Most thermionic reactors are designed to allow the fission gases to escape out of the emitter. A scheme to allow the fission gases to escape is proposed. Because of the low activity of the fission products, this method should pose no radiation hazards

Salmi, E. W.

English

E.W. Salmi

Crossref

VENTING OF FISSION PRODUCTS AND SHIELDING IN THERMIONIC NUCLEAR REACTOR SYSTEMS.

UNT Digital Library

/ VENTING OF FISSION PRODUCTS AND SHIELDING IN THERMIONIC NUCLEAR REACTOR SYSTEMS E. W. Salmi ; Los Alamos Scientific Laboratory University of California Los Alamos, niew Mexico 875'+'* 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, experimental data indicate that reservoirs at several hundred degrees C can be useful. Another scheme is to simply allow the fission gases to escape. Again, because of the low activity of the fission products, this method should pose no radiation hazards. INTRODUCTION 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 U0„ or UC. If UOp 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. -TUNGSTEN CLADDING r-0.1 cm V>yy^yJ'yyyyyy^yyyyyyyJ^yy^'yy^^:^ -3 cm--VENTS TABLE I. TYPICAL EMITTER OPERATING CONDITIONS Emitter surface temperature, °C 1700 Emitter center-line temperature, °C 2500 Fuel power density, w/cm 50 Total fissions/cm-' after 10 h 7.1 x 10' ^35u burnup, ̂  ~ 0.21* Final gaseous fission-product pressure, psi ~ 1000 Time to 2.5% creep of cladding, h 10 19 Fig. 1. Hypothetical tungsten-clad fuel emitter. 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. 1016 DISCLAIMER  This report was prepared as an account of work sponsored by an agency of the United States Government.  Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.  Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof.  The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER  Portions of this document may be illegible in electronic image products.  Images are produced from the best available original document.  FISSION GAS mSSAGE FISSION GASES RELEASED TO ^ METHOD SEPARATE PASSAGES Cs GAP COLLECTOR - F I S S I O N GAS ^CERAMIC TO NCTAL SEAL Fig. 2. Typical thermionic fuel element and possi-ble methods of fission-gas release. the illustration, vents are leading from the support structure to the cesium gap. The cesium gaps of the individual diodes are then connected by separate vents throughout the length of the fuel rod. In Method B, illustrated in the upper half of Fig. 2, the vents bypass the cesium gap and lead to separate fission-gas passatges outside the collector in the A1„0 insulating region. These passages also extend throughout the full length of the fuel rod. -"" - -Fission Gases Enter the Cesium Gap - Method A Both methods have their disadvantages. Experi-ments with Method A show that the gaseous fission products degrade the electrical power output of the diode even after only a short operating time: the presence of xenon or krypton in the cesium vapor at more than 10 or 20 Torr can increase the electri-cal resistivity across the cesium vapor gap and thereby reduce the electrical power output. Also, the presence of foreign gases increases the thermal conductivity across the gap, which, in turn, results in a reduction of the emitter temperature. The final result is the fact that only a very small fraction of the gaseous fission products can be retained in the cesium vapor space. Fission Gases Bypass the Cesium Gap - Method B Method B offers the advantage of not restrict-ing the fission-product pressure to 10 or 20 Torr. This pressure restriction can influence the fission gas storage problem outside the reactor core. There are some disadvantages to the method. The small passages running through the alumina outer sheath might contain cracks connecting the collector and the outer sheath. The gaseous fission products are not purely xenon and krypton but also contain many other elements. In particular, cesium fission pro-ducts are about 70% as numerous as the xenon and krypton products. Therefore, cesium at high pres-sure is present in any cracks in the separate vent-ing system. With approximately 10 V possible across the small passage, there may be electrical shorting between the collector and the outer sheath resulting in very low electrical power output of the fuel element. Another disadvantage of Method B is the pres-ence of many ceramic-to-metal seals in the fuel element (Fig. 