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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.
According to the report "Radioactive Waste Disposal from U. S. Naval Nuclear-Powered Ships", the actual operating results of the U. S. S. NAUTILUS and the U. S. S. SKATE have produced radioactive wastes considerably less, both in intensity of activity and in amount, than those predicted in the Maritime Administration's report for the SAVANNAH. The average gross activity of the primary coolant of the NAUTILUS, 15 minutes after sampling, was 5 x 10'2pc/ml. However, the bulk of this activity was associated with Mn 56 (2.5 hr half life) and F 18 (1.9 hr half life). The gross activity 120 hours after sampling has averaged 3 x 10'' uc/ml. The measured activity of Co 60, Fe 59, and Ta 182 has averaged 5.7 x 10'6 , 1.5 x 10'4, and 7.3 x 10'3 uc/ml re- spectively. Fe 55 apparently does not occur in measurable quantities in the primary coolant of the NAUTILUS. The expansion volume on warm-up for the NAUTILUS is much less than that predicted for the SAVANNAH, averaging about 500 gals (67 cubic feet). The maximum activity on the spent ion exchange resins in the NAUTILUS, and the rate at which these resin beds require re- placement, are likewise much less than the corresponding figures for the SAVANNAH. The total activity on the spent resin beds is reported to be no more than 12.5 curies, with the bulk of the activity (some 10 curies) resulting from Co 60. The beds have required replacement about once every six months. During each warm-up involving the average discharge of 67 ft3 or 1.9 x 10° ml, the NAUTILUS then releases approximately 9.5 x 10'2 curies (measured 15 minutes after sampling). Co 60, Fe 59, and Ta 182 would contribute 1.1 x 10'5 curies, 2.9 x 10"* curies, and 1.4 x 10'2 curies, respectively. Assuming two warm-ups per month, the total activity in the expansion volume liquid wastes from the NAUTILUS during one year would be about 2.3 curies measured 15 minutes after sampling and 0.14 curie measured 100 hours after sampling. In comparison with these amounts, the activity in the fission prod- ucts contained in the spent fuel elements is quite large. Thus the fis- sion products in the fuel elements from a 60 MW reactor which had been in service for one year on a nuclear-powered ship would amount to over 107 curies. It has been stated that the vast majority of the fission product wastes will be stored on land, after chemical separation from unused fuel and useful by-products. However, even a small fraction of this activity could be significant if released in coastal areas. Release of activity to the coastal environment by land based nuclear installations, particularly chemical processing plants, may be difficult to avoid. Thus, under a carefully controlled program designed to limit the re- turn of activity to man to a safe level, the Windscale Works in England is authorized to release over 105 curies per year into the coastal waters of the Irish Sea. The major part of the safe capacity of these inshore areas should then be reserved for land based operations, since nuclear-powered ships could conceivably delay discharge of wastes until outside such areas.
