- Email: [email protected]
Wall stabilization in a possible high-temperature gas-core fission reactor
0013-74X0,78 loOI-0067602CQO
WALL STABILIZATION IN A POSSIBLE HIGHTEMPERATURE GAS-CORE FISSION REACTOR FOM-Institute
J. KISTEMAKER and M. J. P. C. NIESKENS* for Atomic and Molecular Physics, Kruislaan 407, Amsterdam/Wgm.. (Received
13 December,
The Netherlands
1977)
Abstract-It is investigated if graphite as wall material in a gas-core reactor is compatible with a gas mixture consisting of UF, and carbon-fluorine compounds at high temperatures. A possibility for non-corrosive operation exists at temperatures ranging from 2000 to 2500 K at pressures of about 30atm, introducing a mixture of UF, and about 20 to 30 mole % C2F, or C,F,. Another possibility is to introduce UFI as an aerosol in CF4 as a carrier gas. For the latter case a flow-sheet is presented. indicating the possibility of a non-proliferation fission reactor.
High temperature
fission reactor
Gas-core fission reactor
INT’RODUCTION
Corrosion
Non-proliferation
The basic idea which we want to develop is how gaseous uranium-fluorine compounds can be contained in a graphite vessel in the temperature range of 8m2800K at pressures from 1 to 30atm. We do not want to exclude that the wall material might be attacked heavily by fluorine. It is important to choose the right temperature, pressure and composition in the system so that the wall remains intact due to a well-chosen thermodynamic equilibrium between the reacting components. The gaseous species involved in our calculations
Although mankind has known for more than a century the importance of operating at high temperatures to convert low grade heat into electricity, it is remarkable that temperatures above 1000 K are seldom used. All electrical power stations, whether based on coal, oil or nuclear fuel, still use water as the medium for energy conversion. The steam engine of James Watt has a long life. The main reason for this conservatism is that other high vapour pressure media-like mercury, for instance-have been tried but they are so corrosive at 1OOOK that no material lasts long enough. The low vapour pressure alkali’s seem to be of no use for running a turbine, although they might be quite useful for heat transport in high temperature reactors. To reduce low grade heat losses in our energy conversion systems, we should use conversion temperatures of 2000K or higher and extract energy at the upper part of the Carnot cycle as electricity directly by means of magnetohydrodynamic or thermionic conversion. The next step should be the gas turbine. In this case, conversion efficiencies of 55% are predicted. In this paper we want to indicate a new approach to the corrosion problem. We were inspired by the NASA gas-core fission reactor Cl]. This reactor received attention during the last two years because of unique properties connected with its gaseous ‘fuel’. Its critical mass i’s only l&100 kg, if highly enriched uranium is used. The gas can be continuously purified, avoiding reprocessing factories. Transuranic elements like plutonium are continuously recycled as a gas, leaving out the proliferation aspect connected with separate stocks of plutonium.
are U, UF,, UF5, UF4, C,, G
G
Cd, C5, F, F2,
CF, CF,, CF,, CF,, C,F,, C,F, and CIF,. Moreover, we considered the condensation of U, UF,, UF4, UC, UC, and U2C3. In the discussion we estimate the amount of radioactivity in the gas circulation loop of a gas-core fission reactor which is so low that it should not affect the chemical properties of the system.
