Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
RD 8041
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The present invention relates generally to the
art of corrosion prevention in nuclear reactors and is
more particularly concerned with novel nuclear fuel
compositions and with a new fuel system incorporating
material selected from the group consisting of V2O4 or
V2O5 or mixtures thereof; gold, silver, palladium and
mixtures thereof, and CuFe2O4 and CuTio3 and mixtures
thereof, to prevent embrittlement of nuclear fuel
cladding by cadmium.
Nuclear fuel in compacted form suitable for
power reactors is usually enclosed in corrosion-resistant,
non-reactive, heat-conductive containers or cladding
which in assembly may take the form of rods or tubes,
or in the case of Boiling Water reactors even plates. A
plurality of fuel elements of this kind are assembled
in a fixed spaced relation in a coolant flow channel,
and a number of these assemblies form a reactor core
capable of a self-sustained fission reaction. The
core is contained in a reactor vessel through which
water as a coolant is run continuously.
A prime necessity in the operation of a nuclear
reactor is the containment of radioactive fission
products. The cladding serves this purpose, preventing
release of those products into the coolant and, in
addition, preventing contact and chemical reaction
between the nuclear fuel and the coolant. Common cladding
materials include zirconium and its alloys, particularly
~LD Zircaloy-2 and Zircaloy-4.
During operation of a nuclear powered reactor,
a fissionable atom of U-233, U-235, Pu-239 or Pu-241
undergoes a nuclear disintegration producing an average of
two fission products of lower atomic weight and great
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kinetic energy. Some of such fission products, including
iodine and bromine, have been found or considered to
have corrosive effects on the cladding. Thus, cladding
failure resulting from such corrosion has been observed
during operation of nuclear reactors over long periods of
time.
As disclosed and claimed i~ U. S. Patent
3,826,754 - dated July 30, 1974 - L. N. Grossman et al,
assigned to the assignee hereof, certain additives can
be incorporated in nuclear fuels to prevent corxosive
attack on cladding by fission produets. This result is
achieved without offsetting disadvantage by chemical
combination or association of the additives with
deleterious fission products whereby those fission products
are prevented from migrating in the nuclear fuel to reach
the cladding.
This invention is based upon my diseovery
that cadmium, which is produced in only relatively small
amounts in the fission of an atom of U-233, U-235, Pu-239,
Pu-241 or the like, has a markedly deleterious effect upon
common nuclear fuel cladding materials. In particular,
I have found that embrittlement of zirconium alloy
cladding is caused by cadmium in the temperature range
of 300-340C. Thus, sueh destructive attac~ occurs in
the presence of solid eadmium at 300C, liquid cadmium
at 340C and cadmium dissolved in liquid cesium at any
temperature in that range. Still further, the presence in
nuclear fuel of the immobilizing additives of the prior
art does not prevent or limit this embrittling effect
of cadmium.
This invention is additionally based upon my
diseovery that CuFe2O4 and CuTio3 individually and in
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V2O4 and V2O5 individually and in combination and gold,
silver and palladium, individually and in admixture, all
have the capability of reacting with cadmium under normal
boiling water reactor operating conditions and thereby
preventing embrittlement of nuclear fuel cladding by
cadmium in liquid or solid form or in solution in
liquid cesium. Further, I have found that they may be
admixed with a nuclear fuel as a simple additive or
used as a component of a multifunctional fuel additive,
or they may be applied as a coating on fuel pellets
or on the cladding inside surface, or distributed as a
layer between fuel pellets. However, in whatever form
and manner the additive is used for this cadmium-
inerting purpose, it should be so proportioned to insure
that there will not be a substantial amount of cadmium
free to contact and embrittle the fuel cladding. Thus,
in the case of CuTiO3, and V2O5 and their admixtures,
these materials should be used in amount between 0.0025
and 0.025 weight per cent on the basis of the nuclear
fuel material, and preferably in amount about 0.0075
weight per cent on that same basis.
In the case of CuFe2O4 an amount in the range
of 0.0033 to 0.033 weight per cent (preferably about
0.01 weight per cent) should be used.
