[Paper Review] Fields of View for Environmental Radioactivity

Alex Malins, Masahiko Okumura+3 more|arXiv (Cornell University)|Sep 30, 2015

Radioactive contamination and transfer2 references20 citations

TL;DR

This study uses Monte Carlo radiation transport simulations (PHITS) to quantify the field of view—defined as the spatial extent of ground radionuclides contributing to ambient dose rate—for natural radionuclides (40K, U/Th series) and anthropogenic 134Cs and 137Cs fallout. It shows that radiocaesium has a significantly wider field of view than natural radionuclides, which explains discrepancies in KURAMA radiation monitor readings between cars and buses and enables rapid estimation of dose rate reduction potential from land remediation.

ABSTRACT

The gamma component of air radiation dose rates is a function of the amount and spread of radioactive nuclides in the environment. These radionuclides can be natural or anthropogenic in origin. The field of view describes the area of radionuclides on, or below, the ground that is responsible for determining the air dose rate, and hence correspondingly the external radiation exposure. This work describes Monte Carlo radiation transport calculations for the field of view under a variety of situations. Presented first are results for natural 40K and thorium and uranium series radionuclides distributed homogeneously within the ground. Results are then described for atmospheric radioactive caesium fallout, such as from the Fukushima Daiichi Nuclear Power Plant accident. Various stages of fallout evolution are considered through the depth distribution of 134Cs and 137Cs in soil. The fields of view for the natural radionuclides and radiocaesium are different. This can affect the responses of radiation monitors to these nuclides if the detector is partially shielded from the ground within its field of view. The field of view also sets the maximum reduction in air dose rates that can be achieved through local decontamination or remediation measures. This maximum efficiency can be determined quickly from the data presented here for the air dose rate versus the spatial extent of radioactive source on the ground.

Motivation & Objective

• To quantify the spatial extent (field of view) of ground radionuclides contributing to ambient gamma dose rates in environmental radioactivity.
• To explain observed differences in radiation monitor responses between vehicles (e.g., cars vs. buses) during KURAMA surveys in Fukushima.
• To provide a rapid method for estimating the maximum achievable reduction in air dose rates through localized land remediation.

Proposed method

• Monte Carlo radiation transport simulations using the PHITS code to model gamma ray transport in an infinite half-space geometry with soil (ρ = 1.6 g/cm³) and air (ρ = 1.2×10⁻³ g/cm³).
• Simulation of isotropic photon emission from uniformly distributed 40K, 232Th, 235U, and 238U series radionuclides and exponentially depth-distributed 134Cs and 137Cs in soil.
• Calculation of ambient dose equivalent rate (H*(10)) at various heights above ground and for varying source radii to determine the field of view as the area contributing 68% of the total dose rate.
• Use of field of view data to estimate the maximum possible reduction in dose rate achievable via decontamination, incorporating decontamination factors (DF) for realistic remediation scenarios.
• Validation of field of view effects on detector response by simulating car and bus geometries with engine shielding, comparing dose rates in different environmental conditions.

Experimental results

Research questions

• RQ1 How does the field of view of natural terrestrial radionuclides (40K, U/Th series) compare to that of anthropogenic 134Cs and 137Cs fallout in soil?

Key findings

• The field of view for 134Cs and 137Cs fallout is significantly wider than for natural radionuclides (excluding 235U series), due to differences in gamma ray energy and source depth distribution.
• The field of view for radiocaesium narrows as caesium migrates deeper into the soil, reducing its spatial influence on dose rate.
• The field of view for 134Cs and 137Cs increases with height above ground, while that for natural radionuclides remains relatively constant.
• In low-dose areas dominated by natural radionuclides, the bus engine shields a larger fraction of the field of view, leading to a higher dose rate ratio (car to bus) than in high-dose areas dominated by radiocaesium.
• For a 10 m radius area, 68% of the ambient dose equivalent rate from 134Cs and 137Cs fallout originates from within that region, meaning perfect decontamination of this area would reduce the dose rate by 68%.
• With a decontamination factor (DF) of 20, the residual radioactivity is 5%, leading to a maximum achievable dose rate reduction of 64.6% for radiocaesium contributions.