Appendix I DOCUMENTATION and EVALUATION of MODEL VALIDATION DATA USED in SCENARIO S
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XA9642930 Appendix I DOCUMENTATION AND EVALUATION OF MODEL VALIDATION DATA USED IN SCENARIO S A. Rantavaara, H. Arvela, M. Suomela ( SteftBkANK 1.1. INTRODUCTION The scenario on Chernobyl-derived environmental radiocaesium in southern Finland in the years 1986-2036 was used for model testing in the IAEA/CEC Co-ordinated Research Programme on the Validation of Environmental Model Predictions (VAMP). This report gives the scenario description and the observed values of the test quantities, and their derivation from radiocaesium measurement data. Also radiation doses of the test persons are estimated. The modelling task was to assess both the amount of deposited 137Cs and its concentrations in foodstuffs, animal feeds and the human body in the test region during the specified time periods. The test area comprised nine southernmost provinces of Finland, an area of 176 000 km2 with a population of 4.3 million (Fig. I.I). The test persons were 20 year old adults and a child, ten years old when the deposition occurred. Three time periods were considered, namely the first twelve months since the deposition at the end of April 1986, 4.6 years (until 31.12.1990), and 50 years. The test region is characterized by varying environmental and food production conditions. Domestic products from agriculture and also products from aquatic and terrestrial seminatural ecosys- tems contribute to the diet of people. The weather conditions of southern and middle boreal zones add variation and some complexity to the scenario. The year has four seasons, the growth period is short but intensive, and field crops yield one harvest per year. The northern boreal zone and also the southwestern archipelago were outside the test area. The pathways related either to reindeer or to coastal brackish water fishes were not included in the modelling tasks. Input information (Section 1.2) for the test scenario includes measured concentrations of 134Cs and 137Cs in ground level air, and deposition densities based both on rain water collection and ground contamination data. General information on the test region, population, and production of foodstuffs is also given. Most radioactive fallout received in Finland after the Chernobyl nuclear accident originated in the first major release and spread over the test region between April 27 and 29, 1986. Most radioac- tive material was deposited with heavy showers which caused an uneven areal distribution (Fig. 1.2). Deposition continued with varying intensity until May 12. At the end of April the growing season had not yet begun in Finland. Estimation of the mean deposition density was one of the modelling tasks. To avoid a biased starting point for foodchain assessments due to modellers' own deposition estimates, the mean deposition density of 137Cs in each subregion used in the scenario was given as input information. The model validation test was started as a blind test. All regional information in the scenario description was coded, and the first maps of subregions were schematic to hide the actual test area. The blind test idea was gradually cancelled in the course of the exercise. The geographical region S was revealed after deposition estimates were made. At the same time, sources of information in the open literature became available to the modellers. After the blind test phase, communication was encouraged between modellers and the compiler of the scenario description at the Finnish Centre for Radiation and Nuclear Safety (STUK) to avoid unnecessary misinterpretations of input information. Clarifications and additional information provided in the course of model calculations are included in the present scenario description. Test data (Section 1.3) include observed values for the endpoints of model calculations and their evaluation. The observed data include deposition, concentrations in animal feeds and in foodstuffs of agricultural and wild origin, human intake and whole body contents. For radiation dose comparisons (Section 1.4) the following pathways were assessed: ingestion, inhalation, and external radiation from the cloud and from ground contamination. The ingestion pathway was further divided into food types to compare their relative importance to the dose. The internal dose was estimated from measured whole-body contents and from intake of foodstuffs and inhaled air. For all test quantities and dose estimates, the arithmetic mean and its 95% confidence interval were calculated for the entire region S. 91 Choosing a large region in Finland for a test scenario offered some advantages. The origin of consumed food was generally known. Finland is the northernmost country in the world which is almost self-supporting for agricultural produce. Foodstuffs of wild origin are domestic, too. The ready access to comprehensive statistical information on the population and on production of foodstuffs was