AN INFORMED USER PERSPECTIVE ON MODEL APPLICATION

In this intercomparison exercise, the assessment scenario interpretation was the same for all three models (the same assessor applied all three models) and there was consistency in the approach adopted by the assessor for making decisions regarding model parameterisation. Therefore, differences in the model predictions were largely due to differences in the default parameters used within the models (as discussed in section 5.6.1.) and the assessors‟ interpretation of the models and their accompanying guidance documentation.

From the perspective of an informed user, there were some notable differences in the ease with which the models could be understood and used. These include considerations of both the operation of the model and the model outputs.

The ERICA Tool was the most straightforward model to use because the software has a well-designed Graphical User Interface (GUI) that uses simple screens to guide the assessor

through each stage of the assessment and „wizards‟ to enable more complex steps (such as creating a new organism) to be performed. The „help‟ function within the ERICA Tool is comprehensive and, for the purposes of conducting the assessment presented here, largely removed the need to refer to the guidance documentation (Beresford et al., 2007d). This is likely to be a reflection of the extent to which informed users were involved in the

development of the ERICA Tool.

RESRAD-BIOTA has a purpose-built GUI and integral „help‟ function but the process of setting up and running the model was less intuitive than in the ERICA Tool and the „help‟ function was relatively limited. As a result it was necessary to rely heavily on the

supporting documentation (USDoE, 2004) to guide the assessment process. However, this documentation was written for an earlier version of RESRAD-BIOTA so there were some inconsistencies between the instructions in the documentation and the operation of the software. A potential issue in the operation of RESRAD-BIOTA is the lack of clarity over whether units are fresh weight or dry weight but, for the site considered in this assessment, the soil moisture content was 14% so the users‟ decision to treat the units as fresh weight or dry weight would have made a relatively small contribution to the uncertainty in model predictions. Although there were areas of ambiguity in the application of RESRAD- BIOTA to the assessment of the Drigg dunes, it should be noted that this is the only one of

the three models for which training courses are available for users26. It is likely that this

additional level of support for users would clarify the areas of ambiguity highlighted here.

R&D128/SP1a uses the familiar user interface of Microsoft® Office Excel but there is little information within the workbook to guide the assessment process and it was necessary to rely heavily on the supporting documentation (Copplestone et al., 2001a; Copplestone et al., 2003). The input sheet is relatively easy to follow and the units are clearly defined.

However, the lack of equations for calculating the surface area and volume of ellipsoids is an important omission within the supporting documentation since the method for assigning DPUCs to user-defined organisms is dependent on the assessor calculating the surface area:volume ratio.

All three models produce assessment outputs that can be saved as separate files for future use. The R&D128/SP1a output is a Microsoft® Office Excel workbook for the assessment whereas the ERICA Tool and RESRAD-BIOTA output Hyper Text Markup Language

26 _{In 2009, CEH (in association with the Institut de Radioprotection et du Sûreté Nucléaire (IRSN), the}

Environment Agency and Westlakes Scientific Consulting Ltd) was awarded a Natural Environment Research Council (NERC) knowledge exchange grant to develop and deliver a radiological environmental assessment training programme. This will include training in the use of the ERICA Tool.

(HTML) files. Although the HTML files are easier to print out and keep as a hardcopy record of the assessment, the manipulation of data into a form suitable for further data analysis is more time-consuming than working with the data outputs from R&D128/Sp1a. As with the input data, an important omission from the RESRAD-BIOTA output files is the clarification of whether the units quoted are for dry weight or fresh weight.

The model outputs can provide activity concentration, internal dose rate and external dose rate results for each organism under assessment but only the ERICA Tool and

R&D128/SP1a do this by default. In RESRAD-BIOTA it is necessary to run the model twice, once with the ingestion pathway activated within the model (resulting in reported dose rates for each organism which are the total dose rate, i.e. external and internal dose rates combined) and once with the ingestion pathway deactivated (to obtain the external dose rates for each organism). The assessor must then calculate the internal dose rates for each organism by subtracting the external dose rates from the total dose rates. Whilst this is not a complicated procedure, it does detract from the usability of the model. However, it may be questionable as to if the intended users (USDoE site operators) would need to consider contributions to total dose rate.