Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates generally to
materials of construction of nuclear reactors and
is more particularly concerned with a novel method
of enhancing the ability of zirconium-base alloys to
resist corrosive attack under boiling water reactor
operating conditions, and with unique structural
components produced through the use of that method.
This invention is related to that
disclosed in Canadian Patent Application
Serial Wo. 245,659 filed February 13, 1976 in the
names of Allan J. Kiesler, Alan C. Rockwood and
Peter G. Frischmann which concerns the concept of
heat treating a zirconium-base alloy body to cause
redistribution of the intermetallic particulate phase
with resulting substantial increase in resistance to
pustular corrosion under boiling water reactor service
conditions. The aforesaid Canadian patent application
particularly relates to a zone heat treating process
and apparatus based on the concept of traversing the
length of a workpiece with a hot zone of fixed length
in which the maximum temperature is maintained by
regulation of power input automatically in response
to fluctuations in infrared radiation from a portion
of the workpiece axially spaced from the hot zone.
Important requirements for materials used
in boiling water nuclear reactor construction include
low absorption for thermal neutrons, corrosion and
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stress-corrosion resistance and mechanical strength.
Zirconium-base alloys sufficiently satisfy these
requirements that they are widely used for such
purposes, "Zircaloy-2" (containing about 1.5 percent
tin, 0.15 percent iron, 0.1 percent chromium, 0.05
percent nickel and 0.1 percent oxygen) and "Zircaloy-4"
(containing substantially no nickel and about 0.2
percent iron but otherwise similar to Zircaloy-2)
being two of the important commercial alloys commonly
finding such use. These alloys, however, are not
nearly all that one would desire, particularly in
respect to accelerated pustular corrosion which occurs
under boiling water reactor normal operating conditions
and results in spalling of thick oxides from channels
and thickening of oxides on fuel rods. The spalling
of oxide flakes leads in some instances to development
of high radiation fields in locations where the flakes
collect; in addition, the extra loss of metal thickness
due to the accelerated oxidation process requires an
undesirable increase in design allowances for corrosion.
Efforts heretofore to solve this particular
problem have to our knowledge met with no success, ;
although the general subject of corrosion of such
alloys has long been of active interest to experts
in the field. Thus, in U.S. Patent 3,005,706, dated
October 24, 1961-D. Thomas et al, it is proposed that
from 0.03 to 1.0 percent of beryllium be added to
zirconium alloys intended for use in conventional
boilers, boiling water reactors and similar apparatus
to enhance corrosion resistance to high temperature
water. Similarly, in U.S. Patent 3,261,682 dated
July 19, 1966-U. Rosler and Patent ~o. 3,150,972
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dated September 29, 1964-U. Rosler cerium and/or
yttrium and calcium, respectively, are proposed as
zirconium alloy additions in like proportions for the
same purpose. Accounts and reports of the long-term
results of such compositional changes are sparse,
however, and commercial zirconium alloys do not include
these additional constituents.
This invention, which i8 predicated on my
discovery and new concept to be described, provides
an answer to the accelerated pustular corrosion
problem in the form of a heat treatment process which
is expected to at least approximately double the
corrosion-limited lifetime of zirconium-base alloy
boiling water reactor structural components. More-
over, this result can be obtained consistently and at
relatively small additional cost, particularly
through the use of the novel zone heat treating process
and apparatus disclosed and claimed in the above-
referenced Canadian Patent Application Serial ~o.
245,659.
The foregoing surprising corrosion-
resistance properties were discovered through the use
of an accelerated test which provides a good corre-
lation with in-reactor performance data. Thus, the
test specimens were subjected to high temperature
(about 500C), high pressure (about 1500 psi) steam
in autoclave experiments running from 22 to 24 hours,
and then visually examined and measured for weight
gain.
My discovery is that there is a strong
correlation between a particular microstructural
~ characteristic and resistance to corrosion in boiling
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water reactor environments. In particular, I have
found that corrosion resistance at least equal to that
obtained in accordance with the concepts disclosed in the
above referenced Canadian patent application Serial No.
