THE PATH FORWARD

What is our path forward? Although both IAEA-TECDOC-1344 and the DOE/NRC Working Group specify activity levels that define high risk radioactive sources, neither approach is perfect. Different choices are clearly possible on such matters as the measure of disruption for RDDs involving a and b emitting radionuclides of concern. Moreover, there is significant uncertainty in the tool we have used to model dispersal. Nonetheless, we plan to share the DOE/NRC Working Group methodology with our international partners in order to reach consensus as rapidly as possible on an appropriate categorization scheme.

I believe that with a concentrated effort we can reach consensus on the definition of high risk radioactive isotopes in the near term. Above all, I believe that it is essential that the categorization of high risk sources be simple and implementable within national regulatory systems. Categorization is the cornerstone on which the entire international system of control of high risk sources will be based. We need to get it right.

DISCUSSION

V. KOSENKO (Ukraine): When defining the activity levels for aemitters, did you take chemical toxicity into account?

C. PAPERIELLO (USA): No, chemical toxicity was not taken into account in our modelling. However, in the USA, protective actions for uranium hexafluoride are based on chemical toxicity.

P.T. BURDUKOV (Russian Federation): Has the DOE/NRC Working Group you referred to considered the risk of higher radiation doses in Iraq due to the combustion of large quantities of oil if the USA takes action against that country?

C. PAPERIELLO (USA): In our modelling, we used a variety of materials to convert the various isotopes into different forms. We considered dispersal based on the physical and chemical form of the radioisotope and the type of explosive available. The numbers that I showed you represent some fairly conservative bounding values for the various things that you could do with the sources. As you are well aware, typically cobalt and iridium are in the form of solid metal. In some cases it is completely solid and in other cases there are either pellets, in the case of some cobalt sources, or there is actually flake material in the case of iridium. I am aware that in some sources there are iridium wires. So the codes allowed us to vary the chemical and physical form of the material because it would then cause a different percentage of the material to become airborne. In my presentation I said that the material must be a particulate that is small enough, firstly, to become airborne and secondly, to be respirable. Therefore we could take into account the respirable form of the material, and obviously caesium tends to be used in a salt, in which case it is very dispersible and soluble, and in some cases, particularly in newer or in small sources, it is absorbed onto a zeolite and, at least in the USA, it is fired and, in my experience, it will not leach out of the zeolite and the zeolite is not respirable, but it can spread and it will be an external risk.

R.V. ARUTYUNYAN (Russian Federation): Regarding the categorization of radioactive sources, if we use dose criteria, the proposed limits greatly depend on our understanding of possible damage, perhaps 100 mSv as a limit. This would be the point at which the radiation implications are not factual but purely theoretical in terms of a non-threshold approach. But in terms of accidents these zones are not determined so much by dose, but the oblique damage that has been mentioned here. So you have criteria that go from 1 mSv, which in some countries is the dose limit from which you have to take steps — under normal conditions, of course. But the reaction of the population and society is such that measures have to be taken on territories where one is already talking about millisieverts, and on this depends the scale

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of damage and the activity which one takes into account as causing serious problems.

Another point is that, when we categorize the source, we have to think in terms of having one source lower than the limit, and it is better to talk in terms of a hundred small sources than one very large one.

C. PAPERIELLO (USA): I agree. When you model or when you devise a categorization scheme, you are going to have to base it on something other than an arbitrary number. It is going to be based on a decision to ignore certain things — in other words, we will look at direct dose and ignore contaminated land or we will look at contaminated land too, or we ignore the direct dose and we only look at contamination — and then you need a criterion. The criterion we chose was one that was already in use in the USA for nuclear power plant accidents with non-malevolent causes. It is in the range of doses that under the ICRP 82 criteria/criterion would suggest an intervention that falls within that range. Obviously, you can set the contamination levels so ridiculously low that just dumping fertilizer on the ground would constitute a radiological dispersal device. I do not think the number we chose is too far out of line with what we understand as the international intervention levels, which are somewhere between 10 and 100 mSv/a. After all, you are talking about a lifetime of exposure on that land.

T.C. KOTZÉ (South Africa): What are the criteria on the basis of which a decision whether or not to evacuate a city, or part of a city, would be taken following the explosion of a dirty bomb — the increase in additional deaths from cancer?

C. PAPERIELLO (USA): In the USA there are no criteria specific to dirty bombs. We would probably use current Protective Action Guides (PAGs) developed for nuclear reactor accidents. The intermediate PAG for a one year exposure is 20 mSv. The long term PAG is 5 mSv/a.

H.D.K. CODÉE (Netherlands): I conclude from your presentation that only very large radioactive sources could do serious radiological harm. Most such sources are under control. Can you give an estimate of the number of sources that pose a real danger?

C. PAPERIELLO (USA): Very few of the radioactive sources in use are suitable for RDDs — in the USA, one in 1000–10 000, and that does not include the millions of exempt sources in devices like smoke detectors and the millions of short lived medical diagnostic sources used every year. Moreover, many radioisotopes (for example ¹⁴C and ³⁵S) are never used as sealed sources.

K. GRYSHCHENKO (Ukraine): In my view, the public reaction to a dirty bomb will depend less on how powerful the radioactive source was than on how the incident is treated in the media.

C. PAPERIELLO (USA): I agree. In the USA, the NRC and other federal agencies are taking this matter very seriously and working out plans for communicating with the public before and after a dirty bomb incident. I think the public must be given clear, correct information; any attempt to mislead people will only create mistrust.

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SOURCES: EXPERIENCES FROM THE