CARACTERISTICAS POSITIVAS CARACTERISTICAS NEGATIVAS Son jóvenes

34

uncovered after 100 000 years. The main features of the Design Scenario for radionuclides released in the solid phase are summarized in Fig. 10.

FIG. 10. Design scenario: solid releases.

4.2.2. FEP Screening

As a check to ensure that all potentially relevant Features, Events and Processes (FEPs) have been considered in the scenario, a list of potentially relevant FEPs has been selected and screened for the scenario, on the basis of information provided in the assessment context, system description and the scenario description. A FEPs list based on that developed in the ISAM programme [12] and subsequently updated has been used. The screened FEP list for the Design Scenario is presented in Appendix IV. Text is provided to explain why each FEP has been included (indicated by a ‘Yes’) or excluded (indicated by a ‘No’) from consideration in the Design Scenario based on information from the assessment context, system description and/or scenario description.

Since the near field (i.e. waste, waste form and engineered features) is a significant component of the borehole disposal system, the relevant FEPs identified in the FEP list (i.e. FEP 2.1.1 to 2.1.5) have been further broken down into 53 FEPs. These are presented and screened in Appendix V.

4.3. DEFECT SCENARIO

Therefore the scenario assumes that not all components of the near field perform as envisaged in the Design Scenario, resulting in the earlier release of radionuclides from the near field.

A range of possible defects involving one or more of the near field barriers (i.e. capsule, containment barrier, disposal container, disposal and disturbed zone backfill and casing) can be identified. These are summarized and screened in Table 13. Four Defect Scenario variants are identified:

• D1: all welds are satifactory due to QA/QC except for the closure weld in one 316 L disposal container. All other near field barriers as per Design Scenario.

• D2: All welds are satifactory due to QA/QC except for the closure weld in one 304 waste capsule. All other near field barriers as per Design Scenario.

• D3: degraded/incomplete disposal/disturbed zone cement grout. All other near field barriers as per Design Scenario.

• D4: all welds are satifactory due to QA/QC except for the closure weld in one 316 L disposal container and one 304 waste capsule. The faulty capsule is in the faulty container. All other near field barriers as per Design Scenario.

4.3.2. FEP screening

The screened FEP list for the Defect Scenario is presented in Appendix VI. Text is provided to explain why each FEP has been included (indicated by a ‘Yes’) or excluded (indicated by a ‘No’) from consideration in the Defect Scenario based on information from the assessment context, system description and/or scenario description. In addition the detailed near field FEPs are presented and screened in Appendix V.

36 TABLE 13. POSSIBLE DEFECTS CONSIDERED DescriptionConsidered in defect scenario calculations (variant number)Justification All welds okay due to QA/QC except for the closure weld in one 316 Ldisposal container. All other near field barriers as per Design Scenario.

Yes (D1) Cannot be ruled out. For the mass production of welded structures under strict QA/QC procedures, the probability of an individual undetected, through-wall defect is of the order of 10-3 to 10 -4 [32]. Because of the potential for more challenging conditions for welding and inspecting the disposal container, the higher end of this range (10-3 ) is assumed for the GSA. The probability that the weld on 1 of the 50 disposal containers in the borehole contains a defect can be estimated based on a binomial distribution, and is found to be 0.05 for an individual probability of 10-3 . The probability that 2 out of the 50 disposal containers in a borehole will contain defects is 0.0012 and is considered to be too small to be of concern here. All welds okay due to QA/QC except for the closure weld in one 304 waste capsule. All other near field barriers as per Design Scenario.

Yes (D2) Feasible, although considered to be lower consequence than D1. A similar probability for an individual defect (i.e. 10-3 ) is assumed, and for the probability that only 1 out of the 50 waste capsules will contain a defect. Missing/degraded/incompletecontainment barriercement grout. All other near field barriers as per Design Scenario.

No, but missing cement grout is covered by a ‘What-if’ calculation presented separately from the Defect Scenario results

It is considered that the absence of cement grout would be noted due to the reduced weight of the container and the rattling of the capsule in the container (so multiple incompetence would be required for this case to arise). Degraded/incomplete grout is feasible but considered to be low consequence in comparison to D3. Missing/degraded/incompletedisposal/disturbed zone cement grout. All other near field barriers as per Design Scenario.

Yes (D3) Cannot be ruled out. Probability assumed to be c. 1%. Consider more rapid chemical and physical degradation of cement grout than for Design Scenario. Case of missing cement grout is covered under ‘What-if’ calculation presented separately from the Defect Scenario results. Missingcasing. All other near field barriers as per Design Scenario.

No No credit is taken for the casing in the Design Scenario. All engineered barriers are missing. No, but to be covered by ‘What-if’ calculations presented separately from the Defect Scenario results

Very low probability. All welds okay due to QA/QC except for the closure weld in one 316 Ldisposal container and one 304waste capsule. The faulty capsule is in the faulty container. All other near field barriers as per Design Scenario.

Yes (D4) Based on probabilities of 10-3 for individual weld defects and of 0.05 that 1 out of the 50 disposal containers and waste capsules contain a defect, the probability that the defected waste capsule is inside the defected disposal container is 0.05 × 0.05 ÷ 50 = 5 × 10-5 . Although of low probability, the consequences of this scenario could be high (since there could be immediate release from the waste package) and warrants analysis. This is considered to be the most likely two-barrier failure scenario. All welds okay due to QA/QC except for the closure weld in one 316 Ldisposal container and degraded/incomplete disposal/disturbed zonecement grout. All other near field barriers as per Design Scenario.

No, but covered by ‘What-if’ calculations presented separately from the Defect Scenario results

Considered to be low probability since it requires two independent failure events.