Electrode probe for use in aqueous environments of high temperature and high radiation

The present invention is addressed to an electrode for evaluating electrochemical potentials which has a robust structure particularly suiting it for employment within the rigorous environment of the reactor core of a nuclear power facility. The electrode of the present invention is comprised of five major segments: a metal cap electrode, an elongate annular metal jacket, an alumina retainer (i.e. an insulator), an annular metal transition sleeve, and a positioning and signal transfer assembly. The metal cap electrode has a tip and annulus extending therefrom formed from sidewalls having an interior surface. The elongate annular metal jacket has an annular retainer securing mouth at one end and an outlet at its opposite end. The alumina retainer has a recessed cap securing portion nestably secured to a portion of said cap electrode interior sidewall surface, and an oppositely-disposed recessed jacket securing portion nestably secured to the interior of said annular jacket retainer securing mouth. The retainer further has an access channel penetrating therethrough from said cap to said jacket securing portion. The annular metal transition sleeve has one end placed at the jacket outlet, the opposite end at the positioning and signal transfer assembly. A first insulated electrical conductor in electrical connection with the cap electrode extends through the retainer access channel and through the annular metal jacket to the jacket outlet. Finally, the positioning and signal transfer assembly is associated with the transition sleeve outlet for providing fit-up for said sleeve and for conveying electrical signals from said conductor. The elongated annular metal transition sleeve is interposed between said jacket and said positioning and signal transfer assembly.

BACKGROUND OF THE INVENTION 
The nuclear power industry long has been engaged in a multitude of studies 
and investigations seeking improvement in the stamina and reliability of 
the materials and components forming a reactor based power system. One 
such investigation has been concerned with intergrannular stress corrosion 
cracking (IGSCC) which heretofore principally has been manifested in the 
water recirculation piping systems external to the radiation intense 
reactor core regions of nuclear facilities. Typically, the piping 
architecture of these external systems is formed of a stainless steel 
material. Generally, these studies have determined that three factors must 
occur in coincidence to create IGSCC promotional conditions. These factors 
are: (a) a sensitization of the metal (stainless steel), for example, such 
as caused by a chromium depletion at grain boundaries which may be caused 
by heat treatment in the course of normal processing of the material or by 
welding and like procedures; (b) the presence of tensile stress in the 
material; and (c) the oxygenated normal water chemistry (NWC) environment 
typically present in a boiling water reactor (BWR). By removing any one of 
these three factors, the IGSCC phenomenon is essentially obviated. Such 
removal particularly has been accomplished with respect to the latter, 
oxygenated environment factor through employment of an electrochemical 
potential monitoring approach combined with an associated hydrogen water 
chemistry (HWC) technique providing for a controlled addition or injection 
of hydrogen into the aqueous coolant environment. 
Electrochemical potential monitoring is carried out employing paired 
electrochemical half-cell probes or electrodes which are mounted within 
the recirculation piping or in an external vessel which has its water 
source from the reactor water in the recirculation piping. The electrodes 
are accessed to the external environment through gland type mountings or 
the like. Where, as in the instant application, the electrode system of 
interest involves the potential from a metal corrosion electrode, then the 
reference electrode can conveniently be a metal-insoluble salt electrode, 
if the metal salt couple is chemically stable and if appropriate 
thermodynamic data is available. Accordingly, one of the thus-mounted 
probes which is configured as a reference electrode may be based, for 
example, on a silver/silver chloride half-cell reaction. Once the 
reference electrode half-cell is defined, the cell is completed with the 
sensing cell portion based upon a metal such as platinum or stainless 
steel. Calibration of the reference electrode and/or the electrode pair is 
carried out by thermodynamic evaluation and appropriate Nernst based 
electrochemical calculations in combination with laboratory testing within 
a known environment. 
Half cell electrodes developed for use in reactor circulation piping 
traditionally have been configured with metal housings, high temperature 
ceramics, and polymeric seals such as Teflon brand 
polytetrafluoroethylene. These structures have performed adequately in the 
more benign and essentially radiation-free environments of recirculation 
piping. 