2). Should any of these break, the high-pressure gaseous fission products could enter the cesium vapor gap and reduce or terminate the power output. Because of this latter possibility and because of the large number of seals, the separ-ate venting system should preferably be limited to the 10- or 20-Torr pressure regime. From these considerations, a separate fission-gas venting system does not appear to offer major advantages. FISSION-GAS STORAGE PROBLEMS Assume that the fission products, after vent-ing, are finally stored in an outside reservoir. Fission products that may be gaseous at an emitter temperature of ~ 2000°C are listed, together with their atomic percentages, in Table II. Note that the rare gases represent ~ 12fo of the fission pro-ducts while the total high-temperature gases amount TABLE II. GASEOUS FISSION PRODUCTS AT ?000°C AND THEIR PART OF TOTAL GAS RELEASE Fission Products Kr and Xe ' Br and I Rb and Cs As, Se, Te Amount of Species, At. % 12.2 2.5 9.0 i*.3 Total 28.0 to almost 30°/o. In assessing whether all these gaseous products can escape from the emitter, some understanding of the escape mechanism may be useful. A microphotograph of UOp fuel after several thousands of hours of operation at ~ 2000°C is shown in Fig. 3; the dark spots are voids and the gray material is UOp. Many dark spots have a lenticular shape and seem to have developed a trail through the UOg. Fig. 3. X75 Microphotograph of UOp fuel after prolonged operation at 2000°C. According to fission-product escape theory voids are formed during operation, move up the temperature gradient through the fuel, sweep up the gaseous fission products, and eventually deposit the gases in the center of the fuel. In other words, the fission products do not have to diffuse long dis-tances. This would imply,that all fission products with a substantial vapor pressure at ~ 2000°C could be swept up by the lenticular voids and escape from 1017 0 the fuel as readily as either xenon or krypton. Experimental data on these problems at present are not abundant. 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 v;hich shows the results of a few simple calculations. A 100-kWe thermionic reactor with TABLE III. GENERATION OF RARE GASES IN TYPICAL FUEL EMITTERS 2k 23 Reactor thermal power, Ml'/ Fissions produced during 10 h 1.1 x 10' Total number of xenon and krypton atoms 2.6 x 10 Gas volume at STP, liter 9-8 Gas volume at 20 Torr and 600''c, liter 1.2 x 10-̂  Diameter of equivalent sphere, m 1.32 10^ efficiency would have a thermal power of 1 MW. In 10.000 h of operation there vrould have been 1.1 x lÔ 't fissions. Assuming that 12% of the fission products are rare gases, there v/ould be 2.6 x 10^3 atoms of xenon and krypton or O.kk 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 II90 liters rather than ~ 10 liters. A sphere of this large volume would have a diameter of 132 cm or of nearly h ."3 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 problems exist: • 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. k. 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. > INSIDE REACTOR >-> Fig . h. 100 200 300 4CX) TIME, days 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 lCH-5 disintegrations/sec or ~ 50jOOO Ci. However, this trap was assumed to be small and located within the reactor shield. The rare gas value is 1.1 x lO-'-S disintegrations/sec which represents 30jOOO 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 90̂'i of the activity of the rare gases is due to •'-33xe and -̂ 35xe (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 activity. The 133xe is about six times more active than 3Pxe; however, the 8l-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 3''Xe. 