A radiological assessment of the consequences of the disposal of high-level radioactive waste within the seabed sediments in deep oceans has been performed within the framework of the International Seabed Working Group This was conducted under the auspices of the Nuclear Energy Agency of the Organisation for Economic Cooperation and Development Three types of calculation were made

  1. The base case, or ‘normal’ scenario the waste was assumed buned at its prescribed depth with all the barriers behaving as anticipated

  2. Several ‘abnormal’ scenarios, where one or more components of the system behave abnormally

  3. Scenarios of transportation accidents, occurring m coastal areas or in the deep seas the consequences of such accidents were analysed, as was their probability of occurrence, given a special ship design

The assessments were made using a both a deterministic and a stochastic methodology This makes it possible to estimate not only the most likely doses resulting from each scenario, but also the range of uncertainty associated with this estimation, even the uncertainty in the available data.

No significant differences were found between the two sites (GME and SNAP), although the characteristics of their sediments are slightly different For the base case, the peak dose to the maximally exposed group of individuals is of the order of 2 8 × 10−9 Sv a−1 for a complete repository of waste arising from lob metric tons of heavy metal, occurring 150 000 years after disposal. The uncertainty on this value calculated by the stochastic analysis, ranges between 3 × 10−15 and 3 × 108 a Sv a−1 The highest dose calculated in the stochastic analysis was 2 5 × 105 Sv a−1 in a sample of 500 runs. The collective doses integrated to 105 years are of the order of 22 × 103 man Sv and to 107 yr of 28 × 104 man Sv

For abnormal scenarios analysed deterministically, it was found that the seabed option was extremely insensitive to a large number of components of the system. In particular, for a properly emplaced penetrator, the corrosion and leaching properties of the waste are not significant. Only three scenarios were found to increase the doses significantly

  1. emplacement of penetrators at a depth < 10 m in the sediment;

  2. existence of an upward pore water velocity in the sediments;

  3. change in the radionuclide retention properties of the sediments

Doses arising from transportation accidents could be significant, especially for accidents in coastal waters (in the order of 6 5 × 105 Sv a1 per metric ton of heavy metal of waste lost m the sea, with an uncertainty range of 3 7 × 10−6 to 11 × 10−3 Sv a1 per metric ton of heavy metal).

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