Over the recent past, investigators have sought to expand the 
electrochemical potential (ECP) monitoring procedures to the severe 
environment of the fluid in the vicinity of the reactor core itself for 
the purpose of studying or quantifying the effect of hydrogen-water 
chemistry adjustment in mitigating irradiation assisted stress corrosion 
cracking (IASSC) as well as IGSCC. Within the reactor core, the monitoring 
electrode can be mounted, for example, with otherwise unemployed or in 
tandem with the traveling instrumentation probe (TIP) of available local 
power range monitors (LPRM) and the like. The monitors are located in 
severe, high temperature and radiation (typically 10.sup.9 R (rads) per 
hour gamma, 10.sup.13 R per hour neutron) environments. Probe structures 
of earlier designs are completely inadequate for this reactor core 
environment, both from a material standpoint and with respect to the 
critical need to prevent leakage of radioactive materials to the 
environment outside of the reactor vessel. One probe, however, that has a 
robust structure adequate for use in the rigorous environment of the 
reactor core of a nuclear power facility is disclosed in commonly-assigned 
U.S. Ser. No. 07/345,740, filed May 1, 1989, now U.S. Pat. No. 4,948,492. 
A critical feature of such probe design is the alumina (sapphire) post and 
post cap which are located inside of the crucible. Because of space 
limitations, the manufacturing processes which are performed inside the 
crucible, such as metalizing and brazing, cannot be controlled as well as 
the manufacturer would like. Inadequate metalizing and brazing at this 
interface often are not detected until the electrode fails prematurely 
either during final testing or in the field. 
One of the most critical aspects of monitoring and controlling the 
electrochemical potential is during the introduction of hydrogen to the 
reactor. Therefore, the development of reference electrodes that can 
withstand the extreme radiation and the harsh high temperatures, high 
pressure aqueous environment and provide electrochemical potentials are 
directly determined by the introduction of the hydrogen, can provide 
process and operational control leading to elimination of IASCC in the 
core region and, if installed in the piping, control of the IGSCC in the 
reactor recirculation system. 
BROAD STATEMENT OF THE INVENTION 
The present invention is addressed to an electrode for evaluating 
electrochemical potentials which has a robust structure particularly 
suiting it for employment within the rigorous environment of the reactor 
core of a nuclear power facility. 
The electrode of the present invention is comprised of five major segments: 
a metal cap electrode, an elongated annular metal jacket, an alumina 
retainer (i.e. an insulator), an annular transition sleeve, and a 
positioning and signal transfer assembly. The metal cap electrode has a 
tip and annulus extending therefrom formed from sidewalls having an 
interior surface. The elongated annular metal jacket has an annular 
retainer securing mouth at one end and an outlet at its opposite end. The 
alumina retainer has a recessed cap securing portion nestably secured to a 
portion of said cap electrode interior sidewall surface, and an 
oppositely-disposed recessed jacket securing portion nestably secured to 
the interior of said annular jacket retainer securing mouth. The retainer 
further has an access channel penetrating therethrough from said cap to 
said jacket securing portion. A first insulated electrical conductor in 
electrical connection with the cap electrode extends through the retainer 
access channel and through the annular metal jacket to the jacket outlet. 
Finally, the positioning and signal transfer assembly is associated with 
the jacket outlet for providing support for said jacket and for conveying 
electrical signals from said conductor. An elongated annular metal 
transition sleeve is interposed between said jacket and said positioning 
and signal transfer assembly. 
Advantages of the present invention include a probe structure adapted to 
operate under the rigorous environment of the reactor core of a nuclear 
power facility. Another advantage is the ceramic/metal construction of the 
electrode for providing a sealing architecture that has multiple seals to 
prevent leakage of radioactive materials to the ambient environment of the 
reactor. These and other advantages will be readily apparent to those 
skilled in the art based upon the disclosure contained herein.

DETAILED DESCRIPTION OF THE INVENTION 
While having utility in a broad variety of industrial monitoring functions, 
the electrode structure of the instant invention finds particular utility 
operating under the rigorous environment of the reactor core of a nuclear 
power facility. No elastomeric seals or polymeric components are present 
in its structure which incorporates a sealing architecture of the highest 
integrity. In the latter regard, a brazed and welded assembly consisting 
only of ceramic and metal parts forms the structure of the device. The 
electrode finds employment either as a standard or reference electrode, or 
as a sensing electrode depending upon the material used in forming the 
active electrode area. For a detailed discussion in connection with the 
above, reference is made to Physical Chemistry, by G. W. Castellan, 
Chapter 17, "Equilibria in Electrochemical Cells", pp 344-382, 
Addision-Wesley Publishing Co., Reading, Mass. (1964). 
