Basic conditions for exposure scenario

In this project a potential accident scenario was defined as a source term, the associated probability and a set of meteorological and geographic data. Radiation doses/effects have been determined by applying environmental dispersion models for the accidental releases to give activities of nuclides released into environmental media and then applying dose/effects functions. In the following, meteorological and geographic components of the scenario preparation are described.

A hypothetical site has been assumed for the exercise by the participating experts to demonstrate the process of assessment of radiological environmental impact from potential exposure. A map of the site is illustrated in Figure 6. There are cities, urban areas, sea, land and a river. The polar coordinate system was superimposed on the site map to facilitate firstly introduction of the site data for performing calculations and secondly to associate the results of modelling with the certain objects on the map. Based on these overlapped schematic maps, the site data were introduced into the modelling as land fraction, population, spatial intervals and wind directions. Information about population, size of areas and distances from the point of release is summarised in Table 1.

TABLE 1. SITE DATA DESCRIPTION

# Category Population Area,

Meteorological data comprising 8760 weather records from a meteorological station of an operating NPP had been provided for this project by CIEMAT. This set of data describing weather condition during one year with one-hour interval measurements of the wind direction, wind speed, atmospheric stability, and precipitation rate has been available to all participants.

In this exercise, use of the real measurement data from a Spanish meteorological tower involved some special processing of these data due to a few gaps in data records, very low wind velocity (less than 0.1 m/s) etc. In total less than 5% of the whole set of meteorological data needed special processing.

The full list of the parameters of the data set includes:

 Month, Day, Hour;

 Wind speed and direction at 80 m and 10 m altitude;

 Temperature at 80 m and 10 m altitude;

 Humidity (relative) and pressure (mmHg);

 Solar irradiation;

 Precipitation (mm);

 Pasquill-Gifford Category.

FIG.6. Map of the site

1 2 8

1 2 3 4 5 6 7 8 910

1

1213

1

15

16 3

4 56

Sector numbers No. Category PopulationDistance, kmSector number 1 City 250 0001514 2 Town 15 0008 12 3 Village 502.515 4 Urbanization 156.53, 4 5 Urbanization 203.58 6 Urbanization 100 7 6 7 Individual housing (not shown in this figure) 60 1 - 55 – 16 8 Megapolis 2 000 000 507

Computer tools and models used by participants in this project could incorporate limited sets of input parameters selected from the available meteorological array. For example, the WinMACCS code used dates, wind direction and velocity at a given height, stability category and precipitation. Processing of data was necessary for categorization of wind direction into sectors according to the polar grid coordinates, the data preparation in adequate units etc.

FIG.7. Wind rose at 10 m altitude

The mixing heights (see Table 2) required for the WinMACCS code were specified for each of the four seasons of the year and for 0:00-12:00 hour and 12:00-24:00 hour periods with the range from 500 m for 0:00-12:00 hour intervals during the autumn season till 1900 m for 12:00-24:00 hour intervals of the winter season.

TABLE 2. SEASONAL MIXING HEIGHTS, m

Time periods 0:00-12:00 12:00-24:00

Winter 700 1050

Spring 650 1890

Summer 600 1900

Autumn 500 1400

3.2 SOURCE TERM

The source term used in this exercise was generated using the data published in the State-of-the-Art Reactor Consequence Analysis (SOARCA) project [29]. This project has focused on providing a realistic evaluation of accident progression, source term, and offsite consequences for two NPPs: the Surry Nuclear Power Plant (two 800 MWe PWRs) and the Peach Bottom Nuclear Power Plant (two large BWRs). The SOARCA project had been the most

contemporary, comprehensive and sophisticated analysis for which detailed results were publicly available and could be accessed by the participants of ENV-PE exercise.

SOARCA Surry considered five selected accident scenarios. For the ENV-PE exercise, the project participants selected the source term calculated for a short-term station blackout in the SOARCA Surry study [18]. Atmospheric release starts at 25.5 hours and ends at 48 hours after the initiating event. An estimated frequency of this scenario is 1x10^-6 to 2x10^-6 a^-1.

