Design and Performance Assessment of HLW Disposal Canisters

pdf NTB 24-20 Design and Performance Assessment of HLW Disposal Canisters(15.88 MB)

Regulatory guidelines require the development and use of disposal canisters as an indispensable component of the multi-barrier system of a deep geological repository for high-level waste. The primary long-term safety function of the canisters is to provide complete containment of radio­nuclides for a certain period of time.

In this report, a potential method for manufacturing and sealing disposal canisters based on present day technology and satisfying all relevant requirements is presented. The provisional canister design consists of a cylinder with flat lids. Depending on the waste type to be disposed of the canisters are approximately 4 to 5 m long and have a diameter of approximately 0.7 to 1.1 m. The canisters are forged from a bespoke low carbon steel alloy. Both lids will be welded on the cylindrical body using electron beam welding. The bottom lid, which is welded prior to loading the canister with the waste, will undergo a global heat treatment in a furnace for stress relief. The top lid, which is attached with the closure weld to seal the canister after waste loading, will undergo a local heat treatment to relieve residual stresses. Both welds will be appropriately inspected for potential flaws using non-destructive examination methods.

The regulatory requirement for the canister lifetime during disposal is at least 1,000 years. The current provisional canister design was developed with the aim to demonstrate that this require­ment can be satisfied with a significant safety margin. The prediction of the canister lifetime is based on the principle that the evolution of the repository is well understood, such that the behaviour of the canister material can be described, and the potential failure mechanisms can be assessed. The assessment of the structural integrity of the canister is achieved based on a standardised fracture mechanics methodology using a set of conservative assumptions, namely:

• the canister will have a reduced wall thickness, in accordance with a conservative corrosion rate,
• the canister material will exhibit a reduced fracture toughness, in accordance with conserva­tive assumptions related to hydrogen embrittlement,
• the maximum anisotropic lithostatic stresses expected at repository depth act on the canister,
• the residual stresses calculated from the modelling of the weld process and the post-weld heat treatment are present in the weld region,
• and various types of weld flaws have escaped detection and are present in the weld.

Even based on these conservative assumptions, an engineering critical assessment approach demon­strates that the current provisional canister design can satisfy the regulatory lifetime require­ment with a significant safety margin.