Referring to FIG. 1, the structure of the electrode probe of the present 
invention is seen to be comprised of five principal components: metal cap 
electrode 10; alumina retainer 12; annular metal jacket 14; annular 
transition sleeve 36; and positioning and signal transfer assembly 16. 
Electrical signals are transferred from metal cap electrode 10 through 
positioning and signal transfer assembly 16 to the outside via electrical 
conductor 18. 
Referring to the various components in more detail, metal cap electrode 10 
can be seen to be formed of tip portion 20 and annulus 22 that extends 
therefrom and which defines a cavity having interior surface 24. Materials 
of construction for metal cap electrode 10 will determine the function of 
the electrode device of the present invention. For typically-encountered 
boiling water reactor (BWR) applications, use of stainless steel in 
constructing metal cap electrode 10 enables the electrode probe to measure 
the ECP of stainless steel in any given environment. Metal cap electrodes 
fabricated from other materials could be used to form similar sensing 
electrodes for measurement of ECPs of different metals. The second general 
category for the electrode device of the present invention involves the 
use of platinum in fabricating metal cap electrode 10. Such an electrode 
device in HWC environments enables the use of the electrode device as a 
reference electrode (provided the hydrogen concentration is known) or it 
can be used to calibrate other reference electrodes (e.g. an Ag/AgCl 
reference electrode). Thus, it will be seen that the architecture of the 
electrode probe of the present invention provides design flexibility 
enabling it to be adapted to function both as a sensing electrode as well 
as a reference electrode, while retaining the same overall construction 
advantages. 
In order to provide electrical isolation of metal cap electrode 10 from 
other metal components forming the electrode probe, alumina retainer 12 is 
used to support metal cap electrode 10. Alumina retainer 12 desirably is 
formed of sapphire, which is a single crystal form of alumina. Sapphire 
material not only provides requisite electrical insulation, but also, by 
virtue of its single crystal structure, is highly resistant to attack by 
water within which it is immersed and, importantly, it exhibits no grain 
boundaries. High purity alumina, ruby, or other materials, of course, can 
be used as those skilled in the art will appreciate. Alumina retainer 12 
is seen to be formed having recessed jacket securing portion 26 and 
oppositely-disposed recessed cap securing portion 28. Cap securing portion 
28 is nestably disposed within metal cap electrode 10 and in sealing 
engagement with cavity interior surface 24. Advantageously, interior 
surface 24 of annulus 22 is brazed to retainer cap securing portion 28, 
e.g. by use of silver braze. In this regard, it will be appreciated that 
all ceramic surfaces to be brazed are metallized, e.g. with tungsten and 
plated with nickel or platinum in order to ensure adequate wetting of the 
surfaces to be attached by the braze filler metal or alloy. In fact, use 
of multiple layers of metal coating, especially on interior surface 24 of 
recessed cap securing portion 28 of alumina retainer 12, can be practiced 
as is necessary, desirable, or convenient in conventional fashion. The 
braze seal between metal cap electrode 10 and alumina retainer 12 should 
provide a hermetic seal for ensuring integrity of the electrode probe 
structure and to ensure against leakage of radiation to the outside 
environment. To this end, the attachment regions of retainer 12 desirably 
are painted with tungsten oxide paint, fired, and then nickel or platinum 
plated. 
Recessed jacket securing region 26 of alumina retainer 12 is nestably 
disposed within annular metal jacket 14. Again, surface metallization and 
brazing with silver braze or the like is practiced for joining attachment 
region 26 to jacket 14. Retainer 12 also has access channel 30 which runs 
its extent from cap securing portion 28 to jacket securing portion 26. 
Again, it will be appreciated that a hermetic seal needs to be formed. 
Jacket-securing portion 26, then, preferably is metallized, fired, and 
plated. 
Annular metal jacket 14 has alumina retainer region 32 for joining with 
retainer jacket attachment portion 26. In the construction architecture 
depicted at FIG. 1, retainer 12 is formed to have land 34 against which 
jacket 14 rests. Retainer 12 also is formed to have land 35 against which 
cap 10 rests. It should be observed that the dimensional tolerances for 
all components to be joined is such that snug interengagement results, 
thus minimizing the volume to be filled by the braze metal used in joining 
the various components forming the electrode device of the present 
invention. 
