Specific issues in safety assessment

Some of the issues raised in Section 2.5.3 on regulatory guidance for safety assessment involve topics that have been discussed for many years and have a strong judgmental component with respect to their implementation in the analysis of long-term repository performance. In the present section, these and other particularly subtle issues are discussed at more length.

2.5.4.1 Timescales of concern

HSK-R-21 states that the doses and risks related to a sealed repository "shall at no time" exceed the values set in Protection Objectives 1 and 2 (Section 2.3.2). This leads to a potential diffi-culty: The better the disposal system is, the further into the future will any significant release occur, and the more difficult will it be to estimate doses and risks with confidence when they eventually arise. It is important, therefore, to understand the evolution of the hazard posed by radioactive waste so as to determine over what time period the geological disposal system should perform and, also, to focus safety assessments on the time period during which the waste poses an unusual hazard.

A useful measure of the potential hazard of radioactive material is the radiotoxicity index (RTI), which is defined in Appendix 3. Fig. 2.5-1 shows the RTI of the total radionuclide inventory of the three waste categories to be emplaced in the proposed repository as a function of time⁵⁴. This is compared with the RTI of the natural radionuclides contained in 1 km³ of Opalinus Clay ("OPA") and with that of a volume of natural uranium ore corresponding to the volume of the SF / HLW / ILW emplacement tunnels. In the latter case, three uranium concentrations (uranium ore grades) are considered. These are 3 %, which is the average uranium concentration of the small uranium ore body of La Creusa, Switzerland, 8 %, which is a representative value for the Cigar Lake uranium deposit in Canada, and 55 %, which is near the upper end of observed concentrations in uranium ore bodies (see also Appendix 3). Fig. 2.5-1 shows that after one million years, the radiotoxicity of even the most toxic waste, the spent fuel, has dropped to well below that of a volume of natural uranium ore sufficient to fill the SF / HLW / ILW emplacement tunnels.

Another way of putting the potential hazard of spent fuel in perspective is to compare it with that of the quantity of natural uranium that was used to produce the fuel. This was done in a study by Hedin (1997), where it is noted that it takes about 8 tonnes of natural uranium to produce 1 tonne of nuclear fuel suitable for PWR/BWR reactors. Fig. 2.5-2 shows that, after about 300 000 years, the radiotoxicity of spent fuel has dropped to that of the natural uranium from which it was produced (assumed to be in equilibrium with its daughters)⁵⁵.

Both Figs. 2.5-1 and 2.5-2 indicate that the timescale over which the spent fuel presents a hazard that needs special attention is of the order of about one million years. Thus, it is considered that the disposal system should provide effective isolation of the spent fuel from the human environment also for about this period of time. Note that this does not imply a necessity for complete containment within the waste packages; the surrounding geological media are part of the isolation system. The remainder of this report will compile arguments to support the state-ment that a disposal system providing the required isolation capability for at least one million years is, in fact, feasible. The analyses are complemented with arguments that the good performance of the system will continue beyond one million years for at least another few

54 Time is measured from the end of waste emplacement (see Chapter 4 for details).

55 The production of one tonne of enriched uranium, suitable for PWR/BWR fuel, from eight tonnes of natural uranium also results in radioactive products which are removed during uranium milling and extraction, and seven tonnes of depleted uranium produced during uranium enrichment. These materials are the responsibility of their respective producers.

NAGRA NTB 02-05 30

million years. Additionally, more qualitative illustrations are provided for the different evolu-tionary stages of the repository for the period beyond these few million years. However, these illustrations are considered to be of far less importance than the detailed analyses for the period when the waste still presents a higher hazard and they are, therefore, kept rather short.

