4.2. DESIGN SCENARIO
30
TABLE 11. TEMPORAL EVOLUTION OF THE NEAR FIELD COMPONENTS FOR THE DESIGN SCENARIO
Near field component
Temporal evolution Source
container
In many cases the source container will still be intact at the time of disposal, due to proper quality control and quality assurance procedures during the conditioning of the sources. However, this cannot be guaranteed, nor can the longevity of the source container be guaranteed. Consequently, it is assumed that the source container will have failed prior to disposal. It is assumed that the radionuclides in the source container are available for potential release only once the capsule that surrounds the source container is breached.
Capsule A number of different types of corrosion can occur including general and localized (e.g. pitting and crevice). These mechanisms can be enhanced by high chloride concentrations in water and oxidising conditions. More rapid breaching of the capsule can be expected to result from localized corrosion in unsaturated conditons. Corrosion of capsule is assumed to start only once the disposal container and the associated containment barrier has been breached by water (see below).
Containment barrier
Physical and chemical degradation of the cement grout will start only once the disposal container has been breached and the cement grout is contacted by water. Initially the hydraulic conductivity might decrease due to carbonation, however with time it will increase due to the physical (e.g. cracking) and chemical (e.g. calcium leaching and sulphate attack) degradation of the cement grout due to contact with flowing water. Chemical degradation generally results in a decrease in the cement grout’s sorption capacity.
Disposal container
See discussion concerning capsule for corrosion mechanisms. Corrosion of disposal container is assumed to start before the corrosion of the capsule.
Disposal zone backfill
It is assumed that any shrinkage or jointing cracks that might form in the cement grout backfill do not act as significant water flow or radionuclide migration pathways. Initially the hydraulic conductivity might decrease due to carbonation, however with time it will increase due to the physical (e.g. cracking) and chemical (e.g. calcium leaching and sulphate attack) degradation of the cement grout due to contact with flowing water, especially once the borehole casing starts to fail (see below). Chemical degradation generally results in a decrease in the sorption capacity of the cement grout.
Disposal zone plug
Assumed to behave in the same manner as the disposal zone backfill.
Casing Processes such as embrittlement, cracking and biodegradation are assumed to result in the failure of the HDPE casing. References [30], [31] suggest HDPE lifetimes in the region 100 to 400 years. However, there is considerable uncertainty over lifetimes and it is therefore conservatively assumed that the casing fails immediately following closure.
Disturbed zone backfill
Assumed to behave in the same manner as the disposal zone backfill.
Closure zone backfill
The closure zone backfill will be subjected to surface erosion at an assumed rate of 3 × 10-4 m/y. The characteristics of the native soil/crushed rock used to fill the first 5 m of the closure zone from the ground surface is assumed to remain constant. The cement grout used to fill the remainder of the closure zone is assumed to behave in the same manner as in the disposal zone.
4.2.1.3. Post-institutional control period
Due to the corrosion of the stainless steel disposal containers and the subsequent corrosion of the capsules, water eventually contacts the source container, which is assumed to have failed prior to disposal. The radionuclides in the source could be in a number of different physical and chemical forms (Table 12) and so release of radionuclides could occur in the liquid or gas phase.
For radionuclides released in the liquid phase, transport from the source through the various components of the near field can occur by advection, dispersion and diffusion. The relative importance of these processes depends upon the hydrogeological conditions at the site. Migration through the near field is limited by decay/in-growth and sorption of the radionuclides onto the cement grout in the near field. On leaving the near field, the radionuclides migrate through the geosphere by advection, dispersion and diffusion and are subject to decay/in-growth and retardation due to sorption onto the rocks. Flow can be through pores or fractures and diffusion can occur into stagnant water in the rock matrix depending upon the characteristics of the geosphere (Section 3.2). Again the relative importance of these geosphere processes depends on the hydrogeological conditions at the site. The groundwater is assumed to be abstracted from the geosphere via an abstraction borehole that is drilled at the start of the post-institutional control period. The borehole is assumed to be 100 m down the hydraulic gradient from the disposal borehole (Section 3.2) and used for domestic purposes (drinking)
and agricultural purposes (watering of cows and irrigation of root and green vegetables) (Section 3.3).
The water is not treated or stored before use. The main features of the Design Scenario for radionuclides released in the liquid phase into the unsaturated and saturated disposal zones are summarized in Figs 6 and 7, respectively.
FIG. 6. Design scenario: liquid releases for unsaturated disposal zone.
FIG. 7. Design scenario: liquid releases for saturated disposal zone.
Waste Disposal Zone Closure Zone
Watertable 100m
>30m
50m
10m
Groundwater Flow Direction
Contaminant Migrates through the Unsaturated Zone via Percolating Meteoric Water Disposal
Borehole
Water Abstraction from Borehole at 266 m y
Contaminant Plume
Water Abstraction Borehole
3 -1
Waste Disposal Zone Closure Zone
100m
>30m Disposal
Borehole
Water Abstraction from Borehole at 266 m y
Groundwater Flow Direction
Watertable
3 -1
Contaminant Plume 50m
Water Abstraction Borehole
32
TABLE 12. PHYSICAL AND CHEMICAL FORMS OF RADIONUCLIDES IN DISUSED SOURCES Radionuclide Physical/chemical form
H-3 Often tritium gas or liquid as H2O
Co-60 Metallic form in thin discs or small cylindrical pellets. Very low solubility.
Ni-63 Solid, electrical deposition on metal foil.
Kr-85 Gas.
Sr-90 Oxide or titanate form.
Often silver plated for medical applications.
Ceramic or glass bead or rolled silver foil for other applications.
Cs-137 Only used as a salt (often caesium chloride).
Sometimes ceramic form for weak sources (very low solubility).
Pb-210 Solid, mainly carbonate and sulphate.
Ra-226 (+ Rn gas) Very reactive alkaline earth metal in form of salts (e.g. bromides, chlorides, sulphates or carbonates). All soluble.
Pu-238 (+ Rn gas) Used in RTGs, and for neutron generators and calibration. Sources typically have Pu oxide in ceramic.
Pu-239
Am-241 Chemical characteristics similar to rare earth metals. Americium oxides normally used.
For neutron sources, fine americium oxide powder used mixed with beryllium powder. Often in pellet form.
Sometimes in sintered form.
The failure of the containers and capsules allows radioactive gases to be released, which are assumed to migrate up the borehole through the closure zone into the biosphere. It is conservatively assumed that a dwelling is constructed on top of the borehole (without intruding into the disposal zone of the borehole) at the start of the post-institutional control period, resulting in the gases migrating directly into the dwelling and being inhaled by the occupants. The main features of the Design Scenario for radionuclides released in the gas phase into the unsaturated and saturated disposal zones are summarized in Figs 8 and 9, respectively.
FIG. 8. Design scenario: gas releases for unsaturated disposal zone.
FIG. 9. Design scenario: gas releases for saturated disposal zone.
The combination of the assumed surface erosion rate (3 × 10-4 m/y – see Section 3.3) and the depth of the disposal zone from the ground surface (30 m – see Section 3.1.2) results in the waste being
Closure Zone
Waste Disposal
Zone 50m
>30m
Watertable Disposal
Borehole
Dwelling Constructed after Loss of Institutional Control
Upward Migration of Gases into Dwelling
Closure Zone
Waste Disposal Zone
50m
>30m
Watertable Disposal
Borehole
Dwelling Constructed after Loss of Institutional Control
Upward Migration of Gases into Dwelling
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.