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-gas source would produce about 5OO 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 assumcu an escape lifetime from the emitter of one week. Because the two 1018 Xe (I6in) Q^- 1.17 0.53 0.61 0.250 x/ '3''" . Xe (2.3d) 0.233 Q/j-0.428 0.081 # 100 200 300 400 500 600 700 TEMPERATURE.'K 133 135 Fig. 5. Decay schemes for Xe ajid -̂  Xe. 133 n 5 isotopes of greatest interest, Xe and -̂ -̂ Xe, have half-lives of 9 h and 5-3 days, respectively, the large uncertainty in the value of the escape life-time can significantly affect the calculated radia-tion level. Experimental investigations of escape rates out of emitters are required before a satis-factory value can be assigned to the radiation level. In any case, the storage of* fission gases in a lightly shielded gas chamber can pose design problems. Absorption in a Charcoal Trap Normally, absorption traps are thought to operate at very low temperatures; however, at pres-sures of 20 Torr a charcoal trap can be useful even at moderate temperatiires. For the example of a 1 MW thermal reactor, the voluma of charcoal required to store the xenon and krypton at 20 Torr can be cal-culated as a function of temperature. As shown in Fig. 6, this charcoal volume is considerably smaller than the original volume of 1.2 x 103 liters with-out charcoal as shown in Table III. At a tempera-ture of 100°C the charcoal volume is about 15 liters. Although 15 liters is not a trivial volume to shield, it may represent a compromise solution. At 100°C, the use of heat pipes may prove feasible for con-trolling the charcoal trap temperature. The tvro methods discussed above of handling fission gases have their disadvantages. If the charcoal-trap temperature rose appreciably, the fission-gas pressure could Increase and pose a hazard to reactor operation. On the other hand the ligbtly shielded gas-chamber system could pose a radiation hazard after reactor shutdown. Figure 7 Fig. 6. Amount of xenon absorbed in charcoal as a function of temperature. g 9 Kg. 7. Rm-gat .ctivity toUowlnii r.«lor .butdm, utn 1 ! y , „ . of cp.r.tio.. Fig 10 20 30 40 50 60 TIME AFTER REACTOR SHUT DOWN, days 7. Rare-gap activity following reactor shutdown after 1.2 years of operation. shows the rare-gas activity expected following reactor shutdown after 1.2 years of operation. The curve shows that after one month the activity has fallen by a factor of 100. At this time the activity is due mainly to -'Kr which has a ten-year half-life. Although this activity is small compared to the previously cited values, it is a 5-R/h radiation source at a distance of 1 m. With a ten-year half-life it does not disappear suddenly. If poorly shielded, it might pose a hazard for any crew work-ing near the reactor after shutdown. # 1019 Escape of Gases to Outer Space Values for fission gases escaping to space have been calculated, Table TV, and show that the number of rare-gas atoms escaping per second would be 7.5 X 10-'-5. These escaping atoms have an acti-vity of 1.5 X 10^^ disintegrations/sec, which represent 0.5 Ci leaving the reactor per second. Although at first glance this value seems high, the radiation hazard is actually small. Assume that the gases are emitted by an orifice to form a radioactive cloud filling 2rT radians. As the cloud expands, the atom density at a distance of 1 m from TABLE IV. ACTIVITY OF RARE GASES RELEASED TO SPACE Atom escape rate, atom/sec Activity of escaping atoms, disintegration/sec Radiation leaving reactor, Ci/sec Atom density at 1 m, atom/cm Atom mean free path, km Dose rate a i m from cloud, R/h Oc Moles of Kr released per year, mole/yr 1975 projection for Kr release, mole/yr 7 1 1 -7 5 X 10-5 X 10 0.5 5 X 10 200 -10-fl X 10'3 ~ 500 this low-density expanding radioactive cloud, the radiation dose would be ~ 10"° R/h and certainly should pose no radiation hazard. Another aspect of this method of disposal of fission gases is simply that radioactivity is being released. The isotope that is worrisome is the long-lived ''Kr. This 1-MW reactor would release about 7 X 10"3 moles of ''Kr per year. This value must be contrasted with other sources of "^Kr. At present, the 5}̂ j. obtained from the reprocessing of power reactor fuel elements is allowed to escape out of the air vents. No attempt is made, at present, to prevent their escape. Assuming that 100% of the 85Kr is released, the escape rate in a few years is projected to be about 500 moles per year. By con-trast, the 1-MW reactor is only a very small source. Conclusions This study led to the following conclusions: • Release of rare-gas fission products to space is desirable, and • Their release does not constitute a safety hazard. If however, for some reason, the gases should not be released to space, design variations are possible to store the rare-gas fission products and to solve the associated radiation problems. REFERENCE: 1. "Gas-Cooled Fast Breeder Reactor," GA-9359, Quarterly Progress Report, Feb. 1 - April 30, 1969. 1020 

