JUSTIFICATION, OPTIMIZATION AND PRIORITIZATION OF REMEDIAL MEASURES

Justification and optimization are fundamental principles of radiation protection, together with dose limitation. In order to decide whether remedial action is justified, an exposure estimate is usually carried out, and in case the estimated effective dose of the population exceeds a reference level, remedial measures are considered justified from a radiological point of view.

An effective dose of 1 mSv/a in addition to the regional natural background is internationally accepted as a reference value for remediation at uranium production legacy sites [13]. However, the need for remedial measures is not only governed by radiological conditions at a site.

Protection of water resources under toxicological aspects, geotechnical hazards or development plans for contaminated land can also constitute reasons for remedial action. Note that the associated decision processes are always site specific.

Detailed investigations of the environmental and radiological situation at the Mailuu Suu legacy site were firstly carried out by Vandenhove et al. [10] using funding from the EU TACIS program, followed by dose and risk assessments by the authors of this paper under the DHMP project [11, 12]. Further investigations with a focus on water supply to the local population were carried out by BGR [4] and Corcho–Alvarado [5]. The results of all these studies can be summarized as follows:

1) Apart from one waste dump (WD 5, reportedly consisting of unprocessed low-grade ore) and some isolated radiation hot spots due to damaged or incomplete covers, no waste sites were identified which lead to an acute radiological risk to the public that would justify remediation from a radiological point of view. WD 5 was an exception as a residential house was built on its plateau, leading to effective doses clearly exceeding the 1 mSv/a reference value;

2) Dust and external (gamma) radiation did not significantly contribute to the effective dose of the population due to the fact that tailings ponds are mostly covered. Gamma exposure from uncovered waste rock piles was not critical either due to the remote location of the dumps and dose rates smaller than 1 µSv/h;

3) Only small increases of uranium concentrations were temporarily observable in the Mailuu Suu River (below 10 µg/l), which is not critical both from a radiological and toxicological point of view. BGR estimated release rates of solute uranium in the order of a few 100 g/d, which would lead to an increase of the uranium concentration in the Mailuu Suu River in the order of 1 µg/l. Likewise, the incremental activity concentrations of other natural radionuclides did not lead to noticeable effective doses [12]. In some tributaries such as the Kulmen Sai creek, concentrations may be higher [4], but due to the lack of realistic exposure scenarios this could not justify remediation.

Investigations of the central water supply to the population of Mailuu Suu have shown that the central drinking water distribution system is safe but monitoring will need to continue [5];

4) The most obvious and severe impact of radioactive waste facilities is erosion of tailings and waste rock located on the banks of the Mailuu Suu River and its tributaries. While the resulting effective doses are again small and would not provide justification from a radiation protection point of view, erosion of mine wastes is not considered good practice and concerns over uncontrolled water-borne dispersal of contamination to neighboring Uzbekistan may lead to concerns over transboundary impacts;

5) In general, landslides and mudflows were identified as potential hazards for the site in that they may block riverbeds and/or impact the health and property of the population.

Some waste rock dumps (e.g., WD 1 located on the steep banks of the Kulmen Say River) were identified as potentially subject to mudflows. Again, radiation doses would be negligible due to the strong dilution of any contamination originating from these dumps;

6) A major hazard was the seismic instability of the tailings pond TP 3, located in the Mailuu Suu River valley. The tailings of TP 3 comprise a significant radioactive inventory among all legacy waste sites of Mailuu Suu, and failure of the containment would have caused the sudden release of radioactive tailings into the Mailuu Suu River, potentially leading to transboundary impacts. Concerns were also raised that mobilization of the so-called ‘Tektonik’ landslide from the slopes behind TP 3 could threaten the stability of the entire structure. The actual impacts of a tailings release were difficult to quantify and obviously would depend on the exact failure and release mechanisms. However, the possibility of this scenario made remediation of TP 3 a priority of the DHMP.

Based on the findings on environmental impacts and existing risks, the contractor categorized the 13 waste dumps and 23 tailings facilities into one of the following groups:

1) No action required;

2) Remediation measures are recommended, but are of lower priority and may well be implemented when sufficient funding is available;

3) Intervention measures should (and can) be carried out quickly, requiring a relatively small budget, such as fencing or placing a simple cover on isolated radiation hot spots;

4) Temporary stabilization measures, mainly riverbank strengthening in river sections with strong erosion, are urgently required;

5) Long term solutions are needed to sustainably remove a major environmental and/or health risk, such as relocation of wastes to a safer disposal site.

For waste facilities in groups 3 through 5, remediation options were conceptually developed and a preferred option selected in an optimization procedure. In applying the ALARA principle, i.e., keeping radiation exposures “as low as reasonably achievable, economic and social factors being taken into account” [14], the following decision criteria were used. Note that some of the criteria are typically conflicting:

— Cost and funding constraints;

— Technical feasibility and available infrastructure;

— Environmental, social and radiological impacts;

— Time required for implementation;

— Effectiveness in reducing or removing the hazard;

— Stakeholder preferences and public acceptance.

A major constraint imposed on the decision process by the funding agency (IDA, World Bank Group) were the limited time available for implementation of the remedial measures under the project (funding was available by the end of 2012) and the indicative upper limit of implementation costs of approximately US$ 8.5 million [15] for all remediation activities.

Table 1 summarizes the remediation measures that were proposed in the feasibility phase of the DHMP by the contractor [11].

In summary, the only waste facility where radiological considerations provided sufficient justification for remedial action was WD 5. For all other objects where remediation was recommended, this was justified by insufficient geotechnical stability and the risk of damage to containment structures with subsequent release of contaminated material and/or waterborne erosion and dispersion of wastes and associated potential transboundary issues with neighboring Uzbekistan.

TABLE 1. PROPOSED REMEDIATION MEASURES AND PRIORITY RANKING

Waste facility Impact, hazard Proposed remediation solution TP 3 (left bank of Mailuu Suu

TP 2 and 13 (Aylampa Sai) Strong erosion, TEMPORARY