Experimental Studies of Actinides

41 Pages·2005·1.38 MB·English

LA-10340
UC-4
Issued: June 1985
LA—10340
DE85 015368
Experimental Studies of
Actinides in Molten Salts
James G. Reavis
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 responsi-
bility fcr the accuracy, completeness, or usefulness of any information, apparatus, product, or
process disclosed, or represents 'hat its use would not infringe privately owned rights. Refer-
ence herein to any specific commercial product, process, or service by trade name, trademark,
manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-
mendation, 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.
Los Alamos National Laboratory
Los Alamos, New Mexico 87545
!IT"ISUnOS OF THIS DOCUMENT IS ONLIMfTW
t>
EXPERIMENTAL STUDIES OF ACTINIDES
IN MOLTEN SALTS
by
J. G. Reavis
ABSTRACT
This review stresses techniques used in studies of molten salts containing multigram
amounts of actinides exhibiting intense alpha activity but little or no penetrating
gamma radiation. The preponderance of studies have used halides because oxygen-
containing actinide compounds (other than oxides) are generally unstable at high
temperatures. Topics discussed here include special enclosures, materials problems,
preparation and purification of acticide elements and compounds, and measurements of
various properties of the molten volts. Property measurements discussed are phase
relationships, vapor pressure, density, viscosity, absorption spectra, electromotive
force, and conductance.
I. INTRODUCTION portant differences. The actinides are radioactive, and
work with them requires special health protection. A
A. Perspective particular hazard of working with plutonium and en-
riched uranium is that a critical mass may be assembled
The importance of the actinides to society in the next inadvertently.
100 years can hardly be overemphasized. The com- Many of the techniques and examples discussed in
munications media have publicized widely the potential this report apply equally well to work with nonactinides
destmctiveness of energy release from actinides- Less and have been described elsewhere, but it is hoped thai
widely known is that the useful energy derivable from there will be sufficient new material in this discussion to
economically recoverable actinides (uranium and reward the reader.
thorium) is 5 to 20 times as great as the energy derivable Although 15 elements (including actinium) are in the
from all economically recoverable fossil fuels.1 This fact actinide series, only about 6 have been used in molten
establishes the economic importance of developing effi- salt studies. The list of anions in molten salt studies is
cient processes for preparing thorium, uranium, pluto- similarly limited. The combinations found in an ex-
nium, and their compounds. Beyond that economic tensive compilation25 of phase diagrams is listed in
importance, the study of the actinides is technically Table I as an example of the cation/anion combinations
fascinating. The actinide series is analogous to the studied at high temperatures. In addition to the oxides,
lanthanide series, with similarities and differences be- fluorides, and chlorides listed, !4 other phase diagrams
tween homologs and interesting and unexplained were given for bromides, sulfates, oxychlorides, phos-
changes in properties during progression through the phates, silicates, molybdates, and tungstates of the ac-
respective series. tinides. These 14 other diagrams are included in the last
The actinides are chemically active metals, only column of Table I. This distribution is typical of that of
slightly less active than the alkaline-earth and rare-earth actinide-anior. combinations in other molten salt
elements. Molten salt techniques in studies of actinide studies. Because oxides are not generally classified as
compounds often resemble those used in studies of rare- salts, this report deals primarily with halides of the most
earth and alkaline-earth compounds, but there are im- abundant actinides simply because other compounds
isolated by 1950. The other five actinides were dis-
TABLE I. Actinide/Anion Combinations Listed in a
covered after 1950.
Compilation of Phase Diagrams
Actinide Oxide Fluoride Chloride Total
C. Availability of Actinides
Cm 1 0 0 1
Np 1 0 0 3
Chemical studies (including molten salt studies) of
Pu 4 3 9 16
the actinides are limited by the restricted availability of
Th 36 11 0 53
the elements. Besides govemmentally imposed regula-
U 34 30 10 80
tions, there are certain physical problems, such as lim-
ited rate of creation and isolation of the elements,
coupled with short half-lives. Only microgram quan-
tities of transcalifornium elements will exist in the
have not been studied at high temperatures. One reason foreseeable future. Another problem is intense radiation
for this lack is that the oxyanion compounds of actinides emitted by most actinides and their daughter elements,
requiring shielding and containment Such facilities are
are often unstable.
