Fukushima updates; March 18.

I’m going to focus this post on radiation dosimetry – because radiation dosimetry is what really matters in terms of deciding whether anybody can actually get hurt. So far, nobody around Fukushima has been hurt by radioactivity, although of course tens of thousands are still dead or missing because of this great tragedy.

It doesn’t matter what you do or don’t do to the reactors or the used fuel, or what condition they’re in – at the end of the day, the radiation dose to the public is how we measure the effect of this incident on the public and its potential for harm.

To be honest, I’m really not concerned much with what the dose rates are in the plant itself.

The men and women who work there understand dose rates and health physics quite well. They routinely work in areas of elevated above-background dose, and they know how to work safely in those environments. They understand how to measure and quantify the radiation field in the working environment, and the accumulated doses that they’re personally receiving.

They understand how to manage shielding, exposure time, radiation measurement and dosimetry in order to get the work done safely and effectively.

Even with abnormally significantly elevated radiation fields in some areas as a result of these incidents, they still know how to work safely. If the radiation dose rate in some particular area is so highly elevated that it cannot be entered safely for any length of time at all, then they won’t be entering it.

It’s pointless to scare the public with elevated on-site dose rate measurements. They’re not working on the site. Leave that for the people with health physics training. I’m much more interested in off-site dose rate measurements, personally, as those are the measurements that are actually of relevance to the public.

Off-site radiological dose rates

The KEK accelerator physics complex in Tsukuba (165 km from Fukushima) has a webpage showing their real-time measurements of the environmental gamma dose rate (counted with a GM tube). They’re currently measuring 0.17 μSv/h, at the time I write this, which has been fairly constant over the last few days, except for a brief, narrow spike up to 0.6 μSv/h, which they observed on the 16th.

(NOTE: Just to avoid any ambiguity, “m” means milli, 10^-3. “μ”, or “u” if you don’t have access to the Greek alphabet in your software, means micro, 10^-6. And when I say milli I’m quite sure I mean milli, and when I say micro I’m quite sure I mean micro. Because these terms are important, personally, I make damn sure to get them right.)

Each year, a resident of the United States receives an average total dose from background radiation of about 3.1 mSv. This is the radiation dose from natural background sources; from natural radioactivity in the Earth, and cosmic rays from space. That’s equal to 0.354 μSv/h.

In practice, the average dose that a person receives each year in the United States is significantly higher than that natural background dose, about twice that, once you’ve added on the dose from medical imaging and things like that.

The radiation dose rate being measured in Tsukuba right now, after the Fukushima accident, is less than half of the average natural background radiation dose rate that a person receives in the United States. This includes all sources of radiation in Tsukuba, including natural geological radioactivity, cosmic radiation, and any radioactivity released at Fukushima, as well as any ionising radiation from the particle accelerators at KEK, which is what these sensors are actually intended to monitor.

That brief, narrow spike seen in the radiation field measured at KEK doesn’t really concern me. The radiation dose you’ll receive if you hang around in that area for an extended period of time is the area under the graph – the integral – over that period of time. For such a short, sharp spike, the overall potential dose is still quite small.

In order to quantify the potential harm from a significant release of radioactivity, it would make more sense to “filter” that dose-rate data from the detector as a rolling average, to make it simpler to make a more straightforward interpretation of the potential to receive any significant radiation dose.

KEK is also measuring the concentration of ¹³¹I and the short-lived fission product ¹³²Te in the atmosphere and reporting regular updates to this data online. The concentrations we’re looking at here are extremely small – on the order of 10 microbecquerels per cubic centimeter – but they are concentrations which they are able to accurately measure at KEK, using a high-volume air sampler and a high-purity germanium gamma-ray spectrometer.

This site gives us a continually updated log of the gamma-ray dose rate in Tokyo.

The environmental gamma-ray dose rate measured in Tokyo between 11 pm and 12 am, averaged across one hour, on March 18th, was 0.0471 μSv/h. This radiological monitor in Tokyo returned its highest reading yet on the 16th, from 05:00 to 05:59, at a dose rate of 0.143 μSv/h.