Jacket 14 can be formed of nickel or similar metal. Alternatively, it can 
be formed of a Kovar material or a similar corrosion resistant alloy for 
improving the matching of coefficients of thermal expansion between 
retainer 12 and jacket 14. Kovar materials are a group of alloys having a 
characteristic of expansion making it compatible with that of the alumina 
materials of retainer 12. One representative Kovar material comprises Fe 
53.8%, Ni 29%, Co 17%, and Mn 0.2% (Hackh's Chemical Dictionary, Fourth 
Edition, page 374, McGraw-Hill, Inc., 1969). Heretofore, this group of 
alloys were employed in radio tube and thermostat construction where 
bonding of glass was required. Kovar alloys have been known for quite some 
time. Broadly, they contain from 17-18% cobalt, 28-29% nickel, with the 
balance being mostly iron. Their ductility and lack of embrittlement under 
conditions of ordinary use including heating and annealing make them 
useful, such as in sealing glasses, as further expounded upon by Kohn in 
Electron Tubes, pp 448 et seq. Some advantage can be achieved by the use 
of another material (Alloy 42) which has a similar coefficient of 
expansion as Kovar, but contains no added cobalt. 
In the design indicated in FIG. 1, the lower end of annular metal jacket 14 
is attached to annular transition sleeve 36 at juncture 38 by use of 
tungsten inert gas (TIG) welding techniques. Again, a hermetic seal needs 
to result when joining jacket 14 to sleeve 36. Annular transition sleeve 
36 is seen to terminate with outlet 40 where it is attached to coaxial 
cable assembly 42, such as by TIG welding. Conducting wire 46 of coaxial 
cable assembly 42 passes through cable insulation 48 for attachment to 
electrical conductor 18 via welded foil 52. Foil 52 which joins electrical 
conductor wire 18 and conducting wire 46, provides sufficient slack to 
minimize manufacturing and operational stresses in the wiring which could 
result in loss of electrical continuity. Electrical conductor 18 is 
insulated from annular transition sleeve 36 and annular metal jacket 14 by 
ceramic insulator 54A. 
The main difference between designs depicted in FIGS. 1 and 2 are the 
specific manufacturing arrangement of the positioning and signal transfer 
assembly 16 and the application of a bell shaped annular metal jacket 14. 
In FIG. 1, the positioning and signal transfer assembly 16 consists of 
conducting wire 46, cable insulation 48, coaxial cable assembly 42, and 
the lower portion of the annular transition sleeve 36, including outlet 
40. The FIG. 2 positioning and signal transfer assembly 16 consists of the 
same parts defined in FIG. 1 as well as two additional parts, a seal 
assembly and a cable adapter. Seal assembly 68 manufactured by 
GE/Reuter-Stokes of Twinsburg, Ohio, consists of a ceramic to metal seal 
to the coaxial cable, and a ceramic insulator, and a nickel tube sealed to 
the ceramic insulator. The cable adapter 44 is needed to increase the 
effective coaxial cable outside diameter when the bell shaped annular 
metal jacket 14 is used with the larger outside diameter annular 
transition sleeve 36. 
In the alternative design depicted at FIG. 2, the lower end of transition 
sleeve 36 is seen to terminate at outlet 40, where it is attached to the 
stainless steel cable adapter 44, such as by TIG welding. Cable adapter 44 
is seen to terminate at outlet 50 where it is attached to coaxial cable 
assembly 48 and seal assembly 68 where it terminates by welding to the 
inside of the distal end of the nickel tube of the seal assembly. The 
outside of the nickel tube of the seal assembly 68 is attached to the 
electrical conductor wire 18 via welded foil 52. Electrical conductor wire 
18 is insulated from annular transition sleeve 36 by ceramic "T" insulator 
54B. 
