Implications of Voidage for Post-closure Safety of a Geological Disposal Facility

Radioactive Waste Management Limited (RWM) is undertaking a programme of work to consider further the implications of voidage for post-closure safety of a geological disposal facility (GDF) for higher activity waste. In this context, voidage is defined as macroscopic voids, within or around waste packages in the Engineered Barrier System (EBS) of a GDF.

Wastes are being packaged for disposal in the UK now, and RWM provides advice to waste packagers regarding how waste can be packaged such that it is in a form that is compatible with a range of UK-relevant geologies that could in due course be selected to host a GDF. The disposability assessment process currently seeks to minimise voidage in waste packages, with the associated intent to minimise any potential impacts for GDF performance; packaging processes deployed by waste packagers need to address voidage minimisation.

As part of its programme of work, RWM recently commissioned a study to provide a peer-reviewed evidence base as input to a multi-year programme of work that will in time enable it to consider an update of the advice it currently provides to waste packagers in relation to in-package voidage. A related report has recently been published; further work is currently ongoing.


The recently-published study identifies where and how the presence of voidage may affect the mechanical evolution of a GDF, and coupled hydro-chemical processes. It then considers how the effects may be beneficial or detrimental to safety performance, and for which host rocks and disposal concepts the effects may be most significant.

Options for Managing Voidage

For some waste package types voidage can be beneficial to performance or is not an issue in terms of safety; while for others voidage may need to be managed to ensure it does not adversely affect the post-closure performance of a GDF. The report describes a range of options for managing voidage, with the preferred approach varying dependent on the waste package type. In particular, the report describes:

  • the safety arguments that can be made in support of accepting voidage in waste packages;
  • where it may be appropriate to limit voidage in waste packages, and potential limits;
  • where the presence of voidage in waste packages may be beneficial and how this may be factored into the potential limits;
  • the practicalities and implications of options for void filling packages immediately prior to disposal in a GDF; and
  • the implications of voidage for GDF siting.

The findings of the study have been independently peer reviewed, with one set of reviewers commenting, “The report is unique as it is probably the first report trying to tackle the issue as a whole. In this respect it is a reference document”.

The study report 'Implications of Voidage for Post-closure Safety of a GDF' can be downloaded from the RWM bibliography.

The image, courtesy of RWM, shows an impression of part of an underground vault in higher strength rock containing unshielded intermediate level waste.