Note: Descriptions are shown in the official language in which they were submitted.
7~7
,
APPARATUS AND METHOD FOR REPROCESSING AND SEPARATING
SPENT NUCLEAR FUFLS
Background of the Invention
The present invention relates generally to a method and
apparatus for the reprocessing and separating of spent nuclear fuels
contaminated with fission products in a molten metal solvent. More
particularly, the present invention is directed to a method and
apparatus for reprocessing and separating spent fuel,elements in a
separation apparatus housing a molten metal solvent~ and including a
porous filter member to remove the molten solvent and compounds soluble
therein from the separation apparatus.
A molten metal sol:vent, usu~lly tin, has been used as the
solvent and reaction medium in the reprocessing and separation of spent
nuclear fuel elements~ including actinide fuels, e.g., the removal of
fission products and other impur;ties from spent uranium-plutonium and
thorium-uranium (plutonium) fuels in oxide, metal, or carbide form.
Initially, the spent fuel is declad9 if necessary, then put ;nto a
~solution of mol~en tin maintained at a temperature of about 1600C.
If the fuel ;s an oxide a carbothermic reduct10n process is necessary.
20 For this purpose, the separat~on vessel housing the molten tin is
comprised of a material which is a source of carbon, preferably
graphite. Carbon dissolves in the molten tin and reacts with the
actinide and fissio~ product oxides3 converting them to a metallic
solution and generating CO gas. For a uranium-plutoniu~ fuel the
principal reaction is represented as:
7~'7
2 -
(U,Pu)02(s) ~ 2 C(jn Sn solution)
(U,Pu)(jn Sn SOlution) ~ 2 ~0(9)
During dissolut;on of the fuel, volat~le f;sslon products are
released and swep~ out of the separation vessel by the CO, while the
more refractory fiss;on products remain behind in the molten tln
solvent. Although the volatile fission products have varying
volatilities in the molten tin9 they are all removed in one step~
Along with the volatile fission products a significant portion of the
molten tin solvent also evaporates and is removed from the vessel.
Thereafter, the actinides (in solution) are separated from the
majority of the fission products remaining in the molten tin in a
nitriding reaotion. A non-oxidizing nitrogen containing atmosphere is
introduced into the vessel, resulting in the formation of actinide
nitrides in the molten tin. For a uranium-plutonium fuel the reaction
lS is represented by the ~ollowing equation:
(U'Pu)~in Sn solution) ~ N(2)(9~ ~ (U,Pu)N(s) (2)
During the nitriding process, molten tin may also evaporate and escape
from the separation vessel. Once nitriding is oomplete, the solid
actinide nitrides are separated from the molten tin and the fission
products remaining in the tin solvent.
U.S. Patents No. 3,843,765 dated October 22, 1974 to Anderson
et al, and 3,843,766 dated October 22, 1974 to Anderson et al. are
illustrative of such molten tin separation methods and apparatus~
The above-cited patents disclose that the actinide nitrides
are separated from fission product impurities in the separation vessel
by including mechanical means to physically move one from the otherr
However9 the necessary hardware to aohieve this kind o~ physioal
separation is cumbersome to incorporate in with a separation system
operating at liquid tin temperatures. Additionally9 the hardware is
very costly.
1~2~7~7
Summary of the Invention
Accordingly, and object of the invention is to provide an
apparatus and method for reprocessing and separating spent nuclear fuels
inclu~ing actinide fuels and fission products, wherein actinide fuels
are separated from fission products at eleYated temperatures.
Another object of the invention is to provide an apparatus and
method for separating and reprocessing spent nuclear fuels, wherein
actinide fuels are separated from fission products without the use of
mechanical means to separate the ~wo.
Yet another obje~t of the invention is to provide an apparatus
and method for separating and reprocessing spent nuclear fuels, wherein
a transport system to physically move the actinides from the fission
products is unnecessary.
