85 II-2. DESCRIPTION OF SOURCES OF ACTIVITY The model POSEIDON can deal with three types of radioactive releases: (1) atmospheric fallout; (2) point sources associated with routine releases of nuclear facilities, located either directly at the coast or inland at river systems; (3) point sources associated with accidental releases. For coastal discharges occurring into large (‘regional’) boxes, it may be useful to provide a more detailed description in the area close to the release point. For that purpose, ‘coastal’ release boxes can be added to the regional box system. These coastal boxes are nested into the regional boxes, and their physical characteristics (e.g. depth, sedimentation, etc.) can di?er from those of the adjacent regional boxes. POSEIDON also has the possibility to deal with o?shore release points (e.g. for evaluation of the impact of sunken vessels, nuclear submarines, and o?shore waste dumping). In that case, it is also possible to use a so-called ‘local’ box. II-3. NUMERICAL SOLUTION The problem is described by a set of ordinary di?erential equations (see Eqs. II-2–4 above), which may be written in a vector-matrix notation as: ?? ?? = ?? + ??? (II-5) where C is the concentration vector; A is the coe?cient matrix that includes water ?uxes between boxes, parameters of sediments, etc., and Qre is the vector for the release term. Step-like variations of the release in time are assumed and the Matrix Exponential Method [II-4] is used to solve this system. II-4. APPLICATION TO THE BALTIC SEA The model was customized for the Baltic Sea as shown in Figure I-2. Volume and average depth for each new box was calculated based on the bathymetry of the Baltic Sea, details of which were provided by the SMHI. The Baltic Sea compartments were connected with the North Sea compartments as described in the MARINA Project [II-5]. Boxes with depths larger than 60 m were divided into two layers (surface and bottom) for a rough description of stratification in the Baltic Sea. These boxes are shown in blue in Figure I-2 and water ?uxes between boxes were calculated by averaging over 10 years the 3-D currents provided by the SMHI. River runo? was also taken into account for the largest 16 rivers and total river runo? was 484 km3/year [II-6]. The simulations were carried out for the period 1945–2010 and the sources of 137Cs are global deposition from weapons testing, deposition from the Chornobyl accident and releases from the Sellafield and La Hague reprocessing plants. The global atmosphere deposition due to bomb tests was estimated for boxes 1–61 of Figure I-2 from Ris? Research Reactor measurements and deposition due to fallout was estimated for boxes 62–81 taken from the Leningrad NPP measurements. The atmospheric deposition due to the Chornobyl accident was taken into account [II-7] and the release of 137Cs from Sellafield (into box 15) and from La Hague (into box 26) was taken into account [II-8].