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DESCRIPTION OF THE NRPA BOX MODEL
The Norwegian Radiation Protection Authority’s (NRPA) box model uses a modified approach
for compartmental modelling [I-1–3] which allows for dispersion of radionuclides over time.
The box structures for surface, mid-depth and deep water layers have been developed based on
the description of polar, Atlantic and deep waters in the Arctic Ocean and the Northern Seas
and site specific information for the boxes generated from the 3-D hydrodynamic model
NAOSIM [I-4]. The structure of the NRPA’s box model is presented in Figure I-1.
The NRPA box model includes the processes of advection of radioactivity between
compartments, sedimentation, di?usion of radioactivity through pore water in sediments,
particle mixing, pore water mixing and a burial process of radioactivity in deep sediment layers.
Radioactive decay is calculated for all compartments and the contamination of biota is further
calculated from the radionuclide concentrations in filtered seawater in the di?erent water
regions. Doses to humans are calculated on the basis of seafood consumption, in accordance
with available data for seafood catches and assumptions about human diet in the respective
areas. Dose rates to biota are developed on the basis of calculated radionuclide concentrations
in marine organisms, water and sediment, using dose conversion factors. Its structure, for the
Baltic Sea only, is presented in Figure I-2.
The equations of the transfer of radionuclides between the boxes are of the form:
???
??
= ? ??,?????? ? ??? +??,??? ?
?
??? ? ??,?????? ? ??? + ??,??? ? ????? (? ? ??) + ??; ? ? ??
?
??? (I-1)
?? = 0; ? ? ?? (I-2)
where ki,i = 0 for all i, Ai and Aj are activities (Bq) at time t in boxes i and j; ki,j and kj,i are rates
of transfer (y?1) between boxes i and j; ki is an e?ective rate of transfer of activity (y?1) from
box i taking into account loss of material from the compartment without transfer to another, for
example due to radioactive decay; Qi is an input of radionuclides into box i (Bq/y); n is the
number of boxes in the system, Ti is the time of availability for box i (the first time when box i
is open for dispersion of radionuclides) and ? is a unit function:
?(?) = ?1 ? ? ??0 ? < ??
(I-3)
The times of availability:
?? = ?????(??,??)???
? ??,?
?,?
(I-4)
