TECHNICAL FIELD

The invention relates to the field of zirconium alloys for nuclear fuel components for light-water reactors. The new alloy is a further development of zirconium alloys containing alloying elements from the group tin, iron, chromium, nickel, niobium, vanadium, silicon, and oxygen.

BACKGROUND ART, PROBLEMS

It is known to use zirconium alloys as construction material for nuclear fuel components such as cladding tubes, boxes, spacers, guide tubes, etc. The general requirements on these zirconium alloys are that they should have a low neutron absorption cross section, good mechanical properties such as strength, ductility, creep resistance and good corrosion resistance in a light-water reactor environment, and a low hydrogen absorption in connection with the corrosion. A very large number of alloys have been developed to attempt to fulfil these requirement as well as possible or to attempt to obtain an alloy which is considerably improved with respect to any of these requirements. The alloys which are most used as construction material are Zircaloy 2 and Zircaloy 4, which consist of zirconium containing 1.2 - 1.7 per cent by weight tin, 0.07 - 0.20 per cent by weight iron, 0.05 - 0.15 per cent by weight chromium, 0.03 - 0.08 per cent by weight nickel, 0.09 - 0.16 per cent by weight oxygen and ^~ 1.2 - 1.7 per cent by weigh tin, 0.18 - 0.24 per cent by weight iron, 0.07 - 0.13 per cent by weight chromium, and 0.09 - 0.16 per cent by weight oxygen, respectively.

According to US 4 649 023, zirconium alloys are known contai- ning 0.5 to 2.0 % niobium, up to 1.5 % tin and up to 0.25 % of a third element such as iron, chromium, molybdenum, vanadium, copper, nickel, or tungsten. This alloy is, in principle, a zirconium-niobium alloy with addition of tin up to

1.5 % and with a small addition (<0.25 %) of a third sub¬ stance. For zirconium-niobium alloys containing more than 0.5 per cent by weight niobium, a phase transformation to β phase is performed at 610°C.

The ductility of the material in β phase is worse than in α phase, which means that a structural part in a material with a lower phase transformation temperature is given worse proper¬ ties at a heavy increase in temperature in the core, such as at a "Loss of Coolant Accident".

US 4 233 149 describes a zirconium alloy consisting of zir¬ conium and 0.05 - 3.0 % tin, 0.001 - 4.5 % hafnium, 0.001 - 1.0 % iron, 0.001 - 1.0 % chromium, 0.001 - 1.0 % nickel, 0.05 - 0.5 % oxygen, 0.001 - 0.05 % nitrogen, 0.001 - 0.2 % of one or more elements from the group copper, cobalt, cadmium, manganese, aluminium, titanium, silicon, carbon, phosphorus, molybdenum, bismuth, vanadium, antimony, niobium, tungsten, and boron. This alloy is intended to constitute anode material for anodizing aluminium. The alloy is produced to improve the current yield, the abrasion resistance and the corrosion resistance in electrolytes such as sulphuric acid, for anodi¬ zing aluminium and not a zirconium alloy intended to be used as construction material for nuclear fuel components.

From FR 89 13997 it is known to add to zirconium - 2.5 - 10 % niobium alloys 0.05 to 1.0 % of an element from the group iron, chromium, molybdenum or vanadium to obtain an alloy which is especially suited for manufacturing spacers for nuclear fuel elements. This alloy has a high niobium content, which leads to a low transformation temperature to β phase and the disadvantages described for US 4 649 023.

According to US 5 122 334 it is known to add small quantities of vanadium to zirconium-gallium alloys.

A zirconium alloy for nuclear fuel components is described in US 4 981 527 and consists of zirconium with 0.1 - 0.35 % iron,

0.07 - 0.4 % vanadium, 0.05 - 0.3 % oxygen, less than 0.25 % tin and less than 0.25 % niobium. The low contents (<0.25 %) of tin and niobium are stated to be of importance for achieving a good corrosion resistance of the alloy. However, the creep properties of this alloy are very inferior compared with, for example, Zircaloy 2 or 4. The reason is that the alloy has a low content of alloying additives and is therefore soft.

The patent specification CA 859 053 describes a zirconium- niobium-beryllium alloy which may be alloyed with up to 10 per cent by weight of at least one element from the group tin, copper, iron, chromium, molybdenum, vanadium, tungsten, tantalum, nickel, yttrium, antimony and tellurium. The alloy contains 0.005 - 1.0 per cent by weight beryllium, which has a very low neutron absorption cross section but which is exceedingly poisonous and difficult to handle when manufac¬ turing an alloy.

SUMMARY OF THE INVENTION

The present invention relates to a zirconium alloy, for components included in nuclear fuel elements, with improved properties with respect to the absorption of hydrogen released during the corrosion, in combination with good strength and creep properties.

The invention is an improvement of known zirconium alloys based on zirconium with alloying addition of at least tin, iron, chromium, nickel and silicon, and is based on the realization that the hydrogen absorption for a zirconium alloy based on zirconium-tin-iron-chromium-nickel-silicon can be considerably reduced by small additions of vanadium.