available in oniy a few commercial and university labo-
ratories outside the small number of government labo-
ratories built specifically for studies of actinides. Order-
B. Historical Perspective
of-magnitude values of isolated and purified supplies of
the most abundant actinides and naif-lives of their most
Only four actinides (actinium, protactinium,
useful isotopes are listed in Table II. The actinides not
thorium, and uranium) exist in nature in concentrations
listed in Table II are available only in microgram or
detectable without the most sophisticated techniques.
submicrogram quantities and probably will never be
Actinium was discovered in 1899 by Devierne and,
studied extensively by molten salt techniques. Studies of
apparently, independently in 1902 by Geisel but was not
actinium, berkelium, and californium will be limited
isolated in pure form until 1947, when milligram quan-
tities were separated from neutron-irradiated radium.6 because of unavailability and because of their intense
radioactivity. The radioactivity of americium and
Thorium, discovered by Berzelius in 1828, was used
curium discourages their study. Protactinium is unique
commercially before 1900. Protactinium was dis-
in that it is naturally occurring and has a long half-life
covered in 1918 and was first isolated in milligram
but is very expensive to recover because the richest
amounts in 1927. Uranium, discovered in 1789, was
mineral source contains only a few parts per million of
well known to scientists before 1900. The remaining
the element. About 125 g of the element was isolated
actinides are all synthetic and were unknown before
from about 60 tons of ore by workers7 in the UK. This
about 1940. Plutonium was first isolated in visibu.
quantity is sufficient for experimentation using ordinary
quantities as an oxide in September 1942. Neptunium,
techniques; however, its high cost dictates conservation.
amencium, curium, berkelium, and californium were
TABLE II. Availability and Half-Lives of the Most Abundant Ac-
tinides
Element Quantities Available Prevalent Isotope Half-Life, Yr
Ac milligrams 227 22
Th megagrams 232 1.4 X 1010
Pa grams 231 3.2 XW
U megagrams 238 43 X 10»
Np kilograms 237 2.1 X 10*
Pa megagrams 239 2.4 X104
Am grams* 241 458
Cm grams* 244 18
Bk milligrams 249 0.86
a milligrams 249 360
*It is planned that kilograms of compounds of ameridum and enrinm will be
isolated, bat batch sizes of only 10 g or less are amenable to studies of molten
salts without extensive radiation shielding.
n. SPECIAL ENCLOSURES REQUIRED FOR AC- Selecting the enclosure to be used for an actinide
TINIDES project is mostly left to the experimenter and his em-
ployer. This is not to say that the experimenter has
A. Health and Safety Standards much freedom in this matter. Managers of various
laboratories apparently differ widely in interpreting re-
Anyone wishing to conduct experiments with ac- gulations. The nature of the enclosure required is de-
tinides or actinide compounds must face governmental termined in part by regulations, in part by the nature of
regulation of their possession and use. The ex- the operation, and in part by the amount of actinide
perimenter may have to deal with the International involved. Aqueous chemistry experiments involving
Atomic Energy Agency, the United States Nuclear Regu- micrograms of even very highly active members of the
latory Commission and Department of Energy, state series are performed in chemical hoods, whereas larger
agencies, and local government agencies.8"10 Some of the quantities of the same element in powder form must be
regulations limit mere possession of these elements, handled in an enclosure of much higher integrity.
much less their use. The various regulatory agencies will Within certain guidelines the safety of each series of
insist that certain guidelines be obeyed before operating experiments must be evaluated independently to deter-
licenses are issued, and these guidelines will dictate mine the enclosure or hood requirements.