Additional sbjects, advantages and novel features of the
invention wlll be set forth in part in the descript;on which follows7
and in part will become apparent to those skilled in the art upon
examination of the following3 or may be learned by practice o~ the
invention. The objects and advantages of the invention may be realized
and attained by means of the instrumentalities and combinations
particularly pointed out in the appended claims.
To achieve the foregoing and other objects~ and in accordance
with the purpose of the present invention as embodied and broadly
described herein, the apparatus ~or reprocessing and separa~ing spent
nuclear fuels employs a molten metal solvent. A separation vessel is
provided which includes a reaction region to house the metal solvent,
and a reflux region positioned above and adjacent to the reaction
region. The two regions are temperature-independent~ capable of
operating at different temperatures Disposed within a separation
vessel wall member, in a position adjacent to the re~lux region~ is an
aperture. A porous filter member is disposed within a separation vessel
wall member, de~ining the bottom of the separat;on vessel in support;ng
relationship to the metal solvent. Means for varying the temperature of
-- 4 --
the separation vessel are included. Additionally, means for evacuating
the separation vessel and means ~or introducing various atmospheres into
the vessel are provided.
In a further aspect of the present invention, in accordance
with its objects and purposes~ the method for reprocessing and
separating spent nuclear fuels including actinide fuels, volatile and
non-volatile fission products, utilizes a molten metal solvent. A
separation vessel is provided which includes a reaction region housing
the metal solvent, and a reflux region positioned above and adjacent to
the reaction region. The separation vessel further includes an aperture
disposed within a vessel wll member in a position adjacent to the reflux
region, and a porous filter member disposed wi~hin a vessel wall member
de~ining the bottom of the vessel in supporting relationship to the
metal solvent. Spent nuclear ~uels are introduced into the separation
vessel, followed by the introduction of a non-oxidizing
ni~rogen-containing atmosphere. Solid actinide nitrides are formed
within the metal solvent, while the non-volatile fission products remain
in solution. The reflux region is subsequently pressurized sufficiently
to force the molten metal solvent through the porous filter, leaving the
actinide nitrides behind in the reaction region of the separation vessel.
Inclusion of the porous filter member enables the separation of
the solidified actinide nitrides from the molten metal solvent and the
non-volatile fission products remaining in solution without the need to
provide a transport system to collect the solid actinide nitrides, and
then physically transport them away from the molten solution.
Additionally, substantial cost savings are realized by elimina$ing the
expensive mechanical transport system. The separation of the present
invention proceeds successfully at temperatures required to maintain the
metal solvent in a molten state.
Descript;on of the Drawing
The accompanying drawing, which is i ncorporated and forms a
part of the specification~ illustrates an embodiment of the invention9
and, together with the Description, serves to explain the principles of
the invention.
7~7
-- 5 --
Figure 1 ;s a schematic diagram of a spent nuclear fuel
reprocessing and separation apparatus.
Description
Spent nuclear fuels; including actinide fuels in oxide, carbide
or metal form, and volatile and non-volatile fission products are
successfully reprocessed and separated utilizing a separation vessel
housing a liquid metal solvent in a reaction region of the vessel, and a
reflux region positioned above the reaction region. One embodiment of
such a separation apparatus is illustrated in Fig. 1. Separation vessel
lO is formed from wall members 12 made of a carbon~containing material.
Preferably~ vessel walls 12 should: (1) be refractory and capable of
containing the system at temperatures well above reaction temperatures;
(2) inert to actinides at the reaction temperatures and solvent
conditions employed9 (3~ possess low neutron absorption characteristics;
and (4) be a source of carbon for the carbothermic reduckion oF actinide
oxides. Exemplary carbon-containing materials include carbides,
hydrocarbons and graphite~ Graphite is the preferred material.
