I
ROBERT E. ADAMS, WILLIAM E. BROWNING, Jr., and ROBERT D. ACKLEY Oak Ridge National Laboratory, Oak Ridge, Tenn.
C o n t a i n m e n t of
Radioactive Fission Gases by Qynamic Adsorption Contamination of the atmosphere can be controlled by a disposal system based upon adsorption of gases by porous solids
Tm
continuous release of fission product gases is a characteristic of circulating fuel nuclear reactors not found in heterogeneous solid fuel nuclear reactors. For reactor operation this is advantageous in that these fission gases can be removed from the system, and the concentration of xenon-135, a reactor poison, can be maintained a t l o ~ v concentrations. The yield of fission gases through fission of uranium-235 is high, and consequently the radioactivity of the gas mixture is of such magnitude that direct disposal into the atmosphere is prohibited. Special provisions must be made for the disposal of these gases. Two methods for the disposal of fission gases, other than direct release into the atmosphere, have been reported. O n e method, involving complete containment in gas storage tanks, has been applied a t the Shippingport Reactor Site ( 3 ) . A second method involving the absorption of fission gases by kerosinebase solvents has been proposed (5). Theoretical
I n the process of dynamic adsorption, the fission gases, krypton and xenon, are physically adsorbed from the carrier gas stream onto the surface of a solid
adsorbent such as activated charcoal. '4 state of equilibrium exists a t every point, and fission gas molecules will be desorbed from the surface a t the same rate as others are being adsorbed from the gas stream. While the fission gas molecules are not permanently adsorbed, this adsorption process effectively increases the time required for a fission gas molecule to pass through a portion of the adsorber system relative to the passage time required for the carrier gas molecules. The longer the fission gases can be detained in the system, the lower will be the level of radioactivity issuing from the adsorber system because of radiodecay. T h e disappearance of the fission gas atoms through the process of radiodtscay renders the system self-regenerating. To analyze the experimental data a theoretical treatment of the dynamic adsorption process was developed ( 2 ) . I t is assumed that an adsorber column is divided into a number of theoretical chambers*3 ' ' and as gas enters each chamber it is instantly distributed and brought to adsorption equilibrium through the theoretical chamber. T h e rate of removal of the fission gas in each chamber may be represented as:
dP - FAY P dt k'nz P.td
If X' differential equations for the -Y chambers are solved simultaneously, setting k' equal to k/p,,d, the solution of the general equation for the Al-thchamber is :
where = partial pressure of fission gas, atm. = standard pressure (1 atm.) = amount of fission gas injected
into first chamber, cc. at S T P = number of theoretical chambers = flow rate of carrier gas, cc. min.
= time after injection of fission gas
k
rn k'
pulse, min. adsorption coefficient, cc./gram = weight of adsorbent, grams = static equilibrium adsorption coefficient derived from linear portion of isotherm, cc./gramatm. = dynamic
T h e time, t,,, to reach maximum partial pressure, pm,x, is obtained from Equation by setting dP/dt = o.
v
VOL. 51, NO. 12
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DECEMBER 1959
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VENT YOOD
TO CISSION GAS
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
For most adsorber systems the value of
.V will be large, and, consequently, the quantity (Ar - l ) / N will be approximately equal to unity and may be disregarded. Because the values of m and F are fixed in an experiment, it is apparent \vi11 from Equation 3 that the time, t,, vary with any change in the coefficient k. Any condition that affects the adsorption process may be studied by experimentally measuring time, t,,,, for a system under various conditions and observing the resulting variation in the value of the coefficient k. Experimental
The factors influencing adsorption of fission gases in a dynamic system have been studied using a radioactive tracer technique with krypton45 or xenon-133 tracer. This method consists of injecting a brief pulse of the tracer (0.2 mm. of mercury partial pressure) into the carrier gas stream flowing through an experimental adsorber system. The effluent gas mixture is then monitored, and a distribution curve of beta activity in the gas stream is recorded. From these distribution curves various properties of the system can be studied. The various components of
R A D t O A C T I V E F I S S I O N GASES EO
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2 3 4 MOISTURE CONTENT OF CHARCOAL (wt. 70)
5
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Figure 4. Water vapor adsorbed on charcoal surface decreases the dynamic adsorption coefficient Columbia G charcoal, 8-1 4 mesh; oxygen corrier :gas; partial pressure; 25' C.
distribution curve predicted from the theoretical treatment. T h e basic property of a solid adsorbent is its ability to attract gas molecules to its surface physically and to hold these molecules for some finite time. This residence time is related to the difference between the kinetic energy of the gas molecule and the energy of adsorption. Any factor that increases
krypton tracer at 0.2 mm. of mercury
the kinetic energy of the gas molecule would be expected to reduce the residence time of the gas molecule on the adsorbent surface. The result of this decrease in residence time would appear as a decrease in time, t,,,, with an accompanying decrease in the value of the adsorption coefficient k . 'The temperature of the system would be expected to affect this energy bal-
-
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ance. Data presented in Figure 3 support this assumption. T h e value of the coefficient k , for both krypton and xenon is strongly affected by increasing temperature. Any foreign gas molecules adsorbed on the surface of an adsorbent would be expected to influence the adsorptiondesorption equilibrium of the fission gases merely by occupying space on the surface. Adsorbed moisture is one example. All of the solid adsorbents exhibit an attraction for water molecules to some degree. Figure 4 indicates the interference of adsorbed moisture with the value of k for activated charcoal. Other solid adsorbents such as alumina or silica gel would exhibit a much greater interference by adsorbed water. For this reason, complete drying of the solid adsorbent and the gas stream will significantly increase the efficiency of the adsorber system. The sweep or carrier gas used to transport the fission gases will also interfere with the fission gas adsorption. I t is desirable that the carrier gas not be adsorbed, but as all gases are adsorbed to some extent, the least ad-
60
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200 400 600 KRYPTON PRESSURE IN OXYGEN (rnm)
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Figure 6. Increasing partial pressure of krypton in oxygen carrier gas reduces value of the dynamic adsorption coefficient. In most applications the fission gas partial pressure will not exceed several millimeters Columbia G charcoal, 8-1 4 mesh; experimental temperature, 23' charcoal dried at 150' C.
