19 
sediment layers (0-30 cm) ranged between 1 900 of Np-237 in the seabed of the Baltic Sea was 
Bq nr 2 and 3 900 Bq nr 2 . estimated at 0.02 TBq. 
5.7 Np-237 
Neptunium-237 is an artificial radionuclide 
produced in nuclear reactors as a result of the 
irradiation of uranium-235 and uranium-238. 
Large amounts are found in spent nuclear fuel. 
The activity concentrations of Np-237 in surface 
sediments (0-20 cm) of the Baltic Sea were very 
low: 0.2-6.5 millibequerels per kg dry weight. The 
total amounts of Np-237 (mBq nr 2 ) at 26 sampling 
stations are shown in Figure 7. The total inventory 
5.8 Pu-238 
Results on plutonium-238 were reported from 11 
stations in the southern Baltic Proper and four 
stations in the northern areas of the Baltic Sea. In 
the southern Baltic Proper, the activity concentra 
tions of Pu-238 ranged from 6 mBq kg -1 to 250 
mBq kg -1 dry weight in the surface sediments 
(0-15 cm) and the total amounts were generally 
from 0.5 Bq nr 2 to 4.1 Bq nr 2 . Only at one station 
in the southern Baltic Proper was the total amount 
of Pu-238 regularly higher (4.8-9.1 Bq nr 2 ). In the 
Figure 7. 
Total amounts of 
Np-237 (mBq nr 2 ) 
at different sampling 
stations in the Baltic 
Sea at the beginning 
of the 2000s. 
Long-lived radionuclides in the seabed of the Baltic Sea