Modelling of the radioactive daughter elements in soil to assess radiation impact due to contaminated irrigation water by parents of 231Pa, 226Ra, 238U, 237Np and 239Pu

Radioactive wastes may contain radionuclides of decay chains with parent and daughter elements as well. In some cases the transfer parameters of the daughter radio-elements differ significantly from the parent's ones and therefore the dose contributions in the different pathways might be important to assess separately. The international co-operation has compared 5 different site specific models by a scenario of using contaminated water for drinking and irrigation. Altogether 5 radionuclides from the investigated 10 ones contained radioactive daughter elements. In general, for long term studies in biosphere the steady-state formulations of processes are adequate to assess the contamination, except the infiltration into the deeper soil and sediment layers. According to the results of modelling and computer simulations, the following daughters are to be assessed separately from their parents with respect to the radiation impact: for parent 231Pa the daughter 227Ac, for parent 226Ra the daughters 222Rn, 210Pb and 210Po and for parent 237Np the daughter 233Pa. The dose contributions of the daughters from parents 238U and 239Pu are less significant during the considered 100-10000 years. In case of a contamination of irrigation water with 1 Bq m-3 of the parent radionuclide 231Pa, more than 90 % of the external dose from soil exposure comes from the daughter 227Ac (annually 15 nSv). By a similar contamination with 226Ra as a parent radionuclide, 60 % of the ingestion dose is due to food contamination with the daughters 210Pb and 210Po (approximately 150 nSv a-1).