Patent Number: 051223309
Section: summary

This invention relates to an apparatus and method for monitoring corrosion to members in the core of a nuclear reactor. This invention further relates to an apparatus and method for detecting general and nodular corrosion occurring to nuclear reactor fuel cladding, i.e., the material used to contain the fuel in a shape such as a rod or plate-like shape. BACKGROUND OF THE INVENTION Nuclear reactors are used today in numerous ways, e.g., to generate electric power and for special tests. Nuclear power plants employ a nuclear reactor. The fuel is a radioactive material which undergoes a fission reaction. Nuclear fission is the splitting of the atomic nucleus, usually into two or three larger particles, two or more neutrons, and other smaller particles with the release of a relatively large amount of energy. The average amount of energy released in the various fission reactions is about 200 M.sub.e V (million electron volts), which is dramatically higher than the energy produced from a chemical reaction such as oxidation which is used in fossil-fuel plants when coal or oil is burned. During nuclear fission in energy production, an exceedingly large amount of energy is released and excess neutrons are produced which permits a chain reaction. These factors make it possible to design nuclear reactors which are self-sustaining, i.e., wherein reactions occur with the continuous release of energy. Some current light-water cooled nuclear power plants utilize uranium oxide as a fuel. Solid cylindrical rods, instead of plates, are the most common shape for the fuel. For most reactor applications, these fuel rods are protected by a cladding of aluminum, stainless steel, zirconium or zirconium alloy and are assembled into a unit. Other typical constituents of reactors include aluminum-clad uranium metal for plutonium production reactors, stainless steel-clad UO.sub.2 dispersed in stainless steel for propulsion reactors, and zirconium or stainless steel-clad UO.sub.2 pellets for central station power reactors. Zirconium is used in nuclear technology because of its properties which include insolubility in water and cold acids, corrosion resistance, low neutron absorption, and low toxicity. In addition to these desirable properties, zirconium also exhibits good strengths at high temperatures, corrosion resistance to high velocity coolants, avoidance of formation of highly radioactive isotopes, and resistance to mechanical damage from neutron radiation. An important application for zirconium is as the base metal in an alloy known as Zircaloy II, comprised of 1.5% tin, 0.35% iron-chromium-nickel, 0.15% oxygen, and the balance zirconium. This alloy is widely used in water cooled nuclear reactors because of its excellent corrosion resistance up to about 350 degrees in H.sub.2 O, and its low neutron cross-section. The term "Zircaloy" is a trademark for alloys of zirconium and nickel which are used as cladding for nuclear fuel elements and for other reactor applications. "Zircaloy II" is a particular Zircaloy. Even though corrosion-resistant alloys are used to clad the fuel, radiation damage and corrosion can still occur to these alloys. Corrosion is usually the destruction, degradation or deterioration of material due to the reaction between the material and its environment. In a restricted sense corrosion consists of the slow chemical and electrochemical reactions between a metal and its environment; in a broader sense corrosion is the slow destruction of materials by chemical agents and electrochemical reactions. The severe environment in the core of a boiling water nuclear reactor can include temperatures of 290.degree. C. or greater, pressures of 1,000 psi or greater, and radiation of 10.sup.9 rads per hour gamma and 10.sup.13 rads per hour neutron. The Zircaloy alloys are among the best corrosion resistant materials when tested in water at reactor operating temperatures, about 290.degree. C., but without exposure to nuclear radiation. The corrosion rate under these conditions is very low and the corrosion product is a uniform, tightly adherent, black ZrO.sub.2 film. In actual service, however, the Zircaloy is irradiated and is also exposed to radiolysis products present in reactor water. The corrosion resistance properties of Zircaloy deteriorate under these conditions and the corrosion rate thereof is accelerated. The deterioration under actual reactor conditions of the corrosion resistance properties of Zircaloy is not manifested in merely an increased uniform rate of corrosion. Rather, in addition to the black ZrO.sub.2 layer formed, a localized, or nodular corrosion phenomenon has been observed especially in boiling water reactors. In addition to producing an accelerated rate of corrosion, the corrosion product of the nodular corrosion reaction is a highly undesirable white ZrO.sub.2 bloom which is less adherent and lower in density than the uniform corrosion product of black ZrO.sub.2. The increased rate of corrosion caused by the nodular corrosion reaction will be likely to shorten the service life of the tube cladding, and also this nodular corrosion will have a detrimental effect on the efficient operation of the reactor. The white ZrO.sub.2, being less adherent, may be prone to spalling or flaking away from the tube into the reactor water. On the other hand, if the nodular corrosion product does not spall away, a decrease in heat transfer efficiency through the tube into the water is created when the nodular corrosion proliferates and the less dense white ZrO.sub.2 covers all or a large portion of a tube. It is, therefore, desirable to be able to detect or monitor the corrosion, e.g., nodular corrosion or general corrosion, to the fuel cladding and other members in a reactor core. It is, therefore, an object of this invention to provide an apparatus and method of monitoring corrosion to members such as nuclear fuel cladding in the core of a nuclear reactor. It is another object of this invention to provide such an apparatus and method for monitoring of corrosion to such members by measuring potential changes in a sensor located in the nuclear reactor core. Furthermore, it is an object of this invention to monitor corrosion on a scored surface area on the sensor, e.g., to monitor nodular corrosion that is promoted by such scoring. It is also an object of this invention to provide such an apparatus and a method employing the apparatus wherein the sensor is comprised preferably of zirconium, and even more preferably of a zirconium alloy or "Zircaloy" alloy. It is even further preferred that the zirconium, zirconium alloy or Zircaloy alloy be from the same lot as that used to form, for example, the fuel cladding. SUMMARY OF THE INVENTION The method and apparatus of this invention provides means for monitoring corrosion to members in the core of a nuclear reactor, by measuring electrical potential changes in a sensor located in the reactor core. Nuclear radiation causes increased atom motion and metallurgical changes such as formation of vacancies, precipitates, or recrystallization that causes increased resistivity in the sensor, and accounts for some of the potential change in the sensor. Therefore, the potential change from radiation induced changes in resistivity is separately measured in the sensor and factored out to give the potential change