Safety assessment for geological disposal

Safety and feasibility of geological disposal

The safety assessment is an essential element of demonstrating that geological disposal of high-level and long-lived radioactive waste is a safe and feasible long-term solution. All of the arguments used in this demonstration are collected in a so-called safety case. A solid safety case is a prerequisite for obtaining a license for a geological repository.

The main activities concentrate on the possible geological disposal of high-level and long-lived radioactive waste in the Boom clay formation in NE-Belgium in support of the Belgian radioactive waste management programme of ONDRAF/NIRAS. The current focal point of the research programme of ONDRAF/NIRAS is safety and feasibility case 1 (SFC1), which is an important milestone that should be handed over to the Federal Agency of Nuclear Control (FANC) in 2013.

Safety assessments generally consists of three main parts:

  1. The development of a sound methodology and the testing of its application
  2. The analysis of possible evolutions of the repository system and identification of a relevant set of evolution scenarios
  3. The analyses of the radiological consequences (in terms of dose) for the cases identified in step 2

The most striking aspect of safety assessments for geological disposal is the timescale to be considered, which goes up to one million years. The applied methodology is being developed in close collaboration with ONDRAF/NIRAS with guidance available from IAEA and NEA reference documents. Methodological topics are treated in the European project PAMINA as well, in which both SCK•CEN and ONDRAF/NIRAS participate.

Repository concept: the supercontainer

The current repository concept is based on the supercontainer, being the engineered waste package entity in which the vitrified waste canisters or spent fuel assemblies are placed (see Figure 1). The main barriers hindering the leaching of radionuclides in this concept are the waste matrix, the overpack, the concrete buffer, and not in the least, the barrier provided by the clay formation.

Figure 1. Schematic diagram showing a longitudinal section through a disposal gallery containing a supercontainer with vitrified HLW. (figure courtesy of ONDRAF/NIRAS)

The functioning of the disposal system is described and explained in terms of safety functions:

  • Isolation
  • Engineered containment
  • Delay and attenuation of the releases

The analysis of the evolution of the repository system and the identification of altered evolution scenarios is based on the availability of those safety functions, provided by one or more of the system's components (see Figure 2).


Figure 2. Example of the timescales during which the primary safety functions should be effective. (figure courtesy of ONDRAF/NIRAS)

The evolution of the main components of the disposal system is studied in terms of their chemical, mechanical, physical and hydraulic behaviour (with ample attention to coupled processes) using dedicated numerical modelling tools. In particular, the influence of radiogenic heat on the system's behaviour is a point of concern. These detailed process simulations are necessary to demonstrate that the favourable characteristics of the repository system as a whole can be relied upon during the required time scales.

The evaluation of the radiological consequences for people in the far future is based on integrated analyses considering the slow release of radionuclides from the waste matrix, the radionuclide migration –which is dominated by diffusion– through the buffer and the clay barrier, advective transport through the aquifer layers surrounding the clay and transfers into the biosphere. Figure 3 gives an example of the doses calculated via a water well pathway in case of disposal of spent nuclear fuel.

Figure 3. Doses calculated via a water well pathway in the case of disposal of spent nuclear fuel in a repository located in the Boom Clay layer. Beware of the bilogarithmic scale of the axes.

Research domains

The safety assessments of geological disposal strongly rely on results obtained in the following research domains: waste form behaviour, container corrosion studies, radionuclide geochemistry, clay characterisation, hydrogeology, geochemical and coupled transport modelling, geomechanical and THM modelling, and biosphere impact studies.

Leaflet:

Safety evaluations for the disposal of high-level and long-lived radioactive waste in clay formations (573 kB)

Presentations:

Waste & Disposal: Category B and C waste (wmv) (23 MB)

Contact: Marivoet Jan