By Dan Herring
“The choice is not if we are willing to be in the midst of a nuclear revival, the choice is what to do about it now that we are.” - The Nuclear Option, CNBC, June 2009
This is the first and second part of a three part article. The nuclear industry is expanding, and the heat-treating community needs to keep pace. This article 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.
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.
The World View of Nuclear Energy
Fig. 12. Nuclear share of electricity generation for selected countries – 2004
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.
- 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.
- 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%.
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).
Heat-Treating Applications in the Nuclear Industry
Types of Components
Fig. 13. Fuel bundle (pressure tubes)
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.
Zirconium Pressure Tubing
Fig. 14. Process flow chart for heavy-water reactor (HWR) pressure tubes
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
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)
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)
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)
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.
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.
Safety Valve Springs
Fig. 21. DIN 1.8159 (SAE 6150) 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.
State of the Industry
Fig. 1. World net electricity consumption by region, 2010-2025 (billion kilowatt-hours)
The reality around the globe is that nuclear energy is here to stay not necessarily because we’re convinced it is the best option, that it does not carry with it short- and (very) long-term consequences or it is easy, but because it is the most sustainable energy source capable of meeting near-term energy demands (Fig. 1). In most countries concerns over safety, pollution in the form of waste and proliferation continue to create controversy as to how and when to proceed.
In March 2007, for the first time in more than three decades, the U.S. Nuclear Regulatory Commission granted an Early Site Permit (ESP) for a new nuclear power plant. The ESP is the first stage toward a new plant and allows Exelon Generation Company to start construction work in Clinton, Ill., to determine if the site is suitable for a nuclear power plant. This is the first permit granted under a new licensing process that was established in 1989 but had not previously been used. The initial application was filed in September 2003. Decisions on two other plant applications are expected in early 2010 with another pending application still in the early stages.
Fig. 2. World energy usage by type and region
In the U.S. alone, nuclear energy has the potential to supply 40-50% of the country’s total energy demand for the next 40-60 years. These figures include the expected 21% energy growth in the U.S. by 2030. Worldwide, nuclear energy has the potential to supply as much as 20% of the world’s energy needs. Nuclear energy along with renewable energy will have the greatest impact on reducing our dependence on oil, natural gas and coal (Fig. 2).
Fig. 3. Number of nuclear reactors in operation by country – June 2009
As of June 2009, there are 436 nuclear power plants in operation in 31 countries (Fig. 3) with an installed electric net capacity of nearly 370 GW and 48 plants in 15 countries with additional installed capacity of another 42 GW under construction (Table 1).
In its March 2009 report, the International Atomic Energy Agency (IAEA) significantly increased its projection of world nuclear generating capacity. It now anticipates at least 70 new plants in the next 15 years, with up to 750 GW in place by 2030 – much more than projected in 2000 and over 100% more than actually produced in 2008. The change is based on specific plans and actions in a number of countries, including China, India, Russia, Finland and France, coupled with a change in outlook due in part to the Kyoto Protocol. This would give nuclear power a 17% share in electricity production in 2020, up from the present day figure of 6% (Fig. 4). The fastest growth continent for implementation of nuclear energy is Asia.
Fig. 4. World energy usage by type
In addition, 16 countries with existing nuclear power programs (Argentina, Brazil, Bulgaria, Canada, France, Russia, China, India, Pakistan, Japan, Romania, Slovakia, South Korea, South Africa, Ukraine, U.S.) have plans to build new power reactors beyond those now under construction.
In all, over 100 power reactors with a total net capacity of almost 120,000 MW are planned and over 250 more are proposed. Rising energy prices and environmental (greenhouse) constraints have combined to put nuclear power back on the agenda for projected new capacity in both Europe and North America with Asian capacity growing rapidly to keep pace with their fast growing energy demands.
Brief Overview of Nuclear Power
Fig. 5. Nuclear fission reaction
To provide the power for an electric generator, nuclear power plants rely on the process of nuclear fission. In this process, the nucleus of a heavy element, such as uranium, splits when bombarded by a free neutron in a nuclear reactor. The fission process for uranium atoms yields two smaller atoms, one to three free neutrons and an amount of energy (Fig. 5). Because more free neutrons are released from a uranium fission event than are required to initiate the event, the reaction can become self-sustaining, creating the familiar chain reaction under controlled conditions and yielding a tremendous amount of energy.
In the vast majority of the world's nuclear power plants, heat energy generated by burning uranium fuel is collected in ordinary water and is carried away from the reactor's core either as steam in boiling-water reactors (BWR) or as superheated water in pressurized-water reactors (PWR).
Boiling-water and pressurized-water reactors are so-called light-water reactors because they utilize ordinary water to transfer the heat energy from reactor to turbine in the electricity generation process. In other reactor designs, pressurized heavy water, gas or other cooling media transfers the heat energy.
Boiling Water Reactor (BWR)
Fig. 6. Boiling water reactor
Fig. 7. Pressurized-water reactor
In a typical commercial pressurized-water reactor (Fig. 7) the sequence of events are: (1) the reactor core generates heat; (2) pressurized-water in the primary coolant loop carries the heat to the steam generator; (3) inside the steam generator, heat from the primary coolant loop vaporizes the water in a secondary loop, producing steam; and (4) the steam line directs the steam to the main turbine, causing it to turn the turbine generator, which produces electricity. The unused steam is exhausted to the condenser where it returns to liquid form. The resulting water is pumped out of the condenser with a series of pumps, reheated and pumped back to the steam generator. The reactor’s core contains fuel assemblies that are cooled by water, which is force-circulated by electrically powered pumps. Emergency and other safety systems are similar to those in a boiling-water reactor.