As an alternate to the designs shown in FIGS. 1 and 2, a third alternative 
is possible. By elimination of cable adaptor 44 in FIG. 2, but maintaining 
the seal assembly 68, it is possible to achieve an entire assembly without 
a break in the O.D. dimension similar to the O.D. dimensions in FIG. 1. In 
this third alternative, the "bell-shaped" annular metal jacket 14 is 
replaced by a constant O.D. annular metal jacket. Conductor 18 suitably 
can be made from nickel, Kovar, platinum, or other material which is 
electrically conductive. While an electrical conductor can be insulated 
directly, the preferred structures depicted at FIGS. 1 and 2 show annular 
electrical insulator 54A disposed within annular jacket 14 and annular 
sleeve 36. Electrical insulator 54A preferably is made from a ceramic 
material, such as alumina, in order to ensure electrical isolation of 
electrical conductor 18. While the proximal end of electrical conductor 18 
is electrically connected to assembly 16, via foil 52, the distal end of 
electrical conductor 18 passes through the annulus formed within jacket 14 
and sleeve 36, thence though access channel 30 provided in retainer 12 to 
cap electrode holder 10A. Conductor 18 terminates as a nail lead and is 
welded or brazed directly to the interior side of tip portion 20 of cap 
electrode 10. 
The alternative cap arrangement is depicted in FIG. 3. It will be observed 
that tip portion 10 is formed to have elongated threaded section 56 to 
which electrode cap extension 58 is screwed. It will be observed that 
electrode cap extension 58 is formed as having tip portion 60 and annulus 
62 which defines cavity 64. Interior surface 66 of annulus 62 is threaded 
for matching the threads of section 56. Conductor 18 then penetrates into 
cap electrode holder 10A for providing electrical connection thereto. 
Electrode cap 58 is formed from platinum or other suitable reference 
electrode metal and cap electrode holder 10A is formed from Kovar plated 
with platinum. The alternate cap arrangement can provide an advantage by 
way of the Kovar cap electrode holder 10A, which provides a better match 
of expansion coefficients with the sapphire or alumina retainer 12, than 
would platinum directly. The platinum electrode cap extension 58 screwed 
to interior surface 66 provides a sufficient surface area to obviate any 
holidays in the platinum plated Kovar cap electrode holder 10A. 
With respect to performance specifications of the inventive electrode 
probe, the probe is designed to operate at temperatures ranging up to 
600.degree. F. and pressures of up to about 2,000 psi. When metal cap 
electrode 10 is formed of platinum for producing the reference electrode 
device, the novel electrode device exhibits a voltage that is within 
.+-.0.020 volts of the theoretical value for the platinum reference 
electrode. In use as a reference electrode platinum plated cap, the 
inventive electrode probe is capable of measuring ECPs to within .+-.0.010 
volts in constant water chemistry. In attaching cap 10 manufactured of 
Kovar and platinum plated, it should be silver brazed to W/Ni or W/Pt 
coated insulator securing portion 62. 
Referring to FIG. 3, five sensing electrode probes were fabricated in 
accordance with the precepts of the invention utilizing stainless steel 
for metal cap electrode 10 and these probes subjected to laboratory 
testing utilizing a standard Cu/Cu.sub.2 O/ZrO.sub.2 reference electrode. 
The aqueous medium for testing was provided by an autoclave within which 
temperature and water chemistry were controlled. The test was carried out 
at a water temperature of 274.degree. C. and in conjunction with a 
sequence of aqueous conditions wherein certain dissolved gases were 
introduced. A first such dissolved gas was hydrogen, as labeled along the 
elapsed time portion of the figure as represented at 70, and represents 
hydrogen water chemistry. Thereafter, as labeled along the elapsed time 
portion of the figure as represented at 72, oxygen was injected into the 
aqueous medium, thus subjecting the probes to normal boiling water 
chemistry. Finally, additional hydrogen was injected as represented at 74. 
As the potential of the reference electrode can be calculated, its 
potential under the various water conditions can be subtracted from the 
voltage obtained, thus enabling a measurement of the ECP of the stainless 
steel electrode probes. The results of the five probes evaluated are 
represented in FIG. 4 and can be seen to be very close in value. It will 
be observed that a shift in the ECP results by virtue of the water 
chemistry involved. It is this shift that is monitored during use of the 
sensing electrode probes for determining the water chemistry of the 
aqueous medium being tested. The expected shift in ECP can be seen by 
reference to FIG. 4. 
Since certain changes may be made in the above-described apparatus without 
departing from the scope of the invention, the description and 
accompanying drawings shall be interpreted as illustrative and not in a 
limiting sense in accordance with the precepts of the invention disclosed 
herein.