So, that most recent figure from Tokyo is 13% of the average natural background radiation dose rate in the United States. One banana dose is something like 0.1 μSv, so what we’re measuring in Tokyo at the moment comes in at just under 0.5 banana per hour. (One banana per hour, and you’re going to triple that dose rate.)

The highest figured measured at all in recent days, 0.143 μSv/h, is equal to 40% of the average natural background in the United States.

The radiation level in Saitama, outside Tokyo, is also being recorded and charted on the web. As of 21:00 on the 18th of March, they report a dose rate of 0.058 μSv/h. The maximum value reported, at 1 am on March 17, is 0.067 μSv/h. These figures are 16% and 19% of US natural background, respectively.

Two things are apparent from this data.

(a) Japan has very low levels of natural background radiation compared to the continental United States. (This is interesting in itself! It’s probably a combination of both low elevation providing shielding from cosmogenic radiation and a relatively low abundance of uranium and its daughter products in the ground.)

(b) The background ionising receive dose rate that people receive across Japan has not been elevated significantly at all, at least outside the immediate vicinity of the plant, as a result of the Fukushima damage.

There’s an extremely interesting, valuable plot of dose-rate measurements on this page. Units are μSv/h.

(ASIDE: If you can’t read Japanese – I can’t – a little bit of Google’s automatic translation goes a long way in helping you sort through this important data.)

If we look at the 5 monitoring sites closest to the 30 km radius marked on the chart, we see that the last three measurements marked on the chart, for each of those sites are 52, 52, 52, 140, 140, 150, 40, 45, 45, 8.5, 9.0, 8.7, 1.6, 1.6, and 2.0 μSv/h.

This tells us that there is detectable radioactivity which is moving in a narrow plume in the atmosphere – it is not distributed out isotropically, which is indeed exactly what you would expect from thinking about the meteorology.

This chart of compiled radiation measurements also tells us a very similar story.

At that 30 km radius, the average dose rate from that monitoring station which reports the high outlier values – the one corresponding to the location of the plume – is 143 μSv/h.

I wonder what radionuclides are present in that plume? The presence of ¹³¹I, ¹³²Te and ¹³³Xe would tell us that this radioactivity has come from a reactor, the absence of these short-lived fission products will tell us it has come from used fuel in the pool. A little bit of gamma spectroscopy, and we would have the answers.

The presence of these radionuclides as measured at KEK confirms that at least a tiny bit of radioactivity has been released from the reactors themselves.

That’s fairly high, but it’s not obviously high enough to hurt people. If you stood in the location of that plume for an entire week, you would receive 24 mSv over the course of a week – which is a dose figure which would be consistent with a relatively-high-dose nuclear imaging procedure using something like ²⁰¹Tl to make an image of a tumor, or something like that.

If we remove those three outliers corresponding to the plume location from the above set of numbers and we take the mean of the remaining values, this gives us a rough idea of the mean dose rate elsewhere along the 30 km radius, outside the location where the source term of radioactivity is passing in a plume of wind. That mean value, then, is 26 μSv/h.

If you were standing in that radiation field, 26 μSv/h, for five hours per day every day for a year, you would reach a total annual dose of 47 mSv, which is just above the allowed occupational radiation dose – above natural and non-occupational background – of 50 mSv per year, for people working around radioactivity, such as nuclear power plant employees. (This is the limit set in the United States by the NRC; I’m not sure what the corresponding dose limit is in Japan, but it will be something loosely similar.)

But it’s well worth remembering that that radioactivity that is present now, in very low levels, will not be sticking around for a whole year. It is dispersing rapidly, and it drops away exponentially as you move away from the Fukushima site. As we move further out from the 30 km radius marked on that map, the dose rates recorded are all at harmless levels, consistent with background radiation dose rates experienced by people in the United States and elsewhere across the world.

In Ramsar, Iran, the natural background radiation dose rate is unusually high, at 260 mSv per year in some places. That is 30 μSv/h, which is higher than the mean value of about 26 μSv/h measured at these monitoring stations 30 km west of Fukushima, as I described above.

The people of Ramsar experience a background radiation dose significantly above that which most other people across the world experience – but they do not seem to experience any ill health effects at all from this.

I hope all the above helps to put these dose rates in context.

