A team of European radiological protection specialists has developed a method to calculate benchmark values to help establish whether the local population should be required to take shelter or be evacuated or relocated following a nuclear accident. The study, published in the journal Health Physics, shows that these levels are more restrictive in wet environments than in dry ones.
When a nuclear emergency happens, urgent measures are implemented, such as ordering the population to take shelter or evacuating them, followed by long-term measures, such as relocating people to safe areas or allowing them to return home. The decision to choose one or another of these measures is made on the basis of 'intervention levels' or radiation doses (measured in millisieverts, mSv), which are different in various European countries.
Now, the working group 'Emergency Preparedness and Action Level' (EPAL), a team of radiological protection experts trying to harmonise European responses to nuclear emergency, has presented a methodology in the journal Health Physics for unifying these criteria and providing guidance in decision-making.
"This study shows the need for coherence between the varied protection measures in the urgent phase and the long-term phase. If, for example, the intervention level for relocating people is set at a very low level with respect to the level for taking shelter, you could end up with the absurd situation of people being relocated to other places who had not previously been advised to take shelter, and had consequently been exposed to the passage of the radioactive cloud," says José Manuel Martín Calvarro, head of the Emergency Planning Department of the Nuclear Safety Council (CSN) in Spain and a member of EPAL.
The team has developed a 'two-step' method to help coordinators following a nuclear accident. The first step calculates benchmark doses and quotients so that, in the second step, the authorities can ensure that they have adopted the most appropriate protection measure.
The two-step method
The first step involves calculating the effective benchmark doses for two 'source terms': an accident at a nuclear-type reactor and the rupturing of steam generator pipes in a more advanced reactor. Two international codes are used as calculation tools to help decision-making in a nuclear emergency (RODOS, developed by the FZK Centre in Karlsruhe, Germany, and the Romanian version, RO-CODE).
These tools are used to calculate the dose at various distances, under different pollution dispersal conditions (according to varying weather conditions and the dilution factor) in dry and rainy conditions, and for four integration periods.
The periods are: P1 which covers the two days following the accident (the time for which most of the EU countries estimate the dose for taking shelter), P2 from days three to 30, P3 from the second until the twelfth month, and P4 from the second to the fiftieth year. The dose quotients are then obtained for the four periods with respect to P2.
"Possibly the most significant part of this study is that it has determined that, for the same composition of isotopes from the source term, the quotients are just a function of the composition of the discharge, regardless of its quantity," says Martín Calvarro.
The study also shows that the quotients do not vary much with distance, but they do when dry or rainy conditions are simulated. When it rains during the discharge period, the dose deposited on the land is proportionally much greater than that entering the body through inhalation or from the passage of the cloud, while in dry settings the inhaled dose is the most significant.
The values obtained in the first step serve as a benchmark so that, in the second step, the authorities in each country can select the dose value of a protective measure and use this to confirm its coherence with the other protection measures. The benchmark value can be adopted either on the basis of national regulations or those proposed by international bodies.
The International Commission on Radiological Protection (ICRP) and the International Atomic Energy Agency (IAEA) recommend a maximum dose of 100 mSv per year. The maximum level established in most countries for taking shelter in the first two days following a nuclear accident is 10 mSv.
The study does not specify the benchmark doses for distributing iodine tablets to the population, a regular protection measure for reducing the risk of thyroid cancer following a nuclear accident. However, it does indicate that these values are not related to the other intervention levels.
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