The seafloor in the test area, approximately 500 m from shore, is sandy, with rocks and abundant patches of seaweed. Each test chamberis filled with local sand and seawater, and the sample is placed about 2.5 cm beneath the sand surface before the chamber is lowered to the ocean floor. with a nominal power rating of 50 W, and each con- C. Residence Time of Soluble Simulated Seawater. in and cracking of the disk surfaces. Disk PMC-142 Plutonium released from a heat source in seawater and disk PMT-113, manufactured at Mound Research Corporation (MRC) from PuQO, enriched Plutonium would be expected to form a hydrated oxide as a radiocolloid and subsequently to precipitate on nearby surfaces because of the slightly alkaline, 7.6 to 8.4, pH. Therefore, we measured the average residence time of plutonium addedto the simulated seawater (pH 8.2) in the test aquaria by adding 1 mg of 239py (in 1 ml of 1 M H2S0O,4) to the 75 liters of simulated seawater, and periodically determining the plutonium concentration in the water. The same batch of water was used for two experiments, and a fresh batch was used in a third (Table X). The ‘plutonium disappeared from the waterat a rate that followed an exponential decay curve with a half-life of roughly 40 h. There was no change in the pH. Much of the plutonium was deposited on the aquarium walls above the water surface by spray: from bursting of the air bubbles that maintain water circulation. These experiments prove that soluble plutonium does not remain in simulated seawater for a significant time. This measured residence half-time should be applied cautiously to situations involving PuO, heat sources, because the chemical form of the plutonium released from these sources is unknown. D. PMC Dissolution or Release Rates. Thedisks reacted similarly to the seawater, showing gradualcorrosion of the molybdenum coating of the individual PuO: particles from which they were made. This corrosion caused accelerating release of plutonium to the water, accompanied by crumbling manufactured at LASL, was immersedfor 548 days, in 160, was immersedfor 417 days. The average plutonium release rate from PMC- 142, during 548 days, was 2.3 wCi/m? of initial surface per second (Table XI). However, during the final month of immersion,the rate was 7.4 uCi/m?-s, illustrative of the rate increase with time. The release rate from PMT-113 over 417 days, averaged 7.0 uCi/m? ofinitial surface per second. (Table XI). During the last several months of immersion, however, the rate was 11.6 to 17.4 «Ci/m?-s. Figure 6 illustrates the plutonium found in the water as a function of time, in terms of microcuries per square millimetre of initial nominal surface which was 5600 mm? for each disk. Metallography of the disks, after their removal from the aquaria, showed extensive reaction between the seawater and the molybdenum metal network, accompanied by swelling in the reaction zone and loosening of plutonia particles. The reaction layer approximated about 10% of the thickness of PMC142 and about 30% of the thickness of PMT-113. The particle size distribution of 19 g of particles recovered from PMC-142, or 14%of the original 138g disk, was 88% larger than 177 wm and 98% larger than 45 um. Thesize distribution was typical of the uncoated plutonium oxide particles used in making 1. In Simulated Seawater. Two PMC disks were immersed in simulated seawater maintained at 8 to 12°C and a pH of 8.0 to 8.2. These disks were full size, 0.55-cm-thick, 5.44-cm-diam, production types TABLE X the disks. This unchangedsize distribution is consistent with the fact that even the larger amount of plutonium found in the seawater, from PMT-113, was only 1%of the total plutonium weight, and with the fact that seawater corrosion caused no detectable particle shrinkage. 2. In Fresh Water. We measured the plutonium RESIDENCE TIME OF PLUTONIUM IN SIMULATED SEAWATER Seawater Ratch tained approximately 100 g of 80% enriched 238Pu, Half-Life ksh 144 40 133 37 144 40 released during 326 days from a PMC disk (PM-148- T) that was immersed in 75 liters of fresh water at 10°C. This was a production-grade, 29-W disk from MRC. Therelease rate was linear for the first 190 days, with a slope of 15.6 uCi/m2-s (Table XI). From that time on, the release rate gradually decreased (Fig. 7). The disk edges began to disintegrate about 12 gy rR: