SE440962B - NUCLEAR FUEL ELEMENT WITH IMPROVED RESISTANCE TO VOLTAGE CORROSION CRACKING ON THE COVER - Google Patents

Description

ïßa fi ißiw M g 2 refrigerant and moderator are available. Common encapsulation materials are stainless steel, aluminum and aluminum alloys, zirconium and zirconium alloys, niobium, certain magnesium alloys and other materials. Destruction of the enclosure, ie. loss of leakage tightness, can contaminate the refrigerant or moderator and associated systems with radioactive long-lived products to such an extent that the operation of the plant is disturbed.

Problems have arisen in the manufacture and operation of nuclear fuel elements which utilize certain metals and alloys as encapsulating materials due to mechanical or chemical reactions which take place with these encapsulating materials under certain conditions. Zirconium and zirconium alloys are very good nuclear fuel enclosures under normal conditions, as they have a low neutron absorption cross section and at temperatures below about 398 ° C are strong, ductile, extremely stable and non-reactive in the presence of demineralized water or water vapor, which is commonly used. such as coolant and moderator in reactors.

However, the use of fuel elements has shown problems with brittle fracture of the enclosure due to the combined action between the nuclear fuel, the enclosure and fission products formed during the fission reactions. It has been found that this undesired behavior is due to localized mechanical stresses on the fuel housing due to different sizes of surface expansion and friction between the fuel and the housing. Fission products are formed in the nuclear fuel by the fission chain reaction during operation of the nuclear reactor and these fission products are released from the nuclear fuel and are present at the surface of the enclosure. These localized stresses and strains in the presence of certain fission products, such as iodine and cadmium, can cause damage to the enclosure by stress corrosion cracking or embrittlement with molten metal.

In the cavities of a closed fuel element, hydrogen can be generated by slow reaction between the enclosure and residual water inside the enclosure and this hydrogen can be built up to 1351 essence 3 a content which under certain conditions can cause localized hydration of the enclosure with consequent localized destruction of the mechanical properties of the enclosure. The enclosure can also be adversely affected by such gases as oxygen, nitrogen, carbon monoxide and carbon dioxide within a wide temperature range.

The zirconium enclosure for a nuclear fuel element is exposed to one or more of the gases mentioned above and the fission products during the irradiation in the nuclear reactor and this takes place despite the fact that these gases may not be present in the reactor coolant or moderator and may have been excluded from the surrounding atmosphere during the manufacture of the housing and the fuel element.

Sintered refractory and ceramic compositions, such as uranium dioxide and other compositions used as nuclear fuel, emit measurable amounts of the aforementioned gases upon heating, for example during the manufacture of the fuel element, and further emit fission products during nuclear fission chain reactions. Particulate refractory and ceramic compositions, such as uranium dioxide powders and other powders used as nuclear fuel, have been found to release even greater amounts of the aforementioned gases during irradiation. These released gases can react with the zirconium enclosure, which encloses the nuclear fuel.

In view of the above, it has been found desirable to minimize the attack on the enclosure of water, water vapor and gases, in particular hydrogen, which are reactive with respect to the enclosure, from inside the fuel element throughout the time the fuel element is used in the operation of the nuclear power plant. installations. For example, attempts have been made to find materials which react chemically rapidly with water, water vapor and other gases and eliminate these from the interior of the enclosure, and such materials are termed "getter materials".