THEORETICAL To calculate the equilibrium composition, we used the computer program developed by Nieskens rt al. [2]. Since we are interested in the behaviour of a graphite wall, it is assumed that graphite is in equilibrium with gaseous carbon. Thermodynamic data of the various species are discussed in Ref. 2. The largest uncertainty is introduced by the enthalpies of formation at 298 K of gaseous UF, and UF,. as can be seen from Table 1. Because data set I is based upon a less reliable calculating technique, we think data set II to be the more reliable one. Moreover, this latter set of data agrees better with recent results of Hildenbrand [4] and other computed data available at this moment at the National Bureau of Standards [8]. Differences between our data set II and these most modern
* Present address: Royal Dutch Shell Laboratory Amsterdam, Process Research Oil, P.O. Box 3003, Amsterdam-N, The Netherlands. 67
KISTEMAKER
AUI) NIESKENS:
WALL Sl’ABILI%Ai-ION
Table I
numbers. which arrived too late to be used by us. are. however. much less than the differences between our data sets I and II. The results given in Figs lb and 2b can therefore be considered to be quite satisfactory as a first estimate. Concerning the gaseous compounds UF,. UF2 and UF no data are available. RESULTS
.4ccbrding to Hassan and Deese [S], UF, is stable up to a temperature of 1500 K at a pressure of 10 atm (Fig. la, dotted lines). Our results using data set I are represented by the solid lines in Fig. la. From the results at other pressures, it can be concluded that our results are in good agreement with Ref. 5. provided that the mole fraction of uranium is small (< 10m4). From the tabulated equilibrium constants for the reaction UF,(g) s U(g) + 4F(g)
in equilibrium with graphite. Assuming a homogeneous gas-phase at temperatures below T,,, the partial pressure of IJF, exceeds the vapour pressure of condensed UF, (dotted lines). Therefore the calculation is repeated allowing for condensed UF,, assuming thermodynamic equilibrium between condensed and gaseous UF,. Condensed UF, and graphite are considered to hc pure phases (solid lines). It is obvious that at high temperatures all UF, IS converted and that graphite reacts with the fluorine liberated. If. however, the feed consists of UF, the graphite wall ma) be stable, provided that no lower U F compound (e.g. UF,) is formed.
(Table 15 of Ref. 5)
it can be derived that Hassan and Deese [S] omitted the enthalpy of evaporation at 298 K of uranium in the formula:
where GO and HO are the Gibbs-free energq and enthalpy at a pressure of 1 atm. respectively and T is the temperature. The results at a pressure of IOatm. using data set II. are given in Fig lb. The difference between both figures is remarkable, although the enthalpies of formation at 298 K of gaseous UF, and UF, deviate only about 20 kcal/mole. No substances condense [YJ.
To show the dependency of the system with respect to changes in data. we calculated the equilibrium composition employing both data sets. The results for the equilibrium of UF6 with graphite at a pressure of 25 atm are shown in Figs 2a and Zb. Also. IIOW the difference between both figures is remarkable. UF6 disappears completely if data set II is u~d. At temperatures aho\c .<_ ;t homogcneti)il\ ca\ phase is
Fig. I. (a) The decomposition of LiI-‘,. urmg data XI 1 The gaseous rpeaes involved in the calculation are 11. IJF-,,. 1IF 5_I‘F,. F and F2. The dotted lines represent data calculated b? Hassan and Deese [5]. All compounds hevmg it molt frxtion belou IO -I3 are omitted. Cb) The dccompovtion of OF,,. using data set II, See Parr (a) for
KISTEMAKER
AND
NIESKENS:
69
WALL STABILIZATION increase of solid feed
carbon
i’
0.2
mole /mole
la1
Q+J-
Temperature= 2000K
w
Total
pressure = latm
I
OL-
/’
\
/’
03
I
/’
0
/’
015
,’
025
/’
-0s Boo
six
2ccQ
2100
2803
035
uF,*czF:
/’
----
dataset I
-
data set II
mole fraction
C,F, in the feed
Fig. 3. The stability of a graphite wall in equilibrium with a gas consisting of UF, and C,F,
0
Fig. 2. (a) The equilibrium of UF, with graphite, using data set 1. The dotted lines represent data assuming a homogeneous gas phase at T< T,; then the partial pressure of UF, exceeds the vapour pressure of condensed UF,,. (b) The equilibrium of UF, with graphite, using data set II. See part (a) for the meaning of the dotted lines. In order to check whether the graphite wall might be stabilized by introducing a feed consisting of UF, and CF4, C2F, or C,F4. we calculated the equilibria for different feed concentrations, using both data sets. The results at a temperature of 2000 K and a pressure of 1 atm are shown in Fig. 3. It can be concluded that it is theoretically possible to stabilize graphite in contact with a gas containing UF,, but only at the expense of converting an unsaturated carbon fluorine compound into CF+ However, these unsaturated carbon-fluorine compounds are thermodynamically unstable. Neither uranium nor UF, nor any uranium carbide mentioned above condenses [2].
