Appendix CRF - Part 3 - Northamptonshire County Council

Radiological Assessment 0820-2
Version 2
The assessment model developed as part of the SNIFFER methodology was based on
the disposal of LLW at non-hazardous waste sites (SNIFFER 2006a,b), and the
default values for an effective geological barrier were set to a thickness of 1 m and a
hydraulic conductivity of 1 x 10 -9 m s -1 . The assumption was that this barrier would
be provided by an artificial mineral layer between a basal liner and the natural
geological barrier. The presence of a natural geological barrier (unsaturated zone) is
allowed for in the assessment model, but typical characteristics of this zone mean that
it is likely to be less resistant to leachate transport than the artificial geological barrier
and effectively redundant in terms of assessing potential doses via the groundwater
pathway.
In the case of the landfill cap, the default values used in the SNIFFER methodology
assume that the cap is initially 95% efficient in terms of preventing infiltration into
the landfill, and that the cap will gradually degrade over a period of 60 years from the
time of emplacement until it is no more effective than a soil layer. Also, the
SNIFFER methodology assumes that it is only the effectiveness of the cap that limits
infiltration of leachate into groundwater after the end of the operational period – the
liner is assumed to become ineffective at the time the cap is emplaced.
At the ENRMF, the basal liner in the area considered for disposal of radioactive
wastes is constructed with a 2 mm thick high density polyethylene (HDPE)
geomembrane, and at least 1.5m thickness of artificially emplaced geological barrier
(Upper Lias clay sourced locally). This clay is placed with a maximum design
permeability of 3x10 -10 m/s.
The final cap for the ENRMF comprises a gas drainage layer, clay regulating layer,
geotextile protector, geosynthetic clay liner, LDPE geomembrane liner and soil cover.
The cap design will aim for a minimum effectiveness of 99%.
Following capping the assessment model assumes that there will be a minimum of 60
years management of the site, which will include monitoring of leachate levels within
the waste. In practice the management period will be considerably longer. This will
enable the effectiveness of the cap and the bottom liner to be assessed and for
mitigation measures to be taken if there is evidence of damage or deterioration. It is
therefore reasonable to assume that the design performance of the cap and liner will
be maintained during the management phase and that degradation will not take place
until after the withdrawal of control.
3.2.3 Distribution of waste
The SNIFFER methodology does not require the actual volume of waste to be
specified, because the dose calculations are based on the disposal of 1 MBq of each
radionuclide. However, to determine the concentration of waste that might be
excavated after site closure, the methodology assumes that all of the disposals at a
particular site could be in part of a cell as small as 10 m 3 .
The proposed disposals of LLW at the ENRMF could form a significant proportion of
the material disposed of to the selected cell. The SNIFFER methodology has
Galson Sciences Limited 24 14 July 2009
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