Note: Descriptions are shown in the official language in which they were submitted.
--3--
BACKGRO~ND OF THE INVENTION
, .
Radioactive waste has arisen from two major
sources: production of nuclear weapons and production of
nuclear enërgy. The waste can take at least three forms.
By far the largest volume is liquid waste from commercial
nuclear ~nergy ~enexating plants. To recover unused
uranium and/or plutonium, the spent fuel rods are dis-
solved in nitric acid. After removal of these actinides,
the strong acia wastes are neutralized and stored in
steal tanks. The problem has been that the tanks corrode
with subsequent leakage of high-level radioactive liquids
into the biosphere.
One can convert the radioactive liquids to solid
oxides but this physical form can also be aispersed fairly
easily. These powders are generally referred to as cal-
cines. The level of radioactivity from calcine is very
high and of the order of 1.5 million rads (R3 per hour
as a dosage. After storage for a hundred years, the level
will have dropped tb 5800 R per hour but 1000 years storage
i5 indicated before an acceptable dose-rate for humans
arises. However, the above refers only to sub-uranic, or
fission product wastes. I the actinides such as uranium
and plutonium are not removed, then the wastes must be kept
in secure storage for about 250,000 years before they can
be considered safe for human exposure.
The volume of commercial waste (high level waste
- HLW~ is enormous. About 74 million gallons have existed,
ox will exist once the stored spent-fuel rods are processed.
Because of the lack of a really satisfactory disposal method
for HLW, a major part of the spent fuel rods have been
st~red under water in underground bunkers. The United
States has sufficient uranium stockpiled so that recovery
of unused uranium from the spent fuel.rods is not critical.
'' '~ .
~13~)4
Howeve~, this practice cannot continue indefinitelyn Some
of the liquid waste already produced has been converted
to calcine. There is about 3.9 million (M) cubic feet
of unprocessed liquid waste which will form some 585,000
cubic feet of calcine.
The second form of radioactive waste consists
of actinide waste which has been separated from HLW and
other sources. It amounts to about 1.8 M cubic feet of
liquid waste. The third form of radioactive waste, weapons
waste, amounts to about 75 M gallons, or about 9.6 M cubic
feet. This waste is of lower radioactivity level than that
of HLW from reprocessing of commercial fuel rods, but that
of separated actinide waste is much high~r in radioacti~i~y
emissions level.
The use of glass for containment of high-level
rad~oactive waste has been under development for many
years. There are many attractive features of this mode
of encapsulationO They include a rigid incorporation of
the radioactive ions, or species, by dissolving them into
the melt to form the glass structure. They are then not
free to move as long as the glass st:ru~ture is maintained.
Glass is not subject to grain growth, surface ox~dation,
and o-ther factors common to crystalline solids. However,
there are six critical properties required for any glass
in this application. These include:(l) minimal tendency
to devitrify, (2) low hydrolytic leach rate~ ~3) high
solvency power, ~4) relatively low melt temperatures, (5)
low tendency to form crystals from the added waste com-
ponents, and (6) low softening point and viscosity of the
melt.
Devitrification refers to the proclivity of
an amorphous solid (glass~ to beco~e crystalline. All
'glass will devitrify provided that the internal temperature
_5
o~ the glass b~d~ is r~ised to a certain point ca,lled
the devitrification temperature. The devitrification
process is exothermic; that is, it releases heat, so
that when devitrification starts, it is self-sustaining.
The devitrification product COlISiStS of microcrystals so
that the mass is friahle and easily dispersed~ It is
therefore important to maintain the amorphous state for
the HLW encapsulation ~pplication. The problem is that
the incorporated HLW is a heat source through natuLal
fission pxocesses plus absorption of energy from the em-
itted radiation by the glass matrix. 'Internal temperatures
of up to 850 C. have been observedO Thus all of the prior
glasses used for this application have devitrified when
the incorporated HLW has heated the glass to its devitri-
! fication temperature during storage. This remains a severe
problem for which theie has been no solution heretofore.
Since the HLW-glass is to be stored for prolonged
times as a solid mass, the hydrolytic leach rate, as a
loss of the surface of the glass bociy, is important,, Ordi-
nary window glass has a relatively high leach rate of
5.3 x 10-4 gm/cm2~hr in boilin~water. A good waste~glass
must have a value of at least 150 times smaller than this.
