Searching for clues - Interview with the GRS radiation protection supervisor on the radiological consequences of Fukushima

19.02.2021

Physicist Dr Thorsten Stahl heads the Radiation and Environmental Protection Department of GRS, is the company's radiation protection supervisor and was part of the team with which GRS provided technical support to the Federal Government during the accident at Fukushima. In this interview, he explains what makes determining the long-term health effects of the accident so complex and how he assesses a possible discharge of contaminated water from a radiological point of view.

Mr Stahl, when we talk about the consequences of the accident ten years after it happened, we also have to ask whether or how many of those affected will still suffer health-related problems in the years to come. The fact that international organisations such as the WHO and UNSCEAR came to the conclusion several years ago that there was no or only a very small additional cancer risk despite the accident surprised many people and was also discussed controversially in some cases. We would therefore like to have this explained in more detail. To start with, can you explain to us when and why radiation can cause cancer?

The "why" can be easily explained, in a somewhat simplified way. The ionising radiation emitted by radioactive substances can cause damage in our cells, especially to the genetic material. Metaphorically speaking, a beta particle, such as is emitted during the decay of caesium-137, hits a DNA molecule and changes it. Such DNA damage occurs in our bodies all the time and can also be triggered, for example, by the UV component in sunlight. Usually, the DNA damage is repaired; it can also happen that the cell dies as a result. Sometimes, however, this is not the case and then cancer can develop due to a malignant mutation. The higher the dose, i.e. the amount of ionising radiation, the higher the probability that such a case will occur.

How high does such a radiation dose have to be for a person to have a high probability of developing cancer?

This brings us to the "when", which is a little more complicated. It is known for sure that ionising radiation can cause cancer. Nevertheless, in the case of an individual disease, it is usually impossible to say for sure whether it was caused by radiation, because there can also be other reasons for such a malignant mutation and the time until the cancer appears can be very long. You can't tell from the cancer what caused it, so to speak. What can be said with some certainty is that from a larger group of people who have all received a certain dose, on statistical average a certain number will later fall ill. For comparatively high doses of radiation, this has been observed, among other things, through research into the health effects of the atomic bombs dropped at Hiroshima and Nagasaki. With lower doses, it is at some point very difficult or even impossible to assess. The lower limit for a dose received in a very short time is around 100 millisieverts. That is about 25 times the radiation exposure that we receive here in Germany on average per year. With this dose, it could be statistically proven that our general cancer risk, which is about 40 % in Germany, increases by about one percentage point. With lower doses, the probabilities are too low to be able to separate them statistically from the general cancer risk - but of course illnesses can occur even then. However, determining cancer rates in a case like Fukushima on this basis is anything but simple.

Why is that so?

In the case of Fukushima, various aspects come together. It starts with the fact that, as I said, you can only establish a statistical connection between a dose and the risk of cancer later on. So first I have to know exactly how many people received which radiation doses during the accident. Since the people who lived in the vicinity of the plant, unlike the workers at the plant, did not wear dosimeters, these doses must be estimated. For example, data from measurements of environmental radioactivity on site or weather data are included. This was also done in large studies for Fukushima, and these dose estimates are then the basis for being able to say something about the cancer risk of the population.

But surely this harbours the risk that the calculations are too optimistic and that the consequences are then quantified as being lower than they actually are? How can one be sure that the figures published by international organisations are realistic?

It has to be said that many experts from several countries are involved in such studies, each of them contributing different expertise to the discussion. From my point of view, it is also important that in such studies it is explained in detail how exactly certain values are arrived at - for example, the dose values I just mentioned. What measurement or weather data was used as a basis? What calculation models were used? Were uncertainties taken into account in a sufficiently conservative way? If all this is transparent, the experts can take a critical look at it. As far as I can see, this has been the case with the major studies, and I am not aware of any fundamental criticism from experts.

You have mentioned various aspects that make it difficult to assess the cancer risk at Fukushima. What are these?

