Zirconium-based alloy elements used in nuclear reactor cores

Details Zirconium-based alloy elements used in nuclear reactor cores Zirconium-based alloy elements used in nuclear reactor cores

2001-10-22

2004-08-17

Jenkins, Daniel (Department: 1742)

Alloys or metallic compositions

Zirconium or hafnium base

Reexamination Certificate

active

06776957

ABSTRACT:

TECHNICAL FIELD
The present invention relates to metallurgy, more particularly to zirconium-based alloys used in the active core of nuclear reactors.
BACKGROUND ART
Zirconium-based alloys find application as construction elements of the active core of nuclear power reactors operating on thermal neutrons, such as fuel claddings, pipes for process channels, and other construction elements.
Quite a number of requirements are imposed upon the alloys mentioned above, that is, as to corrosion resistance in water and high-temperature steam, strength characteristics, resistance to oxidation, hydrogenation, radiation growth, and creep. In addition, the alloy must possess high processability.
Known in the present state of the art is a zirconium-based alloy containing 1-4 wt. % niobium and 0.1-0,2 wt. % oxygen and consisting predominantly of a martensitic transformation of the &bgr;-phase and a finely dispersed secondary phase rich in niobium (cf. GB Pat. A No 997761).
Products made from said known alloy feature but an inadequately broad complex of anticorrosion properties, including inadequately high resistance to nodular corrosion in boiling water.
Another zirconium-based alloy is known to comprise (on a weight percent basis): niobium, 0.5-1.5; tin, 0.9-1.5; iron, 0.3-0.6; chromium, 0.005-0.2; carbon, 0.005-0.04; oxygen, 0.05-0.15; silicon, 0.005-0.15, the structure of said alloy being a metallic matrix hardened with niobium- and iron-containing intermetallics having the following volumetric content of the sum of intermetallics: Zr(Fe,Nb)
2
+Zr(Fe,Cr,Nb)+(Zr,Nb)
3
Fe being at least 60% of a total content of the iron-containing intermetallics and a distance therebetween equal to 0.322±0.09 &mgr;m (cf. RU Pat. A No 2032759).
Products made from said known alloy feature high strength characteristics, resistance to radiation growth, creep, and rust-proof quality. However, corrosion in a water medium affecting the products made of said known alloy results in forming a thicker oxide layer than is observed in the proposed alloy.
One more zirconium-based alloy is known to comprise (on the weight percent basis): niobium, 0.8-1.3; iron, 0.005-0.025; silicon, below 0.012; carbon, below 0.02; oxygen, below 0.16, zirconium being the balance (cf. EP A 0720177 A1).
This technical solution as being the closest to the herein-claimed one as to technical essence, is elected to be the prototype.
Products made of the alloy known from the prototype possess but an inadequately broad complex of anticorrosion and mechanical properties. A reduced niobium and iron content prevents preparing a structure that imparts high corrosion resistance to the alloy, especially resistance to nodular corrosion, as well as high strength and creep- and radiation growth resistance.
DISCLOSURE OF THE INVENTION
The principal object of this invention is the provision of a zirconium-based alloy for the components of the active core of nuclear reactors, the products made of said alloy possess stable properties, such as corrosion resistance, hardness, resistance to radiation growth and creep, and substantially high resistance to nodular corrosion, whereby the service life of the products in the active core of a nuclear reactor is substantially extended.
Said object is accomplished due to the fact that a zirconium-based alloy for the components of the active core of nuclear reactors, comprising niobium, iron, oxygen, carbon, and silicon, according to the invention, further comprises nickel, with the following ratio of the components (on a weight percent basis):
niobium
 0.5-3.0
iron
0.005-0.02
oxygen
 0.03-0.12
carbon
0.001-0.02
silicon
0.002-0.02
nickel
0.003-0.02
zirconium
being the balance,