Granite, an igneous rock, has a leach rate of about 4~6
x 10-6 gm/cm2/hr while that o marble is about 1.2 x 10 5
gm/cm2/hr. Since the waste-glass is to be stored in under-
ground rock vaults, its hydrolytic leach rate ou~ht to be
less than the surrounding roc]c.
When the HLW is to be added to the glass melt,
all of the components need to be dissolved. Many of them
are refractor~ oxides such as CeO~, Zr02 and Ru02. A
high solvency power of the melt is thPrefore needed. In
j most glasses, the addition of excess oxides to the glass
~331~4
--6--
melt tends to cause formation of insoluble crystallite.s
as specific compounds which begin to recrystallize and
grow larger. When the rnelt is cask, the crystals, as a
second phase, form centers of internal strain, thereby
causing the glass to develop cracks and become friable.
Hence it is also desirable if the glass exhihits little
or no tendency for internal crystallite formation.
Furthermorel the processing temperatures
required for production of glass need to be relatively
low for ~uclear waste encapsulation, preferably not over
1400 C. Conservation of energy is one reason for this
limitation while another is that the containers intended
for actual storage o~ the waste-glass cannot stand process-
ing temperatures in excess of this value~ Finally, the
glass melt also needs to have a low viscosity so that
added waste oxides can be dispersed into the melt more
easily.
The best glass known heretofore for the nuclear
waste encapsulation application, a zinc borosilicate (ZBS)~
was developed especially for this purpose. A melt is
produced at 1400 C. which has a viscosity of less than
200 poise. Up to 45~ by weigh~~of the HLW oxides can be
dissolved into the melt. The hydrolytic leach rate is
lower by an order of magnitude than most commercial glasses.
Unfortunately, HLW-ZBS ~lass devitrifies at 750 C. and
so~tens at 570~ C. Refractory waste oxides such as Ru02,
CeO2 and ZrO2 do not dissolve at all well into the melt
and crystallites of Zn2SiO4, SrMoO4, NdBSiO5 and Gd2Ti207
are among the crystalline compounds observed to ~orm in
the glass or devitrified product.
t ~ ~.31~0~
--7--
SUMM7~RY O~ THE INVENTION
I have found the use of a molecular glass, based
upon a polymexized phosph~te o~ aluminum (PAPj, indium
or gallium and made according to methods already given
in U.5~ Patent No. 4,~49,779 and U.S. Patent No. 4,087,511,
overcomes all o~ the prior objections to use of glass as
a high-level nucleax waste encapsulation agent. ~his
~LW glass product could not be made to devitrify, disso~ved
all of the oxides found in calcine, including the difficultly
soluble ones~ did not form microcrystallites în the melt
or subsequent glass-casting, and possessed a hydrolytic
etching xate to boiling water even lower than that of HLW-
~B5 slass.
In accordance with the present invention, a pre-
cursor compound, M(H2P04)3, is prepared according to methods
of U.S. Patent No. 4,049,779, where M is a trivalent metal
selec~d f ~ ~e group consisting of aluminum, indium and
gallium. Advantageously, the impurity level is carefully
controlled so as not to exceed 300 ppm. total. The pre-
cursor crys1-als may be washed to remove excess phosphoric
acid as desired. HLW is added to the crystals and the
mixture is then heated at a co~rolled heating xate to
induce solid state polymerization and to form a melt a~
1350 C~ in which the ~W oxides dissolve rapidly. When
aluminum was used, the resulting HLW-PAP glass had a hydro-
lytic leach rate to boiling water some 15.8 times lower
than HL~ZBS glass, The melt dissolved all components of
the HLW and no crystallite formation was noted in the melt
or in the finished glass form. The softening point of
HLW-PAP glass is 650 C. It has a high thermal conductivity,
a low thermal expansion which above 350 C. has been observed
t~ become negative, possesses a low cross-section for
absorption of radiation, and apparently does not re~uire
~r
~3
I,. rmal annealing to relieve internal stress generated
during casting ~f the melt to ~orm the glass, like other. ~~
pxior known glasses.