carbon must be added to the aerosol depending on the decomposition of UFI into UFa. This decomposition of UF, cannot be calculated because no data on gaseous UF, are available. A flow sheet of the gas-core reactor is presented in Fig. 5. Gaseous UF, is converted in UF4 with hydrogen: UF,
+ H, Q UF,(S)
+ 2HF
which is a known industrial process [6]. After the passage of the reactor, UF4 has to be converted into UF, to prevent condensation of UF, in the recycle loop. The boiling point of UF, is about 17OOK. In this conversion reactor the heavy components with a mass of about 350 have to be separated from the light components including CF, and the various fission products in a more or less fluoridized state (mass
DISCUSSION From the foregoing it can be concluded that the best way to stabilize the graphite wall is to use a feed consisting of UF,. A way to do this is to use an aerosol of UF, with CF, as a carrier gas. From Fig. 4 it can be seen that CF, remains stable up to 2000 K at a pressure of 25 atm. It is possible some
Fig. 4. The equilibrium of CF., with graphite. The gaseous species involved in the calculation are C,, Cz, C,, C,, Cgr F. F,, CF, CF,, CF,. CF,, C,F,, CIF, and C2F,.
KISTEMAKER
AND
NIESKENS:
gas-core
WALL
STABILIZATION
reactor 2OCOK
Ct.
!
FISS Prod traces
hem
exchanger
decompressor
Fig. 5. Flow-sheet
of a gas-core reactor. Schematic flow-sheet of a non-proliferatmg reactor. In principle heat efficiencies of SOS,, are possible.
about 100). High temperature gas centrifugation seems most appropriate. It is obvious that a lot of problems are to be solved if such a fuel cycle will work. One of them is the heat extraction (300MW thermal) from the nuclear reactor, which has a volume of the order of magnitude of 1 m3 for highly enriched fuel. There are probably two methods to be considered:
(a) Heat conduction, using high pressure He gas as a coolant of the walls. In this case a special gascooled grate construction out of graphite has to be used to enlarge the cooling area to about lOOm’, giving a heat flow of 300 W cm-* see- ‘. Such a design was proposed in principle by Diaz [7]. The carbon walls act as moderator for fast fission neutrons (see Fig. 5). (b) Convective heat transport with the reactor gas itself as a heat carrier (MHD or thermionic heat extraction). Another problem is the purification of the gas. Cleaning the gas on a stationary basis once per hour means that about 15 g of fission products have to be removed every hour, from a reactor with a capacity of 300 MW thermal (2000 K, 30atm, 1 m3). In that case, the reactor gas entering the recycle loop contains 0.2 volume ‘?!, fresh fission products (1.5 x 10-5g/cm3), corresponding with a radioactivity of 5000 C/cm3 producing 20 J/cm3 set-‘. After 5 min the radioactivity of this gas has already decreased by a factor of 6. If we suppose that half of this activity is in fi rays and half in the emission of y rays of about 1 MeV one can estimate, taking into account the recombination rate of positive and
gas-core
tissmn
negative ions, that the ionized fraction of the gas is negligibly small (< IO-“). This means that the purifier sees a ‘normal’ gas mixture. In the purifier CF4 has to be separated from the fission products, which means that 10 kg of fission products have to be removed per month. This amount is less bulky than the quantity of used fuel elements which an equivalent conventional light water reactor produces (250 kg/month). We want to emphasize that this waste should not contain heavy atoms like U or Pu. They are recycled in loop 1, containing IJF, and PuF, gas. Thus the problem of the long living actinides in the waste of reprocessing plants is solved in the most elegant way and the proliferation problem connected with Pu automatically disappears. CONCLUSION Using UF, as the graphite wall gaseous mixture amounts of C2FZ
a material for a gas-core reactor, can be stabilized by introducing a consisting of UF, and the right or C2F,. However, these unsatur-
ated carbonfluorme compounds are thermodynamically unstable. A better way to stabilize the graphite wall is probably to introduce the uranium as UF,. e.g. aerosol in Cl-‘4 as a carrier gas. These aerosols evaporate immediately on arriving in the gas-core reactor (2000 K). An interesting aspect of the fuel cycle of such a gas core reactor is that all fissionable materials stay inside the loop, thus giving a possible solution for a non-proliferation reactor. Only certain quantities of relatively short lived fission products (T$ < 30 years) will have to be handled outside the loop.
KISTEMAKER
A~knowlrdg~,ilc,,~rs-
We thank
ANI)
NIESKENS:
Dr A. J. H. Bocrhoom.
