Appendix CRF - Part 3 - Northamptonshire County Council

Radiological Assessment 0820-2
Version 2
therefore been revised to account for waste deposited throughout the cell rather than
in a particular volume. For the radiological assessment, it is again not necessary to
assume the volume of waste, because the calculations are based on a unit disposal of 1
MBq, homogeneously dispersed throughout the cell. Assumptions regarding waste
activity are required to convert the calculated radiological capacity into waste
volumes and to determine whether the assumption concerning homogeneity is
reasonable.
3.2.4 Leachate concentration
There are significant uncertainties associated with modelling the release of
radionuclides from radioactive waste and into leachate. The mechanisms by which
this release would occur depend on the type of waste (e.g., waste composition, how it
is contaminated and how it is packaged), on the conditions within the landfill (e.g.,
pH, Eh, degree of saturation) and on the radionuclides concerned (e.g., whether they
are readily sorbed). Even with detailed mechanistic models of waste behaviour,
significant variability (due to heterogeneities in the wastes and landfill conditions)
and uncertainties (due to lack of information about the processes involved) would
remain.
In the SNIFFER methodology, these uncertainties are treated by means of
conservative assumptions:
For scenarios involving waste excavation, it is assumed that the entire
radionuclide inventory remains in the solid waste and that there are no losses
to leachate.
For scenarios involving leakage of leachate, it is assumed that the entire
radionuclide inventory is available for dissolution into leachate at site closure,
with the concentration in leachate determined by the appropriate sorption
coefficient (Kd).
The assumption regarding the partitioning of radionuclides between waste and
leachate would be conservative even if sorption coefficients could be determined for
the actual wastes and conditions within the landfill, because not all of the radioactive
contamination would be on the surface of the waste and available for immediate
dissolution. Furthermore, because of the difficulties in determining sorption
coefficients, the default values are set to zero, effectively meaning that in the
SNIFFER methodology the entire radionuclide inventory enters the leachate at site
closure.
For the radiological assessment of the ENRMF, alternative assumptions have been
made for the scenarios involving leakage of leachate:
For pathways involving contamination of soil (including irrigation using
contaminated groundwater), the assumption that the entire radionuclide
inventory is available for dissolution into leachate at site closure is retained.
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