Questions & Answers

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Posted: 07/06/2010

Topic:
Modelling exposure to Acacia trees (EMRAS II Little Forest Burial Ground Scenario)

From:
Marko Černe
Jožef Stefan Institute
Slovenia

Question:
Acacia has it's roots 0-2m deep in the waste (part in Zone 1 and part in Zone 2) and has also aboveground part. How to use occupancy factors in this case? The major part of the tree is above the soils, so in that case we will use on soil, but the dose from ionising radiation here is a consequence of underground direct contact and aboveground part, but according to permitted occupancies for tree reference organisms (ERICA help) we could use occupancy factor just on soil? Does this approach include also the roots? How to deal with such cases?

Answer:
This question is in regard to one of the IAEA EMRAS II programme Biota Modelling Group (WG4) scenarios - Little Forest Burial Ground, Australia. The Zone 1 referred to in the question is contamination at >1 m depth into which it is assumed the Acacia roots.

 An interesting question as you are correct in that the ERICA Tool considers plants to be on the soil surface and in the case of trees the assumed geometry is the trunk (specifically the ICRP Reference Pine Tree trunk geometry as define in ICRP Publication 108). Therefore, the default tree reference organism within the ERICA Tool may not model this scenario very well especially with regard to external dose rates. It would be a fair criticism of the currently available models to say that perhaps the dosimetry for plants requires some further thought. However, scenarios such as this are proposed to make you think and identify problems and (hopefully) solutions. It's not the purpose of the exercise to tell participants what to do - but there are (probably) ways you could try to get around the limitations of the ERICA Tool - although you may then need to consider how to interpret the results. RESRAD-BIOTA has a default geometry which is stated as appropriate for plant roots (default geometry 1 with dimensions 0.2x0.2x0.2 cm and allows any organism type to me modelled as in soil); the Environment Agency R&D128 spreadsheet model also probably has a default geometry more analogous to plant roots. Note that the default CR values in both these models can be highly conservative - read the documentation before using.

Answered by Nick Beresford 08/06/2010

Posted: 25/05/2010

Topic:
Effect of occupancy factors on estimated wholebody activity concentrations

From:
Marko Černe
Jožef Stefan Institute
Slovenia

Question:
How does the geometry or occupancy factor within the ERICA Tool influence the CR value?

Answer:
There is no transformation of CRs according to geometry or occupancy factors - these are used by the ERICA Tool to determine dose rates only. Occupancy factors influence external dose estimates only. However, perhaps the guidance within the ERICA Help is to some extent misleading as it suggests the user can use the occupancy factors to model organims which do not spend 100 % of their time in a given ecosystem. However, if for example an occupancy factor of 0.4 is used the external dose rate would be reduced to 40 % of that if the organism was assumed to spend 100 % of its time in the ecosystem under consideration, whereas, the internal dose estimation (and wholebody activity concentration) would be the same (as if the animal spend 100 % of time in the ecosystem).Note that the Area Factor (AF) parameter in RESRAD-BIOTA is used in the estimation of internal dose rate (although the reported tissue activity concentration appears to stay the same regardless of the value of AF).

Answered by Nick Beresford 07/06/2010