The important role and performance of engineered barriers in a UK geological disposal facility for higher activity radioactive waste

Highlights

• UK radioactive waste will be disposed of in a geological disposal facility (GDF).

• Radionuclides may be contained for a period of up to 3 million years by a GDF.

• The engineered barriers employed are highly dependent on the wasteform and geology.

• Barriers may serve multiple roles e.g. backfill, redox control, pH buffering etc.

• These roles are critical to ensuring radionuclide release is minimised.

Abstract

The effective management of radioactive waste is a necessary prerequisite to the use of nuclear energy. The UK's policy for the long-term management of higher activity radioactive waste (HAW), and potentially spent nuclear fuel (SNF), is disposal in a deep underground geological disposal facility (GDF). A GDF will isolate HAW from mankind until the radioactivity has decayed to levels where any risk to future generations is acceptably low. It is likely, therefore, that a GDF will need to safely contain radioactive materials for hundreds of thousands of years. The necessary isolation will be provided by a combination of natural (geological) and engineered barriers. A multi-layered engineered barrier system will provide the defence-in-depth that is required to give the public confidence in the long-term performance of the GDF. This paper identifies the significant role each engineered barrier or “layer” plays in ensuring that long-lived radionuclides remain isolated from the biosphere and receptors within the vicinity of a GDF. Receptors include human and animal populations, and the natural environment. The paper also explores the characteristics and performance of a number of suitable candidate materials for use in the UK GDF engineered barriers. An indication of the lifetime of potential barriers under conditions pertinent to each of the UKs proposed geological settings is given. As the performance of the engineered barriers will be vital to the GDF post-closure safety case, several areas for further work are proposed.

Introduction

The long-term management of radioactive waste has not been straightforward in the UK. Public concern over the long-term behaviour of radioactive waste and its possible impact on the health of future generations has led to the rigorous evaluation of all disposal options (Committee on Radioactive Waste Management, 2006). International consensus is that the safest, most comprehensive option for the long-term management of higher activity radioactive waste (HAW) is deep geological disposal (Nuclear Energy Agency, 2008). The UK, Welsh and Northern Irish Governments’ initially supported this approach, with the intention to dispose of HAW in a geological disposal facility (GDF), and this has since been written into UK and Welsh policy (Department for Environment, Food & Rural Affairs, 2008). As of April 1, 2019, the UK reported over 100,000 m³ of waste that is classed as HAW (Department for Business Energy & Industrial Strategy, 2019). This contains a small fraction of low-level waste (LLW) unsuitable for near-surface disposal, 102,000 m³ of cementitious intermediate level waste (ILW) and 2150 m³ of vitrified high-level waste (HLW). The latter includes vitrified fission products from the reprocessing of spent nuclear fuel (SNF) and corresponds to approximately 95% (80, 000, 000 TBq) of the total inventory activity; ILW possesses ~5% (4,100,000 TBq) whilst the entire LLW inventory contains only 32 TBq (Department for Business Energy & Industrial Strategy, 2019).

Geological disposal refers to the emplacement of solid radioactive wasteforms in an engineered underground facility, within a stable geological setting, that provides long-term containment and isolation from the biosphere, without impacting on background levels of radioactivity (International Atomic Energy Agency, 2011; Nuclear Energy Agency, 1995). A GDF is an engineered structure, at a depth of between 200 and 1000 m. The minimum depth is required to ensure that the repository will not be affected by forecasted glacial and surface erosion, whilst depths below 1000 m offer little advantage because of increased construction costs and engineering challenge. A GDF is required to isolate HAW from mankind until the radioactivity has decayed to levels where any risk to future generations is acceptably low. It is likely, therefore, that a GDF will need to safely contain radioactive materials for hundreds of thousands of years. This critical isolation function will be provided by combination of geological and engineered barriers. In providing the containment of radionuclides from the biosphere, a multi-layered engineered barrier system shall, as well as facilitating significant radionuclide decay, provide a defence-in-depth that provides the public with confidence in the long-term performance of the GDF. In the UK, three types of geological setting have been identified as suitable generic hosts, comprising: high strength “hard” rock (HSR), lower strength sedimentary rock (LSSR) and evaporites. The interaction between the geology and the engineered barriers is a significant factor in the development of the long-term post closure safety case. Hence a detailed understanding of the interaction between the geology and the engineered barrier materials is essential. This paper explores the role of the engineered barriers and the performance of engineered barrier materials, commenting upon the current level of understanding of the likely lifetime of these barriers under GDF conditions.