Metallic zirconium nuclear applications

Materials for nuclear power industry and their properties

It has been well said that the 20th century was called the atomic century. It was the last century when nuclear power engineering was born and widely developed. 

Nuclear power industry is a branch of energetics dealing with production of electrical and thermal power by transformation of nuclear power. 

Structural materials of the nuclear reactor core work under hard conditions, because in addition to being exposed to high temperatures, significant static and dynamic loads and aggressive environments they are also subjected to radiation of high-energy particles, especially of neutrons. Therefore, over and beyond standard constrains for structural materials of power plants the reactor core materials must also have the following characteristics due to the specifics of operating in radiation: 

1. Minimal cross-section of absorption of neutrons;

2. Good thermophysical properties, in particular: 

• high heat-conduction coefficient, 
• low heat absorption, 
• low coefficient of thermal expansion, etc.;

3. High stress-related characteristics:

• strength and ductility during short-term tests, long-term strength, etc.;
• thermal and radiation resistance, ensuring reliable operation under conditions of high temperatures, loads and radiation;

4. High corrosion and erosion resistance in operating environments;

5. Handling ability: deformability, weldability, casting properties, machinability, etc., which makes it possible to manufacture parts of the required shape and size.

Application of zirconium in production of nuclear power.

Alloys based on zirconium purified from hafnium meet all above mentioned requirements. Zirconium for reactors must contain no more than 0.01% hafnium. 

Thus, in the second half of the twentieth century, thanks to its remarkable nuclear characteristics zirconium turned from a rare exotic metal into “the number one structural material of the atomic age.” 

Zirconium is an indispensable component in production of nuclear power. It is primarily used for manufacturing  fuel pin cladding (or fuel rods) of nuclear reactors. 

A fuel pin (fuel rod) is the main structural element of the core of a heterogeneous nuclear reactor, containing nuclear fuel. 

Nuclear reactor fuel rods are long metal tubes that contain small cylindrical pellets of fissile material, usually uranium oxide.

Fig.  Fuel rods of nuclear reactor cores

Fuel elements. Types.

The most widely used type of fuel elements with zirconium cladding is the fuel rod. In a fuel rod the cladding is a tube with a diameter of 6-26 mm and a wall thickness of 0,3-0,9 mm. 

As a rule, fuel rods are placed in the reactor in the form of a reactor fuel assembly – (FA). Each fuel assembly is a fuel element cluster. 

Since the length of fuel rods is quite long (more than 1 m), spacers and end parts, are used to secure them in a strictly fixed position, which are usually made of the same material as the cladding. 

The number of fuel rods in a fuel assembly can be very different. For example, in carrier vessel reactors it runs into several tens (in WWER type reactor-1 91 pcs., WWER type reactor-2 127 pcs.). In reactors of this type each fuel assembly is placed in a casing made, of a zirconium alloy, the same as the cladding. In WWER type reactor -1 and 2 the Zr-2.5% Nb alloy sheet is used for the casing; in PWR reactors the Zircaloy-2 alloy is used. The operating conditions for fuel assembly parts are easier if compared to claddings, since, as a rule, they do not have external loads.

In pressure tube reactors the number of fuel rods in the fuel assembly is significantly less than in carrier vessel reactors (in PHW-CANDU type reactors from 7 to 28 pieces). 

There are several reasons why zirconium is an ideal material for cladding of uranium pellets: this metal is extremely resistant to corrosion and high temperatures and also absorbs almost no neutrons, produced by nuclear fission. The latter is necessary to start and maintain a chain reaction in the reactor core and stable energy production. 

Properties of zirconium and its alloys

The most valuable nuclear property of zirconium, beeing the key for its wide use in nuclear reactors, is the small thermal-neutron capture cross-section, which is 0.18-10-24 cm2 for a reactor-grade metal. Only carbon, beryllium and magnesium have lower thermal- neutron capture cross-sections.

However, the combination of low neutron absorption with other properties necessary for structural material of the core of nuclear reactors makes zirconium more preferable. Along with low neutron absorption, zirconium is characterized by relatively low activation when radiated by neutrons, beeing second only to aluminum and magnesium. 

A zirconium reactor was installed on the first American nuclear submarine Nautilus in the 50s of the 20th century. Later it turned out that it was more profitable to make the claddings of fuel elements of zirconium, rather than stationary parts of the reactor core.

Zirconium production was increasing from year to year, and the rate of this growth was unusually high. It must be said that in the decade from 1949 to 1959 world production of zirconium increased 1000 times.

By 1962, 21 of 79 power reactors, operating at that time in all countries of the world, used zirconium and its alloys for fuel rod cladding. And already in 1969, the number of power reactors, in which zirconium alloys were used for fuel rod cladding or other structural elements of the core was more than 80.

Fuel rod claddings are considered the most critical structural parts of a nuclear reactor core, since they operate under very difficult thermal, mechanical and chemical conditions.

Fuel rod claddings serve to protect fuel from corrosion when interacting with the reactor heat carrier, and to prevent contamination of the reactor circulation loop with nuclear fuel particles and fission products, and at the same time they are intensive heat exchange surfaces. These tasks determine the requirements for the structural material of claddings.