C.;
4
o.2 0.1 2.5
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Figure 5. Carrier gas employed to transport the fission gas through an adsorber influences the dynamic adsorption coefficient
3.5
Columbia G charcoal, 8-14 mesh; krypton partial pressure, 0.2 mm. of mercury; charcoal dried a t 100' C.
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0.9 TEMPERATURE ("C.) 0.8
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24 32 40 48 56 ADSORBER BED LENGTH (ft.)
Figure 7. Ratio of f b to t,,, of the adsorber increases
64
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becomes larger as the length
Solid line predicted from theoretical treatment
2.5
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Figure 8. Comparison of various solid adsorbents on the basis of krypton adsorption from an oxygen carrier gas stream shows superiority of activated charcoal Tracer a t 0.2 mm. of mercury partial pressure; all adsorbents dried prior to VSB
sorbable gases are more attractive for use as carrier gases. The interference of the more common carrier gases was studied by noting the relative effect on the value of the coefficient k (Figure 5). Helium is only slightly adsorbed and is very attractive for use as a carrier gas. Freon-12 is not recommended as a carrier gas. It was used only to demonstrate the drastic reduction in the adsorption coefficient when the carrier gas is more strongly adsorbed than the fission gas. In most applications the amount of fission gases contained in the carrier gas will be small, probably not exceeding 1 mm. of mercury partial pressure. Assuming that the carrier gas is only slightly adsorbed, the adsorption surfaces available to the fission gases are numerous. However, if the concentration of fission gas in the gas stream is large, the number of fission gas molecules competing for an adsorption site would be increased and self-interference would likely occur. This effect was demonstrated by employing mixtures of oxygen and normal krypton in varying compositions as the carrier gas and krypton-85 as the tracer. T h e self-interference is indicated by the reduction in the value of the coefficient, k (Figure 6). T h e geometry of the adsorber system influences the relationship between breakthrough of a fission gas, t b , and the time at which the fission gas is a t a maximum concentration in the carrier (Figure 2). Ideally, gas stream, t,,, for fission gas disposal, a particular
1 470
molecule of fission gas should not emerge from the adsorber system prior to time t,,. This condition cannot be realized in practice; therefore, the time interval between tb and t,,, should be controlled to as small a value as possible. The ratio of t b to t,,, increases with adsorber length but is relatively independent of adsorber diameter. Figure 7 displays this ratio as a function of adsorber length. Various solid adsorbents have been evaluated for possible use in fission gas adsorption systems. Various types of solid adsorbents were compared on the basis of their adsorption coefficient for krypton in oxygen carrier gas (Figure 8). Obviously, activated charcoal is superior to other solid adsorbents for fission gas adsorption. Application
A fission gas disposal system utilizing activated charcoal was designed to process fission gases released during operation of the 5-Mw homogeneous reactor a t the Oak Ridge National Laboratory (4). This system has been in successful operation for over one year. Fission gases are removed from the reactor system by an oxygen stream dried by passsage through a cold trap and then passed into the adsorber system. T h e inlet of the system is composed of empty pipes to provide a delay time in which the very short-lived isotopes of krypton and xenon are removed from the gas stream by radioactive decay. Heat generation by beta decay in the first
INDUSTRIAL AND ENGINEERING CHEMISTRY
sections of the charcoal unit is reduced in this manner. Two parallel charcoal units are used to process the contaminated oxygen stream. Each unit, composed of 40 feet of 0.5-inch pipe, 40 feet of 1-inch pipe, 40 feet of 2-inch pipe, and 60 feet of 6-inch pipe connected in series, contains 520 pounds of charcoal. Heat removal and radiation shielding are accomplished by placing the system in an underground, water-filled pit. Minimum containment of krypton for 6 days and xenon for 60 days is provided by this system when operating at 25' C., with an oxygen flow of 2 liters per minute from the reactor (7). The effluent, composed of oxygen, krypton-85, and stable isoptopes of krypton and xenon, is dispersed into the atmosphere through a gas disposal stack. literature Cited (1) Adams, R. E., Browning, W. E.,
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ORNL-CF-58-4-14. Oak Ridge Natl. Lab., 1958. E., Bolta, C. C., ORNL-2116.OakRidneKatl. Lab.. 1956. (3) LaPointe, 'J. R., BFown, R. D.; IND. ENG.CHEM.50, 980 (1958). (4) Leland, T . W., ORNL-CF-55-1-12, Oak Ridge Natl. Lab., 1955. (5) Steinberg, M., Manowitz, B., IND. ENG.CHEM.51, 47 (1959). RECEIVED for review April 6, 1959 ACCEPTED August 10, 1959 Division of Industrial and Engineering Chemistry, Symposium on Chemical Considerations in Circulating Fuel Nuclear Reactors, 135th Meeting, ACS, Boston, Mass., April 1959. (2) Browning, W.
