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