Fig. 2.5-1: Radiotoxicity index (RTI) of spent fuel (SF), vitrified high-level waste (HLW) and long-lived intermediate-level waste (ILW) as a function of time

As a comparison, the RTI of the natural radionuclides contained in 1 km³ of Opalinus Clay ("OPA") and in a volume of natural uranium ore corresponding to the volume of the SF / HLW / ILW emplacement tunnels are also given. In the latter case, three uranium concentrations are considered. These are 3 %, which is the average uranium concentration of the small uranium ore body of La Creusa, Switzerland, 8 %, which is a representative value for the Cigar Lake uranium deposit in Canada, and 55 %, which is near the upper end of observed concentrations in uranium ore bodies. Note that the levels for 1 km³ of Opalinus Clay and 3 % U coincide.

2.5.4.2 The role and treatment of the biosphere

The key role of the barrier system (i.e. the EBS together with the host rock) is to limit radionuclide release to the biosphere, and the performance of the barrier system is an

"adjustable parameter" within certain bounds via siting and design. In contrast, the role of the biosphere in a typical safety assessment is to provide a "measuring stick" to convert radionuclide releases from the barrier system into a dose, and the corresponding scale is only marginally adjustable. In addition, the possible evolutions of the barrier system for a well-sited repository and a well-chosen host rock can be bounded with reasonable confidence over about one million years into the future, and its performance can be evaluated with a reasonable level of reliability over this period (Fig. 2.5-3). The same is not true for the factors that must be taken into account for evaluating the meaning of any radionuclide releases from the host rock and for

10¹⁰

RTI

Time [a]

10¹

10⁰ 10² 10³ 10⁴ 10⁵ 10⁶ 10⁷ 10⁸

Cigar Lake, 55 % U

Cigar Lake, 8 % U 1 km OPA / La Creusa, 3 % U³

10⁹ 10⁸ 10¹¹ 10¹² 10¹³ 10¹⁴ 10¹⁵ 10¹⁶

HLW SF

ILW

31 NAGRA NTB 02-05

modelling radionuclide transfer in the biosphere and uptake by man; i.e. processes in the surface environment and, especially, human behaviour that affect the radiological exposure pathways.

For these factors, the timescale over which reliable statements can be made are of the order of only a few hundred years in the case of the natural environment and human populations and, perhaps, a few tens of years in the case of individual human behaviour. Moreover, much of the uncertainty concerning these factors is irreducible. In spite of this, there are approaches available that allow an evaluation of a range of future scenarios regarding the potential harm that might result from a repository and to measure this against safety standards.

Fig. 2.5-2: Radiotoxicity index of 1 tonne of representative Swiss spent fuel (BWR) with a burnup of 48 GWd/t and of 8 tonnes of natural uranium

To fabricate 1 tonne of fuel, about 8 tonnes of natural uranium are required. The different fractions in the nuclear fuel cycle include, in addition to the natural uranium and the fuel, the depleted uranium and the uranium daughters that are separated in the uranium mill (not shown in this figure). Adapted from Hedin (1997).

The role and treatment of the biosphere in long-term safety assessment in the light of this uncertainty and taking into account its limited importance relative to the barrier system has been discussed extensively in international fora. The international consensus that has developed suggests that a reasonable approach is to separate the assessment of the biosphere from that of the barrier system, as proposed, for example, by a NEA ad hoc working group (NEA 1999c) and to develop a range of credible illustrations for the biosphere, thereby exploring the uncertainty related to the biosphere (Sumerling et al. 2001). This is the approach taken in the current safety assessment. For the vast majority of the release calculations (i.e. those that focus on the barrier system) the same stylised biosphere situation is chosen to convert releases into dose. The sensitivity of calculated doses to uncertainty related to the biosphere is then investigated in stand-alone calculations for a broad range of biosphere situations.

