Engineering

Based on the narrow diameter design developed under the IAEA’s AFRA project [9], the reference design for the near field evaluated in the GSA is assumed to comprise a series of engineered components which are described below, illustrated in Figs 2 to 4, and summarized, together with their post-closure safety related functions, in Table 4. Alternative dimensions to those given below are considered in the variant calculations presented in Section 6.2.

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FIG. 2. Schematic representation of the borehole site. Reproduced courtesy of Van Blerk [25].

FIG. 3. Cross-section through the disposal borehole for the reference design evaluated in the GSA. Reproduced from Ref. [8].

TABLE 4. NEAR FIELD COMPONENTS FOR THE REFERENCE DESIGN EVALUATED IN THE GSA

• No safety function since it is assumed that the source container will have failed prior to disposal.

Capsule Standard stainless steel (Type 304) capsule containing the source container

• Until breached, isolates source from water, animals and humans;

• Until breached, prevents escape of gas from source;

• Once breached, limits release of radionuclides available for release from the capsule until it has been corroded.

Containment

• Physical barrier – can inhibit disruption of the disused source by surface erosion, human intrusion, and biotic intrusion;

• Physical barrier – once the disposal container has been breached, can limit flow of water around the capsule due to low permeability;

• Physical barrier – once the capsule has been breached, can act as low permeability barrier to the migration of radionuclides from the borehole in liquid and gaseous phases;

• Cement can passivate corrosion of stainless steel capsule and reduce chloride levels in water through formation of calcium chloride;

• Chemical barrier – once the capsule has been breached, can act as sorption barrier for radionuclides released;

• Chemical barrier – once the capsule has been breached, can act to regulate the availability of radionuclides for release into water through its impact on the solubility of radionuclides . Disposal

container

Type 316 L stainless steel • Until breached, isolates source container, capsule and containment barrier from water, animals and humans;

• Once it and the capsule are both breached, the disposal container can limit the fraction of radionuclides available for release into the borehole until the entire container has been corroded.

Disposal zone

• Physical barrier – can inhibit disruption of the disused source by surface erosion, human intrusion, and biotic intrusion;

• Physical barrier – can limit the flow of water around the disposal container due to low permeability;

• Physical barrier – once the disposal container and capsule have been breached, can act as low permeability barrier to the migration of radionuclides from the borehole in liquid and gaseous phases;

• Cement can passivate corrosion of stainless steel capsule and reduce chloride levels in water through formation of calcium chloride;

• Chemical barrier – once the disposal container and capsule have been breached, can act as sorption barrier for radionuclides released;

• Chemical barrier – once the capsule has been breached, can act to regulate the availability of radionuclides for release into water through its impact on the solubility of radionuclides.

Disposal zone plug

Sulphate-resistant cement grout plug at base of borehole

For unsaturated systems:

• Physical barrier – once the disposal container and capsule have been breached, can act as low permeability barrier to the migration of radionuclides from the borehole in liquid phase;

• Chemical barrier – once the disposal container and capsule have been breached, can act as sorption barrier for radionuclides released;

• Chemical barrier – once the disposal container and capsule has been breached, can act to regulate the availability of radionuclides for release into water through its impact on the solubility of radionuclides.

For saturated systems:

• Physical barrier – until the casing starts to degrade will limit the flow of water up into borehole due to low permeability.

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TABLE 4. NEAR FIELD COMPONENTS FOR THE REFERENCE DESIGN EVALUATED IN THE GSA AND THEIR POST-CLOSURE SAFETY RELATED FUNCTIONS (cont.)

Near field

• Until degraded, restricts the flow of water into the disposal zone in saturated systems.

