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Nuclear-powered ships will spend a part of their time in areas well removed from man, where perhaps the disposal of limited amounts of wastes would not result in ready return to man through the food chain. These ships will also spend part of their time in regions of high human activity, such as harbors and harbor approaches. The release of even very small amounts of radioactive materials in these latter environments might result in the return to man, through the food chain, of undesirable amounts of activity. For this reason even the relatively small amount of activity present in the primary coolant is considered deserving of con- sideration. PREDICTED NUMBER OF NUCLEAR-POWERED SHIPS In order to evaluate properly the capacity of the various parts of the marine environment to receive radioactive wastes from nuclear- powered ships, it is necessary to have a reasonably correct prediction of the number of such ships which will be operating on the world oceans within the next several decades. Efforts to obtain such an evaluation have not resulted in consistent estimates, within even an order of mag- nitude. For this reason the evaluation presented herein is based on an arbitrary number of 300 nuclear-powered ships of the 60 MW class. It is believed that this figure should safely apply to the next ten to fif- teen years. Thus, adequate time is available to revise the conclusions made here on the basis of more dependable estimates of the probable number of nuclear-powered ships which will, in the foreseeable future, operate on the world oceans. Undoubtedly, better information than is now available on the allowable exposure of man to radiation, and on the processes of dispersion, uptake, and concentration in the marine envi- ronment, will become available over the next ten years, making such a re-evaluation desirable in any case. GENERAL APPROACH TO THE PROBLEM The fate of radioactive material introduced into the marine envi- ronment is dependent upon the following considerations: 1. The physical and chemical form in which the material occurs at time of introduction, together with any relatively rapid changes in the physical and chemical character of the introduced material which occur when it is brought into contact with sea water. The subsequent disper- sion by physical processes and re-concentration by the biota and bottom materials will be greatly affected by the physical and chemical form in which the wastes occur in sea water. 2. Initial mechanical dilution of the wastes by the receiving waters, which will depend upon the manner of introduction. Thus a liquid waste will be subject to greater initial mechanical dilution if introduced as a strong jet into the body of the receiving waters, than if introduced as a gently flowing stream on the surface. Large initial mechanical dilution is important in reducing the density difference between the initial con- taminated volume and the surrounding receiving waters. This reduction in turn favors subsequent turbulent diffusion.
3. Advection of the waste material away from the source region, and simultaneous turbulent diffusion, which lead to reduced concentra- tions of the radioactive components in the water. 4. Uptake of the activity onto suspended silt and bottom sediments, which removes some of the radioactive materials from the water, and restricts further dispersion. In deep water such removal would be fa- vorable since material incorporated with the bottom sediments there would be unlikely to return to man's environment. In shallow coastal areas containing bottom living shellfish and bottom feeding commercial fin fish, concentration of radioactivity on the bottom may be unfavorable since these detritus and filter feeders may further concentrate the ac- tivity from the bottom material. 5. Concentration of activity by various parts of the biota, includ- ing shellfish and fin fish important to man as a source of food. Some important fission products and corrosion products are concentrated by marine organisms by factors of 100 to 10,000 over the concentrations of these isotopes in the water. The evaluation of the suitability of any particular marine locality as a receiver of nuclear wastes ideally involves the precise, step by step consideration of all factors affecting the possible return of radio- active material to man. The general procedure is the same whether the evaluation concerns: (a) the selection of suitable disposal areas for packaged wastes; (b) the selection of the position of an outfall discharg- ing low level liquid effluent from a chemical processing plant; (c) the consideration of the suitability of a given harbor or harbor approach to receive low level liquid wastes from nuclear powered ships; or (d) the determination of the suitability of the mixed layer of the open ocean to receive wastes from the ion exchange resins on nuclear powered ships. Understanding of many of the physical and biological processes involved is, however, far from adequate, and further research is recommended to provide a more adequate foundation for the determination of the ca- pacity of any particular marine locale to receive nuclear waste mate- rials without undue risks to man. Figure 1 presents in schematic form such a step by step procedure. The solid arrows between blocks on the diagram indicate the route taken by the radioactive material in returning to man, while the dashed arrows indicate the reverse course taken in the evaluation. The starting point in the evaluation is man. In order to make any evaluation of the problem at all, some maximum permissible rate of exposure must be adopted. The usual basis for the selection of such a maximum rate would be national (or international) published statements of the maximum permis- sible ingestion rate for the various isotopes, for the general population. At the outset it should be recognized that such a maximum permissible rate of exposure is not the most desirable rate. The latter, where tech- nical and economic feasibility allow, should be as close to zero as pos- sible. Thus in making this evaluation, some real, though admittedly extremely slight, risk to the general public is assumed.
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Considerable care should be taken in any evaluation that the po- tential additive effect of exposure to radiation from various sources is given due consideration. The second step in the evaluation is then a consideration of all sources of radiation exposure to the particular seg- ment of the population which would also be potentially affected by the waste disposal operations. On the basis of such consideration a portion of the maximum permissible dose is assigned to sea disposal. The next step is the consideration of the various routes which the radioactivity can take in reaching man from the marine environment. Here an evaluation of the uses man makes of the marine environment is required. The possible danger of direct radiation, the harvest of sea- food, possible contamination of fishing gear and of beach sand are some of the items which should be considered at this stage of the study. The determination of the maximum permissible concentrations of the various significant isotopes in those parts of the marine environ- ment (seafood, bottom sediments, and shore material), which constitute the routes by which radioactivity may return to man from the sea, then follows as the next step in the process. From a consideration of the known factors by which the biota and the sediment concentrate the var- ious radioactive isotopes from the sea water, it is then possible to ar- rive at the maximum permissible concentration of the various isotopes in the sea water. The final steps involve evaluating the changes in the concentration and distribution of radioactivity which may be brought about by (a) exchange between bottom sediment and suspended or dissolved material in sea water; (b) advection and turbulent diffusion, both within a given marine environment and between adjacent environments; (c) initial mechanical dilution, influenced by manner of discharge; and (d) physical and chemical form of the wastes at time of release. The end result is an estimate of the maximum rate of introduction of radioactive material which will not exceed the maximum permissible concentration in sea water. In the present problem of waste disposal from nuclear-powered ships, some modification of this ideal step-wise procedure must be made since we are not dealing with a fixed region of the marine or coastal environment. For this reason it has been necessary for this working group to take a conservative (safe) assumption at each step of the evaluation. Thus, for inshore and coastal environments, when considering the return route to man of radio-isotopes through seafood, the assumption used here is that man receives all his protein require- ment from seafood, and that the highest known concentration factor for 10