Pressurized Heavy Water Reactor (PHWR)
Fig. 8. Pressurized heavy water reactor (PHWR)
This design is also called the CANDU reactor (Fig. 8) because it originated in Canada. It is very similar to the PWR with respect to the pressure system. However, the main difference is that these reactors use natural uranium as the fuel. There is, therefore, not a lot of fissionable U235 in the fuel, so a moderator is needed to control the speed of the neutron fission products. As many neutrons as possible are required to bombard the limited U235 in order to continue the fission chain reaction. Light water, though it slows down neutrons efficiently, also has a tendency to absorb the neutrons. These reactors therefore use heavy water (deuterium oxide, D2O or 2H2O) as the moderator. Deuterium oxide is less likely to absorb neutrons because deuterium already has twice the amount of neutrons as the hydrogen in light water. However, a large quantity of heavy water is needed to have an influence on the neutron speed. Another disadvantage of heavy water is that if it absorbs neutrons it produces the radioactive isotope of hydrogen called tritium. Though heavy water is expensive, costs are cut down by use of natural uranium. Furthermore, the PHWR can be refueled while in operation.
Gas-Cooled Reactors (GCR)
Fig. 9. Gas-cooled reactors (GCR)
Gas-cooled reactors are of European origin (Fig. 9). The coolant is carbon dioxide (CO2), and the moderator is graphite. Carbon dioxide is an effective coolant because it can be heated to high temperatures and can thus maximize heat transfer to the steam-generation process. Also, because it is gaseous, CO2 will not absorb the neutrons that are involved in the fission chain reaction. Gases are poor moderators, however, so graphite is needed because it is inexpensive, readily available and not damaged by high temperatures. The thermal efficiency of these reactors is very high but the fuel efficiency is low.
Light Water Graphite Reactors (LWGR)
Fig. 10. Light water graphite reactors
The Light Water Graphite Reactor is a Soviet invention (Fig. 10). It was uniquely designed to generate power and produce plutonium. The coolant is light water and the moderator is graphite. This coolant/moderator combination is unique to the LWGR. The light water vaporizes as it passes through the reactor core. This steam is used to drive the turbines. A major advantage of the LWGR is that it can be refueled while in operation.
Valves (Pressure, Steam, Safety)
The main steam system used in any power plant provides steam from the source (reactor) to the turbine. The system normally has several other functions:
- To provide the ability to prevent over-pressurization of the steam source (if the source puts out more heat than the turbine can accept)
- To provide the ability to prevent overcooling of the reactor coolant system (if the steam system draws off more heat than the source can provide)
The major components in most main steam systems are:
- Reactor steam line (BWR), steam generator (PWR) or Turbine Steam Separator (GCR).
- Main steam isolation valve – usually an air-operated or motor-operated valve used to isolate the steam source from the turbine.
- Safety valves – large relief valves that will open if steam pressure gets too high (same purpose as the pop valve on your hot water heater).
- Power operated relief valves – large air or motor operated valves that usually lift at a setpoint lower than the safety valves – in order to keep the steam pressure from getting too high.
- Non-return valve – a large valve that prevents backward steam flow in the steam line
Fig. 18. PWR valve locations
Typical PWR components (Fig. 18):
- Pressurizer safety relief (primary)
- Main steam safety relief (powered)
- Main steam safety relief (manual)
- Main steam isolation
- Main feedwater isolation
- Heater drain flow control
- Damped feedwater reverse flow check
- Feedwater regulation
- Aux-feedwater startup flow control
- Steam driven aux-feed pump flow control
- Chemical-injection flow control
Fig. 19. BWR valve locations
Typical BWR components (Fig. 19):
- Main steam isolation valve
- Main steam safety relief
- Damped feedwater reverse flow check
- Residual heat removal low cooling control
- Reactor core isolation cooling control
- Reactor level flow control
- Turbine bypass flow control
- Feedwater pump recirculation control
- Main feedwater regulation flow control
- Feedwater water startup flow control
- Low-pressure core spray flow control
- High-pressure core injection flow control
- Reactor-water cleanup flow control
- Heater drain flow control
- Energy Information Administration (2004 Forecast) http://www.eia.doe.gov/cneaf/nuclear/page/forecast/elec.html
- International Atomic Energy Agency and World Nuclear Association (http://www.iaea.org/programmes/a2/index.html and http://www.world-nuclear.org/info/reactors.html) Updated: 2/09.
- "Public Attitudes Toward America’s Energy Options: Insights for Nuclear Energy," MIT Center for Advanced Nuclear Energy Systems, June 2007.
- "A Nuclear Divide," IAEA Bulletin 50-1, September 2008.
- "Japanese Nuclear Energy Policy and Public Opinion," The Center for International Political Economy and The James A. Baker III Institute for Public Policy, May 2000.
- Nova Machined Products Inc. (www.nova-nsa.com), private correspondence.
- American Seal & Engineering (www.ameriseal.com), private correspondence.
- Tsai, H., "Carbide and Nitride Nuclear Fuels," Encyclopedia of Materials Science and Engineering (1986), Michael B. Bever (Ed.), pp. 493-495
- International Atomic Energy Agency (www.iaea.org)
- Energy Information Administration (www.eia.doe.gov)
- World Nuclear Association (www.world-nuclear.org)
- Nuclear Energy Institute (www.nei.org)
- The Nuclear Fuel Cycle, From Ore to Waste, P. D. Wilson (Ed.), Oxford Science Publications
- Y.P. Lin and J. DeLuc, "On the effects of heat treatment and surface orientation on corrosion and hydrogen ingress of Zr–2.5Nb pressure tube material," Journal of Nuclear Materials, 1999.
- Idaho National Laboratory/Nuclear Power Industry Strategic Plan for Light-Water Reactor Research and Development.