Another attempt has been to coat the nuclear fuel material with different types of material. Nuclear fuel is coated with a ceramic material to prevent moisture from coming into contact with the nuclear fuel material, as disclosed in U.S. Pat. No. 3,109. 39............................................... U.S. Pat. No. 3,085,059 discloses a nuclear fuel element comprising a metal casing containing one or more pellets or pellets of fissile ceramic material and a layer of refractory or glassy material bonded to the ceramic pellet so that the layer is present between the casing and the nuclear fuel to ensure uniform goodness. heat conduction from the pellets to the casing. U.S. Pat. No. 2,873,238 discloses sheathed fissile uranium sheaths enclosed or encapsulated in a metal housing in which the protective sheaths comprise a zinc-aluminum bonding layer. U.S. Pat. No. 2,849,387 discloses an encapsulated fissile body comprising a plurality of open-ended sheathed body sections of nuclear fuel dipped in a melt of a binder to provide effective heat conduction bonding between the uranium body sections and the container (or enclosure). The coating is stated as consisting of any metal alloy with good thermal conductivity, with examples including aluminum-silicon and zinc-aluminum alloys. Japanese Patent Publication No. SHO 47-46559 discloses consolidation of separate nuclear fuel particles in a carbonaceous matrix fuel composite product by coating the fuel particles with a high density, even, carbonaceous coating around the pellets. Yet another coating is disclosed in Japanese Patent Publication No. SHO 47-14200, in which one of two groups of pellets is coated with a layer of silicon carbide and the other group is coated with a layer of pyrocarbon and metal carbide.

The coating of nuclear fuel materials causes reliability problems in that it is difficult to achieve uniform coatings that are free from fault locations. Furthermore, damage to the coating can cause problems with the long-term properties of the nuclear fuel material.

A method has been described for preventing corrosion of nuclear fuel enclosures consisting of the addition of a metal such as niobium to the fuel. The additive may be in the form of a powder, provided that subsequent treatment of the fuel does not cause oxidation of the metal, or the additive may be incorporated into the fuel element such as wire, sheet metal or other form, or may be applied around or between the fuel pellets.

Publication GEAP-4555, February 1974, discloses a composite casing of zirconium alloy having a stainless steel inner liner which is metallurgically bonded to the zirconium alloy, and the composite casing is made by extruding a hollow zirconium alloy hollow blank having an inner cantilever alloy. steel. This enclosure has the disadvantage that the stainless steel develops brittle phases and the stainless steel layer entails neutron absorption losses of 10 - 15 times the losses of a zirconium alloy layer of the same thickness.

U.S. Pat. No. 3,502,549 discloses a process for protecting zirconium and zirconium alloys by electrolytic application of chromium to obtain a composite material suitable for nuclear reactors. A process for the electrolytic application of copper to surfaces of "Zircaloy-2" and subsequent heat treatment to effect diffusion of the electrolytically applied metal is set forth in Energia Nucleare, Volume II, No. 9 (September 1964) on pages 505-508. "Stability and Compatibility of Hydrogen Barriers Applied to Zirconium Alloys" by F. Brossa et al (European Atomic Energy Community, Joint Nuclear Research Center, EUR 4098e, 1969) describes methods for applying different types of coatings and their activity as hydrogen diffusion barrier layers together. with an Al-Si coating as the most promising barrier layer against hydrogen diffusion. Methods for electroplating nickel on zirconium and zirconium-tin alloys as well as heat treatment of these articles to obtain alloy diffusion bonds are given in "Electroplating on Zirconium and Zirconium-Tin" by W.C. Schickner et al (BMI-757, Technical Information Service, 1952). In the American mQuw fi ... n.-a-fun-w flfifl ~ 'fl v ~ * ”” *' 'fw-a »w fa Aff? * IWF than two LhL-šs-Äååu I. _ w- 6 patent 3,625,821 describes a fuel element for nuclear reactors with fuel encapsulation tubes with the inner surface of the tube coated with a metal with a low neutron capture cross section, for example nickel, and with atomized particles of a combustible toxic substance therein. "Reactor Development Program Progress Report", August 1973 (ANL-RDP-19) states an arrangement of chemical getter material with a sacrificial layer of chromium on the inner surface of a stainless steel housing.

There are other patents describing coatings on the inner surface of the housing of a nuclear fuel element. U.S. Pat. No. 3,145,151 discloses a fuel element comprising a hollow, closed pressure vessel containing a metal hydride which loosely holds a core of fissile material and a thin, corrosion resistant metal enclosing the pressure vessel. U.S. Pat. No. 3,053,743 discloses a fuel element having a metal cladding tube coated on the inner wall with metallic nickel or a nickel-chromium-iron alloy and this tube surrounds a core of nuclear fuel pellets with temporarily spaced spacers between the pellets. British Patent Specification 933,500 describes a nuclear fuel element with a deformed cross-section, in which individual particles of fuel are coated on the surface with one or more materials and enclosed in a housing member and subjected to a deformation process to reduce the cross-section of the element.

