I see you've got it, MT, but let me repeat it a little differently for everyone else.

The sievert (or the older unit, rem) measures the effect of radiation on the human body. That way, we don't have to figure out alphas, neutrons, gammas, whatever, separately and then add it all together.

That's one of the details I decided to leave out. It's worth knowing, but the important part is that sieverts measure biological effects.

MT said...

See also;