Separation vessel lO defines var;ous distinct regions: a
reaction region 14 housing a molten metal solvent 16; a reflux region 18
positioned immedia~ely adjacent to and above reaction region 14, and a
condensation region l9 positioned adjacent to and immediately on top of
reflux region 18. Condensation region l9 is a dome~like structure,
removable from the remainder of vessel 10, deFining the top of the
vessel. A reservoir 20 is formed within condensation region l9 at its
intersection witl re~lux region 18. Reservoir 20 is defined by a wall
of insulation 22 and vessel wall member 12,
Es~ablishment of different regions within vessel lO permits the
formation of a thermal gradient therein. Regions 14~ 18 and l9 are all
temperature independent. Generally reaction region 14 is maintained ak
a higher temperature than regions 18 and 19. When reflux region 18 and
condensation region l9 are maintained at lower temperatures, evaporated
metal from solvent 16 enters reflux region 18, condenses therein with
the aid of a series of baffles 249 preferably formed of graphite, and
'7~7
-- 6 --
returns to the solvent in reaction region 14. Regions 14 and 18 can be
maintained at the same temperature, while condensation region 19 is
maintained at a lower temperature. In this event, evaporated metal
solvent continues through region 18 and into reg;on 19. The evaporated
solvent condenses therein and is collected in reservoir 20 as a liquid.
For purposes of the present invention, the metal solvent should
have good solubility for uranium and o~her actinide metals; possess a
vapor pressure suitably low ~o allow condensation of the solvent in the
reflux region; not readily form nitrides; and have a suitably low
neutron oross section. Suitable solvents include lead, zinc~ bismuth,
and tin, as well as combinations thereof. The preferred solvent is tin.
In order to effect the thermal gradient within vessel 10, a
first induction heater 26, connected to a suitable power source, is dis-
posed at the exterior of the vessel in surrounding relationship with
reaction region 14. A second induction heater 28, also connected to a
suitable power source, is disposed at the exterior of the vessel in sur~
rounding relationship with reflux region 18. Each induction heater is
independent of the other, permitting the formation o-f the thermal gradi-
ent within vessel 10. Okher heating and cooling means can be employed
to establish the temperature gradient. Such means include~ bu~ are not
limited to, radiation heating with resistance heaters, electron impact
heating of the vessel, and direct electrical resistance heating applied
to the vessel. A plurality of cooling coils 30 are disposed at the ex-
terior of condensation section 19 in surrounding relationship thereto.
25 r~ circulating medium such as water, steam, air, helium and the like
flows through coils 30 to cool $he condensation section of vessel 10.
Separation vessel 10~ induction heaters 26) 28J and cooling
coils 30 are all housed within a metal vacuum tank 32. Tank 32 can
alternatively be an inert gas chamber. The tank serves as a secondary
containment vessel for the spent nuclear fuels and provides additional
safety in the event of a spill. Top lid 34 of tank 32 is removable,
providing a means for introducing spent fuel into vessel 10. To this
end, a flange 36 joining the vessel wall members whioh define reflux
region 18 and ~ondensation region 19, is removed from condensation
i'Zl~Z'7~
region l9, e.g.; the dome is separated from the remaining sections of
vessel lO. Spent fuel is then added mechanically or in batch~style to
the vessel. A removable graph1te liner 38 may optionally be included
and disposed within the interior of vessel lO. Before each new batch of
spent fuel is reprocessed, a new graphite liner is placed within vessel
lO. This reduces wear on the interior walls of vessel lO.
A conduit 40 pierces bottom wall 42 of tank 32 and wall member
12 of vessel 10. A valve 43 is disposed within conduit 40. Conduit 40
is in communication with pump and vacuum systems (not shown), various
collection vessels (not shown) to coll~ct and hold C0, volatile fission
products, etc. removed from vessel lO, and also a containment vessel
(not shown) housing a non-oxidizing nitrogen-containing gas such as
N2. Electrical leads for induction heaters 26 and 28, piping for
coils 30, and other required auxiliaries are also provided through
bottom wall 42.