resulting from loss of cross-section due to corrosion of the sensor. The corrosion measured includes general corrosion and, in particular, nodular corrosion of fuel cladding. In the method of the invention, corrosion on a member in a nuclear reactor core is estimated by placing a sensor means in close proximity to the member, and in contact with the coolant water circulating in the reactor core. The sensor means is comprised of a generally cylindrical section having an outer surface that is subject to corrosion and radiation, and has a cross-sectional area, A.sub.1. The sensor means additionally has a reference section subjected to radiation but not to corrosion, and having a cross-sectional area, A.sub.2. At least one pair of first probes, separated by a length, L.sub.1, is placed in electrical contact with the cylindrical section. At least one pair of second probes separated by a length, L.sub.2, is placed in electrical contact with the reference section. A current is passed throughout the sensor means to produce a potential gradient in the cylindrical section and reference section. The change in potential in the reference section, and the cylindrical section is measured and used to calculate the subsequent cross-sectional area, A.sub.i, in the cylindrical section between the first probes. Preferably, the cylindrical section and reference section are made from the same material as the member. Most preferably, the cylindrical section and reference section also have the same crystalline texture as the member. The apparatus of this invention has a sensor means comprised of a generally cylindrical section and a reference section. The cylindrical section has sidewall means extending from a base to define an annular channel therein. An inner surface of the cylindrical section faces the annular channel and an oppositely facing outer surface is subjected to the core environment. Preferably, the outer surface has a scored area for promoting nodular corrosion, and a smooth area. The reference section is positioned within and attached to the base so that a current can be passed throughout the sensor means, the sensor means being sealed so that the reference section is not exposed to the water in the nuclear reactor core. The sensor means is preferably comprised of the same material as the member on which corrosion is being estimated, for example, for fuel cladding members zirconium, zirconium alloy, or Zircaloy, or more preferably Zircaloy II, and most preferably the same lot of material as that used to form the cladding is used to form the sensor means. Both the cylindrical section and reference section have current leads and potential probes connected thereto, from within the apparatus. A constant electric current, preferably a direct current having a periodically reversed polarity, is passed throughout the cylindrical section and reference section by way of the leads in electrical connection therewith from within the apparatus. The potential probes are used to measure the potential of the current passing through the cylindrical section and reference section. The potential measured in the cylindrical section corresponds to the subsequent cross-section, or remaining wall thickness of the cylindrical section due to corrosion of the outer surface, and to radiation induced changes in resistivity. By factoring out the potential change due to radiation, as measured in the reference section, the potential change from corrosion alone can be determined in the cylindrical section, and the change in cross-section, or thickness, of the cylindrical section due to corrosion can be calculated from the potential measurements. A more particular embodiment of the apparatus of this invention provides an apparatus for monitoring corrosion of nuclear reactor fuel cladding, comprising: a generally cylindrical sensor member comprised of sidewall means extending from a first end to a second end, the sidewall means extending to a first access opening at the first end and to a second access opening at the second end, the sidewall means having an outer surface, an interior, and a first annular channel extending in the interior from the first end to the second end, and, the outer surface having at least one scored region; a first generally circular end cap means sealably positioned over or attached to the first access opening in relationship therewith so that the first cap retains the first annular channel substantially moisture-free; an elongated rod located inside the first annular channel and attached to the first end cap; a generally cylindrical sleeve means formed of a select metal and comprises of sleeve sidewall means extending from a first sleeve end to a second sleeve end, the sleeve sidewall means extending to a first sleeve access opening at the first sleeve end, to a second sleeve access opening at the second sleeve end, and, the sleeve sidewall means having an interior and a second annular channel extending in the interior from the first sleeve end to the second sleeve end; a generally cylindrical transition member of a select member compatible with both of the sensor member and the sleeve, the transition member comprised of transition sidewall means extending from a first transition end to a second transition end, the transition side wall means extending to a first transition access opening at the first transition end, to a second transition access opening at the second transition end, and, the transition sidewall means having an interior and a third annular channel extending in the interior from the first transition end to the second transition end; the transition member at the first transition end being sealably connected to the second end of the sensor member and the transition member at the second transition end being sealably connected to the first sleeve end of the sleeve means so that the first, second and third annular channels are generally concentric and contiguous, and so that the first, second and third annular channels are retained substantially moisture-free; a first lead in electrical contact with the interior of the sensor member, and a second lead in electrical contact with the rod, the first and second leads insulatively extending from the interior of the sensor member and rod through the first, second and third channels; at least two pairs of probes, a first pair in electrical contact with the interior of the sensor member, and a second pair in electrical contact with the rod for detecting electrical potential, the probes insulatively extending from the interior of the sensor member and rod through the first, second and third channels; and, a second end cap sealably attached to the second sleeve end of the sleeve so that the first, second and third channels are retained substantially moisture free, the second end cap means further comprising means defining at least one opening through which the probes and the first and second leads pass in a moisture-proof seal. Other objects of the invention will, in part, be obvious and will, in part appear hereinafter. The invention, accordingly, comprises the apparatus possessing the construction, combination of elements, and arrangements of parts which are exemplified in the following detailed disclosure; and, method employing the same. For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in connection with the accompanying drawings.