They had a CMD of 0.072 «4m compared with 0.056 there was more plutonium in sections below the surface, especially in the bottom sections. These high um, but they were still very much smaller than the summer agree with the reason postulated to explain with observations that 2°°PuOs is muchless stable concentrations in the subsurface soil during the the low plutonium concentrations in summer rainwater that percolated through the soil; that is, the smaller flow of water in the summerdoes not wash as much plutonium out of the soil as does the greater winter flow. The high concentration in the bottom sections of these summercores,also seen in Fig. 3, is less easily explained. Possibly some of the plutonium is held up by the perforated plate under the soil whenthe flow is low; the holes are only 22% of the surface area. Also, plutonium oxide particles may collect at the soil and air interface under low flow conditions, The seasonaleffect is less marked in cores 7 and 8, which were taken during the second winter, in the 10-um minimum parent particles. This fact agrees than *89puQ..47 This instability seems to be associated with the intense radiation field of the isotope’s alphaactivity. Radiation damageto the ox- ide crystal lattice, reaction with radiolysis products in soil and water, and aggregate recoil may con- tribute to this degradation. Twenty-five of the particles that formed stars, chosen at random on the photographic plates of highly disperse material from the core of soil suppor- ting the large fuel particles, were examined microscopically for agglomeration. Each particle was found to be associated with one microscopically visible soil particle. This indicates that trapping of plutonium dioxide particles by soil is not a matter of filtration of the fine oxide particle, but that thereis chamber containing the fine particles. This may have been caused by gradualfixing of the plutonium dioxide in the soil with the passage of time. Core 7 looks like a winter core except for the bottom section agglomeration of individual plutonium oxide and soil particles. This factor may determine whichparticles will be retained by the soil and which will pass summer core, with small amounts of plutonium in the samesize distribution as the bulk of the soil. which is very high in plutonium. Core 8 is morelike a all sections. Cores from the soil with the large pieces (Fig. 3) show similar, but less pronounced, spatial and seasonal correlations. Part of this blurring of the effects may have been caused by water from the gate valve splashing onto the pieces and causing spalla- tion. Evidence of this complication was seen in the experiment, described earlier, in which the air concentrations during a rain were measured with the dehumidifier off. In that experiment the prerain con- throughit. The soil particles in the agglomerates had Cores 9 and 10 were taken from the chambercon- taining the large pieces of MHFT-12 during the se- cond winter cycle, at locations 20 cm to the left and right of the PPO pieces, respectively. Core 10 was sent to LFE Environmental for particle size and agglomeration studies. Core 9 was divided into 10 vertical sections for plutonium analysis. The top section contained 24 ng, but the other nine sections each contained less than 25 pg of plutonium. This agrees well with the results of core 1, collected during centration was high, probably because the gate valve was opened just before the rain. Unfortunately, the the first summer 25 cm to the left of the heat source lost, and we do not know whether the concentration 2. MHFT-27. As part of the MHW-safety sequential test (SST) program, MHFT-27 a postimpact sphere assembly (PISA), was about 40°o buried in sand in an environmental chamber on January 9. 1974. The temperature of this PISA, which con- sample of the bottom section of core 6 (Fig. 3) was was high at the bottom of this core, as it was in the other cores taken in similar places in the summer. The six half-sections from cores in the two chambers were analyzed at LFE Environmental for particle size distribution and agglomeration with the soil. There were no particles large enough to form stars in a photographic emulsion in either section of core 3 from the chamber containing the fine par- ticles. There were stars in the top section ofcore 4 in both chambers. Theparticle size range was slightly smaller than that in the rainwater that percolated through the soil: 90° of the particles had equivalent diameters of 0.04 to 0.2 um. The lower limit may not be significant, as it is close to the detection limit of the procedure. The particles in the cores from the soil supporting the fine particles were slightly larger than those in the soil supporting the large pieces. pieces. tained 254 g of 80% enriched 238Pu0>2, was monitored by meansof thermocouples welded to the top and bottom of the iridium shell. Initially the temperature was 620°C. During the first rain, the temperature dropped rapidly to 100°C and the sphere sank into the sand until about 90°: of it was covered, probably because rainwater lubricated the sand. The temperature stayed at 100°C for 1.5 days, then gradually increased to about 500°C. This _ pattern of thermal response to a rain changed gradually. After several months, during a rain the temperature remained at 100°C, but as soon as the rain stopped, it rose to 300°C, where it remained for