Venting of Fission Products and Shielding in Thermionic Nuclear Reactor Systems. Page: 1 of 7
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experimental data indicate that reservoirs
Another scheme is to simply allow the
because of the low activity of the fission
no radiation hazards.
A thermionic reactor has an interesting radia-
tion problem generally not associated with other
nuclear systems, that is, the disposal of gaseous
fission products. Almost all designers of therm-
ionic reactors assume that the gaseous fission pro-
ducts must be removed from the core. This assump-
tion is due to the very high operating temperatures
of the fueled emitter in a thermionic diode. In
order to illustrate the problem an idealized (not
real) schematic of an emitter is shown in Fig. 1.
This hypothetical emitter consists of a heavy (~ 1-
mm-thick) tungsten cladding surrounding the fuel
which usually consists of UO2 or UC. If UO2 is used,
a void amounting to ~ 10% of the fuel volume is
created in the fuel after prolonged high-temperature
operation. A typical set of operating conditions
is shown in Table I.
at several hundred degrees C can
fission gases to escape. Again,
products, this method should pose
TAJ3LE I. TYPICAL IfT_- OPlTTNG CONDITION
Emitter surface temperature, *C
Emitter center-line temperature, *C
Fuel power density, W/cm3
Total fissions/cm3 after 10 h
235U burnup, %
Final gaseous fission-product pressure,
Time to 2.5% creep of cladding, h
7.1 x 1019
Experiments have shown that at an operating tempera-
ture of - 1700*C almost all gaseous fission products,
e.g., xenon and krypton, escape from the solid fuel
and accumulate in the central void. After 10,000 h
of operation the gaseous fission product pressure
in the void would be ~ 1000 psi. This high pressure
would induce high hoop stresses in the tungsten
cladding with resultant creep and eventual shorting
of the emitter to the collector. The required creep
time until shorting occurs is ~ 10 h. Obviously,
this is unsatisfactory. Most emitter designs are
affected by this difficulty and therefore allow the
fission gases to escape. The design of emitters
which would permit such escape before the cladding
swells presents some problems. Solutions to these
difficulties have apparently been found, but are
not the subject of this paper.
RELEASE OF FISSION PRODUCTS FROM THE REACTOR
The fission products from the reactor core can
be released by several methods. Two are illustrated
in Fig. 2, which shows a typical thermionic fuel
element of the flashlite design. The fission gases,
on leaving the fueled emitter, enter the emitter
support. In Method A, shown in the lower half of
*Presented at the National Symposium on Natural and Manmade Radiation in Space, Las Vegas, Nevada, Feb.
1971. Work performed under the auspices of the U. S. Atomic Energy Commission.
VENTING OF FISSION PRODUCTS AND SHIELDING IN THERMIONIC NUCLEAR REACTOR SYSTEMS
E. W. Salmi 1
Los Alamos Scientific Lakoratory
University of California
Los Alamos, Iiew Mexkco 8754
Most thermionic reactors are designed to allow the fission gases to escape
out of the emitter. There exist several design variations to allow the gas to
escape from the reactor core. The final disposition of the gas has several
interesting problems. If the gas is put into a chamber, its volume is too large
to be contained in the shield. Fortunately the fission gas do not pose a radia-
tion problem at 100 ft. Because of the long half-life the fission gases could
prevent any close approach to the reactor after it has been shut down.
The fission gases could instead be stored in an absorption trap. An ab-
sorption trap is usually assumed to require very low temperatures, however,
TUNGSTEN CLADDING -0.1cm
Fig. 1. Hypothetical tungsten-clad fuel
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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/m1/1/: accessed April 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.