Safety assessment for near surface disposal

The long-term safety of radioactive waste disposal facilities is usually demonstrated with the support of a safety assessment. This normally includes modelling of radionuclide release from a multi-barrier near surface or deep disposal facility to the geosphere and biosphere.

The main objective of the overall work is to support ONDRAF/NIRAS in ensuring a safe, permanent and sustainable solution for the management of short-lived low and intermediate level radioactive waste (category A waste) in Belgium.

The approach to post-closure safety assessment implemented by SCK•CEN was based on the internationally recognised ISAM methodology, which was developed for near surface disposal of LILW by the IAEA. The methodology considers three main components of the disposal system, i.e. the near field, geosphere and biosphere.

  • The near field comprises the waste and the engineered barriers.
  • The geosphere comprises the geology and hydrogeology of the site.
  • The biosphere incorporates all relevant exposure pathways through which man can be exposed such as inhalation of gaseous radionuclides (e.g. radon) and resuspended dust, direct radiation from contaminated sediments and soils and ingestion of contaminated water and food cultivated on contaminated soil or contaminated by irrigating with contaminated water.

Over long time scales, the optimal properties of concrete for safety will deteriorate as a result of chemical and physical-mechanical stresses. For assessing the safety of radioactive waste disposal facilities, knowledge on the long-term behaviour of cementitious engineered barriers (CEB) under disposal conditions (with interactions between CEB, soil and atmospheric conditions for near-surface disposal facilities) plays a crucial role. SCK•CEN contributes to developing models for assessment of concrete durability.

Leaflet:

Safety assessment studies for near surface disposal of short-lived low and intermediate level nuclear waste (1 MB)

Areas of research

Concrete durability assessment

Cement and concrete are used for many purposes in the disposal of (radioactive) wastes: for incapsulation/solidification of the waste, as backfill and as construction material. Assessment of concrete durability requires a coupled description of migration of water and mass, geochemical reactions and physical-mechanical damage processes. The objective of this research is to develop and evaluate an integrated model able to simulate the long-term durability of cementitious based materials used in (radioactive) waste disposal facilities.

Geochemical evolution of concrete engineered barriers

Near field modelling

Disposal facilities for radioactive waste comprise a series of engineered barriers whose primary purpose is to concentrate and contain radionuclides and other contaminants until their hazard level has decreased to acceptable levels. The purpose of near field modelling (also performance assessment or in a broader context, safety assessment) is to quantitatively evaluate long-term (up to a few ten thousand years or more) radionuclide behaviour under conditions of gradually degrading engineered barriers. At SCK•CEN models are developed and implemented to deal with processes of water flow and radionuclide migration within the engineered components of the disposal facility.


Near field modelling

Geosphere Modelling

Geosphere modelling is concerned with the identification and quantification of pathways for radionuclide migration in groundwater. Typically hydrogeological models are developed for simulating groundwater flow in the vicinity of the disposal facility and for simulating radionuclide migration from the disposal site to a nearby biosphere interceptor such as a drinking water well or a river.


Geosphere modelling

Environmental Systems Modelling

Biogeochemical transport models are developed and evaluated to address migration of radionuclides and major and minor elements in the environment surrounding a near surface disposal site. Analysis of radionuclide and heavy metal migration is also done for areas contaminated as a result of uranium mining and milling activities.


Environmental systems modelling

Fieldwork

We carry out and support field research aimed at characterizing physical/chemical properties of soils and sediments. This is necessary input to the modelling of complex flow and transport processes for disposal relevant systems such as soils and groundwater. Fieldwork is an indispensible component to further guide the development of biogeochemical simulation models, especially to investigate the relationship between cover layers made from soil and underlying concrete engineered barriers.


Field work

Soil water balance modelling: a cross-disciplinary approach

Soil water balance models are used to calculate the redistribution of rain or irrigation water in a soil profile and to determine fluxes of evapotranspiration and drainage. A cross-disciplinary approach is implemented at SCK linking advanced search algorithms and state-of-the-art hydrodynamic water balance models for determination of soil hydraulic properties from field observations.


Soil water balance modelling

Long-term earth surface evolution assessment

Earth surface evolution studies are conducted to evaluate the long-term impact of surface/landscape evolution on the safety and performance of repositories of radioactive waste. For this purpose advanced techniques such as optically stimulated luminescence and thermoluminescence dating techniques are combined with more classical geomorphological methods.


Natural analogue for cover systems

Climate change and the hydrological cycle

Short-term and long-term climate changes will affect various components of the hydrological cycle. Assessments are done with hydrological models to evaluate the magnitude of the impact on the water balance as a result of warmer or colder climate at the Dessel site in Belgium. A tiered approach is being developed at SCK•CEN with Tier 1 using analogue climate stations.



Climate change affects the soil water balance

Integrating uncertainty in hydrological systems modelling

Hydrological models are by essence simplifications of a series of idealized processes whereas they typically contain many unknown parameters that require calibration against some historical record of data. Moreover, input and output data are measured with a certain error. This leads to uncertainty in parameter estimates and model predictions. We develop and apply uncertainty analyses techniques to handle the various sources of uncertainty arising in the model parameterization process and their effects on predictive uncertainty.


Predictive uncertainty for soil water content simulation

Glossary of terms

Performance Assessments - Publication list (113 kB)

Contact: Mallants Dirk