Pure unalloyed zirconium does not have sufficient strength and corrosion resistance in reactor coolants and therefore is not widely used. By alloying it was possible to significantly increase the anti-corrosion and mechanical properties of zirconium to the level required for use as a material for fuel rod cladding. As a result of research many zirconium-based alloys have been developed for manufacturing fuel rod claddings, as binary ones, for example: Zr-0.5% Ta, Zr-0.5% W, Zr-(0.5-5)% Nb, Zr-( 0.5-5)% Cu, and complex alloyed ones, for example: alloys of the Zr-Cu-Fe, Zr-Nb-Sn, Zr-Nb-Cu, Zircaloy-2, 3 and 4, Zr-Cu-Mo etc. Possessing rather high corrosion resistance, some of them are close in strength to steel, for example, alloys of the Zr-Nb-Mo, Zr-Nb-Cu, Zr-Nb-Sn systems.

Application of zirconium alloys

Currently three types of reactor cooling are used in nuclear power engineering:

• steam-water reactor heat carrier пароводяным теплоносителем,  
• carbon dioxide,
• organic coolant.

The most promising alloys for reactors cooled with carbon dioxide at a temperature of 400-550 ° C are alloys of zirconium with copper (up to 2.5%), as well as with copper (0.5-1%) and molybdenum (0.5-1 ,5%). In particular, in the EL-4 reactor (France) fuel rod claddings, made of a zirconium alloy with 2.5% copper, and in reactors of type A-1 and A-2 (Slovakia) – alloys with copper and molybdenum.

The performance of zirconium alloys in carbon dioxide in the temperature range of interest is significantly influenced by the process of dissolution of the oxide film formed as a result of oxidation in the metal and the associated oxygen embrittlement. This factor and corrosion resistance limit the use of zirconium alloys in carbon dioxide at higher parameters.

Zirconium alloys are used to a very limited extent for fuel rod claddings in reactors with organic coolants due to the danger of excessive hydrogenation and embrittlement during lifetime. However, recent research conducted in Canada gives hope for the possibility of expanding the use of zirconium alloys in reactors with organic coolants. The alloy Zr-2.5% Nb is used as a material for fuel rod cladding in WR-1 reactor (Canada), and maximum temperature of the cladding wall reaches 460° C. The lifetime of such fuel rods does not exceed 1.5 years. The use of Ozhenit-0.5 alloy as a cladding material, which is characterized by a significantly lower rate of hydrogen absorption, is believed to make it possible to increase the lifetime of fuel rods to 4 years and to raise the temperature to 480° C.

Zirconium alloys are most widely used in reactors with steam-water heat carrier. Most power reactors are cooled with high-pressure water (regular and heavy), which also serves as a moderator.

Currently, the most widely used for fuel rod claddings are alloys of the Zircaloy type (Zircaloy-2 and 4) and the Zr-1% Nb alloy. Much experience has been accumulated regarding operation of fuel rods with claddings, made of these alloys, and many behavioral features have been found out.

In recent years zirconium alloys have begun to be successfully used for manufacturing of process channels of nuclear reactors. 

Operating conditions of channel tube materials differ significantly from operating conditions of fuel rod claddings and fuel assembly parts. The process channel is a tube, inside of which the fuel bundle is located. The inner surface of the channel is in contact with the coolant flow, and the outer surface is in contact with the intra-reactor environment.

It has already been proven that the efficient and safe operation of nuclear power industry, which produces up to 50% of the world’s electricity, depends entirely on the quality of nuclear fuel, where zirconium alloys are used as the basis for structural material of the reactor core, the share of which in the cost of fuel does not exceed 10%. 

Only zirconium has unique physical and nuclear properties that contribute to the efficient fission of uranium. 

When creating a nuclear-pure zirconium production, the main thing is availability of an efficient and environmentally friendly technology for processing zircon with complete extraction of all valuable components, which allows obtaining competitive high-quality products.

CONCLUSION

1. Nuclear power engineering is an integral part of global industrial production. Nuclear power industry is a branch of energetics dealing with production of electrical and thermal power by transformation of nuclear power. 
2. Nuclear power plants generate nearly a third of the world’s carbon-free electricity and are critical to meeting the goals, associated with combating climate changes.
3. Structural materials of the nuclear reactor core operate under difficult conditions (high temperature, static and dynamic loads, aggressive environments, irradiation by high-energy particles, first and foremost by neutrons), so special requirements are applied to them.
4. Alloys based on zirconium, purified from hafnium, meet the requirements for materials operating under specific conditions of radiation. 
5. Zirconium is an indispensable component in production of nuclear power, it is used as a cladding for nuclear reactor fuel elements (fuel rods). 
6. The type of zirconium alloy used is determined by the type of cooling of the nuclear reactor – steam-water coolant, carbon dioxide and organic coolants.
7. Efficient and safe operation of nuclear power energetics depends on the quality of nuclear fuel, in which zirconium alloys are used as structural material for the nuclear reactor core.