Venting of Fission Products and Shielding in Thermionic Nuclear Reactor Systems. Page: 5 of 7
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the fuel as readily as either xenon or krypton.
Experimental data on these problems at present are
The postulated escape mechanism out of the
emitter can be considered to have a half life,
which has been estimated to be about one week to
one month for all fission products that can escape.
The value of one week is used throughout this study.
After the high-temperature gaseous fission
products escape from the emitter they may condense
on cool surfaces both inside and outside the reactor.
The escaped products can be bled through various
filters or absorption beds, which could either react
with or condense these products into a relatively
small volume located within the reactor radiation
shield. The rare gases, however, may not be trapped
by these devices and may require some other method
of disposal. To illustrate the problem, consider
Table III which shows the results of a few simple
calculations. A lOO-kWe thermionic reactor with
TABLE III. GENERATION OF RARE GASES IN
TYPICAL FUEL EMITTERS
Reactor thermal power, MW
Fissions produced during 104 h 1.1
Total number of xenon and krypton atoms 2.6
Gas volume at STP, liter
Gas volume at 20 Torr and 600*C, liter 1.2
Diameter of equivalent sphere, m
10% efficiency would have a thermal power of 1 MW.
In 10 000 h of operation there would have been 1.1
x 1021 fissions. Assuming that 12% of the fission
products are rare gases, there would be 2.6 x 1023
atoms of xenon and krypton or 0.44 mole. At stan-
dard temperature and pressure the gas volume would
be 9.8 liters. However, in most reactor system
designs a large radiator operates at ~ 600*C in the
immediate vicinity of the reactor so that a large
volume at room temperature may be difficult to
locate. Also, as mentioned earlier, a rare-gas
pressure greater than - 20 Torr may not be accept-
able because of restrictions imposed by the therm-
ionic diodes. Assuming the two worst conditions,
i.e., a pressure of 20 Torr and a temperature of
600*C, the gas volume becomes 1190 liters rather
than - 10 liters. A sphere of this large volume
would hive a diameter of 132 cm or of nearly 4.5 ft.
Such a gas volume is almost an order of magnitude
larger than the reactor and obviously could not be
accommodated inside the reactor radiation shield.
POSSIBLE SOLUTIONS TO STORAGE PROBLEM
Three solutions to the fission-product storage
" Remove the gaseous products from the main
shield to a special shielded tank.
" Absorb the gases in a charcoal trap.
" Let the fission gas escape to space.
Remove the Gases to a Special Shielded Tank
In examining this method, the question of the
existing radiation level arises immediately. To
investigate the problem an example was postulated.
Assume again a 1-MW thermal reactor and a half-life
of one week for gaseous fission products to leave
the emitter and calculate the activity of the
various fission products. Decays per second as a
function of operating time are plotted in Fig. 4.
The top curve shows the fission-product activity
remaining in the reactor; the middle curve plots
the activity of the fission products which escape
from the emitter but are assumed to be collected in
a trap; and the bottom curve shows the activity of
the rare gases which have escaped from the emitter
and are assumed not to be caught in the trap.
Fig. 4. Activity of various fission products
as a function of reactor operating time.
The curves show that a steady state is achieved
after two months of reactor operation. The activity
in the trap would be 1.8 x 1015 disintegrations/sec
or - 50,000 Ci. However, this trap was assumed to
be small and located within the reactor shield. The
rare gas value is 1.1 x 1015 disintegrations/sec
which represents 30,000 Ci. To calculate the roent-
gens this source represents, the activity and decay
schemes of the various isotopes of xenon and krypton
were investigated. More than OD)? of the activity
of the rare gases is due to 13 Xe and 135Xe (Fig. 5).
In each case there is a metastable state; however,
a detailed investigation of the decay chain shows
that the metastable states contribute little to the
activ y. The 133Xe is about six times more active
than 37Xe; however, the 81-keV Y-ray is strongly
internally converted and because of the high absorp-
tion cross section it may be readily absorbed in a
gas container wall. The main contribution to the
outside y-radiation therefore probably comes from
135Xe. If one assumes that the P-rays do not leave
the fission-gas container, and taking into account
the internal conversion of the Y-rays, this rare-Cas
source would produce about 500 R/h at one meter or
500 mR/h at 100 ft.
In some directions around a reactor power supply
this radiation level is acceptable; however, in the
direction of the crew compartment it is excessive.
The activity calculations assumed an escape lifetime
from the emitter of one week. Because the two
100 200 300 400
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Salmi, E. W. Venting of Fission Products and Shielding in Thermionic Nuclear Reactor Systems., report, January 1, 1972; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1036131/m1/5/: accessed May 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.