TABLE 3. RELEASE FRACTIONS FOR PLUME SEGMENTS (ADAPTED FROM [18]) Plume atmosphere have been postulated. The accident sequences initiated within the reactor building have not been considered and the release characteristics have been assumed to be similar to those published in [18, 29]. The release was divided in 24 plume segments⁵ characterized by the release fractions calculated in [18] using the MELCOR code (see Table 3). Plume characteristics include release height, timing for every segment of release, heat contents, plume mass density and mass flow rate. Release properties defining the source term selected for the current exercise are presented in Table 4 (adapted from [18]).

5In the original SOARCA project there were 33 plume segments considered. In this exercise the number of plume segments was limited to 24 because the last nine segments provided a relatively low contribution to the total

TABLE 4. RELEASE PROPERTIES FOR EACH PLUME SEGMENT THAT IS PART OF

Note: ¹ – ENV-PE exercise participants decided to specify the total release height (including temperature and pressure effects at source) as 35m.

Release fractions for each class of radionuclides specify the fraction of the total core inventory released. Plume release time is the start time of every segment from the beginning of the accident and the duration of segments was introduced through a separate parameter (plume segment duration). These two parameters are not independent. Plume heat content is the rate of heat release and the plume release height is the height of release above ground level (constant parameter in this study). Plume mass density is the density of the plume segment in kg m^-3, and mass flow rate is the mass of plume release per second.

Table 5 shows the radionuclides inventory at the scram time of the Surry NPP.

TABLE 5. INVENTORY OF RELEVANT RADIONUCLIDES IN REACTOR CORE AT THE SCRAM TIME (REPRESENTATIVE ELEMENTS IN PARENTHESIS) [18]

No. Name of chemical group Isotope Activity at the time of shutdown, Bq 1

TABLE 5. INVENTORY OF RELEVANT RADIONUCLIDES IN REACTOR CORE AT THE SCRAM TIME (REPRESENTATIVE ELEMENTS IN PARENTHESIS) [18] (cont.)

No. Name of chemical group Isotope Activity at the time of shutdown, Bq 45

The integral release fractions distributed by chemical groups are provided in Table 6 [18].

TABLE 6. INTEGRAL RELEASE FRACTIONS BY CHEMICAL GROUPS [18]

Xe Cs Ba I Te Ru Mo Ce La

0.518 0.001 0.000 0.006 0.006 0.000 0.000 0.000 0.000

Aerosol particles distribution for the selected scenario is presented in Figure 8. For noble gases (Xe group) a uniform distribution of aerosol particles was used (10% in every bin). Table 7 presents the mass median diameter (MMD) and the deposition velocity associated with bins of aerosol groups distribution. The majority of the released contamination is associated with the bins 4, 5 and 6, which correspondent to the size of mass median aerosol diameter around 1, 2 and 3 µm and particle size of the bins are log-normally distributed, except normally distributed alkali metals (Cs) group.

FIG.8. Distribution of aerosol particles by chemical groups and by size

TABLE 7. MASS MEDIAN DIAMETER AND DEPOSITION VELOCITY FOR ALL AEROSOL GROUPS DISTRIBUTIONS [18, 29]

Bins 1 2 3 4 5 6 7 8 9 10

MMD class, μm 0.15 0.29 0.53 0.99 1.8 3.4 6.4 11.9 22.1 41.2 Deposition velocity,

Vdep mm/s 0.54 0.49 0.64 1.08 2.12 4.34 8.37 13.7 17.0 17.0

The size of the building, where the release occurred was assumed as 40·40·40 m. In the WinMACCS code the size of the building is not explicitly required, and it was introduced indirectly through the sigma coefficients. Equations (4) and (5) display the initial values of the crosswind and the vertical sigma as functions of width and height of the building.

𝜎 (𝑥 = 0) =

. = 0.23𝑊 (4)

𝜎 (𝑥 = 0) =

. = 0.47𝐻 (5)

where: 𝜎𝑦 - Gaussian Crosswind Dispersion Parameter; 𝜎𝑧 - Gaussian Vertical Dispersion Parameter; 𝑊𝑏- Width of the building from which release occurred (m); 𝐻𝑏 - Height of the building from which release occurred (m).

4 COMPARISON OF MODELS AND APPROACHES USED IN NATIONAL