WALL
Dr
[2]
H. N. Stein and Dr M. Bustraan for valuable discussions This work was part of the research program of the Foun-
[jl
dation for Fundamental Research on Matter (FOM) and was made possible by a special grant of the Dutch Organization for the Advancement of Pure Rcscarch (ZWO).
REFERENCES
[II
K. Thorn. R. T. Schneider and F. C. Schwcnk. P/II.sPofcwtitrls of'Fissimimq P Itrsmtr.~ for Span’ 25th Int. Astronaut. Congr.. Power urlti Propulsim. Amsterdam. Sept. 1974: J. S. Kendall and R. J. Rodgers. Pmcdiugp / _‘rk 111tcwoc~. Emwq~~ COII~CP sim Euqiu. Cm/I. Washington. Sept. 1977. p. 1706.
its urld
L41
STABILIZ.ATION
71
M. J. P. C. Niakrns. H. N. Stein and J. Ki\temakct. I!tt/. EuM. (‘/l~~/rl. FurIt/. In press. I. N. Godncv
Ztrr-cd.Khitrt. I
and A. S. Svcrdlin. /:I.. I\.!. lilli0l. ~~Ul~lol. 9. 40 ( 19661.
C’c~hh.
D. L. Hildcnhrand. J. c/lc,ri~. P/I~x 66. 4788 (1977). H. A. Haasan and .I. E. Decsc. Tl~c~rltot/r,,~tr~,~~[, Piq’l”‘rrc\ of C’F, (II Hicgh Tcrr~/,l,~trr~r,~,.\. NASACR-2373. lY74 (U.S. National Technical lnformatton Scr\ ice. Sprlngtield. Val. r/(,r ~;~~/III;.\~~/I~~II (‘/~,ru~c. 3rd 161 L’//u~trr~rl\ F_‘,~c~~~k/o/~i~t/r~, cdn. Vol. 17. p. 76X. Lrban & Schwar/enbcrg. Miinchcn-Berlin Wien (lY66). Llni\erbity of Florida. Nucl. Fng. SC. 171N. J. Diu Dept.. Gnins\Ille. Pri\ate communication IX1 Prl\ ate informntion.
ISI
WALL STABILIZATION IN A POSSIBLE HIGHTEMPERATURE GAS-CORE FISSION REACTOR FOM-Institute
J. KISTEMAKER and M. J. P. C. NIESKENS* for Atomic and Molecular Physics, Kruislaan 407, Amsterdam/Wgm.. (Received
13 December,
The Netherlands
1977)
Abstract-It is investigated if graphite as wall material in a gas-core reactor is compatible with a gas mixture consisting of UF, and carbon-fluorine compounds at high temperatures. A possibility for non-corrosive operation exists at temperatures ranging from 2000 to 2500 K at pressures of about 30atm, introducing a mixture of UF, and about 20 to 30 mole % C2F, or C,F,. Another possibility is to introduce UFI as an aerosol in CF4 as a carrier gas. For the latter case a flow-sheet is presented. indicating the possibility of a non-proliferation fission reactor.
High temperature
fission reactor
Gas-core fission reactor
INT’RODUCTION
Corrosion
Non-proliferation
The basic idea which we want to develop is how gaseous uranium-fluorine compounds can be contained in a graphite vessel in the temperature range of 8m2800K at pressures from 1 to 30atm. We do not want to exclude that the wall material might be attacked heavily by fluorine. It is important to choose the right temperature, pressure and composition in the system so that the wall remains intact due to a well-chosen thermodynamic equilibrium between the reacting components. The gaseous species involved in our calculations
Although mankind has known for more than a century the importance of operating at high temperatures to convert low grade heat into electricity, it is remarkable that temperatures above 1000 K are seldom used. All electrical power stations, whether based on coal, oil or nuclear fuel, still use water as the medium for energy conversion. The steam engine of James Watt has a long life. The main reason for this conservatism is that other high vapour pressure media-like mercury, for instance-have been tried but they are so corrosive at 1OOOK that no material lasts long enough. The low vapour pressure alkali’s seem to be of no use for running a turbine, although they might be quite useful for heat transport in high temperature reactors. To reduce low grade heat losses in our energy conversion systems, we should use conversion temperatures of 2000K or higher and extract energy at the upper part of the Carnot cycle as electricity directly by means of magnetohydrodynamic or thermionic conversion. The next step should be the gas turbine. In this case, conversion efficiencies of 55% are predicted. In this paper we want to indicate a new approach to the corrosion problem. We were inspired by the NASA gas-core fission reactor Cl]. This reactor received attention during the last two years because of unique properties connected with its gaseous ‘fuel’. Its critical mass i’s only l&100 kg, if highly enriched uranium is used. The gas can be continuously purified, avoiding reprocessing factories. Transuranic elements like plutonium are continuously recycled as a gas, leaving out the proliferation aspect connected with separate stocks of plutonium.