• Physical barrier – limits the flow of water into the borehole due to low permeability;

• Physical barrier – once the disposal container and capsule have been breached, can act as low permeability barrier to the migration of radionuclides from the borehole in liquid and gaseous phases;

• Chemical barrier – once the disposal container and capsule have been breached, can act as sorption barrier for radionuclides released from the borehole;

• Chemical barrier – once the capsule has been breached, can act to regulate the availability of radionuclides for release into water through its impact on the solubility of radionuclides;

• Cement can passivate corrosion of stainless steel capsule and reduce chloride levels in water through formation of calcium chloride.

Closure zone

• Physical barrier – limits the flow of water into the borehole due to low permeability;

• Physical barrier – inhibits disruption of the disused source by surface erosion, human intrusion, and biotic intrusion;

• Physical barrier – once the disposal container and capsule have been breached, can act as low permeability barrier to the migration of radionuclides from the borehole in liquid and gaseous phases;

• Chemical barrier – once the disposal container and capsule have been breached, can act as sorption barrier for radionuclides released from the borehole;

• Chemical barrier – once the capsule has been breached, can act to regulate the availability of radionuclides for release into water through its impact on the solubility of radionuclides;

• Cement can help maintain high pH conditions which then passivate corrosion of stainless steel capsule and reduce chloride levels in water through formation of calcium chloride.

3.1.2.1.Waste Package

The waste package used for the disposal of disused sealed radioactive sources in the borehole disposal concept comprises the following components (see Fig. 3). It is assumed for the purposes of the derivation of the reference activity levels presented in Section 6 that a total of 50 waste packages are disposed in the borehole.

• The source and its container – the radioactive source material and its container (for example, the radium needles containing the radium salt, or the Pyrex tubes containing tritium gas or tritium oxide). The dimensions of the reference capsule (Table 5) limit the source and its container to be less than 110 mm in length and 15 mm in diameter.

• The capsule – assumed to be a standard stainless steel capsule (Type 304)⁷. The disused source and its associated container are emplaced in the capsule and sealed. No backfill material is used,

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7 Stainless steels are chromiun-containing steels where the Cr provides resistance to corrosion through the formation of a protective (‘passive’) Cr(III) oxide or hydroxide film. There are various classes of stainless steel, a common class being the so-called 300-series austenitic alloys. Two of these alloys have been selected for the waste capsule and disposal container.

which means that apart from the disused source and its container, the capsule is empty. The dimensions for the capsule are presented in Table 5 (alternative dimensions are considered in variant calculations presented in Section 6.2).

• The containment barrier – assumed to be a backfill comprising sulphate-resistant cement grout and filling the void between the capsule and the disposal container. The dimensions for the containment barrier are presented in Table 5.

• The disposal container – assumed to be manufactured from Type 316 L stainless steel with the reference dimensions given in Table 5. Alternative dimensions for the disposal container are considered in variant calculations presented in Section 6.2. These would allow the disposal of source containers of less than 160 mm in length and 140 mm in diameter directly into the disposal container (i.e. with no capsule). As shown in Fig. 3, the container is equipped with a lifting ring to facilitate waste emplacement in the borehole. There are also three centralisers that help to ensure that the container is emplaced centrally and vertically. The centralisers are thin (<10 mm) and do not inhibit the flow of cement grout past the top of the disposal container.

TABLE 5. DIMENSIONS OF THE CAPSULE, CONTAINMENT BARRIER AND DISPOSAL CONTAINER FOR THE REFERENCE DESIGN EVALUATED IN THE GSA

Waste package component

Length (mm) Inside diameter (mm) Outside diameter (mm)

Thickness⁸ (mm)

Capsule 110 15 21 3

Containment barrier 186 21 103 41

Disposal container 250 103 115 6

3.1.2.2. Disposal Borehole

The disposal borehole is 260 mm in diameter and is drilled to a depth of over 80 m. The borehole is fitted with a high-density polyethylene (HDPE) casing for the reference design evaluated in the GSA, although potentially more durable alternative casings such as steel could be considered if it was considered necessary to introduce an additional longer-term isolation barrier. The inner and outer diameters of the casing are 140 mm and 160 mm, respectively, giving a casing thickness of 10 mm.

