been observed in nature when some kinds ofrock,
heated by the sun, are hit by a sudden shower.!
This mechanism for the airborne plutonium observed over the PPO pieces seems to be confirmed by the
striking results (Table I) of an experiment in which
air particulate samples were taken during a rain se-
quence. The average concentration duringthefirst 5
min ofrain was 104 times the prerain concentration.
Ht then dropped continuously until it reached the
prerain level about 1 h after the end of the rain. The
principal cause for the rapid decrease is
gravitational settling of the particles. The rain
washes some of the plutonium out ofthe air but it
cannot be the only removal mechanism because the
decrease continues after the rain has stopped. A
repetition of the experiment, but with dehumidifier
turned off, produced similar rates of decrease, in-
dicating that pickup by the dehumidifier is not the
principal removal mechanism. The results in Table
III were obtained by counting the fiber glassfilters
with a 2-7 gas proportional counter. Some of the
filters were subsequently dissolved, and a more accurate analysis by liquid scintillation counting
showed that most of the initial activity came from
relatively large particles caught on the filter surface
and therefore countedefficiently by the proportional!
counter. Later, the large particles settled out, and
the remaining smaller particles penetrated deeper
into the filter and were countedless efficiently.This
experiment was repeated twice, with Millipore
membranefilters of two different pore sizes, and at
two air collection points within the chamber, in
order to obtain samples for particle size and
agglomeration studies. The airborne plutonium concentrations were similar to those measured in the
first experiment (Table III).
The first rainwater that percolated through the
soil supporting the large fuel pieces contained 1.8
uCi of plutonium. This very rapid plutonium
breakthrough indicated that some of the material
that spalled from the fuel is in the form ofvery small
particles or a colloid. Plutonium ions would not be
stable in solution at the slightly basic pH (about 8).
and if present would be held in the soil by some ion-
exchange type of mechanism.
The amountof plutonium found in the rainwater
that. percolated through the soil supporting the large
pieces is strongly influenced by the climate, as is
shown in Table IV. Each weekly rain is equivalent to
a 32-mm rainfall and deposits 27 liters of water.
Under summer conditions, about 6 liters of rain-
water percolated through the soil. This water contained an average of 0.7 uwCi of plutonium. Winter
rain produced more percolated water, because of
lower evaporation losses, and a higher plutonium
content, the early winter rains yielding as much as
18 wCi of plutonium. It seems that the higher
summer temperatures dried the soil more between
TABLE ITI
AIRBORNE PLUTONIUM CONCENTRATIONS
rains so it became saturated later during a rain.
Because of this lower water flow in summer. fewer
DURING A RAIN IN AN ENVIROAMENTAL
plutonium dioxide particles were washed through
CHAMBER CONTATNING LARGE PTECES OF
the soil. At the beginning of the winter, some ofthe
PLUTONIUM DIOXIDE
Collection
Time
Pu
Concentration
1
24
3
2
5
3x 104
3
5
6x 10
Filter
No.
4
5
6
7
(min)
10
10
15
15
(pCi/m*)
3
4x 10°
3x 10°
9x 10°
8 x 107
particles left in the soil during the summer were
washed through, giving higher results for the first
Remarks
Before
rain
First 5
9
180
10
860
2x 10°
HUMID CLIMATE EFFECTS ON AMOUNT OF
PLUTONIUM CARRIED BY WATER PER-
min of
rain
COLATING THROUGH SOIL
(Large Pieces from MHFT-12)
Last 15
min of
50
Table IV
Climate
Winter
Conditions
0-17 %
Av Percolated
Water per Rain
(liters)
11
Av Pu
fuci)
9
70 - 100% Ri
rain
Summer
20 - 40 °c
6
87 - 96% RH
OP
AKG VEGs
0.7