are U, UF,, UF5, UF4, C,, G
G
Cd, C5, F, F2,
CF, CF,, CF,, CF,, C,F,, C,F, and CIF,. Moreover, we considered the condensation of U, UF,, UF4, UC, UC, and U2C3. In the discussion we estimate the amount of radioactivity in the gas circulation loop of a gas-core fission reactor which is so low that it should not affect the chemical properties of the system.
THEORETICAL To calculate the equilibrium composition, we used the computer program developed by Nieskens rt al. [2]. Since we are interested in the behaviour of a graphite wall, it is assumed that graphite is in equilibrium with gaseous carbon. Thermodynamic data of the various species are discussed in Ref. 2. The largest uncertainty is introduced by the enthalpies of formation at 298 K of gaseous UF, and UF,. as can be seen from Table 1. Because data set I is based upon a less reliable calculating technique, we think data set II to be the more reliable one. Moreover, this latter set of data agrees better with recent results of Hildenbrand [4] and other computed data available at this moment at the National Bureau of Standards [8]. Differences between our data set II and these most modern
* Present address: Royal Dutch Shell Laboratory Amsterdam, Process Research Oil, P.O. Box 3003, Amsterdam-N, The Netherlands. 67
KISTEMAKER
AUI) NIESKENS:
WALL Sl’ABILI%Ai-ION
Table I
numbers. which arrived too late to be used by us. are. however. much less than the differences between our data sets I and II. The results given in Figs lb and 2b can therefore be considered to be quite satisfactory as a first estimate. Concerning the gaseous compounds UF,. UF2 and UF no data are available. RESULTS
.4ccbrding to Hassan and Deese [S], UF, is stable up to a temperature of 1500 K at a pressure of 10 atm (Fig. la, dotted lines). Our results using data set I are represented by the solid lines in Fig. la. From the results at other pressures, it can be concluded that our results are in good agreement with Ref. 5. provided that the mole fraction of uranium is small (< 10m4). From the tabulated equilibrium constants for the reaction UF,(g) s U(g) + 4F(g)
in equilibrium with graphite. Assuming a homogeneous gas-phase at temperatures below T,,, the partial pressure of IJF, exceeds the vapour pressure of condensed UF, (dotted lines). Therefore the calculation is repeated allowing for condensed UF,, assuming thermodynamic equilibrium between condensed and gaseous UF,. Condensed UF, and graphite are considered to hc pure phases (solid lines). It is obvious that at high temperatures all UF, IS converted and that graphite reacts with the fluorine liberated. If. however, the feed consists of UF, the graphite wall ma) be stable, provided that no lower U F compound (e.g. UF,) is formed.
(Table 15 of Ref. 5)
it can be derived that Hassan and Deese [S] omitted the enthalpy of evaporation at 298 K of uranium in the formula:
where GO and HO are the Gibbs-free energq and enthalpy at a pressure of 1 atm. respectively and T is the temperature. The results at a pressure of IOatm. using data set II. are given in Fig lb. The difference between both figures is remarkable, although the enthalpies of formation at 298 K of gaseous UF, and UF, deviate only about 20 kcal/mole. No substances condense [YJ.