Alternately, the HLW can be mixed with the
formed precursor crystals plus phosphoric acid to form
H~ phosphate compounds prior to melting the precursor
crystals to produce the HLW glass composition. Another
method which produces a very stable HLW glass substance
involves the preparation of a solid prefire, by firing
the precursor rrystals at 1100 C. to form a calcine, to
which th~ H~W is added. A melt is then formed at 1350 C.,
which is subsPquently cast to produce the stable HLW
glass block ~or long term storage. Still another alternate
is the formation of the polymexized melt from the pre
cursor crystals, follow~d by casting the melt to form a
glass, to form a glass frit~ The frit softens at 822 C.
and HLW dissolves into ~he melt at 1150 C. rapidly to
form the solidified HLW glass block as a ~inal prodl~ct for
prolonged or permanent storage.
.. . . . .
me present invention, in one aspect, ~hen, resides in
a nuclear waste block for storage of high level
ra~ioactive was~e, said block comprising, in combination:
~olid radioactive waste material dispersed in a polymeric
phospha~e glass selected from the group consisting of
polymeric phosphate glasses having the general formula
M3 P7 22 and polymeric phosphate glasses having the general
formula M(PO3)3, wherein M is a trivalent metal selected from
the group consisting of aluminum, indium and gallium, and
mlxtures of said phosphate glasses.
According to another aspect of the present inv~ntion
there is provided a process of using a polymeric phosphate glass
selected from the group consisting of polymeric phosphate
ylasses having the general fo.rmula M3 P7 22 and polymeric
phosphate glasses having the general formula M (PO3)3,
wherein M is a trivalent metal selected from the group
consisting of aluminum, indium and gallium, and mixtures of
said phosphate glasses, said process comprising the steps of:
forming a mel~ o said glass;
dissolving high level radioactive waste in said
r~lt; and
.
~ ~L3~
- 8a-
allowing said nelt incorporating said radioactive ~ -
waste to cool and solidify into a block;
said block with its encapsulated radioactive waste being
~uitabl~ for prolonged or permanent storage.
,~
~v~
~3~ 0gL --
g . .
.
DESCRIPTIOl~ OF THE PREFERRED l~MBODIMENTS
-
I have determined tha~ a high degree of chemical
durability of non-silicate glasses, such as those based
upon phosphate, sulfate and the like, cannot be attained
unless a precursor is first formed as a separate phase,
heated to induce solid state polymerization of said phase,
to form a melt, to form a polymerized glass. For encapsu-
lation of high-level radioactive nuclear waste, a poiymerized
phosphate of aluminum is required, possessing a high degree
of purity. The precursor compound is prepared by dissolv-
ing an aluminum compound in an excess of phosphoric acid.
Al(OH)3 is preferred as a source of aluminum although other
aluminum compounds can be employed. It is important to
maintain a certain molar ratio of H3P04 : Al in the solu-
tion. The minimum is about 6 : 1 mols per mol but 7 : 1
works much better~ and ratios as high as 9 : 1 have been
found useful. The higher ratios accelerate Al(OH)3 dis-
solution, which may take 3-5 days at the 6 : 1 ratio. After
purification of the resulting solution, controlled evapora-
tion is employed to obtain the precursor ~rystals, Al(H2PO4)3,
with good yield. These crystals, of high crystallinity
and regular morphology, are th~n washed with an organic
sol~ent such as methyl-ethyl ketone or ethyl acetate,
but not'limited to those solvents, to remove excess H3P04
to produce mon~basic crystals uncontaminated by other
chemical species or contained impurities, The presence
of a large excess o~ H3P0~ during evaporation is essential,
during the precursor crystal formation~ to prevent the
appearance of other unwanted phosphates of aluminum which
will not undergo solid state pol~merization when he~ted
to elevated temperatures. Table I shows the analysis of
a typical batch of precursor crystals used to prepare my
new and improved glass for the nucleax waste encapsulation
application.
~3~
--10--
T A B L E
Typical Analysis of Precursor Crystals Vsed to Prepare
Polymeric Glass
Impurity ~ mpurity
Mg 10 Pb -~
Si 50 Cr 3
Fe 20 Mn --
Cu -- Ca . 50
Al major Na . 100
Ni -- Li 15
Sr 3 K 30
Mo ~ -- Ba --
Co ---- V .----
The ~lass product prepared by heating the precursor crystals
has a novel stoichlometxy not described or known heretofore.
