The nuclear industry is expanding, and the heat-treating community needs to keep pace. This group of articles will discuss the current state of the industry including a brief overview of nuclear power and the various styles of reactors and types of components fit for nuclear service.



Editor’s note: This is the first part of three articles, which are also supported by online-exclusive facts and data. As a result, figures and tables are numbered consecutively whether they are included in print or not. If you are seeking a figure not in print, find it in the online exclusive at www.industrialheating.com.

Fig. 11. Forty-eight plants in 15 countries are adding to existing capacity

Specific heat-treating applications will be presented in detail including: annealing of zirconium pressure tubing; stress relief of steam valve bodies and pipe welds; hardening of pressure relief springs and internal valve components; heat treatment of bolts, seal rings and other types of fasteners/retainers; and the sintering of ceramic fuel pellets. Vacuum, atmosphere and induction hardening techniques will be discussed along with design requirements. New developments and industry trends will also be explored.

Fig. 12. Nuclear share of electricity generation for selected countries – 2004

The World View of Nuclear Energy

Today, over 6% of the world’s electricity is produced from nuclear energy, more than from all sources worldwide back in 1960. Public opinion on the use of nuclear energy is divided with strong advocates for and against the technology.
  • In the U.S., those who would reduce the use of nuclear power (39%) slightly outnumber those who would like to increase it (35%), while there are still a significant number who would choose not to use it at all (11%). By contrast, 74% of Americans want to decrease the country’s dependence on oil, and 54% want to decrease the use of coal.[3]
  • In Europe, nearly an identical number of those surveyed supported nuclear energy (44%) as those who opposed it (45%). Meanwhile, the vast majority of the European public agrees that nuclear allows EU countries to diversify their energy sources (64%), decrease their dependence on oil (63%) and reduce greenhouse gases (62%). Concern surrounding nuclear waste is seen as an urgent need (93%), while a number of those opposed to nuclear energy (39%) say that a permanent, safe waste solution would change their opinion.[4]
  • In Asia, the view from Japan is noteworthy. A series of nuclear incidents, the latest being in 1999, has changed public opinion. Prior to these accidents a majority of Japanese (69.9%) supported the existence of nuclear power, though an equal majority (68.3%) were worried about long-term effects. A survey conducted in 1991 was sharply divided on nuclear safety – 51% said it was safe, 49% disagreed. After the incidents, a 1999 survey saw the number who felt nuclear energy was safe fall to 32.1%.[5]
However, the number of nuclear generating units (reactors) and the total commercial nuclear generating gross capacity for each country continues to rise (Table 2 online) with new installations (Fig. 11) adding capacity. Worldwide, the dependence on nuclear energy as the primary source for electricity varies considerably (Fig. 12).

Fig. 13. Fuel bundle (pressure tubes)

Heat-Treating Applications in the Nuclear Industry

Types of Components
Regardless of reactor type, the nuclear industry needs a vast array of components (Table 3 online). This includes pipe, tubing, valves, fasteners, shielding, core components and nuclear material (fuel). Parts are made using wrought and powder-metallurgy methods and are supplied in the form of forgings, castings, bar, plate, rod, wire and near-net shape. Some are metallic – both ferrous and nonferrous – while others are non-metals such as graphite, ceramic and Teflon. High-temperature and/or high-pressure service is not uncommon. What they all share, however, is demanding performance requirements, exacting adherence to stringent specifications, and the need for process control, repeatability and documentation.

The vast majority of these components must be heat treated or otherwise thermally processed. Demand varies from high volumes (fuel and fuel rods, valves) to limited quantities (springs, bolts). Vacuum heat treatment is the most common method employed, even for low-temperature treatments such as stress relief, although many processes can be done in atmosphere, even air. Induction and laser methods are used selectively, and some operations must be done on installed components in the field. Most companies have additional internal standards that must be met, and heat-treatment cycles are proprietary.

Fig. 14. Process flow chart for heavy-water reactor (HWR) pressure tubes

Application Examples

Zirconium Pressure Tubing
The nuclear power industry uses nearly 90% of the zirconium produced each year, principally to manufacture fuel containers commonly referred to as pressure tubes (Fig. 13) or casings. The two principal producers are Cevus-Areva (France) and Allegheny Teledyne Wah Chang (U.S.).

Zirconium is a hard, corrosion-resistant material that is permeable to neutrons and has the ability to confine fission fragments, slow neutrons and efficiently utilize thermal energy, thus improving the efficiency of the reactor. Five principle reactor grades are used (UNS number designations): R60001, R60802, R60804, R60901 and R60904. Vacuum annealing is the heat-treatment option of choice. Annealing temperatures range from 705-760°C (1300-1400°F) on the high end to 515-530°C (960-986°F) on the low end. The material is heat treated after cold pilgering (rolling) into lengths of 6-9 meters (20-30 feet).

