Note: Descriptions are shown in the official language in which they were submitted.
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This invention is directed to an improved electrical
insulation system, and in particular to a zirconia-insulated
system, and the manufacture thereof.
In the operation of nuclear reactors, it is often
desirable to detect or measure what is taking place within
the reactor, using electrical probes, coils and the like.
Owing to the highly hostile environment existing within the
confines of reactors such as the Candu reactors, wherein a
strong field of radiation exists in a water-filled corrosive
environment, and wherein flows of water at high velocity take
place, it has not been found possible to install sensor probes
or electrical sensor coils, owing to the rapid deterioration
of the insulation within the environment. Thus, the usually
alkaline water, at a temperature in excess of 300C rapidly
attacks glass and even quartz.
In the prior art, in Canadian Patent No. 822,775,
which issued September 9, 1969 in the name of Robert D. Watson,
etal, there is shown the provision of a Zircaloy-2 alloy,
comprising 1.2 to 1.7 per cent tin; 0.07 to 0.2 per cent
iron; 0.05 to 0.15 per cent chromium; 0.03 to 0.08 per cent
nickel and the balance zirconium, the alloy being in the
form of a slotted tube intended for use as an electrical
heating element, wherein insulation is provided by utilizing
a zirconia layer thereon.
The formation of the desired beige-coloured
zirconia layer of Zr 2 is obtained in accordance with the
teaching of Canadian Patent No. 770,080, which issued
October 24, 1967 in the name of Robert D. Watson et al.
In addition to the teachings of Watson et al,
as set forth in the referenced patents, it has been found that
a satisfactory adherent layer of zirconia can be formed on
wires and ribbons of many zirconium alloys, including
Zircaloy-2, using the methods formulated by Watson et al, but
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without, in some instances having to resort to the surface
roughening processes adopted by Watson.
Furthermore, it has been found that the benefits
of a zirconia insulative layer may be obtained in the case
of electrical coils of zirconium alloy (hereinafter referred
to as ZA) and for electrical probes of ZA extending through
ZA plates and other ZA surfaces, wherein the assemhly parts
that are intended for use as conductors are assemkled in mutual
short circuiting relation, and also wherein the conducting
portions are assembled in short circuited relation with
elements of the system which are intended for use as
insulators. It has been found that upon heating these afore-
mentioned conductlve and insulative components to a temperature
in the order of 600 to 650C, a highly effective layer of
zirconia is formed, providing electrical insulation, even
where short-circuit conditions previously existed. In the
case even of ZA components press-fitted together, the oxidizing
process can effectively penetrate and insulate the elements
from each other. The zirconia layer thus formed, in addition
to providing excellent electrical insulation serves to support
components in mutually secured relation, owing to the increase
in mutual physical interference provided by the swelling at
the respective surfaces which takes place as the zirconia is
formed. The insulative zirconia surface also is found to be
virtually self-healing within the reactor environment since
the oxide reforms upon exposure to high temperature water.
One envisaged use of the present invention is the
incorporation of a copper or other highly conductive
conductor within a ZA tube. ZA tubes having outer diameters
in the order of one millimeter are available, for use in such
a process. By oxidizing the tube to form a zirconia layer
on the outer surface, there is obtained a superior insulated
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conductor for use within hostile environments. Before
carrying out the oxidation conversion step, the tube may be
reduced in diameter by swaging or by die drawing, to bring
it into intimate gripping relationship with the conductor.
The resulting composite structure provides a conductor having
a lower electrical resistance than a ZA conductor of the same
diamater.
Certain embodiments are described, reference being
made to the accompanying drawings wherein;
Figure 1 shows a zirconium alloy component in
accordance with the present disclosure;
Figure 2 shows a conductive probe and insulative
support plate in accordance with the invention, with
enlarged detail at Fig. 2A;
Figure 3 shows a composite conductor in accordance
with the invention;
Figure 4 is a section view at 4-4 of Figure 3;
Figure 5 is a like view after swaging or
die drawing; and
Figure 6 shows a conductive probe within a
shielding support tube in accordance with the invention.
In Figure 1 there is shown a coil 10 having a
conductor 12 wound into a plurality of turns with an outer
layer of turns 14 and an inner layer 16 of turns of smaller
diameter than the turns 14. The conductor may be of ZA wire
or a composite conductor having a ZA outer surface. The
coil may be wound upon a ZA former 18. The ZA alloy of each
turn is in intimate contact with adjacent turns and with the
former 18, to form a short circuit condition such that the
application of voltage between the ends 20, 22 of the conductor
would result in the passage of a short circuit current from
turn to turn and from the inner turns 14 and the ends 20, 22
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of the conductor to the former 18.
Subjecting the coil to the known oxidation process,
comprising heating in the range 600C to 650~C in air
produces oxidation of the ZA to produce an adherent layer of
zirconia. The zirconia coats all of the wire, so that no
short circuit condition exists between any of the turns or
between the conductor and the former and the application of
normal voltage between the ends 20,22 of the conductor will
result in the passage of a normal operating current around
the turns of the coil.
Referring to Figure 2, there is shown a coil 30 having
leads 32, 34 extending through bores in a plate 36. The
coil 30 and plate 36 are of ZA and the relative dimensions
permit a close but non-interference fit assembly.
- Also shown is a probe 37 of ZA having a contact
point 38 made of a metal which does not oxidize on exposure
to air or water athigh temperature (such as platinum), the
contact point being welded or brazed at 39 to probe 37.
The probe 37 extends through plate 36 ;the relative dimensions
permit a close but non-interference fit assembly.
After completion of the oxidation process the
zirconia layer created on the ZA components serves to secure
them together in an interference fit. The plate 36 now
serves as an insulator. The coil 30 and the probe 37 are
electrically insulated all over, by the layer of zirconia,
except for th~e contact point 38 on the end of probe 37.
It will be evident that components of extreme
complexity may be fabricated in ZA, and subsequently
transformed as desired to provide an insulation layer of
zirconia thereon, even to electrically isolate parts
fabricated in iniimate contact.
Figure 3 illustrates the construction of a
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composite conductor. A highly conductive conductor 40 such
as copper is threaded through a sheath 42 of Zircaloy-2 alloy,
as shown in section in Figure 4.
In Figure 5 the assembly is shown after swaging,
or die-drawing. If then oxidized, the outer surface 45 of
sheath 42 becomes zirconia, and serves to insulate the
conductor 40.
In E'igure 6 is shown an electrode 50 made of a
metal (such as platinum) which does not oxidize or corrode
in high temperature water,having a lead 54 extending through
the bore of a tubular member 52 fabricated of Z~ which has
oxidized on its inner and outer surfaces. The surface oxide
layer serves to insulate the member 52 from the electrode 50
and lead 54 and also from the surrounding environment which
may be a conductive medium such as water. In addition to
supporting the electrode 50, the metallic member 52 may be
connected electrically to act as a shield for lead 54, to
avoid resistive and capacitative coupling to the surrounding
environment.
It will be evident from the foregoing that an
insulation system of unexpected versatility is hereby provided,
wherein the advantages of working in ductile metallic elements,
to close tolerances and in forms of almost unlimited complexity
while yet possessing the potentiality of separating
selected components of the assembly with a highly durable
insulation is herein provided.