another advantage. Producing all radioactivity data at one institution guaranteed comparability of data and facilitated the compilation of the test scenario. STUK delivered the radiocaesium data and other input information to the International Atomic Energy Agency. The approach to environmental measurement programmes at STUK had greatly resembled the content of this kind of model validation tests. The planning of the scenario S started in the middle of the five-year test period. The information on environmental radioactivity in the past was thus necessarily based on existing data. The most challenging features of the test scenario, from a data deliverer's point of view, were the varying environmental pathways and conditions in a large test area, a comprehensive analysis of data and estimation of uncertainties for all test quantities. 1.2. DESCRIPTION OF TEST SCENARIO S 1.2.1. INTRODUCTION This scenario involves the assessment of 137Cs body content and of the total radiation dose due to 137Cs in the environment to people living in an area of varying environmental conditions. The scenario can be seen as being in two parts: (1) a model test in which predictions of l37Cs body content and concentrations in environmental materials can be compared with observed values in the test area; and (2) a model comparison in which predictions of the total dose from l37Cs in the test environment are compared and analysed. Both rural settlements and urban areas are included in the test region, and pathways contributing to dose may include those from both terrestrial and aquatic environments. 1.2.2. ASSESSMENT TASKS 1.2.2.1. General The following subsections contain a description of the calculational endpoints required in this test scenario. The quantities to be predicted are separated into two groups: The first group consists of quantities for which measurements exist and against which model predictions can be tested, the second group consists of quantities (e.g. radiation dose) which can only be predicted but not tested. The latter are included because they are the most common and useful endpoints in radiological assessments. For each quantity, a 95% confidence interval (2.5% and 97.5% lower and upper bound estimates, respectively) should be given to quantify the expected uncertainty in the result. It is anticipated that these values will be subjective confidence intervals, given the nature of the data provided for this scenario. For the quantities requested in Sections 1.2.2.2 and 1.2.2.3, modellers are required to estimate the arithmetic mean for the time-periods specified and for the entire region S, and a 95% confidence interval thereof. 92 1.2.2.2. Calculations for model testing The calculations in the model testing part should be performed for three average representatives of the population of S, i.e., an adult woman (age 20 in 1986), an adult man (age 20 in 1986), and a child (age 10 in 1986). The term 'test persons' refers to these three categories. /.2.2.2.1. Total deposition Estimate the average 137Cs deposition (wet and dry) over the entire region S (Bq m"2). Estimate the total 137Cs and 134Cs inventory of the test region due to the Chernobyl accident after the passage of the contaminated air (Bq). 1.2.2.2.2. n Cs concentrations in food products Estimate the mean contamination of the following food products produced in S: - Leafy vegetables (Bq kg1 f.w.) - Cereals (wheat and rye; Bq kg'1 f.w.) Milk (Bq I/1) - Beef (Bq kg1) Pork (Bq kg'1) - Game (small game and big game; Bq kg'1) - Mushrooms (wild, edible mushrooms, Boletus and Cantharellus types; Bq kg'1 f.w.) - Wild berries (wild, edible berries, Vaccinium type; Bq kg'1 f.w.) - Freshwater fish (average of predatory, non-predatory, and intermediate feeding type species; Bq kg1). Concentrations are requested for products prior to preparation for human consumption, averaged over the specified time-periods and the entire S region. For milk, beef, and pork, estimates of the mean 137Cs concentrations are requested for the months May to September 1986 and for the fourth quarter of 1986 through the fourth quarter of 1990. For leafy vegetables, cereals, game, berries, and mush- rooms, estimates of the mean 137Cs concentrations are requested for the main harvest, picking season, or hunting season for the years 1986 to 1990. For leafy vegetables, estimates are also requested for the months May to September 1986. For freshwater fish, estimates of the mean 137Cs concentrations are requested for the second half of 1986 and the annual catches of 1987 to 1990. 1.2.2.2.3. Human intake Estimate the mean l37Cs intake per day (Bq d'1) of the test persons (woman, man and child) for the month of June 1986, the 4th quarter 1986, and the 2nd and 4th quarters of 1987 through 1990, averaged over the S region. 1.2.2.2.4. l37Cs concentrations in animal feeds Estimate the mean 137Cs concentrations in pasture vegetation, barley, and oats (Bq kg"1 f.w.) for the harvests in 1986, 1987, 1988, 1989, and 1990, averaged over the S region.