Crack Formation in Cementitious Materials Used for an Engineering Barrier System and Their Impact on Hydraulic Conductivity from the Viewpoint of Performance Assessment of a TRU Waste Disposal System

Abstract

The mechanical analysis code MACBECE2014 has been developed at the Japan Atomic Energy Agency (JAEA) to make realistic simulations of the physical integrity of the near field for performance assessment of the geological disposal of TRU waste in Japan. The MACBECE2014 code can be used to evaluate long-term changes in the mechanical behavior of the near field and any subsequent changes in the permeability of engineering barrier components, including crack formation in cementitious materials caused by expansion due to metal corrosion. Cracks in cementitious materials are likely to channel the flow of groundwater and so the represent preferred flow paths of any released radionuclides. Mechanical analysis was conducted using the MACBECE2014 code to investigate the concept of the TRU waste disposal system described in JAEA's Second Progress TRU Report. Simulated results of a disposal system with a bentonite buffer demonstrated that the low permeability of the engineering barrier system could be maintained for long time periods because the physical integrity of the bentonite buffer remained intact even if cracks in the cementitious components had formed locally. Simulated results of the disposal system with a concrete backfill instead of a bentonite buffer showed that crack formation leads to a significant increase in the permeability of the engineering barrier system.

Cement and Concrete for Nuclear Waste Management

“…For bentonite buffer, MACBECE uses the elasto-viscoplastic constitutive model proposed by Ohno et al (2006). The mechanical properties of the bentonite buffer (bentonite 70 wt%, silica sand 30 wt%, dry density 1.6 Mg/m 3 ) used in MACBECE are from the work of Hirano et al (2016).…”

“…This combination of features makes it very difficult to identify which of the non-linear processes will ultimately control concrete degradation and crack development under a given set of service life conditions by expert judgement alone. For example, decalcification caused by the dissolution of the cement hydrate minerals and expansive reactions due to precipitation of secondary minerals and corrosion of steel components can all contribute to crack development in concrete (Okutsu et al 2005;Ferraris et al 1997;Hirano et al 2016;Babaahmadi et al 2015;JAEA 2018). Once established, groundwater flowing into the crack could either cause further decalcification thereby widening the crack and reducing the strength of the concrete (Höglund 2014) or else cause the precipitation of secondary minerals resulting in clogging and/or densification, which could lead to an increase in the strength of the concrete (Babaahmadi et al 2015;Metcalfe et al 2017).…”

A Coupled Modeling Simulator for Near-Field Processes in Cement Engineered Barrier Systems for Radioactive Waste Disposal

“…where α is the corrosion rate (m/y) and β is the corrosion swelling ratio (treated as a magnification). The assumed corrosion rate α is 0.04 µm/y (considering the corrosion of both sides: [36]) and the assumed corrosion swelling ratio β is 3.2 [36], so that γ = 8.8 × 10 −8 (m/y).…”