For a typical preparation, the analysis of washed and
dried crystals was: 98.38% Al(H2P04)3
0.04% ~3P04
1-58% ~2
~pon heating the precursor crystals in a suitable container,
all of the absorbed water is lost by the ~ime the temperature
reaches 17S C. A loss of the three waters of constitution
beyins sequentially at 210~ C. and is complete at 700 C.,
according to the reaction:
(1) m A1(~2P04)3 ~ ~Al(P03)3]m ~ 3 H20~'
where m is an initial degree of polymerization, from m = 1
to m = 4. At about 866 C., the small amount of excess
phosphoric acid is lost as 7 H3P04 3 H20. If a prefire
or calcine is desired, the temperature is held at 1100 -
1150D C. for several hours. If the temperature continues to
rise, a further loss of P~05 is ~bserved above about 1200 C.,
according to the reaction:
~2) 3 [Al(P03)3] m ~ A13P7022 ~ m P205
The loss of P205 accelerates above the melting point of
1325 - 1350 C. and is complete by 1500 C. If the temperature
is held at the melting point, ~he loss continues until the
final stoichiometry given in reaction (2) is attained. This
final stoi~iome~ is maintained while further polymeriza-
tion continues. If ~he polymerization is allowed ~o continue,
the stoichiometry begins to change fur~her and crys~als appear
in the melt, according to the reaction:
(3) n A13P7022 ~ 2 [Al(P03)3]n ~ n AlP04~.
Since crystals are unwanted in the HLW-PAP glass application,
the polymerization time o~ about 36 hours, required for the
appearance o~ AlP04 crystals in the mel~, represents a maxi- ~
mum which can be used. The useful glass composition thus ~:
appears to be: A13P7022. A non-purified, washed precursor,
containing about 4000 ppm of impurities, was analyzed to
be:
98.23% AltH ~04)3
1.76% H20
0.01% H3P04
Upon heating, it behaved in an identical thermal manner
and produced a glass composition, A13P7022, when polymerized
for 16 hours. ~n unwashed batch of precursor crystals was
analyzed to be.
( 2P04)3
10.37~ H20
21,28% H P04
.,~
,,~,....
-12-
Its thermal decomposition behavior was alsv identical t~
that described ahove~ The reac~ion is thus not affected
by the degree of impurity level nor by the presence of
excess phosphoric acid.
The same nominal glass composition may be formed
by precipitatinq Al(P03)3 using metaphosphoric acid , and
then ~iring the product. The precipitation reaction is:
(4) Al(NG3)3 ~ 3 ~PO3 3 3~ 3
Although this method is much to be preferred over the methods
taught in the prior art, such as that of Hatch, Canadi.an
Patents Nos. 449,983 and 504,835, it still suffers ~rom
several deficiencies. Althouyh HP03 is very soluble in
water, it tends to hydxolyze to ~3P04 rather easily so that
the reaction ~4), given above, is difficult to control
without introducing other unwanted aluminum p~osphates into
the meit. In addit~on, contamination by the anion, in
this case nitrate ion N0 , interferes with subsequent
reactions.when the Al(P03)3 is isolated, dried, and then
heated to form the glass melt. The worst. method to use
is the method of Hatch who teahes to combine A1203 and
P04 into a solid mass and then to fire the mass to
fusion and quickly cool it. The resulting glass is
subject to incipient recxystallization and is des~ribed
as a ve.ry slowly water soluble dehydrated phosphate useful
in water purification procedures. If an intermediate is
not isolated, and if said intermediate is not of high
purity, in contrast to the prior art, then the improved
product o~ my new and improved invention does not result.
The products of the prior art inventions suffer fro... lack
of stability to recrystallixation and lack of xesistance
to hydrolytic etching by boiling water, which characterize
and uniquely set apart the product o~ my new and improved
3L~L3~
-13-
invention for encapsulation of high level nuclear waste.
I have determined that it is much better to isolate
the monobasic precursor, fire it to the prefire calcine~
and then to form the glass melt. The prior art has taugllt
to use 3.00 mols H3P04 per mol of aluminum salt, ~ut even
if one uses my improved ratio of 7.00 mol H3P0~ per mol
of Al salt and fires ~his mixture, the glass product remains
inferior and lacks many of the improved properties of my
new and novel invention. Even the properties of the glass
obtained from melting the isolated pxecipitated product
Al(P03~3, remain inferior to those of my new invention.