Another method has been to provide a separate barrier layer between the nuclear fuel material and the enclosure containing the nuclear fuel material, as disclosed in U.S. Pat. No. 3,230,150 (copper foil), German DAS 1,238,115 (titanium barrier layer), and U.S. Pat. No. 3,212. 988 (jacket of zirconium, aluminum or beryllium), U.S. Pat. No. 3,018,238 (barrier layer of crystalline carbon between U02 and zirconium enclosure) and U.S. Pat. No. 3,088,893 (stainless steel foil).

The use of barrier layers has yielded promising results, but some of the publications cited above include the use of incompatible materials, an- »w fi vß fl” ““ '° "5 fšášz: QUALITY, 7.81 02.62 * 1 In the matter of nuclear fuel (e.g. carbon that can be combined with oxygen from the nuclear fuel) or the enclosure (for example, certain metals that can react with the enclosure and change the properties of the enclosure) or with the nuclear fission reaction (for example, by acting as a neutron-absorbing material).

Further attempts to utilize barrier layers are disclosed in U.S. Pat. No. 3,969,186 (refractory metal, for example molybdenum, tungsten, rhenium, niobium and their alloys in the form of a tube or foil such as simple or elaborate layers between the enclosure and the fuel) and the U.S. Pat. No. 3,925,151 (foil of zirconium, niobium or alloys of these substances between the fuel and the housing with a coating of a highly lubricating material on either the liner or the housing).

The present invention relates to a particularly efficient nuclear fuel element for use in the core of a nuclear reactor with improved resistance to stress corrosion cracking of the enclosure due to the action between the fuel material and the enclosure and with improved resistance to destruction of the fuel element by the action of water or water vapor in the fuel element. comprises an elongate composite enclosure container with a zirconium alloy tube containing non-zirconium constituents in excess of 5000 million parts and a barrier layer of zirconium metal, a central core of a body of nuclear fuel material consisting of compounds of uranium, plutonium, torvium or torvium; arranged in and partially filling the container with leaving a gap between the container and the core, as well as an internal cavity at one end of the container and closing means which close both ends of the container. The nuclear fuel element is characterized in that the barrier layer of zirconium metal has an impurity content of at least about 1000 million parts but is less than 5000 million parts and is metallurgically bonded to the inner surface of the zirconium alloy tube and has a thickness of from 5 to 30% of the composite enclosure container thickness. . 7'f8 ^ ï1 fi $ è6'2 '¥ f1 Due to the degree of purity, the enclosure remains soft during irradiation and minimizes localized elongation inside the nuclear fuel element, thereby protecting the alloy tube from stress corrosion cracking or embrittlement with liquid metal. The alloy pipe part of the enclosure is completely unchanged in terms of construction and function compared to what has previously been used for a nuclear reactor and can consist of conventional enclosure materials, for example zirconium alloys.

Methods of making the composite enclosure are also set forth, including: (1) applying a hollow sleeve of the metal barrier layer within a hollow blank of zirconium alloy, explosion bonding of the sleeve to the blank and extruding the composite product followed by tube reduction, (2) applying of a hollow sleeve of the metal barrier layer inside a hollow blank of zirconium alloy, heating of the sleeve and the blank under compression load so as to obtain diffusion bonding of the tube to the blank and extruding the composite product followed by tube reduction and (3) applying a metal sleeve to a sleeve substance of zirconium alloy and extrusion of the composite product followed by pipe reduction.

The metal barrier layer does not cause any significant neutron capture losses, heat transfer losses or problems with incompatible materials for fuel elements according to the invention. The nuclear fuel element can operate in nuclear reactors for a long period of time without the occurrence of splitting or cracking in the enclosure, corrosion of the enclosure or other problems with destruction of the fuel and so that the bond provides a long-lasting connection between the pipe and the metal barrier layer.

