Metal treatment – Stock – Titanium – zirconium – or hafnium base
Patent
1998-04-13
2000-11-21
Wyszomierski, George
Metal treatment
Stock
Titanium, zirconium, or hafnium base
148407, 148421, C22F 118
Patent
active
06149738&
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The present invention relates to the field of fuel boxes of a zirconium alloy for use in nuclear fuel assemblies intended for boiling water reactors, and to a method for manufacturing such boxes.
BACKGROUND OF THE INVENTION
Nuclear fuel bundles for boiling water reactors are usually surrounded by a fuel box. The main functions of the fuel box are to provide mechanical stability to the bundle and to conduct the cooling water so that adequate cooling of the fuel bundle in all its parts is obtained.
The fuel box has a square cross section and surrounds the bundle along all of its length. At the bottom of the fuel box a transition piece is attached, and at the bottom thereof there is a guide intended to be arranged in the assembly supporting plate of the reactor. The box may also be provided with an inner, usually cruciform structure which axially divides the fuel bundle into four sub-bundles.
The shape accuracy of the fuel box is of the utmost importance for its function. It is also important for the box to have good corrosion resistance during reactor operation. The box has large surfaces in contact with the reactor coolant. Flaking corrosion products should not form on the box surfaces since these products may spread radioactivity to various systems in the reactor. There should also be a good margin with respect to weakening of the box wall caused by the metal being transformed into oxide.
During manufacturing of a fuel box, thin rectangular plates of a zirconium alloy constitute the starting material. Zirconium alloys widely used in nuclear reactors are Zircaloy-2 and Zircaloy-4. Zircaloy-2 contains 1.2 to 1.7% tin, 0.07 to 0.20% iron, 0.05 to 0.15% chromium, 0.03 to 0.08% nickel, 0.09 to 0.16% O and Zircaloy-4 contains largely the same alloying elements but lacks nickel and contains somewhat more iron, 0.018 to 0.24%. Also other zirconium alloys for reactor purposes such as, for example, a zirconium-base alloy containing about 1% tin, about 1% niobium and about 0.2% iron, or a zirconium alloy containing about 1% niobium, 1% tin, 0.5% iron and 0.2% chromium may be used as starting material. The alloys comprise incidental impurities, normally in the range of 500 to 1500 ppm.
The box is manufactured by bending two plates into U-shape. The bending is carried out with a conventional method and may be preceded by a heat treatment of the plate to increase its ductility. Two U-shaped plates are turned so as to face each other and are welded together along the folded-up parts of the plates, so as to obtain a box with a square cross section. The shaping of the box into the finished dimension is made by heat-treating the box on a device in a conventional manner.
To improve the corrosion properties of the material, it is known, according to GB 1 537 930 , to heat the material to a temperature exceeding 900.degree. C. so that a phase transformation occurs in the material from hexagonal alpha phase to cubic beta phase, and thereafter to cool the material rapidly, so-called beta quenching. Phase transformation occurs at about 870.degree. C., and above about 930.degree. C. the material is completely transformed into beta phase. The temperature may vary somewhat depending on what alloying elements the zirconium contains, however all zirconium alloys for reactor purposes are low alloy elements so the variation in phase transformation temperature is relatively small.
British patent GB 1 537 930 describes that the plate is to be heated to a temperature of at least 900.degree. C. by allowing the temperature rise from 500.degree. C. to the desired heat-treatment temperature and the heat treatment is to take at most 60 seconds, whereafter the plate is to be cooled at least 200.degree. C. in at most 60 seconds. During cooling of the plate, the material forms an acicular structure, so called Widmanstatten structure.
U.S. Pat. No. 4,238,251 describes heat treatment of nuclear fuel components for improving the corrosion resistance in boiling water reactors.
A fuel box in a finished or almost finished form is
REFERENCES:
patent: 4238251 (1980-12-01), Williams et al.
patent: 4521259 (1985-06-01), Eucken
patent: 5361282 (1994-11-01), Adamson et al.
patent: 5478419 (1995-12-01), Dumas et al.
patent: 5677937 (1997-10-01), Vesterlund et al.
patent: 5887045 (1999-03-01), Mardon et al.
Adamson et al., Zirconium in the Nuclear Industry, ASTM Special Technical Publication 939, Jun. 1985, pp. 284-291.
ABB Atom AB
Wyszomierski George
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