Near-surface disposal optimisation based on knowledge and understanding (SUDOKU)

Objectives

Understanding the behaviour and performance of multi-layer covers and cementitious barriers of near-surface disposal facilities as a basis for optimization of these barriers and ensure long-term disposal safety.

Major highlights (October 2024 - March 2026) 

• The SUDOKU SOTA report (D14.1) was delivered in November 2025, providing a comprehensive summary of the main designs and safety concepts for near-surface repositories across partner countries, and the processes governing the performance of their engineered barriers.

• Initial laboratory-scale experiments were conducted on the Centre de Stockage Manche (CSM) cover topsoil to assess mechanical hydric erosion under simulated intense rainfall. These tests highlighted the impact of both initial and transient soil moisture on erosion rates.

• Testing has been completed for part of the instrumentation intended for laboratory-scale experiments investigating the behaviour of different layers comprising El Cabril’s multilayer cover. Meanwhile, the in-situ monitoring system of the El Cabril cover mock-up has been upgraded and data acquisition has resumed.

• A test procedure for single ring field infiltrometer tests in low-permeability clay has been developed referring to a French standard and supported by a series of field and laboratory-scale tests.

• All Task 4 partners have initiated their degradation protocols (following MS 14) to investigate the chemo-mechanical degradation of mortars and concretes of interest and the impact on radionuclide migration. Also, the experimental set-ups for diffusion tests were prepared and some diffusion tests on non-degraded samples were initiated. Preliminary results were presented and discussed at the Annual Meeting (3-5 March, 2026).

Forward look

• Laboratory scale tests specific to partner’s countries cover design assessing erosion and effectiveness of layers or combination of layers under different scenarios (wetting conditions, slopes, presence, or absence of vegetation etc).
• Analysis of the data collected by the monitoring system of the El Cabril multilayer pilot cover to study the cover’s performance in diverting infiltration.
• Degradation experiments simulating different degrading conditions relevant for near-surface disposal facilities of partner’s countries. In parallel, diffusion tests on non-degraded materials will be conducted to establish the reference diffusion coefficients for the radionuclides of interest.
• First set-up and tests of numerical models to assess the consequences of steel reinforcement corrosion and engineered barrier system materials degradation on the water saturation evolution and change of water flow pathways over time in the repository near-field. Knowledge of saturation state and water transport in the barrier system are the prerequisite for more realistic models of radionuclides transport in the repository near-field.

Description of the WP

The long-term safety of near-surface disposal facilities typically relies on the performance of the multilayer cover and the cementitious barrier system. The mobility of long-lived, mobile radionuclides, often the primary contributors to radiological impact, depends strongly on the properties of the engineered barriers. Specifically, the water flux through the disposal facility, which influences the migration of mobile radionuclides (e.g., ³⁶Cl, ¹⁴C, ⁹⁹Tc, ¹²⁹I), is highly dependent on the durability of these barriers. The impacts of cracking and steel corrosion products on radionuclide transfer in cementitious barriers remain insufficiently characterized and must be appropriately addressed in safety assessments.

The SUDOKU approach combines investigations on multi-layer covers with studies on the durability of cementitious barriers to assess the transport properties of mobile radionuclides, considering the chemo-mechanical evolution of these engineered systems.

The novelty of SUDOKU consists of: (i) the analysis of the hydraulic properties and their evolution within multilayer covers through laboratory and on-site experiments; (ii) the generation of experimental and modelling data on the degradation of reinforced and unreinforced cement-based materials under near-surface disposal conditions; and (iii) the characterization of transport properties for mobile radionuclides in degraded and corroded reinforced cementitious systems.

SUDOKU will provide understanding of the long-term integrity and performance of the multilayer cover under different meteorological conditions contributing to the optimization of this important barrier. The improved characterization of the durability and performance of the EBS, achieved by the work proposed in SUDOKU, will reduce the uncertainties, and improve the Safety Assessment (SA). The use of proven models in combination with on-site and laboratory studies will ensure the necessary reliability of the results and facilitate the determination of the optimal EBS configuration from the safety point of view.

Outcomes

• Understanding the factors controlling water and energy flows, erosion, and associated processes for various combinations of layers and the long-term performance of the multi-layer cover (Innovation for optimisation);
• Improving the understanding of the degradation of cementitious engineered barriers and its subsequent effect on the transfer properties of mobile radionuclides (Scientific insight and implementation safety);
• Providing recommendations for the optimization of repository designs to enhance overall long-term safety (Implementation safety).