A preferred embodiment of the invention relates to a nuclear fuel element with a composite capsule comprising a metal barrier layer consisting of zirconium sponge. The foregoing and other objects of the invention will be apparent to one skilled in the art upon reference to the details of the description taken in conjunction with the accompanying drawings.

Figure 1 is a partial section through a nuclear fuel unit containing nuclear fuel elements made according to the invention.

Figure 2 is a cross-section on a larger scale through the nuclear fuel element according to Figure 1 and shows features characteristic of the invention.

Figure 1 shows a sub-section through a nuclear fuel assembly 10.

This fuel assembly consists of a tubular flow channel 11 with a substantially square cross section, which at the upper end is provided with a lifting loop 12 and at the lower end with a nose part (not shown due to the lower part of the unit 10 being omitted). The upper end of the channel 11 is open at 13 and the lower end of the nose part is provided with coolant openings. A row of fuel elements or rods 14 are enclosed in the channel 11 and are supported therein by means of an upper end plate 15 and a lower end plate (not shown because the lower part of the device is omitted). The liquid refrigerant ordinarily flows in through the openings in the lower end of the nose part, passes upwards around the fuel elements 14 and exits at the upper outlet 13 in a partially evaporated state for boiler reactors and in a non-evaporated state for high temperature pressure reactors.

The nuclear fuel elements or rods 14 are closed at the ends by means of end plugs 18 welded to the housing 17 and may comprise bolts 19 to facilitate the mounting of the fuel rod in the unit. An empty space or plenum 20 is provided at one end of the element to allow longitudinal expansion of the fuel material and accumulation of gases released from the fuel material. A retaining nozzle 24 for the nuclear fuel material in the form of a helical array must be arranged in the space 20 and provides a retaining force against axial movement of the fuel player, in particular during handling of the fuel and the transport element.

The fuel element is designed and constructed to provide very good thermal contact between the housing and the fuel material, a minimum of parasitic neutron absorption, as well as resistance to bending and vibration which in some cases can be caused by flow of the coolant at high speed.

A nuclear fuel element or rod 14 is shown partially in section in Figure 1 and is constructed in accordance with the invention.

The fuel element comprises a core or central cylindrical part of nuclear fuel material 16, which in this case is shown as a plurality of fuel pellets of fissile and / or fertile material, which are arranged in a structural or supporting enclosure or container 17. In some cases the fuel pellets may have different shapes, for example cylindrical pellets or spheres, and in other cases different fuel shapes, such as particulate fuel, can be used. The physical form of the fuel is irrelevant to the invention. Various types of nuclear fuel materials can be used, including uranium compounds, plutonium compounds, thorium compounds and mixtures thereof. A preferred fuel is uranium dioxide or a mixture comprising uranium dioxide and plutonium dioxide.

Figure 2 shows a nuclear fuel material 16, which forms the central core of a fuel element 14 and is surrounded by a housing 17, which in the following is also called a composite housing. The composite enclosure container encloses the core so that a gap 23 is left between the core and the enclosure container when used in a nuclear reactor. The composite housing comprises a zirconium alloy tube 21, which according to a preferred embodiment of the invention is made of Zircaloy-2. The alloy tube has a metal barrier layer 22 bonded to the inner surface so that the metal barrier layer forms a shield or sheath between the alloy tube 21 and the nuclear fuel material held in the enclosure. The metal barrier layer. -. : v; _ ..,. _________.___. _ _ ..

V -.-_ .., _._ f. ztels1lianraspfza1 ll make up about 1 - 30% of the thickness of the enclosure and consists of a material with low neutron absorption, namely zirconium of moderate purity (for example zirconium sponge). The metal barrier layer 22 protects the alloy tube portion of the enclosure from contact and reaction with gases and fission products and prevents the occurrence of localized stresses and strains.