tradeoffs between amounts of actinides to be used and
the complexity of the facility that must be constructed to
handle the material. These guidelines dictate irradiation C. Gloveboxes
levels allowable for various body parts of the operators;
degree of environmental protection from radiological
Gloveboxes have been used for much of the research,
and toxic chemical release during normal operation and
development, and production work with actinides. The
in case of fire, explosion, tornado, and natural disasters;
basic enclosure may be a cube measuring about 75 cm
and other items too numerous to mention here. Among
on each edge, with a window and a pair of elastomer
the well-known sources of these guidelines are the Inter-
gloves about 75 cm long with 20-cm-diam cuffs. Blocks
national Atomic Energy Agency Safety Standards, the
of this sort may be made taller, combined side by side
Scientific Committee on the Effects of Atomic Radia-
and back to back (omitting side or back walls from the
tion of the United Nations, the (US) National Council
combination as appropriate), or both to accommodate
on Radiation Protection, the International Commission
the equipment for the operations. An airlock or ante-
on Radiation Protection, and the Advisory Committee
chamber in which radioactive contamination is kept at a
on the Biological Effects of Ionizing Radiation of the
low level should be provided to avoid escape of radioac-
National Academy of Sciences National Research
tive material to the environment during introduction of
Council. In the US, the Nuclear Regulatory Com-
laboratory equipment or chemicals. The enclosure must
mission regulations appear in the Code of Federal Re-
be "leak free" and must have a system to control the
gulations, Title 10, Chapter 1—Energy. The regulations
internal pressure at a value slightly below ambient
of other US agencies such as the Environmental Protec-
laboratory pressure. The construction materials must be
tion Agency and the Department of Transportation also
suitable for the operation and must conform with stan-
govern handling of the actinides and are listed in other
dards of resistance to fire, earthquake, pressure differen-
titles of the code. Additional information concerning
tial, radiation, and other problems. Early gloveboxes
health and safety standards and practices can be found
were constructed of plywood, ordinary window glass,
in Refs. 8-10.
and obstetrical gloves. The basic glovebox has evolved
to meet the needs of various laboratories, but those
facilities have retained certain design features. Unique
features have evolved from stronger emphasis on dif-
B. Ben?htop and Chemical Hood Enclosures
ferent criteria at different facilities. For instance, one
facility may place emphasis on fire safety, another on
Before about 1940 little thought was given to
resistance to failure at high pressure differential, another
respirator protection of workers handling massive
on operator comfort, another on extreme leak tightness,
amounts of the then commonly available actinides,
and so on.
thorium and uranium. Work with these compounds was
commonly performed on open benchtops without Gloveboxes used at the Los Alamos National Labora-
limitation. During the 1940s, general awareness of the tory will be discussed in some depth as an example, not
nature of radioactivity increased as more intensely because these gloveboxes are the best for all processes or
radioactive actinides became available. Consequently, experiments, but because they meet c exceed US safety
government regulations were set up in an attempt to criteria and are used for molten salt experiments and
legislate safety for workers. pyrochemical processing operations. Figures 1 and 2
Fig-'. A tyuical laboratory al the Los Alamos Plutonium Facility.
STAINLESS STEEL
PIPE NIPPLE-
STAINLESS
STEEL WAL'-
STAINLESS
STAINLESS
STEEL FRAME
STAINLESS STEEL STEEL PANEL
BOX TOP
BOLT
NEOPRENE
GASKET
SAFETY
GLASS— tZj9
LEAD
STAINLESS
STEEL
Fig. 2. Cross section of a glovebox enclosure showing some of the details of construc-
tion. The glovebox shown here has work stations with 75-cm X 75-cm floors and a
height of 80 cm.
show features of these enclosures. The boxes are fabri- residence time for the bulk of the material), it may
cated from stainless steel Type 304 or 316 chosen for the accumulate radioactive decay products that often have
appropriate corrosion resistance. Their welds are greater radiation energy than was emitted by the parent.
polished to the smoothness of the adjacent metal. Floors The standard 0.25-in.-thick window is held in place
have a No. 4 finish; the walls and tops have a 2B finish. by a neoprene gasket that requires a stainless steel frame
Sharp comers are avoided. The radius of the inside or one that requires no frame. The stainless steel walls
corners is 25 mm. These features aid in cleanliness, and floors are approximately 5 mm thick except for
which helps in two ways to reduce the radiation back- stronger floors in enclosures containing very heavy
ground. First, because rough surfaces are difficult to equipment. The gloves are neoprcne or a similar
clean, they accumulate thin layers of the actinides that elastomer and are at least 0.38 mm thick.
