Radioactive Waste Management: Performance Assessment Methodology

The "performance assessment" of a radioactive waste disposal system concentrates on the long-term safety of the disposal system and covers an analysis that:

1. identifies the features, processes and events ('FEPs') that might affect the performance of the disposal system;
2. examines the effects of these FEPs on the performance of the disposal system; and;
3. estimates the timing and amount of any released radionuclides from the disposal system and the migration of radionuclides from the repository to the biosphere.

The calculation of the probabilities of a radioactive release and the future consequences begins with the determination of the sequence of events to be analyzed. These scenarios are determined through a formal process, which basically has four steps:

1. the FEPs that are potentially relevant with respect to the performance of the disposal system are identified and classified ;
2. certain FEPs are eliminated according to well-defined screening criteria as not important or not relevant to the performance of the repository system;
3. scenarios are formed from the remaining FEPs, in the context of regulatory performance criteria;
4. scenarios are specified for the analysis of the consequences of any radioactive release.

For the calculation methodology, generally two different complementary approaches can be applied:

• the deterministic approach. Deterministic or "best-estimate" analyses are performed on separate scenarios and aim to result in best-estimate values of the required variables such as release of radionuclides from the underground repository, or dose rates to the environment. For this type of analyses, state-of-the-art theoretical models are applied that as closely as possible describe the considered phenomena.
• the probabilistic approach. Probabilistic analyses are performed on a resticted set of representative scenarios. They result in average values of the required variables, including the variations around these averages, or probabilities of events to occur.

CORA
NRG has elaborated performance assessments in the framework of the four-year Dutch/Belgian CORA programme (Commissie Opberging Radioactief Afval). This programme concentrated on the METRO design for the retrievable radioactive waste disposal in underground salt formations, and the TRUCK and TRAKTOR designs for a repository in low-permeable clay layers. The programme has been carried out in cooperation with SCK·CEN and the University of Leuven (Belgium).

Performance Assessment of the Asse Mine
NRG assists in the long-term safety performance assessment of the Asse salt mine, where low and intermediate level radioactive waste have been disposed in former salt production chambers. The performance assessment includes the development of several scenarios that concentrated on the closure of the mine, the post-closure evolution of the repository, and the impact of human actions. In addition, an evaluation has been performed of the technical measures to minimize or exclude the release of radionuclides from the mine.

EC Projects
NRG is an active participant in the EC sponsored Framework Programme projects that are related to the performance assessment of different radioactive waste disposal concepts:

• the "SPent fuel Assessment" (SPA) project, a performance assessment of the direct disposal of spent fuel at the Mol site in Belgium;
• the BAMBUS I and II projects "BAckfill and Material Behaviour in Underground Salt repositories", which investigate the performance and the behaviour of the salt backfill in relation to other materials in underground waste disposal units, that are foreseen in geological salt layers;
• the project Bentonite Barriers in Integrated Performance Assessment (BENIPA) aims to assess the state of the art in the application of bentonite as a sealant of radioactive-waste containers (see Figure 1). Bentonite is a highly colloidal plastic clay that is capable to swell considerably in contact with water, thereby sealing the waste containers from the underground galleries, and to absorb radioactive nuclides;
• the "Testing of Safety and Performance Indicators" (SPIN) project, aims at exploring the applicability of various output variables (concentrations, fluxes, etc) as indicators of the performance and safety of radioactive waste repositories.

COMPUTER PROGRAMS
For estimating the safety performance of nuclear waste disposal units, NRG applies a variety of computer programs of which two examples are give below.

The PORFLOW code (www.acrifcd.com) is applied to simulate in detail the release of radionuclides from the underground waste disposal units to the near field and their migration through the surrounding geological layers, that can be granite, clay of salt. An example of the calculated radionuclide fluxes through underground clay formations is given in Figure 1 (FIG.14 from 21082/00.33017 - METRO,TRUCK). This figure has been obtained from the performance assessment for the METRO design for the retrievable disposal of nuclear waste.

The EMOS package (Endlagerbezogene Modellierung von Szenarien) is a more generic computer program for the calculation of the release of radionuclides from different types of underground waste disposal units, the transport though underground geological zones, the release to the biosphere, and the impact of radionuclides on the living environment. EMOS is also capable to perform probabilistic analyses.

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

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