Observed physi~al properties of my new improved
glass, A13P7022, were determined to be:
glass transition point ~g = 7~2 C.
softening point T = 816 C,
devitrification Td = 1053~ C.
melting point TM = 1287 C.
There is an endothermic peak associated with Tsp which is
the "heat of softeningl'. For A13P7022' ~H = 0.201 Kcal/
mole. Its thermal conductivit~is high and lS described
by the e~uation:
~ 5) ~ = 5.1 x 10 3 T(C.) + 0.0023,
where ~ is the thermal conductivity in cal~-cm./C./cm2/sec.
~ increases linearly with temperature and one can calculate
that at 750 C., the expected internal temperature for a
HLW-glass form, my new glass will dissipate 13.8 Kcal.
per cm2 per hour of energy or 14~9 Kilowatts per square
foot of surface per hour. The expansion coefficient o my
new glass is low in relation to prior glasses used in this
a~plication and more nearly matches that oE the metal con-
tainers used for storage. Three parts of the expansion
~L~3~
-14-
curve have been determined.
~6) ~1 = 47 x 10 7 in~/in./C. (T - 142 to 285 C.
~2 = zero (T = 285 to 363~ C.); and
a3 = 32 x 10 7 in./in./C. (T = 363 to 604 C.3.
In this equation a is the expansion coefficient and remains
positive. In another case, I observed a negative value
for ~3, vis:
(7) al =! 30 x 10 7 in.~in./C. (T = 78 to 456 C.);
2 (T = 456 to 480 C.); and
~ 3 = -6.5 x 10-7 in./in./ C. (T = 480 - 750 C.).
These expansion properties can be controlled somewhat by
the polymerization time used. It is quite obvious that
a negative expansion is a valuable- property in a glass which
becomes reheated by the nuclear waste it contains. While
the metal container expands, this glass contracts, thereby
obviating external stress which might crack the glass block
otherwise.
When a synthetic mixture of chemical oxides was
added to the ~13P7022 melt in ~uantities to simulate the
HLW additives, I determined two essential factors, which
set my new and improved glass apart Erom any prior known
glasses used heretofore in the ~ield of nuclear waste en-
capsulation. The first is that the HLW-PAP glass would
not devitrify under any circumstances employed. This
was first observed visually and con~irmed several *imes
b~ differential thermal analysis, an analytical method
commonly used to determine thermal.behavior of glasses.
This is entirely unexpected and unique since my ~lass is
the only one observed to date which does not devitrify
when containing ~ILW~ The second is that the hydrolysis
loss o~ HLW~PAP glass is related to the pol~erization time.
The relation has been determined to he linear and its the
-15-
equation:
(8) Wt = 0.0466 t - 0.806,
where Wt i~' the weight change observed in 10-6 ~m./cm2/hr.
and t is the polymexization time in hours. At 4 hours
polymerization time, a loss of 0.64 x 10~~ gm./cm2/hr~ in
boiling water was observed, whereas at 24 hours polymeri-
zation time, a galn of 0.36 x 10 6 gm./cm2/hr. was determined.
According to Ithe above equation, a polymerization time of
17.0 hours polymerization time ought to give a zero change
in weight. When this was tried, the result was ,a loss,
1.91 x 10 7gm?/cm2/hr~ (4'.6 x 10 gm.,/cm /day~, This is some
15 times lower than that of HL~-ZBS glass. These results
were obtained by measuring the physical dimensions of the
glass bar and immersing i,t in boiling water f~r 96 hours.
The behavior of my new glass is unusual, especiall~for
HLW-PAP glass, as shown in Table II~ These data were
obtained for a HLW-PAP glass rod in which the glass had
been polymeriæed for 17.0 hours before casting the melt.
T A B L E I I
Effect of ~ryin~ Time on Weight Changes Observed or a
17 Hour Polymerized HLW-PAP Gla~ss
Time After Removal ~rom Boiling Weight Gain
Water (96 Hour Immersion) ' --6 2
(x 10 gm./cm /h~.)
O hour 0.315
O . 510
2 " 0.382
3 " ; 0.330
0.216
25 " 0.161
67 " 0.090
72 !' .0 . 082
~3~ 0~
-16-
This behavior indicates that the surface of the HLW-PAP
glass becomes hydroxylated and that the actual weight
loss ~or gain) is really zero (at 17.0 hours polymeriza
tion time). This was estimated by fitting the data of
Table II to an exponential decay equation, starting with
1 hour drying time. The statistical fit is 96% and the
e~uation obtained was:
(9) Wt' = 0.378 exp. - 0.022 t.
