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Waste Acceptance Criteria Development for different LILW disposal systems

S. Konopaskova 1, D. Lukin1, I. Zadakova1 1Radioactive waste repository authority (SURAO), Praha, Czech Republic

E-mail contact of main author: konopaskova@surao.cz

Abstract. In the Czech Republic, there are operated two types of repositories: near surface for disposal of low level waste from NPPs, and repositories for institutional waste, specified as low and intermediate level waste; these are located underground, in former mines of different types. New legislation after 1997 and optimized conditions for final waste form characterization lead to improvement of WAC derivation methods by the means of safety assessment and supported their variety.

Key Words: waste acceptance criteria, subsurface repository, repository for low and intermediate level waste, safety assessment

1. Introduction

This paper describes the procedure of waste acceptance criteria (WAC) development, applied for various types of operated radioactive waste repositories in the Czech Republic. Safety related criteria are derived using the results of safety assessment, considering waste streams, barriers system, and position of the repository in the host structure. Special considerations are included evaluating hydrogeological conditions of the host structure and accessible biosphere. Differences of repositories lead to differences in WAC, as it is presented below.

2. WAC for disposal systems in the Czech Republic

In the Czech Republic, there are operated two types of radioactive waste repositories:

Subsurface disposal of waste from nuclear power plants

Disposal of low and intermediate level institutional waste in mine cavities, some tenths of meters below surface

WAC defined for individual repositories differ in extent, qualitative expression and quantitative parameters thanks to specific approach to their derivation, considering different project, operational and environmental conditions of the repositories.

2.1. Operated repositories and their types

The overview of repositories is specified in Table 1.TABLE I: MARGINS FOR YOUR MANUSCRIPT.

Site Type Volume Waste streams Matrix Host rock

Dukovany, 1993 -

subsurface 55 000 m3 Waste from NPPs Bitumen,geopolymer

Crystalline

Richard, 1964 -

LILW 17 000 m3 Institutional waste, artificial radionuclides

Cement Limestone mine

Bratrství, 1973 -

LILW 1 200 m3 Institutional waste, natural radionuclides

Cement Uranium mine

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FIGURE I: Dukovany repository – a subsurface vault system

FIGURE II: Richard and Bratrství repositoris – underground disposal systems

2.2. WAC structure

WAC are structured according to safety requirements, technical restrictions and administrative requirements defined by law.

Safety related criteria guarantee the compliance with qualitative and quantitative objectives of nuclear safety and radiation protection. These criteria are derived from the results of safety assessment, namely:

Total activity of radionuclides in the repository Volume activity of radionuclides in different waste forms Leachability of the final waste form Activity of radionuclides in non-solidified waste Stress resistance of final waste form solidified by cement and/or geopolymer Dose rate on the surface of waste package

Technical restrictions are done by repository construction and include:

Water presence in drainage system Weight of waste package Integrity and structural stability of the waste package

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Administrative and formal restrictions guarantee the compliance with nuclear and environmental legislation:

Presence of free liquids, pyrophoric and toxic substances, complexing and microbio-logical agents

Waste tracking system and passport

2.3. Derivation of safety related WAC

In the procedure of safety related WAC, there are more aspects that can lead to differences in WAC specifications, done by:

Composition of the radionuclide vector Final waste form and packages properties Repository construction Depth of the repository below surface Hydrogeological conditions of the host structure, and Probable use of the land in communication points

Generally, there is defined a set of scenarios supporting safety case, i. e. operational scenarios and long term scenarios that should help to evaluate probable radiation effects during operations and after repository closure.

Normal evolution scenario is used to define the capacity of the site - volume of the waste as well as its total activity. Scenario components are site specific. For subsurface system, direct infiltration of rainwater and advective flow through disposal units are considered immediately after institutional control period. For underground system, the infiltration is controlled by diffusion and by inflow to fractures in near field and advective flow starts much later thanks to final waste form and filling stability. Safety function of host structure is strongly affected by hydrogeology system as a part of transport pathway in all types of repositories.

Alternative scenarios are used to evaluate disposal system performance by deviations from projected performance. For near surface repository, bathtubbing is considered; in mine systems, possible outflow of contaminated mine water is taken into account.

Intrusion scenarios are the base for limiting volume activities in the final waste form. In subsurface system, evaluation of on site residence and working activities on the site after institutional control are considered. For underground repositories, there is evaluated a contact with waste as a consequence of drilling activities.

Limits of dose rates in the controlled area and on the waste packages are derived by means of radiation protection. In addition, radon intake has to be considered in underground repositories.

Potential emergency situations could lead to non acceptable doses of workers, but the evaluation of accidents is not considered in formulating criteria for radioactive content of the waste.

2.4. Quantitative comparison of safety related criteria

The limits of total and volume activities have been compared for various disposal systems, as it is indicated in FIG. 1. In spite of the fact that the composition of radioactive waste and its activities are not identical in waste from NPPs and in institutional waste, there are some radionuclides present in both types of waste. The results of safety cases lead to lower

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permitted activities and activity concentration in the vault system and the capacity. Underground repositories provide higher capacity for activity of short lived radionuclides as well as for long lived radionuclides, thanks to sophisticated stabilization system, better hydrogeological conditions, as well as to lower potential for inadvertent intrusion.

FIGURE III: Limits of activity in vault and underground systems

3. Conclusions

Normal evolution scenario is limiting for both total and volume activities in mine repositories, Higher capacity of mine system is shown thanks to better engineered barriers performance and/or lower probability of intrusion.

EBS system is more efficient in mine systems – barrier can assure diffusion driven transport for longer periods of time even in the case that the number of fractures in near field is relatively high.

In subsurface system, increasing of life time of barriers leads to higher doses in intrusion and on site scenarios as a consequence of negligible decrease of activity of long lived radionuclides during institutional control period. Volume activities in subsurface system have to be strongly limited, to the values of very low level waste.

2.5. References

[1] INTERNATIONAL ATOMIC ENERGY AGENCY, Derivation of activity limits for the disposal of radioactive waste in near surface disposal facilities, IAEA-TECDOC-1380, Vienna (2003).

[2] KONOPASKOVA, S., et al., “Safety report of Dukovany repository”, SÚRAO, Praha (2012).

[3] KONOPASKOVA, S., et al., “Safety report of Richard repository”, SÚRAO, Praha (2014).

[4] MILICKY, M. et al., Hydrogeological model of the Dukovany site, ProGeo 2012

[5] MILICKY, M. et al., Hydrogeological model of the Richard site, ProGeo 2013

[6] Regulation SONS No. 307/2007 Coll. on radiation protection