Used-fuel radiochemistry

The composition of the radionuclides that are responsible for most of the radioactivity in used nuclear fuel that has been stored in a cooling pool for a few months is very different than in nuclear fuel in a reactor that is operating, or has just been shut down.

Fuel straight out of an operating reactor contains a number of rather short lived, rather high specific activity fission product radionuclides which are of the largest health physics significance in the time immediately following severe reactor accidents.

Some of these short-lived fission products include iodine-131, xenon-133, xenon-135, tellurium-131, tellurium-132, and ruthenium-105. These short-lived fission products were very significant contributions to people’s radiation doses in the environment around Chernobyl in the time immediately following the Chernobyl disaster, for example, when they were dispersed from “hot” nuclear fuel from the reactor.

However, they are not present to any significant level in stored nuclear fuel, because they decay away relatively fast, and they cannot contribute any significant source term into the environment in some sort of accident scenario involving used nuclear fuel which has been stored for a month or three post-defueling.

So, what radionuclides are present in stored fuel? The main ones of interest here are the longer-lived fission products. ¹³⁷Cs, ⁸⁵Kr and ⁹⁰Sr are the most significant ones. Of these, ⁸⁵Kr is a gas, and has the most potential to be readily released from the fuel into the atmosphere. ¹³⁷Cs accounts for most of the radioactivity of the used nuclear fuel, and it is usually the most feared radionuclide in the used fuel inventory, in terms of the potential source term released from an accident with a used-fuel pool.

Fuel-pool water evaporation

“In this house, we obey the laws of thermodynamics!”
— Homer Simpson

With the used fuel heating the water in the Unit 4 fuel transfer pool, how long will it take for all the water to be boiled away? It is actually possible to know this, without any real speculation. The physics is pretty simple.

We know that there is a full core-load of used fuel in the Unit 4 defueling pool, which was put there after the reactor was shut down for inspection on November 30.

(Assumption I’ve made here which may possibly be wrong: That that single core-load of fuel is the only fuel in the pool. Is there additional fuel in the pool? If there is, somebody needs to tell me how much and how long it has been cooling for, so we can re-run the numbers – or you can take what I’ve explained here and re-calculate it for yourself.)

That fuel has been cooling for the last 3.5 months, or approximately 9.2 * 10⁶ s.

After this time, the radiothermal power output of the used fuel is small. Looking at this decay heat chart, we read that the decay heat is approximately 2 MWt. However, this chart is for a power reactor with a thermal power rating of 3000 MW. (And I’ve done a not-really-precise job of eyeballing that chart.)

But the Fukushima-I Unit 4 reactor has an electrical power capacity of 784 MW; that’s about 2352 MW thermal. So, we need to scale back the above figure commensurately; it’s approximately 1.6 MWt, from the entire core load of fuel.

The latent heat of vaporisation of water, at 100 C, is 2260 kJ/kg. (Let’s assume, conservatively, that the water in the pool is boiling; it’s at 100 degrees C, and the only route of energy dissipation from the system is through vaporisation of the water. This also assumes that none of the energy released is stored in the water by means of a rise in the water’s temperature, because it’s already at boiling point, and that there is no functioning mechanism for otherwise cooling that water.)

1.6 MW / (2260 kJ/kg * 1000 g/L) = 0.7 litres per second. 61 cubic meters per day.

The used fuel pool at Vermont Yankee, which is also a GE BWR-4, is 40 feet long, 26 feet wide and 39 feet deep, and is normally filled with 35,000 cubic feet of water. I will make an assumption that the Fukushima I Unit 4 used fuel pool has the same dimensions.

The level of water in the used fuel pool is normally 16 feet above the top of the fuel assemblies. With the water level evaporating at the rate described above, the water level will drop by 2 feet per day.

Uncovery of the fuel assemblies will take eight days. (Beginning from the point where the water level reached boiling point, after active cooling ceased.)

(Working assumption which you may subject to some skepticism: That there is no form of leakage or other water loss pathway from the used fuel pool.)

It seems plausible that a fire hose or something can be used to add water to the pool at a rate equal to this loss rate of 700 ml per second. The Chernobyl-style helicopter drops seem like overkill, and they are doing an effective job of whipping up “Chernobyl again” fear, and images that the newspapers are having a field day with.