Venting of Fission Products and Shielding in Thermionic Nuclear Reactor Systems.

NASA Technical Reports Server

VENTING OF FISSION PRODUCTS AND SHIELDING IN THERMIONIC NUCLEAR REACTOR SYSTEMS
E. W. Salmi
Los Alamos Scientific Laboratory
University of California
Los Alamos, New Mexico 87544
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 lOO 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,
experimental data indicate that reservoirs at several hundred degrees C can
be useful.
Another scheme is to simply allow the fission gases to escape. Again,
because of the low activity of the fission products, this method should pose
no radiation hazards.
INTRODUCTION TABLE I. TYPICAL EMITTER OPERATING CONDITIONS
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. i.
This hypothetical emitter consists of a heavy (NI-
mm-thick) tungsten cladding surrounding the fuel
which usually consists of UO 2 or UC. If UO 2 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.
Fig. I. Hypothetical tungsten-clad fuel
emitter.
Emitter surface temperature, °C 1700
Emitter center-line temperature, °C 2500
Fuel power density, W/cm 3 50
Total fissions/cm 3 after iO 4 h 7.1 x lO 19
235U burnup, % _ 0.24
Final gaseous fission-product pressure,
psi _ lOOO
Time to 2.5% creep of cladding, h 10
Experiments have shown that at an operating tempera-
ture of N1700°C almost all gaseous fission products,
e.g., xenon and krypton, escape from the solid fuel
and accumulate in the central void. After 10,OOO h
of operation the gaseous fission product pressure
in the void would be _ lO00 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 N lO 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.
1016
https://ntrs.nasa.gov/search.jsp?R=19720010083 2020-03-17T03:47:14+00:00Z
(ALUMINA INSULATING LAYER y FISSION WS RSSAGE 
VENTS\ 
N GASES \ 
c, G A P J  ,/ \\FISSION GAS 
COLLECTOR CERAMIC TO METAL SEAL 
Fig. 2. Typical thermionic f u e l  element and possi-  
b le  methods of fission-gas release.  
t he  i l l u s t r a t ion ,  vents a re  leading from the  support 
s t ruc ture  t o  the cesium gap. The cesium gaps of 
the  individual diodes a re  $hen connected by separate 
vents throughout the length of the  f u e l  rod. 
I n  Method By i l l u s t r a t e d  i n  the upper ha l f  of 
Fig. 2, t he  vents bypass the  cesium gap and lead t o  
separate fission-gas passages outside the  co l lec tor  
i n  t h e  A 1  0 insulating region. These passages 
a l so  exte$d3throughout t he  full length of the  f u e l  
rod. 
Fission Gases Enter t h e  Cesium Gap - Method A 
Both methods have t h e i r  disadvantages. Experi- 
ments with Method A show t h a t  t he  gaseous f i s s ion  
products degrade the e l e c t r i c a l  power output of t he  
diode even a f t e r  only a short  operating time: t he  
presence of xenon or krypton i n  the  cesium vapor a t  
more than 10 or  20 Torr can increase the  e l e c t r i -  
c a l  r e s i s t i v i t y  across the  cesium vapor gap and 
thereby reduce t h e  e l e c t r i c a l  power output. 
t he  presence of foreign gases increases the  thermal 
conductivity across the  gap, which, i n  turn,  r e su l t s  
i n  a reduction o f t h e  emitter temperature. The 
f i n a l  r e s u l t  i s  the f a c t  t h a t  only a very small 
f rac t ion  of t he  gaseous f i s s ion  products can be 
retained i n  t h e  cesium vapor space. 
Fission Gases Bypass the  Cesium Gap - Method B 
Also, 
Method B offers t he  advantage of not r e s t r i c t -  
ing the  fission-product pressure t o  10 o r  20 Torr. 
This pressure r e s t r i c t ton  can influence t h e  f i s s ion  
gas storage problem outside the  reactor core. There 
a r e  some disadvantages t o  the  method. The s m a l l  
passages running through the  alumina outer sheath 
might contain cracks connecting the  co l lec tor  and 
the  outer sheath. The gaseous f i s s ion  products a re  
not purely xenon and krypton but a l so  contain many 
other elements. I n  pa r t i cu la r ,  cesium f i s s ion  pro- 
ducts a r e  about 7@ as  numerous as the  xenon and 
krypton products. Therefore, cesium a t  high pres- 
sure i s  present i n  any cracks i n  the  separate vent- 
ing system. With approximately 10 V possible across 
the  small passage, there  may be e l e c t r i c a l  shorting 
between the  collector and the  outer sheath resu l t ing  
i n  very low e l ec t r i ca l  power output of the f u e l  
element. 
Another disadvantage of Method B i s  t h e  pres- 
ence of many ceramic-to-metal sea ls  i n  the  fue l  
element (Fig. 2).  
high-pressure gaseous f i s s ion  products could enter 