and the structure of the alloy further comprises &bgr;-particles of the Nb-phase which are sized less than 0.1 &mgr;m and are uniformly distributed in the &agr;-solid solution, while said phase has a niobium percentage content of from 60 to 95. The structure may also further comprise intermetallic particles Zr—Fe—Nb sized less than 0.3 &mgr;m.
The alloy may be constituted by the following constituents taken in the following ratio therebetween (on a weight ratio basis):
niobium
 0.5-3.0
iron
0.005-0.02
oxygen
 0.03-0.12
carbon
0.001-0.02
silicon
0.002-0.02
nickel
0.003-0.02
zirconium
being the balance,
with the niobium content of the beta-particles in the Nb-phase ranging between 75 and 95%.
The alloy may be constituted by the following constituents taken in the following ratio therebetween (on a weight ratio basis):
niobium
 0.5-3.0
iron
0.02-0.5
oxygen
 0.03-0.12
carbon
0.001-0.02
silicon
0.002-0.02
nickel
0.003-0.02
zirconium
being the balance,
the iron
iobium ratio being 0.05:0.2.
The alloy may be constituted by the following constituents taken with the following ratio therebetween (on a weight ratio basis):
niobium
 0.5-3.0
iron
0.005-0.5
oxygen
 0.1-0.2
carbon
0.001-0.02
silicon
0.002-0.1
nickel
0.003-0.02
zirconium
being the balance,
with the niobium content of the &bgr;-particles in the Nb-phase ranging between 75 and 95% and the &agr;-solid solution being further oxygen-hardened.
The alloy proposed herein, in contradistinction to the prototype, allows one to obtain an optimum structure-phase state which provides for high corrosion resistance in water and steam media, as well as high hardness, creep- and radiation growth resistance.
Making products from the herein-proposed alloy by virtue of more exactly selected ratio between the constituents constituting said alloy enables one to create a definite alloy structure in finished products, which structure comprises &agr;-solid solution of zirconium, uniformly distributed finely divided particles of the equilibrium &bgr; Nb-phase, solid solution of zirconium in niobium having a body-centered cubic lattice with the parameter ‘a’ equal to 3.3-3.35 Å and the niobium content in excess of 75% which corresponds to an equilibrium composition of the &bgr; Nb-phase. The structure of the material may also incorporate finely divided particles of intermetallics Zr—Fe—Nb.
The herein-proposed chemical analysis of the alloy and the presence of the &bgr; Nb-phase therein the particles of which are sized below 0.1 &mgr;m, the niobium content of said phase ranging within 75 and 95%, provides for establishing an equilibrium and superfine structure, thus adding to the stability of in-service characteristics of finished products, especially such characteristics as corrosion resistance and plasticity.
The 75 to 95% niobium content of the &bgr; Nb-phase provides for its state of equilibrium, and particle size distribution below 0.1 &mgr;m. Such a structure of the alloy imparts thereto high corrosion resistance in high-temperature water, and ductility. Taking into account the fact that corrosion resistance is the principal performance characteristics of zirconium products made use of in the active core of nuclear reactors, the alloy structure comprising the &bgr; Nb-phase of an equilibrium composition imparts high corrosion-resistance characteristics to finished products in high-temperature water.
The iron
iobium ratio below 0.2 enables one to additionally isolate particles of iron-containing intermetallics Zr—Fe—Nb sized less than 0.3 &mgr;m and uniformly distributed in the &agr;-solid solution, thus adding to the in-service hardness characteristics of finished products. In addition, the presence of intermetallics Zr—Fe—Nb in the alloy structure increases resistance of said alloy to nodular corrosion under boiling conditions, which is accompanied by thinning of the wall and hydrogenation of the cladding, as well as formation of thick oxide films which reduce heat conductivity thereof. Presence of intermetallics Zr—Fe—Nb in the alloy structure reduces susceptibility of the alloy to nodular corrosion by 1.5-2 times.
With the iron
iobium ratio exceeding 0.2 the specified composition of the &bgr; Nb-phase is not observed, that is, the niobium proportion in the &bgr; Nb-phase is diminished, with the result that stability of anticorrosion properties is adve

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