It is also possible to add small quantities of niobium to the alloy. However, the niobium content should be maintained low to avoid phase transformation at 610 °C.

These alloys based on zirconium-tin-iron-chromium-nickel- silicon and with addition of vanadium or vanadium and niobium have good strength in spite of the fact that the tin content may be lower than the standard value for Zircaloy 2, 4. This is possible because vanadium, besides affecting the hydrogen absorption, also has a solution-hardening effect. Also, the combination of iron, chromium and nickel and, where appli¬ cable, also niobium contributes to increase the strength.

A good high temperature strength is also obtained in the alloy, and by keeping the niobium content low, the phase transformation at 610 °C, which is characteristic of conven¬ tional zirconium-niobium alloys, does not take place.

The zirconium alloy according to the invention comprises, in addition to zirconium and normal contents of impurities for reactor-grade zirconium, also the alloying elements tin, iron, chromium, nickel, silicon, oxygen and vanadium, or vanadium and niobium. The total content of tin, iron, nickel, chromium and silicon is at least 0.54 per cent by weight and at most 2.15 per cent by weight, of which the content of tin is at least 0.3 per cent by weight, the content of iron at least 0.07 per cent by weight, the content of chromium 0.05 per cent by weight, the content of nickel at least 0.03 per cent by weight, and the content of silicon at least 0.005 per cent by weight. In addition, the alloy contains 0.015 to 0.30 per cent by weight vanadium or 0.015 to 0.30 per cent by weight vanadium and 0.015 to 0.30 per cent niobium.

The alloy according to the invention makes it possible for nuclear fuel components such as cladding tubes or spacers to obtain good strength properties in combination with good corrosion resistance and a low propensity to absorb hydrogen in connection with the corrosion process. Absorption of hydrogen means that the material is embrittled when the hydrogen is precipitated in the form of needle-shaped hydrides. This embrittlement reduces the capacity of the material to withstand impacts, vibrations, and the like, which

may arise in operation or when handling the fuel in connection with refuelling or storage of spent fuel.

The composition of the alloy is such that it has a higher phase transformation temperature than the conventional resistant zirconium-niobium alloys which exhibit a decreasing ductility at temperatures exceeding about 600 °C.

The manufacture of nuclear fuel components is usually performed by means of hot-working, for example forging or hot- rolling of a blank. Cladding tubes are manufactured by extruding a blank and then cold-rolling it in several steps with intermediate heat treatments and a final heat treatment. Flat products for manufacturing boxes or spacers are produced by hot- and cold-rolling to a suitable dimension and by heat treatments between the cold-rolling operations, and a final heat treatment.

The manufacture of the alloy according to the invention is performed in the most advantageous manner by allowing all the hot-working and heat-treatment operations to take place in the α phase range. This provides the most favourable effect on the corrosion properties of the alloy. It is therefore advan¬ tageous with an alloy with a low niobium content to avoid initiating the phase transformation to β phase as early as at

610 °C, which lies within that temperature range in which zirconium alloys are normally heat-treated.

It has been found that cladding tubes according to the inven- tion may be manufactured from a zirconium alloy according to the above and that it is favourable to heat-treat the cladding tubes between the cold-rolling operations at a temperature below 600 °C.

However, it is possible to manufacture the alloy according to the invention according to other methods for manufacturing zirconium-base alloys, which may mean that heat treatments are carried out in the α + β phase range or in the β phase range,

so-called β quenching.

The invention will be explained in greater detail by describing a few embodiments.

Example 1

An alloy for cladding tubes intended for boiling or pressurized-water reactors consists of zirconium with 0.5 to 1.7 per cent by weight tin, 0.07 to 0.20 per cent by weight iron, 0.05 to 0.15 per cent by weight chromium, 0.03 to 0.08 per cent by weight nickel, 0.005 to 0.05 per cent by weight silicon, 0.09 to 0.16 per cent by weight oxygen, 0.015 to 0.30 per cent by weight vanadium and impurities in quantities normally occurring for reactor-grade zirconium. The cladding tubes may be manufactured in conventional manner by means of extrusion and a number of cold-rolling steps alternating with heat treatments and a final heat treatment to impart optimum properties to the finished cladding tube.

Example 2

Another alloy composition according to the invention, which may be used for manufacturing cladding tubes, is an alloy consisting of zirconium with 0.03 to 1.0 per cent by weight tin, 0.07 to 0.70 per cent by weigh iron, 0.05 to 0.15 per cent by weight chromium, 0.16 to 0.40 per cent by weight nickel, 0.015 to 0.05 per cent by weight silicon, 0.09 to 0.16 per cent by weight oxygen, 0.015 to 0.30 per cent by weight vanadium, 0.015 to 0.30 per cent by weight niobium, and impurities in quantities normally occurring for reactor-grade zirconium. The cladding tubes may be manufactured by means of extrusion and four cold-working steps with intermediate heat treatments at temperatures lower than 600 °C and a final heat treatment at 450 to 700 °C.