Should any of these break, t h e  
t h e  cesium vapor gap and reduce or terminate the  
power output. 
and because of the  la rge  number of sea ls ,  the  separ- 
a t e  venting system should preferably be l imited t o  
the  10- or  20-Torr pressure regime. 
considerations, a separate fission-gas venting 
system does not appear t o  o f f e r  major advantages. 
Because of t h i s  l a t t e r  p o s s i b i l i t y  
From these 
FISSION-GAS STORAGE PROBLEMS 
Assume tha t  the  f i s s ion  products, a f t e r  vent- 
ing,  a re  f i n a l l y  stored in  an outside reservoir.  
Fission products t h a t  may be gaseous a t  an emitter 
temperature of ., 2000°C are  l i s t e d ,  together with 
t h e i r  atomic percentages, i n  Table 11. Note t h a t  
t h e  r a re  gases represent - 12$ of the f i s s ion  pro- 
ducts while t he  t o t a l  high-temperature gases amount 
TABLE 11. GASEOUS FISSION PRODUCTS AT 2O0O0C 
AND THEIR PART OF TOTAL GAS RELEASE 
Fission Products Amount of Species, A t .  % 
K r  and Xe 12.2 
B r  and I 2.5 
Rb and C s  9.0 
A s ,  Se, Te - 4.3 
Total 28.0 
t o  almost 3C%. 
gaseous products can escape from the  emitter,  some 
understanding of t he  escape,mechanism may be useful. 
A microphotograph of UO 
of hours of operation a? - 2000°C i s  shown i n  Fig. 
3; t he  dark spots a re  voids and the  gray material  
i s  U02. 
and seem t o  have developed a t r a i l  through t h e  U02. 
In  assessing whether a l l  these 
fue l  a f t e r  several  thousands 
Many dark spots have a len t icu lar  shape 
x 75 
Fig. 3. Microphotograph of UO fue l  a f t e r  
prolonged operation a% 2000°C. 
According t o  fission-product escape theory voids 
a r e  formed during operation, move up the  temperature 
gradient through t h e  fue l ,  sweep up the gaseous 
f i s s ion  products, and eventually deposit t he  gases 
i n  the  center of t he  fue l .  I n  other words, the  
f i s s ion  products do not have t o  diffuse long dis- 
tances. This would imply,that a l l  f i s s ion  products 
with a subs tan t ia l  vapor pressure a t  - 2000°C could 
be s w e p t  up by the  l en t i cu la r  voids and escape from 
1017 
the fuel asreadilyaseither xenonor krypton.
Experimentaldataontheseproblemsat presentare
not abundant.
Thepostulatedescapemechanismout 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 lO0-kWe thermionic reactor with
TABLE III. GENERATION OF RARE GASES IN
TYPICAL FUEL E_TTERS
Reactor thermal power, MW
Fissions produced during lO 4 h
Total number of xenon and krypton atoms
Gas volume at STP, liter
Gas volume at 20 Torr and 600°C, liter
Diameter of equivalent sphere, m
1
1.1 x lO 24
2.6 x 1023
9.8
1.2 x 103
1.32
10% efficiency would have a thermal power of 1 MW.
In 10_OOO h of operation there would have been 1.1
x lO 2_ 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 N 6OO°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 greate r than N 20 Torr may not be accept-
able becaus_ 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 ga s volume becomes ll90 liters rather
than _ lO liters. A sphere of this large volume
would have a diameter pf 132 cm or of nearly 4.5 ft.
Such a g_s volum@ 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 PROBL_
Three solutions to the fission-product storage
problems exist:
• 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-N_g 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
fromthe 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.
o
x
)-
h-
h-
2(
f
/
INSIDE REACTOR
TRAP
RARE GAS
,oo 2oo 3ao _o
TIME, days
o
'o_
x
)-
o__
I-
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 iO15 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 i.i 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 90_0 of the activity
of the rare gases is due to 133Xe 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 A_Xe; 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 _-rays do not leave
the fission-gas container, and taking into account
the internal conversion of the Y-rays, this rare-gas
source would produce about 500 R/h at one meter or
500 mR/h at 1OO 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
1018
Xe135m (16m) 0.53
Xel3519.1hI o
97 %
\ . 0.61. / 0.250
Xe 133m (2.3 d)
Qo-o""O" 0.081
Cs_33 I
0
600
400
200
I00
8o
60
40
2c
bJ
_1
o
>
a.
I-
_1
p
IO
8
6
4
2
1.0
O.S
0.6
0.4
0.2
0.!
I00
i i i ,
_IO0 I I I300 400 560 600 760
TEMPERATURE, °K
Fig. 5. Decay schemes for 13_e and 135Xe.
isotopes of greatest interest, 13_e and 135Xe, have
half-lives of 9 h and 5.3 days, respectively, the
large uncertainty in the value of the escape life-