are handled inside the enclosure. Thin films of this sort The gloveboxes described above give adequate
do not have the self-absorption that dense, massive protection for handling large quantities of thorium,
samples have, so a small amount of actinide in the film natural uranium, and highly enriched uranium. They
contributes inordinate radiation to the general back-
are also adequate for handling up to a kilogram of
ground. Second, if the actinide film remains on the wall plutonium containing less than approximately 1% 241Pu
for a long period (longer than the normal glovebox
and having had americium separated from it within the
previous few months. If multikilograms of plutonium velops even a pinho'.e in a glove of an inert atmosphere
are being processed or if the 24lAm decay product has enclosure, instrumentation for detecting oxygen and
been allowed to accumulate several months from decay water leaks responds much more quickly than instru-
of 241Pu present at concentrations of about 1%, the mentation for detecting escape of radioactive particles.
additional radiation emission of 24lAm requires extra
shielding to protect personnel. Such usage should be
anticipated by including 0.25 in. of lead shielding in the D. Remote Handling
walls of the enclosure during fabrication. The detail of
Fig. 2 shows how the lead is covered by a thin layer of The actinide researcher can almost always avoid the
stainless steel welded to the shell of the box to make an use of remotely operated hot cells if the proper isotopes
easily cleanable outside surface. This construction are available and if the amount of actinide can be kept
eliminates exposed cracks at the edge of the lead and small and repurified often to remove highly active decay
makes decontamination much easier when inadvertent products. The small amounts of berkelium and heavier
contamination of the laboratory occurs. Adding lead actinides that are available almost force the use of
glass outside the safety glass easily provides additional microchemical techniques for their studies, so the ques-
shielding for windows. Additional shielding for hands tion of using remote operation and intermediate-shield-
may be provided by lead-impregnated gloves available ing facilities for research is moot.
in thicknesses up to 1.65 mm. When these modifica-
Separating transuranium elements from irradiated
tions are made, the enclosure is safe for handling multi-
sources is a different matter. Transuranium actinides
gram quantities of americium, even in dilute solution
are commonly produced by irradiating actinides in
where self-shielding is minimized. Multigram quantities
high-flux reactors. Many intensely radioactive elements
of M1Np and 2MU may also be handled in such an
are created during irradiation, so the wanted actinides
enclosure if exposure times are kept short. However,
must be isolated in highly shielded hot cells. Most or all
more lead shielding must be added for routine opera-
of these separations are done by aqueous procedures,
tions with these isotopes. Plutonium-238 processing is
but there are arguments" that molten salt processes
routinely performed in the same type of enclosure with
should be developed. The expected benefits of
the addition of about 15-cm-thick hydrogenous shield-
pyrochemical processing include freedom from radia-
ing (either tanks of water or slabs of plastic). The
tion damage of solvents, extractants, or ion exchange
hydrogenous shielding protects against neutrons
resins, and production of a smaller volume of radioac-
produced by alpha-neutron reactions from the intense
tive waste products, whose disposal is quite expensive.
alpha particle emission of 238Pu.
Pyrochemical processing should be considered not only
Glovebox enclosures have been used for many for preparation of research quantities of actinides, but
pyrochemical experiments and processing operations should also assume great commercial importance in
involving molten salts. This type of enclosure is better separating fertile and fissionable fuel from spent reactor
than benchtop and open-hood enclosures when cor- fuels.