The composition of the metal barrier layer of zirconium of moderate purity is significant and gives the metal barrier layer special properties. In general, at least about 1000 ppm (parts per million) by weight and preferably less than about 5000 ppm of impurities are present in the material of the metal barrier layer and preferably less than about 4200 ppm. Of these, oxygen is kept in the range of about 200 to about 1200 ppm. Other contaminants are within the normal limits for commercial reactor grade zirconium sponges and are listed below: Aluminum 75 ppm or less, boron 0.4 ppm or less, cadmium 0.4 ppm or less, carbon 270 ppm or less, chromium 200 ppm or less, cobalt 20 ppm or less, copper 50 ppm or less, hafnium 100 ppm or less, hydrogen 25 ppm or less, iron 1500 ppm or less, magnesium 20 ppm or less, manganese 50 ppm or less, molybdenum 50 ppm or less, nickel 70 ppm or less, niob 10 10 ppm or less, nitrogen 80 ppm or less, silicon 120 ppm or less, tin 50 ppm or less, tungsten 100 ppm or less, titanium 50 ppm or less, and uranium 3.5 ppm or less.

The composite housing in nuclear fuel elements according to the invention comprises the metal barrier layer bonded to the substrate with a very strong bond. Metallographic examination shows that the cross-diffusion between the materials in the substrate and the metal barrier layer is sufficient to form a bond, but no cross-diffusion to a considerable extent from the bonding area is present.

It has been found that zirconium sponge metal which forms the metal barrier layer in the composite enclosure is highly resistant to radiation curing and this enables metal barrier- 'fhšñ fi -W4 "" \' I "", 'Y * "" m "ïw» i, _ “_ ',, 1 ,, ....... -. s.

The ll s zaiiâs fi zk-1 12 layer after prolonged irradiation retains desired strength properties, such as yield strength and hardness at levels significantly lower than conventional zirconium alloys. The metal barrier layer does not harden as much as conventional zirconium alloys under the influence of radiation, and this in combination with the initially low yield strength enables the metal barrier layer to be plastically deformed and equalizes or absorbs the fuel pieces induced stresses in the fuel element during power changes. Voltages induced by the fuel pieces in the fuel element can be caused, for example, by swelling of the fuel pieces of the nuclear fuel at the reactor operating temperatures (300 - 350 ° C), so that the fuel comes into contact with the housing.

It has further been found that a metal barrier layer of zirconium sponge having a thickness of about 5-15% of the thickness of the enclosure and in particular a preferred thickness of 10% of the thickness of the enclosure bonded to a zirconium alloy alloy tube provides a voltage reduction and barrier layer effect, which are sufficient to prevent destruction of the composite housing.

Zirconium alloys suitable for alloy pipes include Zircaloy-2 and Zircaloy-4G Zircaloy-2 contains, by weight, about 1.5% tin, 0.12% iron, 0.09% chromium and 0.005% nickel and is used in to a large extent in water-cooled reactors. Zircaloy-4 contains less nickel than Zircaloy-2 but contains slightly more iron than Zircaloy-2.

The composite housing used in nuclear fuel elements according to the invention can be manufactured by any of the following methods.

According to one method, a hollow sleeve of the zirconium sponge to be used as a metal barrier layer is inserted into a hollow blank of the zirconium alloy to be used for the alloy tube, after which the unit is subjected to explosion bonding of the sleeve to the blank. The composite product is extruded at a high temperature of about 538 - 750 ° C using conventionally fine penetration -1 13 tional extrusion methods for pipe jackets. The extruded composite product is then subjected to a process involving conventional pipe reduction until the desired size of the enclosure is reached.

According to another method, a hollow sleeve of the zirconium sponge to be used as a metal barrier layer is inserted into a hollow blank of the zirconium alloy selected for the alloy tube, after which the unit is subjected to a heating step (for example 750 ° C for about 8 hours) for âproduction of diffusion bonding between the jacket and the substance. The composite product is then extruded using conventional tubular sheath extruding methods and the extruded composite product is subjected to a process involving conventional tube reduction until the desired size of the enclosure is reached.