To show the dependency of the system with respect to changes in data. we calculated the equilibrium composition employing both data sets. The results for the equilibrium of UF6 with graphite at a pressure of 25 atm are shown in Figs 2a and Zb. Also. IIOW the difference between both figures is remarkable. UF6 disappears completely if data set II is u~d. At temperatures aho\c .<_ ;t homogcneti)il\ ca\ phase is
Fig. I. (a) The decomposition of LiI-‘,. urmg data XI 1 The gaseous rpeaes involved in the calculation are 11. IJF-,,. 1IF 5_I‘F,. F and F2. The dotted lines represent data calculated b? Hassan and Deese [5]. All compounds hevmg it molt frxtion belou IO -I3 are omitted. Cb) The dccompovtion of OF,,. using data set II, See Parr (a) for
KISTEMAKER
AND
NIESKENS:
69
WALL STABILIZATION increase of solid feed
carbon
i’
0.2
mole /mole
la1
Q+J-
Temperature= 2000K
w
Total
pressure = latm
I
OL-
/’
\
/’
03
I
/’
0
/’
015
,’
025
/’
-0s Boo
six
2ccQ
2100
2803
035
uF,*czF:
/’
----
dataset I
-
data set II
mole fraction
C,F, in the feed
Fig. 3. The stability of a graphite wall in equilibrium with a gas consisting of UF, and C,F,
0
Fig. 2. (a) The equilibrium of UF, with graphite, using data set 1. The dotted lines represent data assuming a homogeneous gas phase at T< T,; then the partial pressure of UF, exceeds the vapour pressure of condensed UF,,. (b) The equilibrium of UF, with graphite, using data set II. See part (a) for the meaning of the dotted lines. In order to check whether the graphite wall might be stabilized by introducing a feed consisting of UF, and CF4, C2F, or C,F4. we calculated the equilibria for different feed concentrations, using both data sets. The results at a temperature of 2000 K and a pressure of 1 atm are shown in Fig. 3. It can be concluded that it is theoretically possible to stabilize graphite in contact with a gas containing UF,, but only at the expense of converting an unsaturated carbon fluorine compound into CF+ However, these unsaturated carbon-fluorine compounds are thermodynamically unstable. Neither uranium nor UF, nor any uranium carbide mentioned above condenses [2].
carbon must be added to the aerosol depending on the decomposition of UFI into UFa. This decomposition of UF, cannot be calculated because no data on gaseous UF, are available. A flow sheet of the gas-core reactor is presented in Fig. 5. Gaseous UF, is converted in UF4 with hydrogen: UF,
+ H, Q UF,(S)
+ 2HF
which is a known industrial process [6]. After the passage of the reactor, UF4 has to be converted into UF, to prevent condensation of UF, in the recycle loop. The boiling point of UF, is about 17OOK. In this conversion reactor the heavy components with a mass of about 350 have to be separated from the light components including CF, and the various fission products in a more or less fluoridized state (mass
DISCUSSION From the foregoing it can be concluded that the best way to stabilize the graphite wall is to use a feed consisting of UF,. A way to do this is to use an aerosol of UF, with CF, as a carrier gas. From Fig. 4 it can be seen that CF, remains stable up to 2000 K at a pressure of 25 atm. It is possible some
Fig. 4. The equilibrium of CF., with graphite. The gaseous species involved in the calculation are C,, Cz, C,, C,, Cgr F. F,, CF, CF,, CF,. CF,, C,F,, CIF, and C2F,.
KISTEMAKER
AND
NIESKENS:
gas-core
WALL
STABILIZATION
reactor 2OCOK
Ct.
!
FISS Prod traces
hem
exchanger
decompressor
Fig. 5. Flow-sheet
of a gas-core reactor. Schematic flow-sheet of a non-proliferatmg reactor. In principle heat efficiencies of SOS,, are possible.
