2009 Report to Government on National Research and

near-field of the part containing the ILW and LLW and the near-field of the part
containing the HLW and spent fuel. An important question for site selection and
GDF design is how this can be achieved in various geological environments. A
key question is whether it is possible <strong>to</strong> demonstrate that there will be sufficiently
limited interaction between the two near-fields over sufficiently long time periods
for a post-closure safety case <strong>to</strong> be developed. There are a number of <strong>to</strong>pics on
which R&D is required in order <strong>to</strong> address these questions and hence inform a
decision on whether one co-located GDF or two (or more) separate GDFs will be
the most appropriate option.
A.78 ILW disposal concepts considered <strong>to</strong> date in the UK involve the use of large
volumes of cement-based materials in the waste packages, the backfill and in
construction of a GDF. Several concepts for the disposal of HLW and spent fuel
involve the use of ben<strong>to</strong>nite backfill. The cement-based materials will cause
groundwaters that come in<strong>to</strong> contact with them <strong>to</strong> become highly alkaline. The
potential interactions between these high pH groundwaters and ben<strong>to</strong>nite require
further research, as do the interactions between high pH groundwaters and
vitrified HLW.
A.79 At a major workshop on ben<strong>to</strong>nite-cement interaction (NUMO, 2004) it was
emphasised that significantly more and new experiments, and new coupled
chemical diffusion–advection models, are required <strong>to</strong> understand and predict
ben<strong>to</strong>nite behaviour, including swelling and evolution of high gas pressures,
dissolution, cementing and porosity reduction by precipitation of silica, cation and
anion affinities under high pH, and overall barrier system performance. Most
workers considered that low pH alternatives <strong>to</strong> Ordinary Portland Cement (OPC)
and composite cement systems need development, and that systems with lower
cement/ben<strong>to</strong>nite ratios were less likely <strong>to</strong> be problematic than those in which the
cement/ben<strong>to</strong>nite ratio is high or the groundwater chemistry in a GDF
environment dominated by cement interaction (NUMO, 2004).
A.80 The NUMO 2004 executive summary and conclusions, which constitutes their
considered set of suggestions and recommendations for future action, is
extracted below <strong>to</strong> illustrate the recognised issues:
• Additional experiments <strong>to</strong> verify present understanding should be
undertaken.
• Additional information on the long-term evolution of cement-barrier systems
would be desirable.
• Consideration should be given <strong>to</strong> long-term experiments.
• Analogues could yield useful information.
• There are underground labora<strong>to</strong>ry facilities, such as the Mol URL in Belgium,
which have been operating for 20 <strong>to</strong> 30 years and could be used <strong>to</strong> give
useful information on these time-scales.
• It is desirable for new materials <strong>to</strong> be considered in place of cement and <strong>to</strong>
question ‘conventional wisdom’ which states that cement is the best material
<strong>to</strong> use for construction and barrier functions in many circumstances.
• It is desirable <strong>to</strong> develop low pH cements (pH ≤ 11, but not so low as <strong>to</strong>
promote excessively rapid corrosion of steel materials) with particular
CoRWM Document 2543, Oc<strong>to</strong>ber <strong>2009</strong> Page 119 of 151