ReSCA: decision support tool for remediation planning after the Chernobyl accident

Abstract

Radioactive contamination of the environment following the Chernobyl accident still provide a substantial impact on the population of affected territories in Belarus, Russia, and Ukraine. Reduction of population exposure can be achieved by performing remediation activities in these areas. Resulting from the IAEA Technical Co-operation Projects with these countries, the program ReSCA (Remediation Strategies after the Chernobyl Accident) has been developed to provide assistance to decision makers and to facilitate a selection of an optimized remediation strategy in rural settlements. The paper provides in-depth description of the program, its algorithm, and structure.

Appendix

Derivation of algorithm and parameters for partial decontamination of settlement

During the development of ReSCA, the need had emerged to improve the algorithm that describes settlement decontamination (RS remedial action). This remedial action can be very difficult to consider as a part of a remediation strategy when applied to the whole settlement. Settlements vary in size of population, and so do the costs of decontamination work. On the other hand, decontamination practices (IAEA 2001; Golikov et al. 2002; Golikov 2003; Jacob 2003, 2005) showed that decontamination can also be applied only to a part of the settlement depending on its contamination and availability of funds.

The algorithm described here is based on earlier developments (Jacob et al. 2001, Jacob 2005), and it is designed to allow consideration of a step-wise decontamination of a settlement in ReSCA. As such, the algorithm allows evaluation of external dose reduction, averted doses, and costs required when only an arbitrary fraction of a settlement is decontaminated.

The main assumptions of the algorithm are (a) the settlement population is uniformly distributed over the settlement area, (b) the whole territory of the settlement can be split in two parts where part A is the higher-contaminated part (fraction $\theta$ of the settlement area), while part B is the lower-contaminated part (fraction $\left( {1 - \theta } \right)$ of the settlement area, (c) inhabitants of both parts are assumed to permanently residing in their respective parts, (d) average annual doses due to external exposure are related—for both parts—through the ratio X _S of soil contamination levels:

$$ {\frac{{D^{\text{B}} }}{{D^{\text{A}} }}} = {\frac{{q_{\text{s}}^{\text{B}} }}{{q_{\text{s}}^{\text{A}} }}} = X_{\text{S}} . $$

(29)

The population of a settlement is exposed to external radiation depending on location, occupational activities, and life style. The location factor $\Uplambda_{l}$ expresses the dose at location l in terms of that over undisturbed soil, $D$. According to age, social, and professional activities, the population can be split in different groups, e.g. pre-school children or outdoor workers. These groups are characterized by their relative sizes $p_{g}$. People from different groups can spend their time at different locations; this is expressed by matrix $T_{{g,{\text{l}}}}$, which characterizes the time spent by group g at location l. Following these definitions, the average dose for population group g is

$$ D_{g} = {\text{DC}}_{E} q_{S} \bar{\Uplambda }_{g} , $$

(30)

where ${\text{DC}}_{\text{E}}$ is the dose conversion coefficient (mSv a⁻¹ kBq⁻¹ m²), $q_{\text{S}}$ is the average ¹³⁷Cs soil contamination in the settlement, and $\,\bar{\Uplambda }_{g} = \sum\nolimits_{l} {T_{g,l} \,\Uplambda_{l} }$ is the average location factor for population group g.

If decontamination starts from the higher-contaminated part A of the settlement, then the reduction factor for such partial remedial action is by definition [see (5)] by:

$$ R(\theta ) = {\frac{{D_{\text{A}} + D_{\text{B}} }}{{D_{\text{A}}^{*} + D_{\text{B}} }}}, $$

(31)

where

$$ D_{\text{A}} = {\text{DC}}_{\text{E}} q_{\text{S}}^{\text{A}} \sum\limits_{g} {\theta p_{g} \bar{\Uplambda }_{\text{l}} } \quad {\text{and,}} $$

(32)

$$ D_{\text{B}} = {\text{DC}}_{E} q_{S}^{B} \sum\limits_{g} {(1 - \theta )p_{g} \bar{\Uplambda }_{l} } $$

(33)

are the average external doses without remediation in parts A and B, respectively; and

$$ D_{\text{A}}^{*} = {\text{DC}}_{\text{E}} {\frac{{q_{\text{s}}^{A} }}{{R_{\text{RS}} }}}\sum\nolimits_{g} {\theta p_{g} \bar{\Uplambda }_{g} } $$

(34)

is the average external dose in part A after application of the remedial action RS, and $R_{\text{RS}}$ is the reduction factor for decontamination of the whole settlement.