time can significantly affect the calculated radia-
tion level. Experimental investigations of escape
rates out of emitters are required before a satis-
factory value can be assigned to the radiation
level. In any case, the storage o_ fission gases in
a lightly shielded gas chamber can pose design
problems.
Absorption in a Charcoal Trap
Normally, absorption traps are thought to
operate at very low temperatures; however, at pres-
sures of 20 Torra charcoal trap can be useful even
at moderate temperatures. For the example of a 1MW
thermal reactor, the volums of charcoal required to
store the xenon and krypton at 20 Torr can be cal-
culated as a function of temperature. 1 As shown in
Fig. 6, this charcoal volume is considerably smaller
than the original volume of 1.2 x 103 liters with-
out charcoal as shown in Table III. At a tempera-
ture of 1OO°C the charcoal volume is about 15 liters.
Although 15 liters is not a trivial volume to shield,
it may represent a compromise solution. At leO°C,
the use of heat pipes may prove feasible for con-
trolling the charcoal trap temperature.
The two methods discussed above of handling
fission gases have their disadvantages. If the
charcoal-trap temperature rose appreciably, the
fission-gas pressure could increase and pose a
hazard to reactor operation. On the other hand the
lightly shielded gas-chamber system could pose a
radiation hazard after reactor shutdown. Figure 7
Fig. 6. Amount of xenon absorbed in charcoal
as a function of temperature.
o
-o
7
o
lg. 7. RL_-_ activity follcvl_ _tor
I_X_m a_er 1.2 )_=j of _mttm.
,_ 2'o 3_ 4_ 5'0 6o
TiME AFTER REACTOR SHUT DOWN, cloys
Fig. 7. Rare-gap activity following reactor
shutdown after 1.2 years of operation.
shows the rare-gas activity expected following
reactor shutdown after 1.2 years of operation. The
curve shows that after one month the activity has
fallen by a facto_ of leO. At this time the activity
_pis due mainly to Krwhich has a ten-year half-life.
Although this activity is small compared to the
previously cited values, it is a 5-R/h radiation
source at a distance of 1 m. With a ten-year half-
life it does not disappear suddenly. If poorly
shielded, it might pose a hazard for any crew work-
ing near the reactor after shutdown.
1019
Escape of Gases to Outer Space
Values for fission gases escaping to space
have been calculated, Table IV, and show that the
number of rare-gas atoms escaping per second would
be 7.5 x 1015. These escaping atoms have an acti-
vity of 1.5 x 1010 disintegrations/sec, which
represent 0.5 Ci leaving the reactor per second.
Although at first glance this value seems high,
the radiation hazard is actually small. Assume
that the gases are emitted by an orifice to form a
radioactive cloud filling 2_ radians. As the cloud
expands, the atom density at a distance of 1 m from
TABLE IV. ACTMTY OF RARE GASES RELEASED TO
SPACE
^*^- escape rate, atom/sec 7.5 x 1015uvn*
Activity of escaping atoms, 1010
disintegration/sec 1.5 x
Radiation leaving reactor, Ci/sec 0.5
Atom density at 1 m, atom/cm 3 1.5 x 107
Atom mean free path, km 200
Dose rate a 1 m from cloud, R/h _ 10 -8
Moles of 85Kr released per year,
mole/yr
1975 projection for 85Kr release,
mole/yr
7 x 10 -3
5o0
this low-denslty expanding radioactive cloud, the
radiation dose would be _ 10- R/h and certainly
should pose no radiation hazard.
Another aspect of this method of disposal of
fission gases is simply that radioactivity is being
released. _e isotope that is worrisome is the
long-lived "Kr. This lx_MW reactor would release
about 7 x lO -5 moles of OgKr per year. _is value
must be contrasted with other sources of 5Kr. At
present, the 85Kr obtained from the reprocessing of
power reactor fuel elements is allowed to escape out
of the air vents. No attempt is made, at present,
o prevent their escape. Assuming that lO0_ of the
5Kr is released, the escape rate in a few years is
projected to be about 500 moles per year. By con-
trast, the I-MW reactor is only a very small source.
Conclusions
This study led to the following conclusions:
• Release of rare-gas fission products to
space is desirable, and
• Their release does not constitute a safety
hazard.
If however, for some reason, the gases should not
be released to space, design variations are possible
to store the rare-gas fission products and to solve
the associated radiation problems.
REFERENCE:
i. "Gas-Cooled Fast Breeder Reactor," GA-9359,
Quarterly Progress Report, Feb. 1 - April 30,
1969.
1020


Venting of fission products and shielding in thermionic nuclear reactor systems

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