A very important aspect is basically a rather positive finding, namely that the absolute majority of people in the affected areas received a rather low radiation dose. Only for the two most severely affected locations in the Fukushima prefecture was the average dose between 10 and 50 millisieverts, according to a WHO study; for all other affected areas it was in an order of magnitude between 1 and 10 millisieverts. In these dose ranges, the additional cancer risk caused by the radiation can be estimated mathematically but can no longer be separated from the general cancer rate by statistical means.

Does that mean that people could still develop cancer in the future as a result of the accident, without this connection being proven?

Exactly. If, as expected from the WHO and UNSCEAR studies, there is only a very small increase in cancer cases, it will not be possible by statistical means to filter out the cases caused by the accident from the usual annual fluctuation in the general cancer rate. This is further complicated by the fact that certain cancers can also be caused by other environmental influences and lifestyle. One must also take into account that both the earthquake and the tsunami as well as the accident have already led to very serious health consequences, e.g. to an increase in addiction and depression. Whether or to what extent these consequences in turn also cause a subsequent increase in cancer is probably difficult to determine. You would actually need a kind of control group of people who, apart from these additional stresses, live under largely identical conditions to be able to compare this.

Apart from the long-term consequences, many people are currently concerned that TEPCO and the Japanese government may want to discharge more than a million tonnes of water contaminated with radioactive tritium into the Pacific Ocean. What does a radiation protectionist say to that idea?

To put it very briefly: As long as we only consider the tritium here, and under the proviso that we do not yet know much about how exactly it is to be discharged, I do not expect any serious consequences for the region from a radiological point of view. But you are not quite happy with that, are you?

No, you would have to explain that in more detail. The fact that it is supposed to be harmless to release radioactive substances into the environment must seem like a contradiction to lay people.

Yes, I can understand that. To be able to grasp this, you have to know a bit more about tritium. Tritium is also a radionuclide, i.e. a radioactive substance, but for various reasons it cannot be compared with the other radionuclides that play a major role at Fukushima - for example, with the radioactive caesium isotopes or strontium. From a radiation protection point of view, these are much more problematic than tritium.

Why is that?

A very important difference to most other radionuclides is that tritium emits much lower-energy radiation in comparison. The beta radiation of tritium, for example, cannot penetrate the outer layers of the skin. The situation is different if it enters the body in sufficiently large quantities. However, the fact that a discharge of tritium at the planned rate would not entail any relevant radiological risks is also due to the very short biological half-life and the chemical properties.

Why does that play a role?

The tritium is incorporated into water molecules. Incidentally, this is also the reason why it cannot be easily filtered out of the contaminated water on an industrial scale like the other radionuclides - it forms part of the water. The consequence of this is, on the one hand, that it would very quickly become highly diluted in seawater and that there would also be no deposition on the seabed. This is different with caesium, for example, which is still found in relatively large quantities in the seabed near the plant. On the other hand, tritium, being part of water, can be excreted again relatively easily; it has a biological half-life of about 12 days. So even if one were to consume small amounts of water containing tritium with food despite the extreme dilution, half of it would be excreted again after ten days. How strong the dilution effect is in the sea could be seen in the first weeks after the accident. Enormous amounts of radioactive substances were released into the sea. Directly at the plant, this led to very high nuclide concentrations, but a few kilometres away they were already below the limits for drinking water.

So to sum things up: From your point of view, there is nothing that speaks against the discharge?

I can't offer such a general assessment because many aspects play a role. For example, I can understand the local fishermen's concern that they would no longer be able to sell their products. As far as radiation protection alone is concerned, this does not seem critical to me - but with a big "but": Today, a large part of the stored water still contains other radionuclides such as caesium and strontium. TEPCO and the Japanese government have announced that they will not discharge the water until these other substances are only present in concentrations below certain limits. This must then be ensured by appropriate checks.

Thank you very much for the interview!