~xtrapolating the gain from 72 hours to one week gives
a value of 8.9 x 10 9 gm ~cm2/hr.~ that for 2 weeks is
2.1 x 10 10, while the value calculated for 4 weeks is:
Wt = 3.9 x 10 14gm./cm2/hr., as a final weight loss. This
illustrates the fact that the gain change observed for
the glass rod is xeversible and is caused by the boiling
water at the sur~ace of the glass rod. The weight change
then reequilibrates, with time, back to its original value.
In other words, only the surface of the glass is affected
but it reverts back to its original state once the boiling
water is removed. This proves that the change within the
glass matrix is actually zero in accordance with the experi-
ment~lly determined equation (8) for 17~0 hours polymeriza-
ti~n time. ~
~ he changes in weight as related to surface
hydroxylation are affected by the methods of J~Ll~-PAP glass
preparation ~s shown in Table III.
.
~3~
--17--
~ A !'l I, E III
Erfect Or ~ P.~ e.la;s Prc~re~ thsd Upc:~ Rcslst~ncc to Surfr~ce ~Iyarox~,rlntlon
!~atDrl~l Ura to E~ess }!370~ ity cr P~ crlzation Welrht ChDJ~e Gbs~rv~d *
M2'~ 1t Pre,~3t ~la~erl~ (10-6 ~m/c.-~/hr)
loss ~t 72 hrs.
prcc~rssr cr~stals 20 ~ hlg:h4 hr.... ~~ o.66 --
n ~ . n 17 ~. o.o66 - _
n ~ 24 hr . O . ~7
48 hr. 1.38 -- __
w c~ 72 ~ . . 2 . 60 - - _
pref~re ~c~lc~e) 20$ ~ih '72 ~rØ11 -_ t~.l2
n DCrDg 0.20 ___ 0.21
" BoIle ~ ~17 ~- 0.25 -- 0.17
Dcn~ b~. 0.2~
~p hi3h . y hr. 0.21 -~ O.OI7
,~ _ 95 ~ s ~ ~cili~g-;~'.,er
The data in Table IIIshows: (1) the prefire is a better
method and a better material with which to make the melt,
(2) the convert~on of HLW to phosphates is indicated
as a better method to approach zero weight loss, and (3)
purification of the precursor crystals gives a ~LW-PAP
glass form with essentially no~weight change, i.e.,
1.7 x 10 8 gm./cm /hr. or 4.1 x 10 7 gm./cm /day, as a
gain. Undoubtedly, this will revert to zero as the surface
continues to dehydroxylate with time.
The molecular glass has other interesting
properties in regard to the HLW encapsulation application.
The melt dissol~es all metals includin~ the noble metals
(Pt is very slow but Rh and Pd dissolve rapidly). All
o~~~des, or compounds which decompose to form oxides, do
dissolve, including the re~ractory oxldes, CeO2, ZrO2 and
Ru02. No crystal formation has been observed at any time
from HLW additives, unless the polymerization time exceeds
.
,
~3~4
about 36 hours, when AlP04 crystals appear. The melt has
a low viscosity of about 180 poise.
The amount of HLW additives can be varied frorn
about 5~ by weight of the glass, ~13P7022, to nearly 45%
by weight. I prefer to use about 20% - 25% by weight of
HLW additives, although one is not limited to this, as is
well known in the art.
It will be recognized that the instant invention
arises from the application of my novel polymerized molecu-
lar phosphate ~lass to the encapsulation of high-level
radioactive nuclear waste for disposal thereo~. Although
I have given data and results which stemmed from the aluminum
cationic variety of my new glass, other cations can be
employed for the same purpose, using the methods and approaches
given herein as applying to my new and impro~ed invention.
Two trivalent cations which may be substituted for the alumi-
num are In3~ and Ga3~; however, these materials are considerably
more expensive and have a larger cross-section for neutron
capture than aluminum. A particular advantage of using
aluminum is its low nuclear absorption, as compared with
indium and gallium and as compared wtth zinc borosilicate
(ZBS) glasses of the prior ar~. rrhis transparency to nuclear
particles reduces the possibility of radiation damage to the
molecu~ar structure, and ~linimizes the generation of thermal
energy.
