This article discusses specific heat-treated parts that are used in nuclear industry. New developments and collaborative efforts are also discussed.
|Fig. 22. Typical assortment of fasteners used in the nuclear industry|
Specific heat-treating applications will be presented in detail. These include bolts, seal rings and other types of fasteners/retainers. On-site heat treating is also discussed.
Bolts, Nuts, Other Fasteners, Retainers and Gaskets
The nuclear industry uses bolts and fasteners (Fig. 22) to very exacting standards (Table 4). For example, grade-8.0 bolts are manufactured from medium-carbon steel alloy and are vacuum heat treated, which allows the bolts to be used at high temperatures. In addition, special washers with dimples on the face compress as a connection is tightened, allowing measurement by a feeler gauge and monitoring of the tension applied to a joint. Substandard bolting, for example, grade 8.2 fasteners made of low-carbon martensitic steel, softens at temperatures over 260°C (500°F), making them unusable.
Seal rings have sophisticated microstructures and stringent requirements regarding grain size and flow lines (Fig. 23). Seals range in size from under 50 mm (2 inch) diameter to over 1400 mm (55 inches). They are formed from sheet and plate in thicknesses from 3.5 mm (0.140 inch) to over 12.7 mm (0.500 inch) thick and by machining them from forgings. They can be used in a variety of service applications (Table 5).
|Fig. 23. Typical metal seal ring|
Field Heat Treating
The need for heat treating also extends to installed components “on-site.” This primarily consists of preheating for welding, post-weld stress relief (PWSR), hydrogen bakeout and thawing of frozen lines and vessels. The types of heat treatment include:
1. Weld preheating
a. Reduces thermal stress
b. Compensates for heat losses
c. Slows the rate of weld hardening
d. Reduces porosity
e. Improves the microstructure of the HAZ (heat-affected zone) – finer grain size
2. Post-weld heat treating
a. Minimizes residual and thermal stresses
b. Tempers the material
c. Removes diffusible hydrogen
3. Refractory dry-out
a. Improves refractory strength
b. Prevents spalling (refractory separation)
c. Eliminates moisture (water)
d. Reduces thermal stress
New Developments - U.S. Research and Development
The dual issues of energy security and climate-change mitigation are driving a renewed debate over how to best provide safe, secure, reliable and environmentally responsible electricity. The combination of growing energy demand and aging or non-existent electricity generation infrastructure suggests major new capacity additions will be required in the years ahead.
Recent analysis by the Electric Power Research Institute (EPRI) shows that all low-emission electricity technologies will be required to satisfy anticipated goals for reduced CO2 emissions – renewable energy, nuclear energy, clean coal with CO2 capture and sequestration, and updated energy efficiency for all fuel sources. There is a growing consensus that large CO2 reductions cannot be achieved without a major contribution from nuclear energy.
In the U.S., due in large measure to the Department of Energy (DOE) NP-2010 Program and Congressional support for expanding nuclear energy, the nuclear industry has plans to construct as many as 30 new nuclear plants starting in the next decade. Similar growth is planned and under way around the world.
All of the currently operating nuclear plants in the U.S. and all of the planned additions of nuclear plants in the foreseeable future employ light-water reactor (LWR) technology. However, very little U.S. investment has been made in advancing LWR technology over the last decade. Industry has thus carried the R&D responsibility for LWR technology over these years with short-term needs dominating the research agenda.
The growing realization that a renaissance in nuclear plant construction is looming has caused both industry and the U.S. government to rethink the nuclear-energy research agenda. Starting early in 2009, industry, the DOE and the NRC began discussing the challenges ahead and the ability to meet them in ways that maintain high standards of safety and performance. Obstacles to very large build rates of new nuclear plants were examined and understood, and a renewed focus emerged on the importance of existing plants. The limitations to new plant construction – financing, infrastructure support, licensing, aging workforce, as well as a number of technical issues – all argued for taking a hard look at what would be required to enable further life extension of the existing facilities, even beyond the current license renewals (from 40-60 years). Some of the likely solutions to extended life of the current plants will also apply to the needs of the new facilities, particularly when one considers the challenges associated with very large new plant build rates.
From these considerations emerged:
Vision – Nuclear energy will reduce U.S. and global carbon emissions and enhance the U.S. energy security. Greater U.S. reliance on nuclear energy will improve its international engagement and leadership on nuclear safety and security issues.
Stretch Goals – Life extension of the current nuclear sites beyond 60 years and creation of strong, sustained expansion of advanced light-water reactors (ALWR) throughout this century, proceeding uninterrupted from first new plant deployments in about 2015 to sustained build rates over the next decade.
Strategic Plan – Use of a public-private partnership as well as international collaboration. R&D collaboration is now viewed as a pathway to improved nuclear safety and enhanced nonproliferation globally.
Example of Collaborative Effort in Heat Treatment
The influences of surface orientation and heat treatment on the corrosion and hydrogen pickup of Zr–2.5Nb pressure tube material is an area of active research. Nuclear experts in Europe and the U.S. are embracing a process developed in the Soviet Union to address the “embrittlement” that threatens some of their older reactors. The research centers around a new type of annealing process in which the steel casing of a reactor is heated beyond the temperatures it would normally experience to restore the ability of the aging pressure vessels to withstand sudden shocks.
For new designs, research has shown that a small amount of lanthanum and praseodymium will substantially improve the slow strain rate ductility of certain zirconium-based alloys. In the irradiated condition, these new alloys and certain other zirconium-based materials will see improved load-carrying capacity and extended service life. Alloys containing yttrium or calcium instead of lanthanum or praseodymium are also being investigated.
The corrosion and hydrogen pickup are affected by the alignment of b-Zr with respect to the oxide surface. An enhanced oxide growth is associated with the initially undecomposed b-Zr aligned primarily perpendicular to the oxide surface. A minor effect on oxide growth is also attributed to changes in the dislocation substructure due to heat treatment. A reduced hydrogen pickup is associated with b-Zr initially in the partially or fully decomposed states and aligned primarily perpendicular to the oxide surface. IH
Part 1 of this article can be found in June’s archives, the online exclusive is in July’s archives and Part 2 is in August’s archives.
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: firstname.lastname@example.org; web: www.heat-treat-doctor.com
The Nuclear Renaissance online exclusive originally posted in June contains the figures and references missing from this printed article as well as more detail on nuclear power generation.
Editor’s note: This is the final article in a three-part series, which is 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 (or reference) not found in print, locate it in the online exclusive here.