Optimized Low Heat High Performance Concrete Mix For DGR Shaft Sealing

Abstract

Concrete is expected to be an important part of the shaft seals installed during the decommissioning and closure phase of a deep geological repository (DGR) in Canada. Concrete structures are presently part of the conceptual DGR design of the Nuclear Waste Management Organization for used nuclear fuel. Low alkalinity concretes are favored for use in DGRs as they will minimize the potential for adverse chemical interaction with bentonite-based materials used in these repositories.

A low heat high performance concrete (LHHPC) reference mix design was previously developed and key properties evaluated. This paper presents results of an extensive program aimed to (a) optimize the LHHPC reference mixture, (b) fabricate specimens of LHHPC seal mixes and (c) measure key performance parameters expected to meet the performance targets. This work was part of an ongoing project where a methodology was developed to design optimized LHHPC mixtures using mix ingredients from local and sustainable sources. Site-specific waters with different chemical compositions representative for two potential DGR sites were prepared and used for the curing and testing of various relevant optimized LHHPC properties. Additionally, development of a more compatible LHHPC mix design for use in DGRs also addressed hydrogen gas release due to reaction of free silicon in silica fume with alkaline release during concrete mixing, as well as compatibility with the limestone host rock of the DGR. It also considered availability of mix ingredients including Portland cement and admixtures in the original mix design. The optimized LHHPC mix developed met the performance targets for the application. Based on the findings of the test program conducted for the optimized LHHPC mix recommendations were made for revised performance parameters and targets.