rosive and hygroscopic materials are being used (even if Figure 3 illustrates the cross-section and plan views of
they are not radioactive) because the atmosphere can be a typical research hot cell facility12 with four cells con-
controlled to eliminate undesirable side reactions. Com- taining individual alpha containment enclosures. These
mercial, regenerable air dryers using dual beds of molec- cells are separated by thick steel doors from a corridor
ular sieve material can maintain air atmospheres in used for transferring highly active samples from
enclosures with water concentrations of about 1 ppm. shielded shipping casks to an alpha enclosure or a
Similarly, "boil off' gas from liquid argon or nitrogen storage well. These are typical cells for research with
can be used in a once-through flow system to maintain highly active gamma emitters. Cells of this size can also
atmospheres with oxygen and water concentrations less be used for processing kilogram quantities of irradiated
than 10 ppm. Recirculating purifiers use dual, re- fuel or isotope source material by pyrochemical meth-
generable sorbent beds containing molecular sieves and ods where the reactants are kept in a very dense, com-
molecular sieves loaded with activated copper or nickel pact form. Work with manipulators is extremely time
to maintain atmospheres of this quality. The enclosure consuming and maintenance of hot cells is very ex-
must be virtually leak free. Very sensitive leak detection pensive. Hot cells are also expensive to construct and
instruments such as a commercial helium mass spec- they require much space in expensive buildings that
trometer leak detector must be used to ensure freedom meet criteria for actinide containment. It is usually less
from leaks. The allowable leak rate of enclosures operat- expensive to set up equipment for microchemical
ing at oxygen and water concentrations of 10 ppm are measurements of chemical and physical properties.
orders-of-.nagnitude smaller than the allowable leak These are possible when the pure ac'inides are available
rates for air atmosphere enclosures for intense alpha free from large amounts of other radioactive materials.
emitters such as plutoniurn and americium. If one de- Less time is often required for hands-on microchemical
MONORAB.
HOIST
DOOR DOCK-
TRACKS
EQUIPMENT
ROOM \
\
STORAQE
WELLS—~
oo oo oo
STEEL
DOORS-
Fig, 3. Plan and cross-section views of four hot cells
used for research and small-scale production opera-
CELL •v«S&vSi tions with actinides containing intense gamma radia-
EFIXLTHEARUST' TT XJLJZELL EXlL tion emitters.
SERVICE WINDOW CART FOR
TRENCH SHIELDING CONTAINER
GENERAL MILLS
MANIPULATOR
MAGNETITE CONCRETE
ANLMODEL8
MANIPULATOR
SERVICE
TRENCH
VENTILATION STEEL STORAGE CORRIDOR
DUCT DOOR WELL
10
experiments than for cumbersome operations gamma fields will not be discussed here. This subject is
performed with manipulators. Some measurements in thoroughly covered in many publications, including
molten salt studies, however, are extremely difficult to Refs. 8, 14, and 15. Less well known is the severity of
make by microchemical techniques and may be done radiation damage by intense alpha emissions of some of
more efficiently in a hot cell. Additional information the actinides. The intensity of alpha particle emission by
concerning design and operation of enclosures for han- protactinium, thorium, natural uranium, and the most
dling the actinides and their compounds is given in commonly used isotope of plutonium (mass 239) is low
Refs.8,10,andl3. enough to cause almost negligible effects.
Storage of the less active actinides, even in kilogram
amounts, as solutions or dry compounds in
HI. MATERIALS PROBLEMS polyethylene or glass containers presents no particular
problems (subject to critical-mass limitations). Pluto-
A. Degradation by Radiation nium-238 and more active actinide isotopes, however,
do cause significant container degradation. Even milli-
Irradiation degrading of chemicals, elastomers, and gram amounts of these actinides should be stored in
glass is severe in hot cells where irradiated materials are nonreactive metal containers. Plastics are degraded
handled, but because reprocessing does not extensively quickly, not only by the intense alpha irradiation, but
employ molten salts or pure actinides (except as an end also because of the heat generated locally. Glass vials
product), materials degradation in high-intensity containing fractional gram amounts of 238Pu crack,
probably because of large thermal gradients rather than Two types of containment problems are encountered
because of radiation damage. in molten salt work. One is reaction of the container
The isotopes having higher intensities of alpha emis- with the contents, which contaminates the contents to
sion significantly deteriorate most organic materials. make the reaction product undesirable, or changes the
Although it is practical to use 0.4-mm-thick neoprene property being measured in an experiment, or both. The
gloves on enclosures for H9Pu, 238Pu deteriorates these second type of problem is seepage of the liquid into
gloves rapidly and requires Hypalon-coated gloves at pores or through cracks in the container, not signifi-
least 0.6 mm thick. Hypalon is a modified polyethylene cantly contaminating the contents. The latter problem is
manufactured by Du Pont. Silicone grease may be used often more significant in work with actinides (compared
for days to weeks on ground glass joints and stopcocks of with nonactinides) because of their value and because
an experimental apparatus for handling 239Fu, but 238Pu the actinide must be recovered from the container after
in the same apparatus produces unacceptably rapid the operation. The effort to recover the actinide often is
increases in grease viscosity. Exhaust filters on much greater than the effort to observe or prepare a
enclosures used for handling multigram quantities of compound. Because absolute recovery of the actinides is
powdered compounds of the more intense alpha emit- impossible, the scrap from the original container and
:ers must be fabricated with radiation-resistant glues. If wastes generated during recovery will all have low-level
silicone or other types of oils are used in pressure-relief contamination and must be disposed of by elaborate
devices on such enclosures, they must be checked and very expensive methods.