According to a further method, a hollow sheath of the zirconium sponge selected as the metal barrier layer is introduced into a hollow blank of the zirconium alloy selected for the alloy tube and the unit is extruded using conventional tubular sheath extruding techniques. Thereafter, the extruded composite product is subjected to a process involving conventional pipe reduction until the desired size of the enclosure is reached.

The above-mentioned processes for manufacturing the composite enclosure according to the invention provide economic advantages compared with other processes used in the manufacture of the enclosure, for example electroplating or vapor deposition.

A suitable method of making a nuclear fuel element comprises manufacturing a composite enclosure container comprising a metal barrier layer of zirconium sponge bonded to the inner surface of a zirconium alloy tube, the container being open at one end, and filling the composite encapsulation container with a core capping container. * 14 fuel material leaving a gap between the core and the container and leaving a cavity at the open end, inserting means for retaining the nuclear fuel material in the cavity, attaching a closing means at the open end of the container leaving the cavity in connection with the nuclear fuel and then bonding the end of the composite container to the closure means so that a tight closure between these parts is obtained.

The invention offers a number of advantages that benefit long service life of a nuclear fuel element including reduction of chemical reaction or mutual action of the enclosure, minimization of localized stresses on the zirconium alloy pipe portion of the enclosure, minimization of stress corrosion and elongation corrosion of the zirconium the probability of a cracking of the zirconium alloy tube. The invention prevents further expansion (or swelling) of the nuclear fuel into direct contact with the zirconium alloy tube and this prevents the occurrence of localized stresses on the zirconium alloy tube, initiation or acceleration of stress corrosion of the alloy tube and bonding of the alloy fuel to the nuclear fuel.

An important property of the composite enclosure according to the invention is that the above-mentioned improvements are achieved without a significant increase in neutron losses. Such an enclosure is well accepted in nuclear reactors, since the enclosure does not show the formation of a eutectic in the event of failure of cooling or an accident involving the loss of a nuclear control rod. Furthermore, the composite housing has very little heat transfer loss, since there is no thermal barrier layer for heat transfer of the kind obtained if a separate foil or cladding is inserted into a fuel element.

The composite housing of the invention can also be inspected by conventional non-destructive testing at various stages of manufacture and use. As is apparent to those skilled in the art, the invention described can be varied in a variety of ways. , ._ 9% Now] _ _., VL; _, __- - ,, ._._.._ r

Claims (5)

7íâäf-f- fi âí fl d1 / éï PATENTKRAV Nuclear fuel element with improved resistance to stress corrosion cracking of the housing due to the action between the fuel material and the housing and with improved resistance to destruction of the fuel element by the action of water or water vapor in the fuel element, wherein the fuel element comprises an elongate composite. of zirconium alloy containing constituents other than zirconium in excess of 5000 parts per million and a barrier layer of zirconium metal, a central core of a body of nuclear fuel material consisting of compounds of uranium, plutonium, thorium or mixtures thereof arranged in and partially filling containers leaving a gap between the container and the core, as well as an internal cavity at one end of the container and closing means closing the two ends of the container, characterized in that the barrier layer of zirconium metal has a contaminant content of at least c a 1000 millionths but is less than 5000 millionths and is metallurgically bonded to the inner surface of the zirconium alloy tube and has a thickness of from 5 to 30% of the thickness of the composite enclosure container. Nuclear fuel element according to claim 1, characterized in that the element further comprises a grinding needle means for nuclear fuel material comprising a helical member arranged in the cavity. 3. A nuclear fuel element according to claim 1 or 2, characterized in that the metal barrier layer comprises zirconium sponge. Nuclear fuel element according to one of the preceding claims, characterized in that the nuclear fuel material comprises plutonium compounds and / or uranium dioxide. 7-8 1Û36ï2š 1 l? Nuclear fuel element according to any one of the preceding claims, characterized in that the nuclear fuel material consists of a mixture comprising uranium dioxide and plutonium dioxide.