245,659 can consistently be produced in zirconium-base alloys
by a process including a high temperature solution
treatment and rapid quench followed by a heat-aging
step which results in precipitation of a second
phase in the form of particles ranging from about
100 to 400 Angstroms in size. These particles of
intermetallic material Zr(Cr,Fe)2 in Zircaloy-4
and both Zr(Cr,Fe)2 and Zr2(Ni,Fe) in Zircaloy-2
are segregated in two-dimensional arrays along
grain- and sub-grain boundaries instead of being
in the usual condition of generally uniform distribution
and isolated and separated from each other.
Our concept is to use this discovery to
greatly increase the service life of a zirconium-
base alloy body by preparing it to intermediate or
to substantially finished form as a boiling water
reactor channel, or as a tube for nuclear fuel cladding,
or as a fuel rod spacer for use in a reactor channel,
and heating it to transform substantially completely
from alpha (hexagonal close packed) to beta (body
centered cubic) phase, quenching it to produce a
very fine Widmanstattan or martensitic structure
without intermetallic particles as the iron, chromium -~
and nickel are maintained in solution, and finally
annealing it at a relatively low temperature to cause ;~
3G precipitation of intermetallic particles along the
grain boundaries and sub-grain boundaries.
In principle, it is possible to perform the
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initial solution annealing treatment at a temperature
where the alpha phase transforms only partly to the
beta phase (i.e., a treatment in the alpha + beta phase
field), since such treatments result in dissolution of
the intermetallic precipitates. However, it is our
experience that the usual rapid quenching procedures
(such as a water quench) are not fast enough to retain
the iron, chromium and nickel in solution following these
lower temperature anneals. In these cases, the
precipitates tend to form during the quench
rather than during the subsequent thermal aging.
The zirconium-base alloy body treated in this
manner in addition to having significantly enhanced
resistance to corrosion has desirable mechanical
characteristics attributable to the fine microstructure
resulting from the quenching operation followed by the
heat-aging step.
It is important in carrying out this invention
to avoid processing operations subsequent to the fore-
going heating and quenching steps such as hot and cold
rolling and annealing which will result in elimination
of the two dimensional arrays of precipitate particles
throughout the alloy body. Rehomogenizing of those
particles in any manner can lead to loss of the desired
corrosion-resistance characteristic.
This new concept of mine also differs import-
antly from the prior art notion of subjecting Zircaloy
channels and tubes for use in boiling water eactors
to heat treatment in the beta temperature range at an
early stage of their fabrication so as to eliminate
any undesirable dendritic or other segregate phase.
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Although quenching may have followed such heat treatment,
any beneficial effects in the direction of the present
invention were quickly lost in subsequent hot and cold
working and annealing operations which were a necessary
part of the fabrication schedule and different from
the forming, straightening, grit blasting, pickling
and stress-relief annealing steps comprising the
finishing (as distinguished from the fabrication)
operations, which do not eliminate or diminish the
foregoing beneficial effects.
In its method aspect, this invention comprises
the steps of heating a zirconium alloy body to a tem-
perature in the beta phase region and maintaining it
there until the alpha phase transforms substantially
completely to the beta phase, then cooling the body
below about 400C without precipitating intermetallic
phase dissolved during the heating step, thereafter
reheating to an intermediate temperature to cause
precipitation of the intermetallic phase in the form
20 of particles from about 100 to 400 Angstroms in diameter
along grain- and sub-grain boundaries, Preferably,
the solution heat treatment is carried out at tem-
perature between about 1000C and 1100C in about
three seconds to one minute, these temperatures being
somewhat above the alpha + beta to beta transformation
temperatures of the alloys described above. As a
practical matter, temperatures above 1100C are not
desirable because detrimental grain growth and ex-
cessive contamination may occur. Similarly, there is
nothing to be gained and there is some risk in pro-
longing the solution heat treatment beyond one minute
for the same reasons.
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The quenching step is carried out so as to
bring the temperature of the solution heat treated body
from the beta transformation range to about room
temperature, water being preferred for this purpose
although other media such as oil are within the scope
of this invention. Using water and the apparatus
disclosed and claimed in referenced Canadian Patent
Application Serial No. 2~5,659, quenching rates of more
than 800C per second can be obtained to prevent
precipitation of any significant amount of intermetallic
phase.