periodically to ascertain that the oil has not become so
Misadventures involving container breakage and re-
viscous that the device malfunctions. These effects are
lease of actinides may have grave consequences. If a salt
seen even in enclosures with inert atmospheres. If the
or salt/metal mixture at high temperatures is released
enclosure atmosphere is air containing water vapor at
not only from its primary container but also from the
ambient relative humidities, these effects are ac-
glovebox enclosure, reaction with the atmosphere may
celerated and corrosion of metals and corrosion-resis-
lead to dispersal of finely divided radioactive material.
tant materials becomes serious. There is speculation
This may be inhaled by personnel or may contaminate
that ozone formed in air subjected to alpha radiation
the building severely. If batches of hundreds of grams of
may be important in accelerating corrosion. Some work-
anhydrous fissile material are being handled, they must
ers have attempted to remove ozone from enclosure
be kept from mixing with water or other hydrogeneous
atmospheres by decomposition on MnO to reduce the
2 material, which event could lead to criticality.
rate of corrosion. The lighter, halogenated hydrocarbons
These constraints necessitate choices of materials and
(such as freons) are similarly degraded and produce
designs often more expensive and conservative than
corrosive products.
would be used for containment of nonradioactive
As was mentioned, one way to keep radiation damage materials. Double containment is often specified for
(either to personnel or to equipment and enclosures) to actinides where single containment would be used for
a minimum is to keep actinides and their compounds other elements. Limited-volume, recirculating cooling
confined to the minimum volume (within thermal and water systems prevent flooding of enclosures and
criticality constraints) to provide seif-shielding. Avoid- possible criticality incidents. High-density vitrified
ing thin deposits of alpha emitters on elastomers such as ceramic crucibles minimize loss of accountable material
glovebox gloves is very important. Good housekeeping to crucible scrap. These elements require additional
can hardly be overemphasized. design time and safety analyses.
Chemical reactions with container materials must be
carefully considered before starting work with molten
B. Container Compatibility at High Temperatures salt systems containing actinides. Experiments with
pure molten chloride systems may be performed in
Problems of containing molten actinide salts are very borosilicate (Pyrex) glass at temperatures up to about
similar to problems of containing salts of other active 500°C, but fused quartz or Vycor (96% SiO ) affords
2
metals such as the alkali, alkaline-earth, and rare-earth additional resistance to breakage caused by thermal
elements. The radioactivity of the actinides adds little to shock or to melting if temperature control malfunctions.
the experimental problems, other than the general prob- Quartz may be attacked significantly if air and water are
lems of handling radioactive substances. Much informa- not rigorously excluded from the system, but these
tion about container compatibility has been generated, should be excluded anyway to prevent direct reaction
however, in research on actinide salts, in particular with the actinide halides to form oxides and oxyhalides.