Disposed within separation vessel wall member 12 in a position
defining the bottom of vessel lO, and speciFically the bottom of
reaction region 14, is a porous filter member 46 which is in a
supporting relationship with metal solvent 16. Filter 46 can vary in
size, and may extend to the full width of the bottom of vessel lO. The
porosity of filter 46 must be sufficient to provide support for molten
metal solvent 16 when a pressure of about l atm is appliPd to reflux
region lO and condensation region l9, but yet permit the flow of the
solvent and any compositions soluble therein when a pressure of about
l.l to 1.2 atm is applied to the reflux region. Preferably; the pore
size of the filter material should be about lO to lO0 microns.
If graphite liner 38 is included in separation vessel lO, the
bottom of liner 38 is of a porous nature similar to filter 46.
Preferred filter materials include graphite, stainless steel9 and other
materials which may be used to form separation vessel lO. The preferred
material is graphite. When a pressure of about 1~1 to 1.2 atm of the
non-oxidizing nitrogen-containing gas~ or an inert gas such as argon~ is
applied to separation vessel lO above m~tal solvent 16, the solvent is
caused to flow from the vessel and into a recycle chamber 48 positioned
7~3~7
- 8 -
under vessel 10. Recycle chamber 48 is cooled by the circulation of a
cooling medium such as water through its chamber walls. Me~al solvent 16
as well as the non-Yola~ile fission products are collected in recycle
chamber 48, cooled, and ~herea~ter removed ~herefrom. A drainage tube
49 is removably attached to separation vessel 10, and also removably
attached to recycle chamber 48. Inclusion of drainage tube 49 is
hecessary to separate the cooler recycle chamber from the much hotter
separation vessel.
Metal solvent 16, as well as reaction region 14, are maintained
at a temperature sufficient to solubilize the actinlde fuels, e.g~9 the
reaction region temperature. Preferably3 the reaction region
temperature is abcut 1450 to 1800~C. More preferably~ ~he temperature
is from 1550 to 1700C~ and most preferably it is about 1630C.
Reflux reg~on 18 is înitially maintained at a lower tempera~ure than the
lS reaction region e~9~9 the reflux temperature. This reflux temperature
must be low enough to effect the refluxing of the metal solvent. When
tin is employed as the solvent, a temperature of aboyt 800 to 1200C
is maintained. Preferably, the temperature range is about 950 to
1050C. The temperature of condensation region 19 is lower than
reflux region 18, but above the melting point of the metal solvent. A
temperature range of about 232 to about 1050C is suitableO At these
lower temperatures~ residual metal solvent or volatile fission products
that pass through reflux region 18 are trapped and collected.
The rate of carbothenmic reduction of aotin~de oxides is
dependent on the amount cf ava~lable carbon within the metal solvent
of choice. Carbon ~s only very sl~ghtly soluble in the metals of
choice, especially in tin. However~ w~th the additlon of a first
catalyst to the metal to increase rarbon solubility and permeabilik
the carbotherm~c reduction rate is greatly increasedO To be
effective, the eatalyst must increase carbon solubility in the metal
and not readily form stable carbides~ Suitable catalysts ~nclude
cobalt, nickel~ ~ron, and oombinatisns thereof. The preferred
catalyst ~s cobalt. These elements ~e also part~cularly useful
. . . ~. .
- 9 -
because they are relatfvely non-volattle and do not evapora~e from the
molten tin sol Yent~ For example, kl n is more than one thousand times
as volatile as cobalt. The catalyst ~s present in an amount wherein
the weigh~ percentage of catalyst in molten metal so1vent is in khe
range of from about 0.1 to 20. Preferably, the percentage is from
about 1 ~o 20. Most preferably, ~he percen~age is about 5 to lO.
Prior to introducing the spent nuclear fuel in~o vessel lO, it
may be necessary to declad the fuel, either mechanically or chemically.
The spent ~uel is broken up ~o relatively small pieces (3mm or less~ and
then introduced into vessel lO with a mechanical feeder or in batch
style.
