Model 2: Codes for Severe Accident Progression Analysis
Computer codes have been developed to analyze or predict the progression of severe accidents. Modular Accident Analysis Program (MAAP) was developed
Fig. 3.4 Temperature dependence of release rate constants from UO2 fuel
by U.S. industries while MELCOR was developed by the United States Nuclear Regulatory Commission (US NRC) .
These codes basically calculate the thermal response of the core, dealing with the entire progression from the initiating event to the radionuclide releases to the environment, which is called the “source term.” Therefore, the initial inventory and the release properties for each nuclide are required as input parameters. These values are usually calculated by a burn-up code, such as ORIGEN or CORSOR. The entire progression from the initial event includes damage in the RPV and PCV and consequent leakage of water and steam.
After the accident, another code named “Severe Accident analysis code with Mechanistic, Parallelized Simulations Oriented towards Nuclear fields (SAMPSON)” , developed by Nuclear Power Engineering Corporation (NUPEC), has been improved by Institute of Applied Energy in Japan. The merit of the SAMPSON code is the fact that there is no factor adjusted by the user.
Model 3: Atmospheric Transport Model
Behavior of the radioactive materials released from a nuclear facility differs depending on their chemical properties, weather conditions (e.g., wind direction, wind speed, rainfall, snowfall), and the geography around the plant. Noble gases such as Kr or Xe are transported and dispersed by wind. If upward wind is predominant, the gases will be transported to the stratosphere and delivered across the entire earth by the wind. Gases of volatile radioactive materials such as I2 are also transported by the wind. CsI or Cs oxides can be transported by the wind if these nuclides float in the air as dust particles or attach to aerosols. This is called the “plume” as schematically shown in Fig. 3.2.
If rain or snow falls, some particles will fall to the surface of the earth together with raindrops (wash-out or rain-out) and contaminate the land. Therefore, prediction of the transport of radionuclides, i.e., evolution of the plume, is crucial for protecting local residents from radiation. Note, in contrast, that relatively large particles such as fuel grains are rather difficult to be transported far by the wind, so they tend to fall out by gravity near the NPS.
The time-integrated concentration of the released nuclides in the atmosphere,
χ(x, y, z) [Bq/m3], can be formulated by the Gaussian model as:
where Γ is the release rate at source [Bq/s], U the mean wind speed in the x direction [m/s], h the physical height where the plume comes out . The diffusion parameters, σy and σz, represent the broadening in the transverse and vertical direction, respectively. Their values can be found in the data chart known as the Pasquill-Gifford diagram shown in Fig. 3.5, which categorizes air-stability into 6 classes, A-F, depending on local solar radiation and surface wind speed . One can see from this figure that the lateral spread of the plume is only 1/10–1/100 the
Fig. 3.5 Pasquill-Gifford dispersion diagrams: a horizontal dispersion, ground sources; b vertical dispersion, ground sources. In Japan, an extremely stable class G is added to classes A–F (see Appendix A of this chapter)
travel distance necessary to deliver local effects to the environment. Nevertheless, the atmospheric diffusions are larger than that deduced from molecular collisional diffusion, since the turbulent flows enhance the net diffusion.
Because Eq. (3.2) is only applicable to the simple condition, i.e., flat topography and temporally and spatially constant wind, it is not suitable for the real-time simulation of atmospheric dispersion of radionuclides during emergency. Thus, more sophisticated model is used for this purpose. The System for Prediction of Environmental Emergency Dose Information (SPEEDI)  predicts the atmospheric dispersion and deposition of released radionuclides in the local and regional areas by solving the transport and diffusion equation numerically in which threedimensional meteorological fields and topography are considered explicitly. A worldwide version of SPEEDI (WSPEEDI)  can predict in detail the process of the atmospheric dispersion and deposition of released radioactive materials over the world for overseas accident.
The behavior of radioactive materials released to the ocean is evaluated from transportation and dispersion along the ocean current, dispersion by the tidal stream and wind, precipitation to the bottom of the sea, and intake by fishes and their migration. The compartment model is used for evaluation of the contamination in the ocean. The amount of release directly to the ocean as contaminated water is not included in the assessment of the accident scale.
Model 4: Ambient Dose Rate from the Contaminated Ground
The total release of the radioactive material, that is the integral of the source term with respect to the period of release, can be roughly evaluated from the ground contamination caused by the fallout/rainout/washout after the radiation plume has passed through, based on the following equations.
′ where D and A represent the dose rate [Sv/h] and the radio activity of the surface area [Bq/m2], respectively.
Fig. 3.6 Schematic drawing of evaluation of dose rate based on the ground shine
CFgrd is the conversion factor from ground contamination to the ambient dose rate at 1 m above the ground, [(Sv/h)/(Bq/m2)] shown in Fig. 3.6, while SF is the shielding factor depending on the ground condition, location, or buildings. We determined that SF = 0.7 is a plausible value to be applied in the present situation (see Appendix C). τ is the half life of the radioactivity. tcom, tobs and ts are the times when the species ratio is determined, when the dose rate was measured, and when the radioactive species are released, respectively. Note that the subscript j is the label of the species and 131I (τ = 8.02 d), 134Cs (τ = 752.4 d), and 137Cs (τ = 11019.3 d) in the present case.