After an accidental release of radioactivity to atmosphere, modelling assessments are needed to predict what the contamination levels are likely to be and what measures need to be taken to protect human health. These predictions will be imprecise due to lack of knowledge about the nature of the release and the weather, and also due to measurement inaccuracy. This thesis describes work to investigate this imprecision and to find better ways of including it in assessments and representing it in results. It starts by reviewing exposure pathways and the basic dose calculations in an emergency response assessment. The possible variability of key parameters in emergency dose calculations is considered, and ranges are developed for each. The imprecision typically associated with calculational endpoints is explored through a sensitivity study. This has been done using both a simple Gaussian atmospheric dispersion model and also real-time weather data in combination with a complex atmospheric dispersion model. The key parameters influencing assessment imprecision are identified. These are demonstrated to be factors relating to the release, arising from inevitable lack of knowledge in the early stages of an accident, and factors relating to meteorology and dispersion. An alternative improved approach to emergency response assessments is then outlined, which retains a simple and transparent assessment capability but which also indicates the imprecision associated with the results through incomplete knowledge. This tool uses input from real-time atmospheric dispersion and weather prediction tools. A prototype version of the tool has been created and this has been used to produce example results. The final stage of the thesis describes the use of the new tool to develop ways in which imprecise or uncertain information can be presented to decision makers. Alternative presentational techniques are demonstrated using example results. “Judgement which is needed to make important decisions on imperfect knowledge in a limited time” Definition of the art of politics, attributed to Clement Attlee, British Prime Minister 1945-51

Estimating and visualising imprecision in radiological emergency response assessments

In the event of an accidental release of radionuclides into the atmosphere, dispersion calculations would be used to model the consequences and to assist in determining appropriate countermeasures. Environmental contamination depends on the characteristics of the releases, the trajectory of the radioactive plumes and the deposition episodes. The weather conditions (wind, atmospheric stability, etc.) determine the transport of the radioactive plume in the atmosphere, as well as its vertical and horizontal extension. The plumes are depleted during transport by dilution in the atmosphere and deposition processes. Dry deposition occurs in the absence of precipitation, whereas wet deposition is the dominant process during rainfall or snowfall episodes. The behaviour of the radionuclides in the atmosphere depends on whether they are in gaseous or particulate form, their size and their reactivity. These elements also influence their behaviour in relation to the deposition processes, as well as their absorption and their harmfulness to the human body. The composition of the releases and the characteristics of the radionuclides vary during the release phase according to the facility events that caused them. Simulations are subject to significant uncertainties related to the source, to the meteorological conditions, those due to the atmospheric dispersion models and radiological assessment models. The identification and description of source term and meteorological uncertainties associated with the atmospheric dispersion model-specific uncertainties will enable a comprehensive assessment of the nature and impact of the atmospheric dispersion model (ADM) output uncertainties. It is intended that such uncertainty ranges and distributions will be used subsequently in the propagation of uncertainties through the chain of atmospheric dispersion and radiological assessment models for both historical (for example the accident at the Fukushima Daiichi Nuclear Power Plant) and hypothetical scenarios, to better understand the effect on model output, and thereby appraise the impact on decision making in the context of an emergency response. This document provides guidelines describing uncertainties related to the meteorological conditions, related to the source and related to the atmospheric dispersion models. The first chapter looks at meteorological ensembles as a source of information on meteorological uncertainty for dispersion models. The second chapter questions the use of meteorological measurements to reduce uncertainty. The third chapter provides guidelines describing uncertainties related to the source. The fourth chapter presents a literature review to evaluate the range and distribution of atmospheric dispersion model-specific input parameter uncertainties. The last chapter provides Guidelines for ranking uncertainties in atmospheric dispersion.

/pdf/guidelines-for-ranking-uncertainties-in-atmospheric-4bq3uupe75.pdf

Guidelines for ranking uncertainties in atmospheric dispersion

The objective of the CONFIDENCE project has been to identify, assess and communicate the uncertainties that arise in managing and recovering from a nuclear accident. To do that, it is important to be clear on what uncertainty means, how it arises and how we might analyse it. In fact, there are many forms of uncertainty, some of which we can assess with current tools, but some of which are more nebulous and difficult to deal with. Nonetheless, all need to be communicated to the emergency managers. Starting with a review of different categories of uncertainties, and using examples from the CONFIDENCE project, this paper discusses how the various uncertainties are manifested in nuclear emergency management. It concludes with a simple framework for categorising and analysing them. The overall intention is that such a framework could contribute to a broader discussion on identifying and handling uncertainties with nuclear emergency management actors

/pdf/different-types-of-uncertainty-in-nuclear-emergency-481ixmw0f8.pdf

Different types of uncertainty in nuclear emergency management

/pdf/guidelines-detailing-the-range-and-distribution-of-3344uy5bl6.pdf

Guidelines detailing the range and distribution of atmospheric dispersion model input parameter uncertainties

This paper presents an approach to estimating and displaying the imprecision associated with predictions from early emergency response calculations based on a limited number of off-site measurements and incomplete information about the nature of the release. The method enables key elements of imprecision to be included in the assessment in a simple and transparent manner, notably those arising from alternative weather evolutions, release durations, and factors influencing the amount of radioactivity deposited to ground. The presentation of assessment results incorporating lack of knowledge in a way that is easily understandable is important in the context of emergency decision making, and options for alternative styles of display are presented and discussed. A new system of early emergency response for radiological releases to atmosphere based on this approach is under development.

A method for displaying imprecision in early radiological emergency assessments.

In the very early stages of response to an accidental release of radioactivity leading to environmental contamination, it is likely that only limited measurements of radioactivity in the local environment will be available on which to base decisions concerning protection measures and radiation monitoring activities. Model predictions will be used to aid understanding of the radiological situation and to form a basis for emergency health protection decisions. This paper presents an analysis of the relative importance of contributors to the imprecision associated with emergency response calculations based on a few off-site measurements, using predictions from the UK Met Office's NAME III (Numerical Atmospheric dispersion Modelling Environment version 5.2) dispersion model. The results presented extend those from a previous study in which a simple Gaussian plume model was used and confirm the key parameters contributing to imprecision. The potential extent of the sheltering countermeasure resulting from a hypothetical release in real weather conditions occurring in 2007 and 2008 is also presented.

http://iopscience.iop.org/article/10.1088/0952-4746/30/1/002/pdf

Key factors in imprecision in radiological emergency response assessments using the NAME model

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Key factors in imprecision in radiological emergency response assessments using the NAME model