The aging or precipitation heat treatment is
accomplished by reheating the quenched body to 400C
to 600C for two to four hours and then cooling as
desired to about room temperature. The duration of the
heat treatment will be greater at lower temperature
for the same desired result, and no substantial ad-
vantage is to be gained by prolonging this operation
beyond the time when precipitation of the intermetallic
phase is substantially complete. While temperatures up
to the alpha transformation temperature (about 825C)
may be used, thereis a marked tendency for the desired
microstructure to break down at temperatures above
about 600C with resulting loss of corrosion resistance
in the ultimate alloy body. On the other hand, at
temperatures below about 400C, the intermetallic
material does not precipitate or does so at a rate
much too slow for practical purposes.
In its product or article aspect, the
structural component of this invention is a æirconium-
base alloy and has special utility in a boiling water
- reactor by virtue of its resistance to accelerated
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pustular corrosion. As indicated above, the alloy
contains tin, iron and chromium and may additionally
contain nickel, and it includes the zirconium-iron-
chromium intermetallic compound, Zr(Cr,Fe)2, and may
also contain Zr2(Ni,Fe) in the form of a particulate
precipitate. The microstructure of the article is
characterized by segregation of precipitate particles
of diameter from about 100 to 400 Angstroms in two
dimensional arrays along grain boundaries and sub-
grain boundaries distributed throughout the component.
The novel features of this invention areillustrated in the drawings accompanying and forming
a part of this specification, in which:
Fig. 1 is a partial cutaway sectional view
of a nuclear reactor fuel assembly incorporating
structural members embodying this invention in pre-
ferred form;
FigO 2 is a scanning electron photomicrograph
(2000X) of a conventional zirconium-base alloy,
showing the distribution of particulate intermetallic
phase; and
Fig. 3 is a transmission electron photo-
micrograph (20,000X) of the Fig. 2 alloy following
heat treatment in accordance with this invention.
A primary application of this invention is -~
in the fabrication of nuclear fuel assemblies such as
that illustrated in the partial cutaway sectional
view of Fig. 1. Assembly 10, as illustrated, is typical
of the boiling water reactor fuel assembly design and
consists of a tubular flow channel 11 of generally
square cross section provided at its upper end with
- lifting bale 12 and at its lower end with a nose piece
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(not shown due to the lower portion of assembly 10
being omitted). The upper end of channel 11 is open
at 13 and the lower end of the nose piece is provided
with coolant flow openings. An array of fuel elements
or rods 14 is enclosed in channel 11 and supported
therein by means of upper end plate 15 and a lower end
plate (not shown due to the lower portion being
omitted), and rods 14 are maintained in spaced relation
to each other by spacer grids (not shown) through which
the rods extend located at intervals along the length
of the assembly and secured to the rods 11. The
liquid coolant ordinarily enters through the openings
in the lower end of the nose piece, passes upwardly
around fuel elements 14, and discharges at upper outlet
13 in a partially vaporized condition for boiling water
reactors or in an unvaporized condition for pressurized
reactors at an elevated temperature.
The nuclear fuel elements or rods 14 are
sealed at their ends by means of end plugs 18 welded
to the cladding 17, which may include studs 19 to
facilitate the mounting of the fuel rod in the assembly.
A void space or plenum 20 is provided at one end of
the element to permit longitudinal expansion of the
fuel material and accumulation of gases released from
the fuel material. A nuclear fuel material retainer
means 24 in the form of a helical member is positioned
within space 20 to provide restraint against the axial
movement of the pellet column, especially during
handling and transportation of the fuel element.
The fuel element is designed to provide an
excellent thermal contact between the cladding and the
~ fuel material, a minimum of parasitic neutron absorption,
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and resistance to bowing and vibration which is
occasionally caused by flow of the coolant at high
velocity.