As examples of the invention, I cite:
Example I
To prepare the precursor compound, measure out
970 ml of reagent grade, 85% H3P04 (specific gra~ity of
1.689 gm/cc), although other, lower grades can be used as
well, and add to 1000 ml. of water. Dilute to 2000 ml.
total vo]ume. Weigh out 156.0 gm. of Al(OH)3 and dissolve
in ~I3P04 solution. Heating may be necessary to obtain a
-19~ O~
clear ~olution. Weigh out 5.0 - 10.0 gm. of ammonium
l-pyrrolidine dithiocarbamate (APC) and dissolve in 50 mlc
of water. Add to solution. ~ilter oif the dark grey
precipitate using a a 45 micron filter. Set up a mercury-
pool electrolysis apparatus and electrolyze solution in
a nitrogen atmosphere at - 2.90 VDC at Hg pool for several
hours to remove residual impurities. A minimum of 2 hours
is required be~ore most of the impurities are removed. Evapo-
rate the purified solution slowly~ using a hea sc~rce, to
obtain precur~or crystals plus a liquid. The liquid con~
tains excess H3P04 plu~ water. The exce~s liquor is decanted
and the crystals are washed free o~ exc~ss H3P04, using
methyl-ethyl ketone as a washing agent. Assay the washed
and dried crystalsO
~ dd 20.0 gm. of HLW add.itives per 96.5 gmO of
cry~tals (assuming t~,~ experimental assay to be 83.0%);
the total Yolume used should fill the container used for
heating. Heat at a rate of about 10-12 C. per minute to
cause initial dehydration and polymerization. As the
temperature rises to 1325 C., a melt will form, with a
shrinkage o~ about 80~. More HLW-crystal mix is added
until the container is filled with meltO This takes about
1 hour..-Hold the melt about 16 hours longer to reach a
suitable degree o~ polymerization, and then cast the melt
in a suitable mold to form the final HLW-PAP glass slug,
~or long term stora~e thereo~. No annealing is necessary
but very large pieces may require a minimal annealing.
Molecular glasses require that annealing be done some
8-1~ C~ above the softening point.
Exam~le 2
.
Alternately~ the methods of Example I are
followed except that the HLW is not added at the point of
~ .
3~
-20-
initial firing. The precursor rystals are heated separately
at a rate of about 10 C. per minute to 1100 C. and then
held there for several hours to form a calcine powder. This ,
powder, which is partially polymerized, is cooled and mixed
with HLW at a rate of 0.O gm. of calcine powder to 20.0 gm.
of HLW additives, heated to 1350 C. to ~orm a melt which
is held at t~?is temperature for 17.0 houxs to complete
polymerizatiGn and then cast in final form for long term
storage thereof..
Example 3
Another alternate method is to heat the precursor
crystals to induce initial polymerizatio~ and then to obtain
the melt. The melt is then cast immediately and c0012d.
The resulting giass is ground to obtain a glass frit which
is ~hen used to encapsulate the HLW additives according to
methods o Example 2. In this ~ase, the frit sof tens at
822 C. and is liquid at 1150 C. This ~lt is used for
the encapsulation of ~LW additives according to methods
given aboveO This method has the advantage that much lower
tempera~ures can be used when ~he final casting container
to be used for long term storage cannot withstand the
higher temperatuxes required for producti~n of a direct
melt.
Example 4
The procedure given in Example 1 is followed
except that the crystals are not washed free of excess
H3P04. A portion of the crystals are assayed. The assay
is used to calculate the weight of crystals plus phosphoric
acid needed to obtain 0.20 HLW - 0.80 PAP glass on a weight
basis. ~he HLW, added prior to heating, begins to ~orm
phosphates. Upon heating, phosphate ~ormation is accelerated
-21-
and is complete by the time melt temperature is reached.
The formation of HLW-phosphates accelerates the dissolu-
tion of HLW into the melt, and aids dispersion thereof.
Further procedures of Example l are then followed.
