Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Nonconventional jacket or can material
Patent
1998-04-20
1999-08-17
Wasil, Daniel D.
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Nonconventional jacket or can material
420422, 148672, G21C 307
Patent
active
059404647
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to tubes of zirconium-base alloy suitable for use, in particular, for constituting all or the outer portion of the cladding of a nuclear fuel rod, and also to a method of manufacturing them.
Until now, use has been made above all of cladding made of a so-called "Zircaloy 4" alloy which contains tin, iron, and chromium in addition to zirconium. Numerous other compositions have been proposed, with content ranges that are often so broad that, to the person skilled in the art, they can be seen immediately to be purely speculative.
In particular, various alloys have been proposed with a niobium content lying in a range so broad that their resistance to thermal creep is quite poor at maximum values, whatever the metallurgical treatments used in making the alloy.
Alloys have also been proposed that contain, in addition to zirconium, tin to improve creep resistance, and iron.
An object of the invention is to provide tubes that have simultaneously good creep behavior and good resistance to corrosion, even in a high temperature medium containing lithium, while nevertheless being capable of being manufactured with a low reject rate, and being suitable for use in making cladding or guide tubes for fuel assemblies.
One of the causes of rejects is the formation of cracks during mechanical and heat treatments, giving rise to defects that make the tubes unacceptable. This risk exists particularly for high tin contents.
To achieve the above objects, there is provided a tube of zirconium-base alloy containing, by weight, 0.8% to 1.8% niobium, 0.2% to 0.6% tin, and 0.02% to 0.4% iron, the alloy being in the recrystallized state or in relaxed state, depending on whether it is desired to enhance resistance to corrosion or to creep.
The alloy has a carbon content lying in the range 30 parts per million (ppm) to 180 ppm, a silicon content lying in the range 10 ppm to 120 ppm, and an oxygen content lying in the range 600 ppm to 1600 ppm.
The relatively high niobium content, which is always above the solubility limit (about 0.6%), provides high resistance to corrosion in an aqueous medium at high temperature. If used alone, niobium at such concentrations imparts creep characteristics to the alloy which are of interest but insufficient. Tin, when associated with niobium, improves creep resistance and also resistance to an aqueous medium containing lithium, without running the risk of causing cracks to be formed during rolling if its content does not exceed 0.6%. An iron content of up to 0.4% participates in compensating for the unfavorable effect of tin on generalized corrosion.
The contents given above take account of the way in which tolerances and variations within a single ingot mean that the limits can be reached even for set specific contents lying within a narrower range. For example, set contents of 0.84% and 1.71% Nb may give rise within a single ingot to local contents of 0.8% and of 1.8% depending on proximity to the leading end or the trailing end of the ingot.
In addition to the above-specified elements, the alloy contains inevitable impurities, but always at very low contents.
It has been found that set content values of niobium in the range 0.9% to 1.1%, of tin in the range 0.25% to 0.35%, and of iron in the range 0.2% to 0.3% give results that are particularly favorable.
Because of the relatively low tin content, recrystallization during metal-making can be performed at a relatively low temperature, below 620.degree. C., and that has a favorable effect on hot corrosion resistance and on creep.
The invention also provides a method of manufacturing a tube for constituting cladding for a nuclear fuel rod or a guide tube for a nuclear fuel assembly. The initial alloy-making stage can be that performed conventionally for so-called "Zircaloy 4" alloys. However, the final stages are different, and in particular they make use of recrystallization heat treatments at relatively low temperature only.
In particular, the method may comprise the following steps:
making a bar of zirconium-base alloy h
REFERENCES:
patent: 5023048 (1991-06-01), Mardon et al.
patent: 5254308 (1993-10-01), Garde et al.
Patent Abstracts of Japan--vol. 016, No. 393 (P-1406), Aug. 20, '92 & JP,A,04 128687 (Nuclear Fuel Ind. Ltd.), Apr. 30, '92.
Charquet Daniel
Mardon Jean-Paul
Senevat Jean
Compagnie Generale des Matieres Nucleaires
Framatome
Wasil Daniel D.
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