In the case of gold, silver and palladium, on the
basis of 4000 grams of uranium, contained as a fcr instance,
in a typical Boiling Water reactor fuel rod charge of
uranium oxide fuel, which in 20,000 megawatt days will
generate about 0.1 gram of cadmium, there should be used
between 0.3 and 2.0 grams of yold, or between about 0.1
and 0.6 gram of silver, or between 0.1 and 0.8 gram of
palladium, in accordance with this invention, to nullify or
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make inert the cadmium.
This invention comprises recognition of the
role played by cadmium in embrittlement, and the step
of providing in contact with nuclear fuel material an
amount of cadmium-immobilizing additive effective to prevent
such cadmium embrittlement of nuclear reactor structural
components such as fuel cladding at reactor operating
temperatures.
Thus, the invention comprises an oxide composi-
tion nuclear fuel material in compacted pellet form
containing an amount of additive material selected from
the group of additives, effective to immobilize cadmium
resulting from the nuclear fission chain reactions of
the fuel material, by reacting with the cadmium and
thereby preven reaction of the cadmium with the metal
of nuclear fuel cladding under reactor operating conditions.
In the preferred practice of this invention, the
additive is associated with the fuel in any suitable manner
as by mechanically blending the additive in powder form
with the nuclear fuel material in a similar finely-
divided condition. It is also feasible, according to
this invention, to apply the additive as a coating to
part or all of the surface of a fuel pellet, or it may
be applied as a coating on the inside surface of the
cladding for contact with fuel pellets loaded therein. As
indicated above, it is also contemplated that the additive
in powder form can be disposed in the pellet assembly
as it is loaded into cladding. In any event, it is
desirable that the selected additive material be
distributed in respect to the nuclear fuel material to
insure that substantially all cadmium generated as a
fission product in the operation of the reactor comes
RD 8041
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into contact with and is reacted with the additive during
reactor operation so as to obtain the new results and
advantages of this invention described above.
Generally, the new and highly useful cadmium
immobilization result of this invention can be achieved
in accordance with the use of relatively small amounts
of the selected additive material. Thus, in order to
immobilize the cadmium generated on a basis of uranium
oxide fuel in a reactor operation at 20/000 megawatt days
per metric ton of uranium, for a reactor fuel rod in a
boiling water reactor, in which Q.ll gram of cadmium would
be generated, the respective quantities of additive would
be:
0.16 gram CuTiO3;
or 0.21 gram CuFe2O4;
or 0.16 gram V2O5.
In the best practice presently contemplated, the additive
will be present in association with the fuel in one or the
other of the several alternative ways described above in
such stoichiometric a~ount. Appreciably less than such
stoichiometric amount will leave the way open to some
extent for cadmium embrittlement of cladding, while use
of substantially more than the stoichiometric amount
burdens the system with inert material, using space
that should be occupied by fissile or fertile material.
When the additive is incorporated in the fuel
elements, they make take any desired geometric form or
; configuration, but it is preferred that the nuclear
fuel material be in the form of right cylindrical pellets
which are incorporated in a tubular cladding of a
zirconium alloy. The swelling of the pellets in the
cladding is accommodated by providing porosity in the
fuel pellet or by forming it with dished ends or
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axial openings or the like to accommodate such swelling.
In the case of the metallic additives those
skilled in the art will understand that when part or all
of the metal additive is to be provided in the form of
a coating on nuclear fuel powder particles, or on fuel
pellets, or on the inside surface of the fue~ cladding,
one has a choice of methods of providing that coating.
Thus, the vacuum deposition technique widely employed
in semiconductor production may be used, the gold, silver
or palladium being vaporized in a vacuum of 10 3 to 10 5
torr to condense on the surface of the powder, pellet
or cladding in the vacuum chamber. Alternatively, to
avoid the necessity for a vacuum operation, the desired
coating can be applied by the firing-on technique
involving the application by brushing or spraying of a metal-
organic solution of the selected metal to the material
or surface to be coated, followed by heating in air to
decompose the compound and volatilize the organic consti-
tuents leaving the metal film in place. Whatever method is
employed for this coating purpose, it will further be
understood that it is not necessary to consistently obtain
the new results and advantages of this invention that the
films produced be continuous or nonporous. Also, as
indicated above, it is likewise not necessary that the
proportion or the absolute amount of metal additive
incorporated in the nuclear fuel system be closely
controlled, the principal consideration being that
substantially all cadmium generated as a fission product
in the operation of the reactor comes into contact with
and is reacted with the metallic additive during reactor
operation so as to obtain the new results and advantages
of this invention described above.
