Averting cumulative lifetime attributable risk (LAR) of cancer by decontamination of residential areas affected by a large-scale nuclear power plant fallout: time aspects of radiological benefits for newborns and adults.

The averted cumulative lifetime attributable risk (LAR), the residual dose and highest ground deposition of 137Cs complying with a reference dose level of 20 mSv yr-1 to an individual returning after one year to an area contaminated by unfiltered releases of fission products from a nuclear power plant (NPP) were evaluated by applying an existing exposure model designed to compute age- and gender-dependent time-integrated LAR. The model was applied to four types of nuclear fallout scenarios, partly based on data from the Chernobyl and Fukushima releases and from theoretical source terms from Swedish NPPs. For rapid decontamination measures that achieve a 50% relative reduction in external dose rate within 1 year, compliance with the reference level 20 mSv yr-1 can be attained for an initial 137Cs ground deposition of up to 2 MBq m-2 with relaxed food restrictions. This compliance can be attained at even higher ground deposition (up to 4.5 MBq m-2) if using the strict food restrictions employed in Japan after 2011. Considering longer than 1 year return times it was also found that the benefit of implementing decontamination decreases rapidly with time (2-3 years half-time), especially if the fallout has a high initial 134Cs to 137Cs activity ratio and if the ecological half-time of the external dose rate is short (<5 years). Depending on fallout scenario the averted cumulative LAR for newborn girls by decontamination that is achieved after 5 years is only between 6% and 11% of that obtained by evacuation alone during the same time, indicating a rather limited radiological benefit of decontamination if delayed more than a few years. We conclude that decision makers and emergency response planners need to consider that protracted decontamination measures may have limited radiological benefit compared with evacuation in terms of averted future cancer cases, albeit it may have other societal benefits.

Kerma conversion factors for modern glass buildings in radioactively contaminated areas.

To improve the estimation of external gamma irradiation from deposited radioactivity in urban environments a model of a modern office or residential building with glass facades was set up with eleven different building heights. Kerma conversion factors for the floors inside the building from contamination on different types of surfaces were determined by using the Monte Carlo code MCNP6 for the primary gamma energies 0.3 , 0.662 and 3.0 MeV and for three different environmental scenarios. The kerma conversion factors were expressed as formulas for each possible deposition area for contaminants. The importance of the determined factors was shown by comparing them to previously generally used factors for multistorey house blocks.

Introducing the concept of the isodose for optimisation of decontamination activities in a radioactive fallout scenario.

In the recovery phase after a radioactive release incident, it is important to be able to focus decontamination operations on the areas that contribute most to the radiation dose. Monte Carlo simulations were applied to determine the shielding effect of a building against radiation from various directions, also giving information on the dose contributions at various locations inside the building from specific areas outside. The concept of the isodose was developed to optimise decontamination activities, and was applied as isodose lines to define the smallest areas that lead to a certain dose reduction through decontamination of areas surrounding the building. The shape and position of the isodose lines depend on the building's geometry, wall thickness, and material, and on the observation point inside the building. Calculations have been made with a surface resolution of 1 m2 for four observation points in a modular building, assuming depositions of 137Cs and 60Co on the ground surface and on the roof, as well as 1 cm below the ground surface to represent ground penetration. For example, a ten times as large area would have to be decontaminated to increase the dose reduction from 10% to 30%, if it is assumed that all the contamination is located at a depth of 1 cm.