Validation study of ambient dose equivalent conversion coefficients for radiocaesium distributed in the ground: lessons from the Fukushima Daiichi Nuclear Power Station accident

Abstract

Ambient dose equivalent conversion coefficients (ADC_RCs) for converting a radiocaesium inventory to ambient dose equivalent rates (air dose rates) depend on the vertical distribution of radiocaesium in soil. To access the validity of ADC_RCs, the air dose rate at 1 m above ground and the vertical distribution of radiocaesium in the soil around the Fukushima Daiichi Nuclear Power Station (FDNPS) present between 2011 and 2019 were measured in the current study. ADC_RCs were calculated using air dose rates and three different parameters representing the vertical distribution of radiocaesium in soil: (1) relaxation mass depth (β), (2) effective relaxation mass depth (β_eff) and (3) relaxation mass depth recommended by the International Commission on Radiation Units and Measurements before the FDNPS accident (β_ICRU). When ADC_RCs based on β and β_eff were compared to those based on β and β_ICRU, a positive correlation was found. To confirm the applicability of the ADC_RCs based on the three types of β values, radiocaesium inventories were estimated using the air dose rates and ADC_RCs, and the obtained results were compared to the radiocaesium inventory calculated using soil sample measurements. Good agreement was observed between the radiocaesium inventories estimated using the ADC_RCs based on β and β_eff and measured by investigating soil samples. By contrast, the radiocaesium inventory estimated using the ADC_RCs based on β_ICRU was overestimated compared with that measured by investigating soil samples. These findings support the applicability of ADC_RCs based on β and β_eff in the Fukushima region. Furthermore, the β_ICRU result suggests that differences in soil characteristics between Japan and other countries should be considered for evaluating ADC_RCs.

Appendix

The radiocaesium inventory in the soil at each campaign was decay-corrected on the same day (15 March 2011). The coefficient of variation (CV) of the decay-corrected radiocaesium inventory in the soil (MI₀ (Bq m⁻²)) at the study sites was obtained using Eq. 12:

$$\mathrm{CV}=\sigma /\overline{{\mathrm{MI} }_{0},}$$

(12)

where σ is the standard deviation of MI₀ and $\overline{{\mathrm{MI} }_{0}}$ is the mean of MI₀. The CV was evaluated based on MI₀ obtained at the same site over several different campaigns. The CV was not evaluated based on MI₀ at the site where the campaign was performed once or twice.

To obtain β_eff, the air kerma rate and inventory of ¹³⁷Cs were obtained with Eqs. 13 and 14, respectively.

$${\int }_{0}^{\infty }{A}_{\mathrm{m,0},\mathrm{eff}}\mathrm{exp}\left(-\zeta /{\beta }_{\mathrm{eff}}\right){I}_{\upgamma}C\left(\zeta \right){d}\zeta ={\int }_{0}^{\infty }{{A}_{\mathrm{m},0}}^{\mathrm{^{\prime}}}\mathrm{cosh}\left({\zeta }_{0}/{\beta }^{\mathrm{^{\prime}}}\right)\mathrm{sech}\left\{-\left(\zeta -{\zeta }_{0}\right)/{\beta }^{\mathrm{^{\prime}}}\right\}{I}_{\upgamma }C\left(\zeta \right){d}\zeta ,$$

(13)

$${\beta }_{\mathrm{eff}}{A}_{\mathrm{m},0,\mathrm{eff}}={\beta }^{\mathrm{^{\prime}}}{{A}_{\mathrm{m},\zeta 0}}^{^{\prime}}\left[\left(\pi /2\right)-{\mathrm{tan}}^{-1}\left\{-\mathrm{sinh}\left({\zeta }_{0}/{\beta }^{\mathrm{^{\prime}}}\right)\right\}\right],$$

(14)

where I_γ is the gamma-ray emission rate per ¹³⁷Cs activity (photons Bq⁻¹), C(ζ) is the conversion coefficient of the source intensity of ¹³⁷Cs in soil (photons m⁻²) to the air kerma (Gy) (Saito and Jacob 1995) and A_m,ζ0′ is the activity concentration of ¹³⁷Cs at ζ₀ (Bq kg⁻¹). In the case of type A profile, ζ₀ is infinitely close to zero because the mass depth where the activity concentration of ¹³⁷Cs is the highest is located in top soil layer. The calculation method for β_eff is the same in either of case type A and B profile.