NRG, Service: radwaste: research, consultancy and management- Scenario Development Methodology -. Group: Radiation and Environment (RE). EN-version (NRG, An ECN-KEMA company)

Safe storage and disposal of radioactive waste
In The Netherlands radioactive waste is stored at the COVRA interim storage facility in Borssele. According to the present government policy part of this waste, mainly the high- and medium level long-lived waste, will eventually be disposed of in an underground repository.

Safety assessment based on over covering scenarios
A common method to assess the safety of systems is the development of so-called scenarios that cover all possible developments of the system. In the present context a scenario means a group of similar developments of the system. Scenarios are particularly useful as part of the communication about the safety of the system. The safety of a system is evaluated based on the consequences of one or more representatives of each scenario. Obviously, assessment criteria determine the choice of representatives of each scenario.

Barriers and FEPs used in scenario development
Over time several methodologies have been developed for determination of scenarios for a system. For scenario development NRG applies the barrier concept. For that first the potential barriers between the 'source' of a danger and 'man' are determined. These barriers can be visualised in a multi barrier system (MBS). Then the features, events and processes (FEPs) are determined that may influence the efficacy of the barriers and thus enables 'transport' through a barrier. Such FEPS can be drawn from numerous lists that over time have been collected by different institutes for specific applications.

Scenario development for surface storage of radioactive waste
For the safety assessment of the COVRA facility scenarios have been determined using a HAZOP study. In a HAZOP study systematically the ability of an installation is assessed to endure all kinds of malfunctions, both by internal and external causes. In such a study (combinations of) malfunctions that lead to short-circuiting of barriers are grouped into scenarios. Usually, these scenarios are indicated by their main initiating event (fire, flood, airplane crash, etc.).

Scenario development for underground disposal of radioactive waste
The concept for an underground facility for disposal of radioactive waste is still being developed. In The Netherlands attention mainly has been focussed on suitable salt domes (north) and clay layers (south). Since the Dutch radioactive waste will be stored in the COVRA surface interim storage facility for a long time (up to some 100 years) the determination of a suitable concept is not really opportune. For the safety assessment of an underground repository for radioactive waste NRG developed the so-called PROSA methodology for the determination of scenarios for an underground disposal facility. Starting with one base scenario, in this methodology iteratively further scenarios are being derived.

Iterative derivation of scenarios in the PROSA methodology
Starting point for the derivation of scenarios for an underground disposal facility is a broad knowledge of the system, which enables a good description of the development of the facility and its vicinity in a so-called ''conceptual model'. Based on this one or more relevant FEP lists are selected.
The PROSA methodology for derivation of scenarios is summarised in the following steps:

Based on the available knowledge of the system (conceptual model) a transport pathway is devised. This is a global description of the way in which radioactive matter can be transported from the waste to the human living environment, a specific representative of a scenario.
For this transport pathway the system is examined for places and volumes in the system where radioactivity might be delayed or retained, which can be identified as barrier. Then these barriers are used to build a suitable multi barrier system (MBS) for the scenario concerned.
All system relevant FEPs are classified into four classes: Primary FEPs, Transport FEPs, Variant FEPs and Other FEPs (see on this page Classification of FEPs).
By checking whether deletion of included Primary FEPs, addition of excluded Primary FEPs or interchanges between both classes would lead to relevant changes in the efficacy of barriers, both other representatives of the same scenario - when barrier efficacy is not relevantly changed - or new transport pathways - representatives of other scenarios - are derived.
For each new found transport pathway a multi barrier system must be developed, which brings us back to step 2.

The PROSA methodology is explicitly meant to be 'open': for each FEP that can be thought up it can be checked for each known scenario whether it can be included, or - if it turns out that it is an excluded Primary FEP in a scenario - whether it might lead to a new scenario. As supporting tool for the application of the PROSA methodology the FEP representation table has been developed.

Conceptual model
A conceptual model for an underground disposal facility for radioactive waste is a broad description of the properties of the facility and immediate vicinity, including their possible development (waste characteristics, geometry of the facility, host rock, geology, hydrology, climate, land use, nutrition pattern...). This description is based on all known facts both from location specific investigations as from location independent investigations into certain aspects. A conceptual model serves as basis for estimating the occurrence probabilities of FEPs and thus for the determination of potential transport pathways and their development in space and time.

Classification of FEPs
For application of the PROSA methodology all possible features, events and processes (FEPs) that determine the isolation efficacy of barriers are classified into four classes:

Primary-FEP

FEPs that describe boundary conditions of the system
FEPs that describe transport causing processes

Transport-FEP

FEPs that describe the transport of radioactivity through a barrier

Variant-FEP

FEPs that determine the rate of a Transport or Primary FEP

Other-FEP

included Variant FEPs, with negligible influence in the scenario
excluded Primary FEPs, that are not active in the scenario

FEP representation table
In a FEP representation table for each scenario all FEPs are classified per FEP class with respect to their way of expression for each barrier. This representation enforces re-formulation of FEPs from very diverse sources, thus avoiding one of the weak points of many methodologies for scenario development. Since the class of Primary FEPs is often very broad, per sub-system general boundary conditions and variations thereof are added to the FEP representation table as an additional 'barrier'.

Primary-FEPs	Barriers	Transport-FEP	Variant-FEP	included Variant-FEP	excluded Variant-FEP
Geometry of the mine Stress field in host rock Planned closure procedure	Mine sub-system		Changes in stress field by construction of the mine Deviations in the closure procedure	Changes in mine geometry by gas explosions Differences in thermal expansion	Groundwater intrusion Later underground constructions Sabotage during/after closure of the mine
Radioactive inventory of waste Physical form of radioactive matter	Waste	Radioactive decay and ingrowth	Difference in waste content Difference in waste forms		Production of radioactive gases
Physical properties of the waste matrix Gasproduction by (bio)degradation of waste matrix Gasproduction by corrosion of metals in the waste	Waste matrix	Leaching of radionuclides from the waste matrix Diffusion of nuclides from the waste matrix	Difference in waste matrix Structural changes by mechanical influences Corrosion rate of the matrix Chemical environment in the matrix Nuclide adsorption in the matrix Element specific diffusion in the waste matrix Complexing ligands in the waste matrix	Heat production by radioactive decay Changes in the waste matrix by radiological effects Changes in the waste matrix by heat-effects Thermal effects on microbial gas production Pressure effects on microbial gas production Radiological effects on microbial gas production Biofilm effects on microbial gas production	Production of radioactive gases Nuclear criticality Nuclear explosions Co-disposal of reactive waste
Physical properties of the waste container Gas production by corrosion of the waste container	Waste container	Failure rate of the waste container	Different container types Container corrosion rate Chemical environment around the waste container	Radiological effects on container integrity Thermal effects on container integrity Microbial enhanced cement corrosion

MORE INFORMATION:
NRG, Radiation & Environment
Mr. A.D. Poley, PO Box 25, 1755 ZG Petten, Netherlands
Tel +31-224564333, Fax +31-224568491, Email: poley@nrg.eu

Radioactive Waste Management: Scenario Development & Risk Analysis Methodology