10⁹ 10⁸

RTI

Time [a]

10⁷ 10¹⁰ 10¹¹ 10¹² 10¹³

10¹

10⁰ 10² 10³ 10⁴ 10⁵ 10⁶ 10⁷ 10⁸

239Pu

240Pu

137Cs

241Am

210Po

226Ra

210Pb

229Th Natural uranium with

daughters, 8 tonnes Spent fuel (BWR), 1 tonne

90Sr

238Pu

NAGRA NTB 02-05 32

Fig. 2.5-3: Schematic illustration of the limits of predictability of the different elements of a geological disposal system (NEA 1999c)

2.5.4.3 Treatment of future human behaviour

In the present assessment, doses are calculated to average members of hypothetical critical groups chosen to be representative of the individuals or population groups that might receive the highest doses as a result of the presence of the repository and assuming current living habits and diets. The calculated doses are regarded as indicators of the level of protection, rather than estimates of actual future doses (see ICRP Publication 81 (ICRP 1998) for specific recommendations for the definition of critical groups in relation to solid radioactive waste disposal).

2.5.4.4 Treatment of future human actions

The issues surrounding the assessment of future human actions affecting a deep geological repository are, in some respects, similar to those surrounding the treatment of the biosphere.

Any statement about the actions that humans might take in the far future is largely speculative.

The relationship between the assessment of such actions and the assessment of the quality of a site and a design is problematic because human actions have the potential to create exposure paths that by-pass the normal safety functions of the repository. These issues have been discussed internationally by a NEA Working Group (NEA 1995b), within the NEA IPAG exercise (NEA 2000d) and, most recently, within IAEA Specialists' Meetings (IAEA 2001b).

The ICRP has also given guidance for the assessment of future human actions (ICRP 1998). The Elements to be represented

EBS &

host rock

Hydrogeological system

Surface environment

processes

Radiological exposure modes

Ecological change Geological change

Climatic change

Individual habits Human activities

100 10 000 1 000 000

Human intrusion

Changes acting on these elements

Predictability of changes into the future Time [a]

33 NAGRA NTB 02-05

international consensus on these issues guides the treatment of future human actions in this safety assessment as follows:

• The decisions to dispose of waste in a deep geological repository, and to site the repository in a host rock that does not have any other obvious resource potential, are made to reduce the likelihood of inadvertent human interference.

• It is acknowledged that human technology and society will change over the timescales of relevance for repository safety assessment. These changes are unpredictable. To limit speculation, it is assumed that human technological capabilities and societal patterns observed today, and in the past, provide a reasonable model to assess the safety of the repository in the future. Thus, only future human actions that could be undertaken with present-day technology are considered.

• Exploratory drilling through the repository horizon can be considered as an illustrative case for the purpose of testing the resilience of the disposal system to such an event. The main concerns are whether there is significant degradation of the long-term performance of the disposal system following such an event, and the possible dose to human individuals dwelling near to the site. The ICRP has suggested that doses to such individuals could be viewed against the levels at which intervention would be considered, i.e. in the range 10 to 100 mSv (ICRP 1998). Individuals involved in a drilling that actually intercepts a spent fuel or HLW canister could be subject to very high doses, but this is an unavoidable hazard arising from the decision to concentrate and contain the waste, and should be judged against the generally very high level of protection that the disposal system offers.

• In accordance with HSK-R-21, which states that intentional human intrusion into the repository need not be discussed in a safety assessment, there is no consideration of deliberate acts of intrusion into the repository undertaken with knowledge of the repository location and content. Such actions, which could include retrieval of the waste or other materials and malicious acts, are considered to be the responsibility of future generations⁵⁶. 2.6 Summary and conclusions: Objectives and principles

The key points from Chapters 1 and 2 that determine how a repository system should be deve-loped and then analysed to determine whether it fulfils its safety goals are recapitulated in the following sections and then summarised in two concluding overview tables (Tab. 2.6-1 and 2.6-2). The points are collected, grouped and summarised into the following broad categories:

• Objectives of geological disposal

• Objectives related to the system

• Objectives related to stepwise implementation

• Assessment principles.

2.6.1 Principal objectives of deep geological disposal: Security and long-term safety

Im Dokument TECHNICALREPORT 02-05 (Seite 90-94)