Appendix CRF - Part 3 - Northamptonshire County Council

Radiological Assessment 0820-2
Version 2
t te
P
eff Ptotal
AE runoff
E 1
t f te
Peff total
PAE runoff
1 0 for te < t tf
for t > tf
where qinf is the volume of water entering the landfill through the cap
(m 3 year -1 ).
alandfill is the area of the landfill (m 2 ).
Peff is the potential rate of water infiltration through the cap of the
landfill (m year -1 ).
Ptotal is the total precipitation (m year -1 ).
AE is the amount of precipitation that is lost by evapotranspiration
(m year -1 ).
runoff is the amount of precipitation lost by runoff (m year -1 ).
E0 is the initial cap efficiency (a dimensionless fraction of the
infiltration water initially deflected by the cap).
t is the time after closure (years).
tc is the time cap is emplaced (years).
te is the time cap starts to degrade (years).
tf is the time of cap failure (years).
The HRA (p 31) provides a table of average monthly effective rainfall (rainfall minus
potential evapotranspiration) for the period 1961 to 1990, indicating a mean annual
value of ~ 0.072 m. In comparison with the annual rainfall for the area (~ 0.6
m/year), this value is low for effective rainfall but is considered appropriate for net
infiltration (effective rainfall minus runoff).
Based on the assumptions for infiltration made in the HRA, the initial cap efficiency
is set at 99%. The period of cap effectiveness is assumed to be 60 years and the
period of cap degradation is assumed to be 100 years.
In addition to providing protection to the landfill against infiltration, the cap also
reduces potential doses from external radiation to members of the public living and
working on the cap after closure. For these calculations, the cap is assumed to have a
minimum thickness of 1.5 m.
4.1.5 Operational period
For the purpose of the assessment, it is assumed that the facility will continue in
operation for a further 5 years, and that LLW disposals will take place throughout this
period.
It is proposed that waste will be transported to the site in suitable transport packages
and disposed directly, and that loose waste will not be handled at the site. It is also
proposed that waste will be covered by 0.3 m of soil material. These practices will
ensure that members of the public off-site will not be exposed to groundshine. On-
Galson Sciences Limited 31 14 July 2009
WS010001/ENRMF/CONSAPPCRF 581