Once the spent nuclear fuels have been added to solvent 16 at
the appropriate temperature, carbon from graphite wall 12, dispenser 52
and liner 38 reduces the actinide oxides, evolving CO. As the spent
lS fuels dissolve in the metal solvent, volatile fission products are
released, while non-volat~le fission products remain in solution. At
the reac~ion reg;on ~emperature, a significant amount of solvent from
molten solvent 16 in reaction region 14 is evaporated and enters reflux
region 18 along with the volatile fission products and CO. Within
reflux region 18 the evaporated solvent is caused to condense,.due to
the lower temperature, and is returned ~o reaction region 14. YPry
little of the evaporated solvent reaches condensation region l9.
-- 10 --
Volatlle fission products9 as de~ined hereln, ~nclude those
~ission products l~ste~ in Table I which have very high, moderate, or
low volatilities. Volatil1ties of the vola~ile fiss~on products within
reflux region 18 vary tremendously. At the reflux region temperature
the less vola~ile fission products condense and return ~o molten t~n
solvent 16. These volatile fiss~on products are defined as condensable
fission products. The more vola~ile fission products do not condense in
reflux region 18 or condensation region 19. Instead) they flow from the
vessel through conduit 40 and are separately collected in a collection
vessel. By definition herein, the more volatile fission products are
called non-condensable fission products.
Table I lists the estimated volatilities of fission products at
about 1 627C .
Table I
15Volatilities of Fission Products
A. ~ery high (10 3 tn 1 atm)
I, Br, Cd, Cs, Rb, and Se
B. Moderate (10 6 to 10 4 atm)
Sb and Te
20C. Low (10 8 to 10 6 atm)
Ba9 Eu9 Sm9 and Sr
D~ Very low (less than 10 9 atm~
Ce, Gd~ La, Mo, Nd, Pd, Pm, Pr, Rh, Ru, Tcg Y9 and ~r
. .
During the removal of C0 and the non-condensable vola~le
~ission products, a sp~rglng gas is introd~ced into vessel 10 through
conduit S0 to accelerate carbothermic reduc~ion~ An inert gas such ~s
argon is suitable for this purpose. After the non-condensable volatile
fission products have been removed from Yessel 10, the temperature of
re~lux region 18 is increased from its reflux temperature, eOg., 9S0 to
1050C, ~o ~he ~empera~ure of the reaction region. However,
condensation region 19 is mainta~ned by cooling as necessary at a
~emperature within the range of 232 to 1050C. At the elevated
reaction region temperature, the condensable volatile fission products
will yenerally not condense in reflux region 180 Essentially~ the
condensable volatile fission products, as well as some evaporated tin9
are distilled, condensed and collected in condensation region 19 at
reservoir 20. Althnugh some tin ~s lost ~n this distillation process,
far less is lost than if the removal of all volatile fission produ~ts
occurred without the existence of reflux and condensatiDn regions which
establish the temperature gradient. Add~tionally9 the temperature
gradient permits selective separation oi the volatile fission products
based on their different ~olatilities7 e.g.~ condensable and
non-condensable volatile fission products are separatedO
When d~stillat~on ~5 camplete~ the temperature of re~lux
region 18 is decreased back to the defined reflux temperature of about
95~ to 1050C. Therea~ter, a non-oxidiz~ng nitrogen contain~ng
atmosphere is introdueed into vessel 10. An example o~ such an
atmosphere is pure nitrogenJ The nitrogen atmosphere enters metal
solvent 16 and is dispersed there~n wlth the aid of dispenser 52.
Additionally, the nitrogen containing atmosphere may also be introduced
through conduit 40. A nitrogen pressure of about 1 atm is preferred~
Within solvent 16, solid actinide nitr~des ~ncluding UN, U2N3~ PuN,
3n Pu2N3, ThN and the llke, are formedO The orlginal soluble f~ssion
products remain in solution.
