Appendix CRF - Part 3 - Northamptonshire County Council

Radiological Assessment 0820-2
Version 2
Radionuclide
Specific dose
(μSv y -1 per MBq)
Borehole
1500m
Irrigation
Site
boundary
Drinking
Radionuclide
Specific dose
(μSv y -1 per MBq)
Borehole
1500m
Irrigation
Site
boundary
Drinking
Eu-152 2.77E-32 4.84E-25 Am-241 5.37E-11 1.08E-08
Eu-154 3.28E-35 7.98E-28 Cm-243 1.82E-10 1.63E-08
Eu-155 3.63E-41 1.29E-33 Cm-244 1.29E-09 1.15E-07
Pb-210 1.25E-25 1.43E-18
Table 5.1 Specific doses to members of the public via the groundwater pathway.
Results include doses arising from ingrowth of daughter radionuclides
for 100 years.
The results from the sensitivity studies show that the calculated doses for most
radionuclides are not sensitive to leachate head. This is because the most significant
releases take place after the engineered barriers (cap and liner) have degraded and
radionuclide transport into groundwater is governed by the infiltration rate through
soil and the properties of the geological barrier. Radionuclides with short half-lives
do show some sensitivity to leachate head, because they have largely decayed by the
time the barriers degrade. Calculated doses for these radionuclides are determined by
the relatively small releases while the barriers are effective, and the magnitude of
these releases is governed by the leachate head. The small changes in calculated
doses for longer-lived radionuclides arise because some of the inventory is lost from
the site while the barriers are effective, and this reduces the inventory available for
later release.
Results from the sensitivity studies for cap lifetime again show a dependency on halflife.
Calculated doses for long-lived radionuclides show little variation with cap
lifetime because they are not released in significant amounts during the period the cap
is effective. Radionuclides with shorter half-lives show a sensitivity to cap lifetime;
this affects whether a significant inventory is still available for release once the
engineered barriers have degraded.
Sensitivity studies for cap efficiency show that this has relatively little effect on
calculated doses. This is because it is the barriers at the base of the landfill that have
most effect on the release of radionuclides during the period when the engineered
barriers are effective. In practice, a less effective cap would allow more infiltration
which would lead to an increase in leachate head and potentially to bath-tubbing if the
site was not monitored and managed. The assessment methodology used does not
explicitly model these links and so the secondary effects of changes in cap efficiency
are not apparent in the calculated doses.
In the case of the sensitivity studies on the effects of varying the assessment period,
the reverse of the effects discussed above is apparent – calculated doses for shortlived
radionuclides show little or no sensitivity and those for long-lived radionuclides
are very sensitive. This is because radionuclides are released only slowly to the
Galson Sciences Limited 44 14 July 2009
WS010001/ENRMF/CONSAPPCRF 594