metal/salt systems and fluoride systems. The latter have All salts put into the system must be rigorously treated
been studied extensively in the Molten Salt Reactor by well-known methods to eliminate water and ox-
Experiment program.1617 yhalides. Much less work has been done with actinide
bromide? and iodides and they may be expected to be separate from the actinides in waste recovery opera-
thermally less stable than the chlorides, but quartz is not tions. Beryllium produces neutron radiation problems
expected to contribute significantly to their decomposi- created tcc-u^e of the alpha-neutron reaction that oc-
tion. Fluorides, on the other hand, are expected to react curs when this element is in intimate contact with the
with siliceous materials at elevated temperatures, al- intense alpha emitters. On the other hand, magnesia and
though a report18 on spectra of actinides in mixed alumina are commonly available, present no ap-
fluoride/chloride molten salts did not mention reaction preciable health hazards, and have been studied for
with quartz cells. Platinum and gold have been used as many years, so fabrication techniques for these ceramics
containers for experiments with fluorides, and extensive are well known. Also, magnesium and aluminum are
corrosion studies of fluoride systems containing easily separated from the actinides by traditional waste
thorium and uranium fluorides were carried out in the recovery techniques. For these and other reasons, some-
Molten Salt Reactor Experiment program.1617 The best thing other than the thermodynamically "best" con-
containment for flowing molten fluoride salts in large tainer material is often chosen for molten actinide
systems was provided by Hastelloy N(7.4% Cr, 4.5% Fe, metal/salt mixtures.
17.2% Mo, and 70% Ni) and by titanium-modified The physical properties as well as chemical properties
Hastelloy N (7.3% Cr, 13.6% Mo, 77% Ni, balance Ti).19 must be considered in the selection of fabricated con-
Molten salt studies of actinides in oxyanion systems tainers. If the time of contact with liquid phases is short
have been much less extensive than in halide systems. and the container will be subject to thermal shock, the
Pyrex optical cells showed no evidence of reaction with sintered density should be low to avoid breakage during
L1NO3-KNO3 solutions of several of the actinide rapid thermal cycling. If the heating cycle is slow and the
nitrates at temperatures up to 250°C. Molybdates, liquid contact time is long, the container should be
tungstates, phosphates, and silicates have been studied vitrified (sintered to a density approaching 100% of the
in platinum and gold containers. theoretical density). This vitrification minimizes the
The choices of containers for work with metal/molten surface area available for reaction and minimizes pene-
salt systems are much more difficult than the choices for tration of the wall by the liquid phase, which contains
pure salts. The actinide metals react with many ordinary accountable actinides. Regardless of the density require-
ceramic materials. Table III lists the approximate free ments of the container, undesirable reactions can be
energies of formation (at a common observation tem- minimized by reducing roughness (and therefore reac-
perature) of selected oxides used as containers for ac- tant area) of the container's interior.
tinide metal/salt mixtures. The free energies of forma- Specifications for typical magnesia containers used
tion of oxides that may form from the metals being for routine plutonium metal preparation by molten salt
studied are also included. Examination of the values reductions (see Sees. VI-B and VI-C for process descrip-
listed in Table III leads one to choose as containers the tions) are listed in Table IV. Only recognized significant
oxides of calcium, thorium, lanthanum, cerium, properties are specified because obviously not all
beryllium, yttrium, magnesium, and aluminum, in that properties can be. One manufacturer's product might
order. One must also consider other chemical, histori- meet these specifications and be satisfactory, bul an-
cal, and economic factors governing the availability of other manufacturer's product made to the same speci-
containers made from these materials. Calcium oxide fications might be unsatisfactory because an unknown,
reacts too readily with water, thoria is radioactive, the
unspecified property may be critical to the success of the
rare earths are expensive, and beryllium presents health
process. Thir, factor might contribute to contradictory
hazards. Thorium and the rare earths are difficult to
reports from' different laboratories about the suitability
TABLE III. Free Energies of Formation of Selected
Oxides20 at 725°C (Stated in kcal/g-atom
of Oxygen)
Oxide -AF Oxide -AF Oxide -AF
Ac o 129 SrO 118 ZrO 108
2 3 2 2
CaO 127 MgO 118 CeO 108
2
ThO 123 BaO 111 NpO 102
2 2
La O 122 HfO 110 PaO 101
2 3 2 2
Ce O 122 Li O 110 TiO 91
2 3 2 2
3eO 120 A1 O 109 SiO 83
2 3 2
Am O 120 UO 109 Ta O 78
2 3 2 2 5
Y O 119 Pu O 109 Na O 66
2 3 2 3 2