(SATURN CDT) Developing advanced cementitious sealing materials for deep borehole disposal of high-level nuclear waste

About the Project

Deep borehole disposal (DBD) is rapidly emerging as a credible and potentially superior alternative to a mined & engineered deep geological repository, or GDF (geological disposal facility) as it is known in the UK. Among the most important advantages of DBD are: increased safety through greater emphasis on geological barriers; substantially less expensive - either by reducing the volume of the DGR/GDFor replacing it altogether; accelerating the disposal of spent nuclear fuel (SNF) by enabling earlier implementation, including requiring less time for post reactor cooling. A crucial feature of the operational and post-closure safety cases for DBD is the use of a sealing and support matrix (SSM) within the disposal zone to fill the annular space between the waste package (WP) and the borehole wall. It has 3 main functions:

1. Providing a barrier to ingress of (corrosive) saline groundwaters to protect the components of the engineered barrier system, e.g., waste package (potentially container & overpack) and casing, for as long as possible.
2. Providing mechanical support for the stack of waste packages.
3. As a potential absorbent for any radionuclides that might eventually escape from the waste package.

Emplacement of a cementitious grout at the required depths is challenging under conditions of elevated temperature, high hydrostatic pressure and difficult groundwater chemistry and the time delay in getting the newly mixed grout down to the disposal zone requires a grout with a delayed thickening time.

A decade ago the Sheffield DBD research group developed a cement formulation suitable for use as an SSM in DBD. This special grout, based on a class G oilwell cement, was demonstrated to have a delayed thickening time at elevated temperature of up to 4 hours when organic additives were used [1-5]. This work was pioneering and its significance is recognised by inclusion in IAEA documentation of DBD, but its laboratory nature is seen as being of low technical readiness level (TRL).

The overarching aim of the proposed experimental research project is to take the existing grout formulation as a starting point and to raise the TRL of this key aspect of the safety cases for DBD. Work will be carried in both in the lab and on a larger (1/3 full scale) experimental setup at the site of the Marriott Drilling Group in nearby Chesterfield. This setup is based on a UoS/Marriott design for the disposal of the UK’s inventory of spent fuel from its fleet of AGR NPPs.

The student will learn about nuclear waste management, including treatment, packaging and storage, leading to geological disposal (especially by DBD) and will have access to the IAEA CRP knowledge base with the possibility to attend a workshop and present their research. Wider aspects of the nuclear fuel cycle will be taught during the Saturn course. The student will have a chance to learn about modelling methods though this is only a minor component of this PhD.

A successful outcome to this project would overcome one of the few objections to the adoption of DBD by the international waste management community such that it could become the option of choice for the disposal of high heat generating waste (like SNF) with major ramifications for the future of nuclear power. A successful and greatly expanded nuclear power programme is needed to meet the ever-increasing energy demands from AI data centres as well as meeting the legally binding target if net zero carbon emissions by 2050. A student graduating with a PhD in this key aspect of nuclear waste management can be expected to have excellent career prospects in that sector.