Example 5
The procedure of Example 2 is followed to obtain
a calcine. Both HLW additives and H3P04 are added at a
ratio of 207 mlO of 85% H~P04 per 100 gm. of HLW additives,
to form a final composition of 0.20 HLW-0.80 PAP glass by
weight. The HLW - H3P0~ mixture is thoroughly blended before
it is added to the calcine, and then the final mixture is
heated according to the procedures of Example 2 to form the
melt, to form the final glass composition of ~.20 HLW -
0.80 PAP glass, for storage thereof.
~ . .
Example 6
If a glass frit is to be used, the procedures
of Examples 3 and 5 axe followed exc~ept that the H3P0~
is mixed with the HLW additives prior to addition to the
glass former, and is gently heated to 100 ~ 150 C.j as
required, to induce frothing an~ phosphate ~ormation.
~hen phosphate formation is complete, the HLW-phosphates
are added to the PAP glass-frit to Eorm a 0.20HLW-0.80 PAP
glass composition ~ixture, and the mass is heated at a
rate of 8 , 10 C. per minute to 825 C. where the frit
so~tens. The heating is continued up to 1100-1150 C.
where the melt is held for several hours until the HLW
additives can dissolve and become d;spersed within the melt~
~h* melt is then cast and handled according to procedures
already developed in prior examplesO
Example 7
The above examples give methods suitable for HLW
~L3~
-22-
encapsulation b~ PAP glass using a static or single con-
tainer method. If a continuous method is desired, there
are several alternatives. A glass melting furnace capable
of operating continuously at 1400 C. is set up and made
ready for operation. Such furnaces generally are composed
of a preheat chamber, a melt chamber and a holding tank~
It is essential that the inner faces of each chamber be
lined with impervious (high density) alumina, which is the
only material found to be sufficiently resistant to etching
by the very corrosive melt. A mixture of ~LW additives
and precursor crystals is added to the preheat chamber
to orm a melt. As the volume o~ melt încreases, the melt
moves over into the melt chamber and finally to the hold
chamber. HLW-phosphates are added simultaneously with
the PAP calcine, to form more melt, at a ratio so as to
mai~tain a ratio in tlle general range of 0.20 HLW - 0.80 PAP
glass in the final product. It is essential that the
throughput of the HLW-PAP glass be about 17.0 hours in
order for sufficient polymerization to take place before
the glass-casting is formed. Therefore the rate of addi-
tion of the HLW - calcine powder must be adjusted according
to the size of furnace used so as to obtain about 17.0 hours
of poly~erization time.
The addition o,f HLW aaditives can take 'at least
two forms, as oxides obtained by drying or calcining the
high-level liquid wastes, or as phospha-tes obtained by the
addition of H3P04 to the li~uid wastes, followed by separa-
tion thereof of the radioactive precipitated wastes as
phosphates.
The melt can be formed from precursor crystals (,un~
washed ox 'washed precursor crystals~ or PAP-calcine
powder. When HLW calcine is to be used, it is better to
'use unwashed crystals containing excess ~3P04 to convert
~3~0~L
-23-
the HLW oxides to phosphates in the preheat chamber of the
furnace. If HLW-phosphates are used, then PAP-calcine
can be use~ and added simultaneously to the preheat chamber.
The II~W-PAP glass melt is continuously drawn
from the holding chamber of the glass furnace, the melt
having a residence time of 17.0 houxs before casting into
a suitable container for long term storage thereof.
Example 8
When a glass frit is to be used on a continuous
casting basis, the method to be employed is somewhat dif-
ferent than that of Example 7. The HLW plus glass frit,
or-alternatively the HLW-phosphates plus glass frit are
mixed together in a ratio of about 0.20 HLW - 0.80 PAP
glass frit, but not to exceed about 0.45 to 0.55, and adcled
dir~ectly to a heated container, held at about 1150C. The
addition is fairly slow so as to give the melt enough time
to form. I~ the cannister i5 stainless steel, the addition
rate can be faster then if it is alumina, which has a lower
heat transfer rate from the furnace. After the cannister
is full, the melt is held at 1150 C. so that the total
melt-hold-time is about 17 hours. The cannister is then
cooled slowly and made ready for long term storage, as
is known in the priox art.
While the invention has been described hereinahove
in terms of the preferred embodiments and specific examples,
the invention itself is not limited thereto, but rather
comphrehends all such modifications of, and variatlons and c~
departures from these embodiments as properly fall within
t} - spirit and scope of the appended claims.
