A literature study on nuclide specific soil-to-plant transfer factors for uranium, thorium and radium

In the absence of measurements of naturally occurring radionuclides in foods grown on well-characterized soils with respect to their nuclide vector, soil-to-plant transfer factors are used to estimate the nuclide vectors of the food. This approach allows for the estimation of radiation exposure to humans due to the ingestion of these foods. For such calculations, it is assumed that all nuclides of the same element share the same soil-to-plant transfer factor, meaning they are treated in a nuclide-nonspecific manner. Typically, these transfer factors have been determined primarily for the most easily measurable nuclide of an element. However, exposure assessments might be inaccurate if the transfer of nuclides from soils to plants varies by nuclide of the same element.

In this study, a literature review was conducted to identify nuclide-specific transfer factors for uranium, thorium, and radium. The aim of the study is to explore whether there are differences in soil-to-plant transfer factors among the long-lived nuclides of the same element. The focus was on studies presenting measurements from the same soil and plant material for at least two nuclides of the same element. The following nuclide systems were examined: ²³⁸U – ²³⁴U; ²³²Th – ²³⁰Th – ²²⁸Th; and ²²⁶Ra – ²²⁸Ra. In particular, the latter nuclide pair is critical for assessing radiation exposure due to food ingestion, and differences in transfer factors between ²²⁶Ra and ²²⁸Ra could have significant implications. The analysis of the collected literature data suggests that slight differences can occur across all nuclide systems, and that more attention should be directed towards investigating the transfer factors for ²²⁶Ra – ²²⁸Ra and ²³²Th – ²²⁸Th as observed differences are larger.