RD 8041
11(~4~36
Generally, the new and highly useful cadmium
immobilization result of this invention can be achieved
in accordance with relatively small amounts of gold,
silver or palladium. Thus, as indicated above, 0.3
gram of gold, 0.25 gram of silver and 0.1 gram of palladium
is sufficient for the present purpose in a reactor
operation at 20,000 megawatt days per metric ton of
uranium in a boiling water reactor fuel rod which generates
0.11 gram of cadmium. In the best practice presently
contemplated, the metallic additive will be used indi-
vidually rather than in admixture with another metallic
additive and will be present in association with the
fuel in one or the other of the several alternative ways
described above in amounts between 0.6 and 1.0 gram of
gold, between 0.5 and 0.7 gram of silver and between 0.1
and 0.6 gram of palladium.
From the foregoing description, it will be
understood that this invention achieves the chemical
inerting of reactive fission product cadmium through the
use of selected additives which react with cadmium
under normal nuclear reactor operating conditions to
-~ form stable compounds so that fission product cadmium is
not available or free to react with or attack fuel
cladding or any other metal that it may come in
contact with during reactor operation. In this manner,
the additive which is effective for the purposes of
this invention blocks potential cladding-fission product
reaction and so increases the cladding reliability and
useful life.
In a test conducted for the purpose of confirming
B that cadmium embrittles zirconium alloy cladding material
r~
under elevated temperature conditions, a Zircaloy-2 tensile
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test speciman was broken in argon at 300C after under-
going a 75 per cent reduction in cross-sectional area
and with a plastic strain of about 15 per cent following
a maximum stress of 60,000 psi. Fracture morphology was
ductile.
Then in a repetition of that test but for the
presence of cadmium in contact with the test specimen,
breakage oeeurred as a transgranular cleavage fracture
with zero reduction in area and zero plastic strain at
maximum stress of 40,000 psi before reaehing the yield
point of the speeimen. Many incipient eraeks were observed
in the speeimen on eonelusion of the test.
Similar results to those of the latter test
were obtained in subsequent tests performed in the same
manner but at temperatures between 250C and 350C involving
the use of solid eadmium (below 321~C), liquid cadmium
(above 321C) and cadmium dissolved in liquid eesium
at temperatures both above and below 321C.
In testing the basie new eadmium inerting
coneept of this invention, oxides of variable-valence metals
were equilibrated with cadmium at 350C in evaeuated
quartz eapsules in a thermal gradient furnaee. Either
a reaetion oeeurred or it did not; and where the test
result was positive in this sense, the eompounas formed
were stable up to the approximately 1000C temperature
limit of the furnaee. As indieated above, all of the
seleeted group of materials did so reaet under these
conditions in this test with the apparent formation
formation in the case of the oxide additives of cadmium
oxide and a non-active substanee, such as elemented
eopper or a lower oxide of vanadium respectively. No
such reaction was observed in tests of this kind
RD 8041
36
involving the use o~ either TiO2 or Nb2O5, CeO2 or
depleted UO2.
In the ca~e of the metal additives there was
apparently formed a stable cadmium intermetallic compound.
In testing the basic new cadmium inerting
concept of this invention, copper, gold, silver, palladium,
silicon, chromium, iron, nickel, aluminu, yttrium and
niobium were each equilibrated with cadmium at 350C in
evacuated quartz capsules in a thermal gradient furnace
in a series of tests. Either a reaction occurred or
it did not; and where the test result was positive in
this sense, the compounds formed were stable to relative
high temperature. As indicated above, only gold, silver
and palladium did so react under these conditions in this
test with the apparent formation of cadmium intermetallic
compounds which were stable up to 550C in the case of
palladium, 650aC in the case of silver and 1000C (the
temperature limit of the furnace) in the case of gold.
No such reaction was observed in these tests of the
other metals listed above.