Channel 11, fuel element or cladding 14 and
spacer grids (not shown) are produced in accordance
with this invention by a method which includes in
addition to the usual channel and tube-forming operations
a final heat treatment in which the alpha phase is
transformed substantially completely to beta phase and
the body is quenched and then reheated to a relatively
low temperature to cause precipitation of very fine
particles of dissolved intermetallic phase along
grain- and sub-grain boundaries. The rate at which
the workpiece is heated to the beta phase transformation
temperature range and the temperature level reached in
that range are matters of choice, but both the minimum
time in that range and the minimum cooling rate from
the threshold (965C-990C) of the range are highly
critical. Thus, the new advantages and results of this ~;
invention cannot be consistently obtained unless the
particulate precipitate phase is in the very fine
condition previously described; and I have found that
such condition cannot be established to the extent
necessary to increase by a factor of approximately two
or more the corrosion-limited lifetimes of channels
and cladding unless the time at temperature above the
alpha-to-beta transus temperature is at least about 3
seconds and the cooling ra~e to below about 400C is
rapid enough to avoid precipitation of the intermetallic
phase. The minimum required cooling rate for t~is
purpose is not well established; however, a rate of
800C per second appears to be adequate.
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The atmospheres in which the solution and
precipitation heat treatments are conducted are not
critical. Thus, air is suitable in both and, in fact,
represents the best practice of this invention carried
out on a commercial scale as long as the oxide developed
during heat treatment is removed in final processing.
The present novel method and products are
set forth in detail in the following illustrative, but
not limiting, examples of the best practice of this
invention.
EXAMPLE 1
T test strip of Zircaloy-4 ASTM B352 Grade
RA2 of 80-mil gauge thickness was heated in argon
to 1000C for five minutes and then water-~uenched to
20C. The strip was then cut into two parts, one of
which was reheated to 500C for 24 hours. It was air-
cooled once again to 20C and then both parts were
subjected to transmission electron microscopic exam-
ination. Fig. 3 shows the fine particles developed
during the aging process, no such particles being
present after quenching and before aging. Similar
results were obtained using shorter aging treatments
of about four hours duration.
A sample of the strip material which had
been aged as described above was then subjected to
500C, 1500 psi steam for 24 hours along with a sample
of the same alloy which had not been heat treated,
Visual examination of the two specimens on removal
from the test autoclave on conclusion of this
accelerated corrosion test revealed that substantial
corrosion resistance was obtained through the use
of the heat treatment process of this invention, there
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being only minor, uniform oxide growth on the treated
one while the untreated one was heavily corroded in
the manner characteristic of zirconium alloy bodies
subject for protracted periods to boiling water nuclear
reactor conditions.
EXAMPLE II
A boiling water reactor channel of Zircaloy-4
(ASTM B352 Grade RA2) of 120 mil thickness was heat
treated by passing it through an induction heating
apparatus similar to that described in ref2renced
Canadian Application Serial No. 245,659. The time
within the desired temperature range of 1000 to 1100C
was approximately three seconds. The channel was quenched
by spraying water on its exterior surface below the
elevation of the heating coils. Subsequent examination
by transmission electron microscopy showed that
precipitation of intermetallic particles did not
occur near the external surfaces, and that this material
responded to aging treatments in a similar manner to
that described in Example 1 and illustrated in Fig. 3.
While some precipitation occurred near the inner
(uncooled) surface of the channel, it is believed that
this could be eliminated by further improvements in the
external quenching spray, or by direct spray quenching
of the internal surfaces.
Throughout this specification and the appended
claims where ratios or proportions are stated, re-
ference is to the weight basis unless otherwise specified.
Those skilled in the art will understand from
the above description of this invention in general and
specific terms that the invention is applicable to
zirconium-base alloy strip material as well as to channels
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and other structural components fabricated therefrom.
The important point is that hot or cold working and
annealing operations which tend to rehomogenize the
microstructural segregation produced by the process
of this invention should be avoided in subsequent
fabrication operations. Channels or spacers can,
however, be fabricated from strip processed in ac-
cordance with this invention method without the necessity
for such hot or cold rolling and annealing steps and
without causing such rehomogenization.
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