Session: 12-02 Risk Assessments and Management - Session 2

Paper Number: 133049

133049 - Development of the Ambient Dose Rate Evaluation Methodology Based on Plant Conditions for Rapid Consequence Assessment 

Abstract: 

In a nuclear emergency, it is important to assess accident consequences based on plant conditions. The U.S. Nuclear Regulatory Commission (NRC) has previously issued the Response Technical Manual (RTM-96) with pre-calculated bone marrow dose and thyroid dose of one mile from the plant based on plant conditions. When evaluating dose beyond one mile, conversion factors can be used to calculate dose for different distances, and this type of conversion is also possible based on plant power and release timing. The method for pre-calculating dose described in RTM-96 involves multiplying initial inventories by core release fraction (fraction of each nuclide released from the core to the containment, as defined in NUREG-1465), reduction factor (e.g., fraction at which aerosols are removed within the containment), and the escape fraction (e.g., the leak rate from the containment). In this way, the activity of nuclides released to the environment is calculated. Then, atmospheric dispersion is calculated under the average meteorological condition using the Gaussian Plume Model. Finally, dose is calculated based using dose conversion factors in FGR11 and FGR12 without any protective action. Thyroid dose considers only the inhalation of the plume, while the bone marrow dose considers cloudshine, inhalation of the plume, and groundshine over a day. RTM-96 evaluates only cumulative dose (rem) and does not evaluate the ambient dose rate (μSv/h).

On the other hand, protective action is determined based on ambient dose rates with Operational Intervention Level (OIL) in some countries. For example, if the ambient dose rate is exceeded the value of OIL1, evacuation is required. Therefore, predicting ambient dose rate based on the plant condition contributes to planning, such as determining the extent of monitoring required. This study introduces a methodology to evaluate ambient dose rate, with reference to the approach of RTM-96.

The RTM-96 approach was used as a reference in the newly developed method, multiplying various coefficients with initial inventories to calculate the released activity. However, core release fractions have been updated to reflect the latest knowledge, differing from RTM-96, and reduction factors were set to consider the Filtered Containment Venting System (FCVS). The gaussian plume model, which is used in RTM-96, was also introduced for atmospheric dispersion calculation in this method, considering low wind speed correction. The groundshine per hour immediately after deposition (μSv/h) was computed for dose calculation. Dose conversion factors in FGR12 and a correction factor of 0.7 from UNSCEAR were applied to adjust from protective to practical quantities. Additionally, the following two models described in the MACCS theory manual were considered:

• Case A (progeny model): The environmental release fraction of daughter nuclides was considered independent of the parent nuclides, and it was assumed that each daughter nuclide is released independently.

• Case B (parent model): The environmental release fraction of daughter nuclides was assumed to be equal to the release fraction of the parent nuclides.

In the result of both Case A and B, the contribution of I-132, a daughter nuclide of Te-132, which had not shown a significant contribution to thyroid dose, was significant in the case of calculating the ambient dose rate. In Case A, the ambient dose rate was twice as large as that in Case B in some accident scenarios due to more significant release activity of I-132 than that of Te-132. Thus, for calculating the ambient dose rate in this method, it is essential to consider not only nuclides, such as I-131 and Cs-137, but also, I-132, and the calculation model of daughter nuclides significantly affects the results.

Additionally, an example was shown where the method was compared to the hardened containment venting at the Fukushima Daiichi Nuclear Power Plant. The results were consistent in order of magnitude with the value of monitoring post.

In summary, a methodology to calculate the ambient dose rate based on plant conditions was proposed. This methodology can contribute to the prediction of area that may exceed OIL1 and planning for monitoring during accidents.

Presenting Author: Kodai Wadayama Nuclear Regulation Authority

Presenting Author Biography: Nuclear Regulation Authority Japan | NRA · Division of research for severe accident
Master’s degree from Waseda University

Authors:

Kodai Wadayama Nuclear Regulation Authority
Retsu Kojo Nuclear Regulation Authority