Digging repository deeper may not make it safer, says Nagra

Additional documentation on the engineering feasibility of a deep repository has been submitted by Switzerland's national radioactive waste disposal cooperative Nagra. It claims there are no safety advantages to constructing a deeper repository than originally envisaged.

In January 2015, Nagra proposed that further investigations are carried out at the proposed siting regions of Zürich Nordost and Jura Ost in the third and final stage of Switzerland's plan for selecting sites for two repositories: one for low- and intermediate-level waste (LLW/ILW) and the other for high-level waste (HLW). It also said the four other regions under consideration in the second stage - Südranden, Nördlich Lägern, Jura-Südfuss and Wellenberg - will be placed in reserve.

The six sites were proposed in November 2011 during the first stage of the selection process. Nagra is required to propose at least two regions to host each of the repositories for further investigations in the third stage.

Last September, the Federal Nuclear Safety Inspectorate (ENSI) requested that Nagra submit additional technical documentation related to the optimization of the repository depth in terms of safety. It was required to show whether construction at greater depth involves disadvantages in terms of safety and whether modifying the repository concept would be advantageous for such depths.

In February, two Swiss expert groups released separate reports supporting the repository siting proposals put forward by Nagra. However, they called for further investigations to be carried out in the Nördlich Lägern region. Nagra has already stated, "The geological siting region of Nördlich Lägern for a HLW repository has insufficient underground space in the preferred depth range and there is no possibility of realising the repository at the optimum (from an engineering viewpoint) depth."

Nagra has now provided ENSI with the requested information. It said it has "considered a range of concepts for disposal chambers and sealing sections at different depths". Nagra added, "Variants of potential repository and barrier concepts were also described and compared from the viewpoint of depth of construction." According to Nagra, this information is "particularly relevant with respect to the Nördlich Lägern siting region".

Nagra said the information it has now submitted to ENSI supports its earlier conclusion. "Unless it is necessary, a repository for high-level waste in the Opalinus Clay should be constructed no deeper than 700 metres and a repository for low-level waste no deeper than 600 metres," it said. Nagra is of the opinion that constructing the repositories deeper than this "would bring disadvantages with respect to safety". Building a repository at an optimum depth "allows extreme conditions during construction, operation and possible waste retrieval to be avoided and prevents the geological barrier from being unnecessarily compromised", Angra claims.

Nagra executive board member Piet Zuidema said, "Our analyses have shown that constructing a repository at great depth in the Opalinus Clay is highly challenging." He added, "It is important for safety that we should give priority to conditions that are straightforward in terms of construction."

ENSI is expected to complete a detailed review in early 2017 of the reports and analyses submitted by Nagra. Based on the result of these examinations, the Federal Council is expected to decide by the end of 2018 whether to agree with the location areas proposed by Nagra for consideration in the third stage.

ENSI noted, "Until this decision is made, all six potential siting areas basically remain in the selection process."

Nagra said, "It is impossible to predict today whether ENSI will share this view in its overall final assessment. In order to avoid further delays in the site selection process, Nagra will therefore carry out 3D seismic measurements in the Nördlich Lägern region from autumn 2016 and prepare applications for exploratory boreholes in the region."

Researched and written
by World Nuclear News