about 100). High temperature gas centrifugation seems most appropriate. It is obvious that a lot of problems are to be solved if such a fuel cycle will work. One of them is the heat extraction (300MW thermal) from the nuclear reactor, which has a volume of the order of magnitude of 1 m3 for highly enriched fuel. There are probably two methods to be considered:
(a) Heat conduction, using high pressure He gas as a coolant of the walls. In this case a special gascooled grate construction out of graphite has to be used to enlarge the cooling area to about lOOm’, giving a heat flow of 300 W cm-* see- ‘. Such a design was proposed in principle by Diaz [7]. The carbon walls act as moderator for fast fission neutrons (see Fig. 5). (b) Convective heat transport with the reactor gas itself as a heat carrier (MHD or thermionic heat extraction). Another problem is the purification of the gas. Cleaning the gas on a stationary basis once per hour means that about 15 g of fission products have to be removed every hour, from a reactor with a capacity of 300 MW thermal (2000 K, 30atm, 1 m3). In that case, the reactor gas entering the recycle loop contains 0.2 volume ‘?!, fresh fission products (1.5 x 10-5g/cm3), corresponding with a radioactivity of 5000 C/cm3 producing 20 J/cm3 set-‘. After 5 min the radioactivity of this gas has already decreased by a factor of 6. If we suppose that half of this activity is in fi rays and half in the emission of y rays of about 1 MeV one can estimate, taking into account the recombination rate of positive and
gas-core
tissmn
negative ions, that the ionized fraction of the gas is negligibly small (< IO-“). This means that the purifier sees a ‘normal’ gas mixture. In the purifier CF4 has to be separated from the fission products, which means that 10 kg of fission products have to be removed per month. This amount is less bulky than the quantity of used fuel elements which an equivalent conventional light water reactor produces (250 kg/month). We want to emphasize that this waste should not contain heavy atoms like U or Pu. They are recycled in loop 1, containing IJF, and PuF, gas. Thus the problem of the long living actinides in the waste of reprocessing plants is solved in the most elegant way and the proliferation problem connected with Pu automatically disappears. CONCLUSION Using UF, as the graphite wall gaseous mixture amounts of C2FZ
a material for a gas-core reactor, can be stabilized by introducing a consisting of UF, and the right or C2F,. However, these unsatur-
ated carbonfluorme compounds are thermodynamically unstable. A better way to stabilize the graphite wall is probably to introduce the uranium as UF,. e.g. aerosol in Cl-‘4 as a carrier gas. These aerosols evaporate immediately on arriving in the gas-core reactor (2000 K). An interesting aspect of the fuel cycle of such a gas core reactor is that all fissionable materials stay inside the loop, thus giving a possible solution for a non-proliferation reactor. Only certain quantities of relatively short lived fission products (T$ < 30 years) will have to be handled outside the loop.
KISTEMAKER
A~knowlrdg~,ilc,,~rs-
We thank
ANI)
NIESKENS:
Dr A. J. H. Bocrhoom.
WALL
Dr
[2]
H. N. Stein and Dr M. Bustraan for valuable discussions This work was part of the research program of the Foun-
[jl
dation for Fundamental Research on Matter (FOM) and was made possible by a special grant of the Dutch Organization for the Advancement of Pure Rcscarch (ZWO).
REFERENCES
[II
K. Thorn. R. T. Schneider and F. C. Schwcnk. P/II.sPofcwtitrls of'Fissimimq P Itrsmtr.~ for Span’ 25th Int. Astronaut. Congr.. Power urlti Propulsim. Amsterdam. Sept. 1974: J. S. Kendall and R. J. Rodgers. Pmcdiugp / _‘rk 111tcwoc~. Emwq~~ COII~CP sim Euqiu. Cm/I. Washington. Sept. 1977. p. 1706.
its urld
L41
STABILIZ.ATION
71
M. J. P. C. Niakrns. H. N. Stein and J. Ki\temakct. I!tt/. EuM. (‘/l~~/rl. FurIt/. In press. I. N. Godncv
Ztrr-cd.Khitrt. I
and A. S. Svcrdlin. /:I.. I\.!. lilli0l. ~~Ul~lol. 9. 40 ( 19661.
C’c~hh.
D. L. Hildcnhrand. J. c/lc,ri~. P/I~x 66. 4788 (1977). H. A. Haasan and .I. E. Decsc. Tl~c~rltot/r,,~tr~,~~[, Piq’l”‘rrc\ of C’F, (II Hicgh Tcrr~/,l,~trr~r,~,.\. NASACR-2373. lY74 (U.S. National Technical lnformatton Scr\ ice. Sprlngtield. Val. r/(,r ~;~~/III;.\~~/I~~II (‘/~,ru~c. 3rd 161 L’//u~trr~rl\ F_‘,~c~~~k/o/~i~t/r~, cdn. Vol. 17. p. 76X. Lrban & Schwar/enbcrg. Miinchcn-Berlin Wien (lY66). Llni\erbity of Florida. Nucl. Fng. SC. 171N. J. Diu Dept.. Gnins\Ille. Pri\ate communication IX1 Prl\ ate informntion.
ISI