Three distinct zones can be defined in the disposal borehole (see Fig. 4).

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Type 304 stainless steel has the nominal composition 18-20 wt.%Cr, 8-10.5 wt.%Ni, 1 wt.%Si, 2 wt.%Mn, 0.08 wt.%C, 0.045 wt.%P, and 0.03 wt.%S. Type 316L stainless steel has the nominal composition 16-18 wt.%Cr, 10-14 wt.%Ni, 1 wt.%Si, 2 wt.%Mn, 0.03 wt.%C, 0.045 wt.%P, 0.03 wt.%S, and 2-3 wt.% Mo, where the addition of Mo improves the resistance to localized corrosion and the reduced C content improves resistance to intergranular attack.

8 As used here thickness refers to the wall thickness of the capsule and disposal containers as well as the thickness of the containment barrier.

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Native Soil/

Crushed Roc k

Closure Zone

Disposal Zone Closure Zone

Bac kfill

Casing Disturbed Zone Bac kfill Disposal Zone Bac kfill

Disposal Zone Plug

Waste Pac kages Anti-intrusion barrier

FIG. 4. Illustration of the borehole zones for the reference design evaluated in the GSA.

• The disposal zone – the zone inside the casing in which the waste packages are disposed. The base of the disposal zone is assumed to be at least 80 m from the ground surface. A 0.5 m thick

‘plug’ of backfill slurry is emplaced at the base of the borehole. The borehole backfill slurry is assumed to be sulphate-resistant cement grout. Once the plug material is set, the waste packages are lowered into the borehole, one at a time. After the emplacement of each waste package, backfill material is poured over the waste packages to fill the 12.5 mm thick void between the waste package and the casing wall, as well as a volume on top of the waste package. The layer of backfill on top of the waste package is on the order of 700 mm to 800 mm deep. Together with the waste package, this constitutes a pitch height of about 1 m per waste package. Given that it is assumed that there are 50 waste packages to be disposed, the total thickness of the disposal zone is about 50 m.

• The closure zone – the zone between the disposal zone and the ground surface. Once the waste packages have been emplaced in the borehole, it is assumed that the casing in the closure zone is withdrawn from the borehole from a depth 1 m above the disposal zone. This removes a potential fast transit pathway to and from the disposal zone which might arise once the casing has degraded. An anti-intrusion barrier (for example a metallic ‘drill deflector’) is placed above

the disposal zone in order to deter/prevent human intrusion. The closure zone is then backfilled to a depth 5 m below the ground surface with the same backfill material used for the disposal zone. The final 5 m of the closure zone is then backfilled with native soil and/or crushed rock to the ground surface. It is assumed that the total depth of the closure zone is at least 30 m, which is considered to be a depth beyond which human intrusion is limited to drilling [8].

• The disturbed zone – the zone between the casing and the wall of the borehole. Voids and cracks in the host geology immediately adjacent to the borehole are assumed to be grouted and sealed during the drilling process with the same slurry used for the backfilling of the disposal and closure zones. In addition, an average gap of 50 mm between the casing and the borehole wall is backfilled with the slurry using a pressure grouting technique [9]. As shown in Fig. 3, the casing is fitted with centralisers to ensure that the casing is in the middle of the borehole.

These centralisers are made of thin mild steel plates inserted vertically to ensure that they do not hamper the flow of the backfill slurry.

The design of the borehole disposal concept is a final disposal concept that is not designed to facilitate the retrieval of waste packages once disposed since, once each waste package has been lowered into the borehole, it is backfilled into the borehole with sulphate-resistant cement grout. Following the emplacement of the final waste package, the closure zone above the waste package is also backfilled with sulphate-resistant cement grout. This greatly reduces the possibility of sabotage or theft of the disposed disused sealed radioactive sources.