Implementation

Implementation procedures will be established at future programme stages.

The chain of activities needed to implement the proposed disposal system has not so far been planned in detail. Some tentative choices regarding implementation have, however, been made, consistent with the requirements ensuing from the boundary conditions and the strategic choices.

The decision to opt for phased implementation, whereby different parts of the repository will be constructed, operated and closed separately and according to different time sched- ules. Thus, parts of the repository where radioactive wastes have already been emplaced may be closed off while wastes are still in the process of being emplaced in other parts. Disposal gallery seals will isolate the emplaced wastes from the workforce while other galleries are operational. This choice is consistent with strategic choice No. 6.

The decision to assemble key engineered components of the repository at the surface. This choice minimises underground operations, consistent with strategic choice No. 7. In addi- tion, and also consistent with strategic choice No. 7, the use of supercontainers and mono- liths provides permanent shielding of the wastes, protecting the workforce from the associ- ated external radiation hazard.

The decision to maintain some form of post-closure surveillance and monitoring for as long as reasonably possible. This choice is consistent with strategic choice No. 9.

The disposal system components related to the feasibility of implementation, as well as the implementation procedures themselves, will be specified in detail in future programme stages.

3.5 Tentative definition of safety and feasibility statements

A tentative set of safety and feasibility statements has been developed iteratively. This set of statements, structured in statements concerning system definition and development, safety statements, feasibility statements and statements concerning uncertainties (Section 2.2.4.1), will be a main structuring element of SFC1 (Figure 10), and is outlined in Sections 3.5.1 to 3.5.4. The development of some of the high-level statements down to the lowest levels is illustrated in Sections 3.5.2.1 and 3.5.2.2, whereas the other high-level statements are not developed. The full set of safety and feasibility statements, together with the relevant underpinning, will be the subject of (a) separate report(s).

As shown in Figure 10, the assessment basis, safety and feasibility assessments, and quality assurance procedures are key elements underpinning safety and feasibility statements:

safety and feasibility assessments underpin directly a wide range of safety and feasibility statements and underpin statements on system definition;

the assessment basis

► underpins directly a wide range of safety and feasibility statements and statements on system definition;

► underpins indirectly statements concerning the performance and safety indicators evaluated in safety assessments, through providing evidence that the safety functions will be fulfilled as described by the safety concept;

► underpins indirectly statements concerning the feasibility evaluated in feasibility as- sessments;

quality assurance procedures underpin directly statements concerning quality in long-term safety assessments and feasibility assessments.

Finally, the expected findings of the assessments (no identified uncertainties calling into ques- tion the capacity of the system to fulfil the requirements), completeness checking (all uncer- tainties identified) and the existence of an adequate RD&D plan to reduce or even avoid safety- relevant uncertainties in future programme stages underpin statements regarding the residual uncertainties.

Figure 10 – Tentative organisation of the statements around which SFC1 will be structured.

3.5.1 Statements concerning system definition and development

SFC1 will include a general statement to the effect that the proposed disposal system is properly defined and that its development has been guided by a well-defined and rational step-by-step safety strategy. This general, high-level statement is underpinned by two other high-level statements:

the wastes to be disposed of, including their conditioning characteristics, are adequately known [DS 1] 8;

the application of the safety strategy takes proper account of the boundary conditions of the B&C Programme and of the available science and technology [DS 2].

The underpinning of these high-level statements by lower-level statements will be documented separately.

8

Statement labels refer to the current numbering of statements in ONDRAF/NIRAS’ full set of statements.

Documentation of safety and feasibility case 1

Statement of confidence that safety concept and design show sufficient promise to proceed to the next programme stage

Statements on system definition and development ■ waste knowledge ■ application of the safety strategy Safety statements ■ indicators ■ safety concept ■ quality Feasibility statements ■operat. ■ engineering safety practicality ■quality ■costs Statements on residual uncer- tainties ■no critical uncertainties ■future RD&D Quality measures Feasibility assessment Assessment basis Safety assessment

3.5.2 Statements concerning long-term safety

SFC1 will include a general statement to the effect that the proposed disposal system will pro- vide passive long-term safety if implemented according to design specifications. This general, high-level statement is underpinned by three other high-level statements:

safety concept: the assessment basis provides evidence that the safety functions will be fulfilled as described by the safety concept [SS 1];

performance and safety indicators: results from long-term safety assessments confirm the safety of the disposal system [SS 2];

quality: quality assurance procedures have been applied that favour confidence in assess- ment basis development and in the findings of long-term safety assessments [SS 3]. These high-level statements are, in turn, underpinned by other statements, as further shown, for instance, for statements concerning the safety concept (Section 3.5.2.1) and statements con- cerning performance and safety indicators (Section 3.5.2.2). Statements concerning the safety concept are underpinned directly by phenomenological evidence from the assessment basis, used in the framework of preparatory assessments, whereas statements concerning performance and safety indicators are underpinned by formal safety assessment, itself based on the safety concept, as reflected by the curved arrows in Figure 10.

3.5.2.1 Statements concerning the safety concept underpinned directly by the