Fig. 15. Manufacturing process for RBMK (Russian) pressure tubes

Manufacturing
Current estimates for zirconium metal production indicate capacity at about 8,600 tons per year with demand around 5,000 tons. Demand estimates in five years are projected to be in the range of 6,500 tons and within existing supply capability.

Ingot material is primarily vacuum-arc or electron-beam melted in furnaces conventionally used for reactive metals. Seamless tubes may be made by billet extrusion with subsequent cold working by drawing, swaging or rocking with intermediate annealing. Welded tubing is made from flat-rolled products by an automatic or semi-automatic welding process with no addition of filler metal, and it is cold reduced by drawing, swaging or rocking with intermediate heat treatments as necessary (Figs. 14 & 15).

Despite its work-hardening characteristics, zirconium’s formability by hot and cold operations is considered good. Designs that eliminate severe or abrupt section changes and allow generous radii are used in the nuclear industry. Dies of non-galling material with tolerances and clearances comparable to those used for austenitic stainless steels are employed. As in the case of tube bending, die designs should allow for the spring-back tendency of the material.

Fig. 16. Typical vertical vacuum annealing furnace (schematic courtesy of VAC AERO International)

Heat Treatment
Vacuum annealing (Figs. 16 & 17) is performed on bundles of material in either a vertical (preferred) or horizontal orientation. Small-diameter, closely packed bundles present heat-transfer (conduction) challenges due to “air” gaps between parts and intermittent line contact between the tubes in the bundle. Distortion is a concern that is minimized only by proper fixturing and uniform heating, equalization of heat transfer through the bundle and correct soaking times. Parts should be at temperature for only a minimum length of time, and there is a maximum (threshold) time that must not be exceeded.

Part thermocouples are mandatory, and furnace temperature control is critical given the high aspect ratio (length-to-diameter) of most designs. Diffusion pumps are required, and all-metal hot-zone designs (metal shielding, metallic elements) are mandatory to prevent oxidation, surface contamination and surface defects. Zirconium is a highly reactive element in the presence of oxygen and can absorb hydrogen if present (with disastrous in-service consequences).

Fig. 17. Typical horizontal vacuum annealing furnace (photograph courtesy of SECO/WARWICK Corporation)

Testing
Chemical and product analysis is performed on the materials that must meet the chemical composition requirements for tin, iron, chromium, nickel, niobium, oxygen and other impurity elements. The tensile property is determined by a tensile-test method and conforms to specific tensile strength, yield strength and elongation limits. Steam and water corrosion tests and hydrostatic tests are conducted to determine the acceptance criteria for corrosion and internal hydrostatic pressure, respectively. Burst properties, contractile strain ratio, grain size and hydride orientation of the finished tubing must also be determined.

Fig. 20. Typical valve assembly (photograph courtesy of Control Components Inc.)

In-Service Replacement
Most reactors need to be shut down for refueling so that the pressure vessel can be opened up. In this case, refueling is at intervals of one to two years, when a quarter to a third of the fuel assemblies are replaced with fresh ones.

Heat Treatment
Both individual components (stems, seats) and the entire valve body (Fig. 20) are subjected to a stress relief in vacuum. Vacuum is used to protect the surface finish and to prevent contamination. Valve materials include 17-4 PH, 440 SS, Monel and some exotic alloys.

Fig. 21. DIN 1.8159 (SAE 6150) valve springs

Safety Valve Springs
Pressure-relief valve springs (Fig. 21) are another example of components that benefit from heat treatment. Springs in the size range of 360-600 mm (14-23.5 inches) OD and up to 1,270 mm (52 inches) tall with a wire diameter of up to 57 mm (2.25 inches) thick are manufactured from steels such as DIN 1.8159 (SAE 6150). They are hardened, quenched and tempered to 48-52 HRC to conform to standards such as ASTM A232 (Standard Specification for Chromium-Vanadium Alloy Steel Valve Spring Quality Wire) prior to assembly into the valve. Typical heat treatment consists of austenitizing at 870°C (1600°F) in a direct gas-fired or atmosphere furnace, quenching in oil in the range of 70-120°C (160-250°F) and air tempering. IH

The Nuclear Renaissance online exclusive posted in June contains the figures missing from this printed article as well as more detail on nuclear power generation. Find it on our home page or June archives, or go there by using the Mobile Tag below.

For more information: Contact the author at The HERRING GROUP, Inc., P.O. Box 884, Elmhurst, IL 60126; tel: 630-834-3017; fax: 630-834-3117; e-mail: dherring@heat-treat-doctor.com; web: www.heat-treat-doctor.com