A comparison of a hand calculation tool and a Monte Carlo method for determining skyshine doses from the on-site storage of radioactive waste

A comparison of a hand calculation tool and a Monte Carlo method for determining skyshine doses from the on-site storage of radioactive waste

T A Jones1, R Meadows1, S A Trerise1, F O’Brien1, P A Bryant1,2, V Haemmerli1,3

1University of Bath, Claverton Down Rd, Bath, North East Somerset BA2 7AY, UK
2Arup, 13 Fitzroy Street, London W1T 4BQ, United Kingdom
3Arcadis Consulting (UK) Ltd, 1st Floor, 2 Glass Wharf, Bristol BS2 0FR, UK
Corresponding Author: Pete.Bryant@arup.com

Abstract
When developing radioactive waste stores, due consideration has to be given to the hazard
associated with the storage of the radioactive material. The walls of the stores are normally
designed to provide a sufficient level of shielding so as to reduce the external dose rate to a
sufficiently low level. However, the roofs of such waste stores are typically made relatively
thin. Over the last few years concerns have been raised that gamma radiation escaping from
the roofs of these buildings may reflect off the atmosphere, a phenomenon commonly known
as skyshine. This may result in a radiation exposure pathway and increased dose to member
of the public. A simple ‘hand calculation’ model, based on NCRP (National Council on Radiation Protection and Measurements) Report No 151, was created to calculate this skyshine dose, alongside a Monte Carlo N-Particle (MCNP) program. The models were run for two example inventories over a variety of distances, both initially and 15 years after emplacement. It was found that the simple calculation consistently underestimated skyshine dose by approximately 2 orders of magnitude when compared to the MCNP simulation. This difference was attributed to inherent differences in the way the two models treated scattering effects, which the hand calculation model made no attempt to consider. It was therefore concluded that the hand calculation may not provide a suitably conservative estimate of skyshine dose from radioactive waste stores and care should be taken when using this method for assessing the suitability of the design of the store.


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