In out-of-pile experiments performed with gold, it
was found that 0.1 gram of cadmium was immobilized or
gettered by 2.0 grams of gold at temperatures between
300C and 950ac (upper limit of the test). Actually, on
visual examination, it was observed that only about one-
tenth of the total volume of gold powder was affected,
indicating that a stoichiometric reaction of one-to~one
stoichiometry had taken place.
In using gold, silver or palladium or mixtures
of them in accordance with this invention to fill the
gap between the nuclear fuel and the cladding of a
fuel rod, the metallic material in powder form may be
R~ 8041
11~4~36
packed lightly in place. With the volume of that gap
typically being about 14.5 cc, a gap-filling load would
be about 35.0 grams, which would insure inerting of the
cadmium released at all locations in the fuel rod during
reactor operation.
When it is desired to provide the embrittlement
protection of this invention in locations between fuel
pellets, a 5-mil coating of gold, for example, may be
applied to one of each pair of opposed pellet end
surfaces. Thus, in a typical fuel rod assembly of 100
fuel pellets, each of 0.87 square centimeters end surface
area, a total of about 1.2 grams of gold will be
incorporated in the fuel rod~ As indicated above, this
amount will be in substantial excess of the stoichiometric
equivalent of the cadmium produced in the normal useful
life of the fuel rod in the typical boiling water nuclear
reactor operation, but will not constitute a significant
displacement of fissile or fertile material.
In out-of-pile experiments performed with V2O5,
- it was found that 0.1 gram of cadmium was immobilized
or gettered by 1.6 grams of V2O5 at temperatures between
300 and 950C (the maximum test temperature). Actually,
on visual examination it was observed that only about
one-tenth of the total volume of V2O5 was darkened (i.e.
changed in color from yellow to black), indicating
that a reaction of one-to-one stoichiometry had taken
place.
To test the copper oxide additives a variety
of compounds were equilibriated with cadmium at 350C
in evacuated quartz capsules in a thermal gradient
furnace. These compounds included TiO2, 13 per cent
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il~433t; RD 8041
A12O3 in sio2, 25 per cent A12O3 in SiO2, copper chromate,
copper tungstate, copper molybdate, nickel molybdate
and nickel titanate. Either a reaction occurred or it
did not; and where the test result was positive in
this sense, the compounds formed were stable up to the
approximately 1000C temperature limit of the furnace.
As indicated above, CuFe2O4 and CuTiO3 did so react under
these conditions in this test with the apparent release
of copper in metallic or elemental form through displace-
ment by cadmium in the ferrite and titanate compounds. No
such reaction was observed in tests of this kind involving
the use of any of the other compounds listed above.
In out-of-pile experiments performed with CuYe2O4,
it was found that 0.1 gram of cadmium was immobilized
or gettered by 2.9 grams of CuFe2O4 at temperatures
between 300 and 950C. Actually, on visual examination
it was observed that copper was formed. Thus, only
about one-tenth of the total volume of CuFe2O4 powder
would have reacted by the stoichiometry o~ the cadmium-
copper displacement reaction.
In using CuTio3 or V2O4 or V2O5 or mixtures
of them in accordance with this invention to fill the
gap between the nuclear fuel and the cladding of a uel
rod, the vanadium oxide material in powder form may
be packed lightly in place. ~Jith the volume of that gap
typically being about 14.5 cc, a gap-filling load would
be about 11 grams, which would insure inerting of the
cadmium xeleased at all locations in the fule rod during
reactor operation~ The load of CuFe2O4 would be about
32 grams.
When it is desired to provide the embrittlement
protection of this invention in locations between fuel
~1~4~3~ RD 8041
pellets, a 5-mil layer of CuFe2O4 or V2O5, for example,
may be disposed between each pair of pellets. Thus,
in a typical fuel rod assembly of 100 fuel pel.lets, each
of 0.87 square centimeters end surface area, a total of
about 2.5 grams of CuFe2O4 or 0.9 gram of V2O5 will be
incorporated in the fuel rod. As indicated above, this
amount will be in substantial excess of the stoichiometric
equivalent of the cadmium produced in the normal useful
life of the fuel rod in the typical boiling water nuclear
reactor cperation.
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