Substitution of (32–34) into (31) and accounting for (29) lead to the following expression for the reduction factor of partial decontamination:

$$ R(\theta ) = {\frac{{\theta X_{\text{S}} + \left( {1 - \theta } \right)}}{{{\frac{{\theta X_{\text{S}} }}{{R_{\text{RS}} }}} + \left( {1 - \theta } \right)}}}. $$

(35)

In the above equation, the only unknown parameter is X _S . This parameter can be derived using results obtained in field studies. It was observed (Golikov et al. 2002) that generally the most exposed group in a settlement (group H) consists of outdoor workers living in wooden houses, while the least exposed group (group L) is formed by pre-school children living in part B. For these groups, average location factors were found (Golikov 2003) to be 0.304 and 0.173, respectively. Correspondingly, the average sizes of these population groups were found to be p _H  = 0.34 and p _L  = 0.08.

If decontamination is applied to the higher contaminated part of the settlement (part A), then the size of group H is $\theta p_{\text{H}}$ and the size of group L is $\left( {1 - \theta } \right)p_{\text{L}}$. The ratio of average doses for the groups L and H,

$$ {\frac{{D_{\text{L}} }}{{D_{\text{H}} }}} = {\frac{{q_{\text{s}}^{\text{B}} }}{{q_{\text{s}}^{\text{A}} }}}\,{\frac{{\bar{\Uplambda }_{\text{L}} }}{{\bar{\Uplambda }_{\text{H}} }}} = X_{\text{S}} {\frac{0.173}{0.304}}, $$

(36)

allows assessment of the ratio of the contamination levels in parts A and B, which is necessary to calculate the reduction factor (35).

Finally, assuming that the doses of external exposure follow a log-normal distribution $f_{\text{LN}} (x,\mu ,\sigma )$ with GSD = 1.5 (Golikov 2003; IAEA 2007), this ratio (36) can be estimated using:

$$ {\frac{{D_{H} }}{{D_{L} }}} = {\frac{{\int_{{d_{\text{H}} \left( \theta \right)}}^{\infty } {x\,f_{\text{LN}} (x,\mu ,\sigma ){\text{d}}x} }}{{\int_{{d_{\text{H}} \left( \theta \right)}}^{\infty } {f_{\text{LN}} (x,\mu ,\sigma ){\text{d}}x} }}} \times {\frac{{\int_{0}^{{d_{\text{L}} \left( \theta \right)}} {f_{\text{LN}} (x,\mu ,\sigma ){\text{d}}x} }}{{\int_{0}^{{d_{\text{L}} \left( \theta \right)}} {xf_{\text{LN}} (x,\mu ,\sigma ){\text{d}}x} }}}, $$

(37)

where quantiles of the groups are computed as:

$$ d_{\text{H}} \left( \theta \right) = F_{\text{LN}}^{ - 1} \left( {1 - \theta p_{\text{H}} ,\mu ,\sigma } \right)\quad {\text{and}}\quad d_{\text{L}} \left( \theta \right) = F_{\text{LN}}^{ - 1} \left( {\left( {1 - \theta } \right)p_{\text{L}} ,\mu ,\sigma } \right). $$

(38)

Equation 37 expresses the ratio of doses for an arbitrary fraction of the settlement being decontaminated. This algorithm is currently implemented in ReSCA.

Examples of the reduction factors for a fractional RS remedial action are shown in Fig. 6. The example values have been derived for different values of the RS reduction factor applicable to the whole settlement.

Fig. 6

Reduction factor for partial RS remedial action as a function of the decontaminated fraction of a settlement, for different values of RS in the whole settlement (R _max)