To separate the sol~d actinide nitrides from the malten
solvent and soluble fission products, reflux and condensation regions
18 and 19 are pressured with a gas sueh as nitrogen, and ~ntroduced
through csndult 4a at a pressure of about 1.1 to 1.2 atm. The
h~
~2
~ressurizing effec~ forces the molten solvent and solubl~ fission
products to flow from react~on reg~on 14, through filter 46 and ~nto
recycle chamber 48~ Vessel 10 is then cooled and dismantled e.g., a
separation of the vessel at flange 36 occurs. The remaining sol1d
actinide nitrides Are taken from vessel 10 and processed to yield
usable fuels, e.g., ignited in the presence of oxygen to produce
actin~de oxid sO
~ o accelerate the nitriding process, a second catalyst is
included. The second catalyst chosen must ~ncrease the affinity of
nitrogen in the metal solvent, but not form stable nitrides. Suitable
catalysts ~nclude magnesium, calcium9 strontium, barium, aluminum,
manganese~ vanadium, chrom~um, and m1xturPs thereof. The pref2rred
se~ond catalyst is calcium. Depending on the choice of second
catalyst, it may be added to the metal solvent initially as part of
~he solvent" or ~ust prior to the nitridiny process. This is due to
varying volatil~t~es of the catalysts and the potential loss of the
more volatile seeond catalysts during the carbothermis reduction
process. The weight percentage of second catalyst to metal solvent is
about 1.0 to 1030~ Preferably, the weight percentage is about 1~8 to
2.S. Most preferably, it is about 1Ø By including the second
oatalyst, an approximate threefold rate incre~se in nitriding is
observed.
The following examples are illustrative of the process ~nd
appar~tus of the present invent~on~ and are not to be regarded as
limiting its scope.
Example 1
509 nf reagent grade shot tin~ 109 of U02 (depleted~ and lg
of cobalt ~re added to the separation vessel shown in Fig. 1~ Cobalt
is added as a catalyst otherwlse the carbothermic reduction w~ll take
about 20~30 hours. The reaction re~on of the vessel and the tin are
then heated and maintained at a temperature of about 1600OCD creating
a ~olten tln solvent. ~he re~lux region of the vessel ~s ~ain~ained ~t
lz~7~
- 13 -
,
a temperature of about 1000C. 10 grams of U0z (depleted) are
~dded to the molten t~n solvent. Argon ~s then bubbled through the
molten tin as a sparg~ng gas for two hours: Non-condensable volatile
fission products are collected ~n s~itable cold traps, C0 ~s oxidized
to C02 and trapped ~n an alkaline matrix such as ghe commercial
product Ascarite. The ~emperature of the reflux region is then
~ncreased to about 1600~C. vaporated ~in and the previously
condensable volatlle fission products are collected ~n a collection
vessel. After about 1~ minutes, the temperature of the reflux region
is decreased to about 1000C. N2 (at 1 atm) is introduc~d into ~he
vessel through the porous filter, forming actinide nitridesO After
a~out one hour~ the ~low of N2 through the filter is discont~nued
while N2 is in~roduced into the reflux region at about 1.2 atm. The
molten tin dnd the non-volatile fiss~on products in the molt~n ~in flow
out of the Yessel through the porous filter and collected in a recycle
chamber, leaving the solid actini~e nitrides behind.
Example ~
lOg of ThO2 is separated frQm other spent nuclear fuels
under the same cond~tions as in Example 1.
Example 3
lOg of ~U,Pu~02, i.e.~ a mixture of U02 and Pu02, is
separated from other spent nuclear fuels under the same conditions as
xampl~ 1 . '
~he ~oregoing descript1On of ~ preferred emhodiment Df the
2~ lnvent~on has been presented for purposes of ~llustratlon ~nd
descr~ption~ It ~s ~ntended that the ScopP of the invent~on be defin2d
by the cla~ms appended hereto.
~ ~ ~~- r, . ........ _.. ,,,. _
