Cracking the mystery of cracks
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February 11, 2002—Cracks in nuclear reactor components are a serious concern to the nuclear power industry because they can impact safety as well as reactor performance. It has been long held that they are the result of alloy embrittlement or local changes to the water environment caused by radiation. With the use of state-of-the-art analytical electron microscopy and a new approach to access buried corrosion interfaces for study, however, Larry Thomas and Steve Bruemmer have discovered that the crack advance is promoted by radiation-enhanced, corrosion-induced material changes ahead of the tip of the crack.
While cracking of metal components inside nuclear reactors has been a continuing problem over many years, the ability to directly evaluate cracking mechanisms has been limited. Most research has been performed on non-irradiated components and even in this case only indirect approaches have been employed. With the new techniques developed at PNNL with support from DOE's Division of Materials Sciences, Office of Basic Energy Sciences, these processes can now be directly investigated at the leading edge of crack propagation buried within the material.
Thomas and Bruemmer with the key support of Clyde Chamberlin and other PNNL staff have developed the tools and techniques in laboratory radiation facilities to study the cracking phenomenon at the near-atomic scale. Using high-resolution transmission electron microscopy they have discovered that the metal composition is altered over nanometer dimensions immediately ahead of the crack tip. Nanoscale observations of cross-sectioned crack tips in an irradiated stainless steel core component reveal unexpected morphologies and local chemistries never seen in non-irradiated steels. This composition change is believed to promote crack advance and is caused by locally enhanced atomic movement due to excess space produced by the crack-tip corrosion process and neutron irradiation.
"When materials crack, the leading edge of the crack is often buried deep inside the material and no one has ever been able to look inside at the point where the degradation occurs. Now we can do it with great precision, allowing us to make a quantum leap in our understanding of corrosion and cracking mechanisms in a wide range of materials," said Bruemmer.
A fundamental understanding of the cracking process is an important first step in development of computer simulations to model propagation. It is also the first step in designing new alloys that can resist the corrosion process.
"Most metals and alloys are susceptible to some degree of cracking in corrosive environments such as high-temperature water. The more we learn, the more we can enhance the alloy performance and extend the safe operating life of reactors. We may never be able to prevent cracks from occurring but these new insights could give us the tools to significantly delay the onset of degradation and reduce the probability of failure," said Bruemmer. —by Mike Berriochoa
Media contact: PNNL Media Relations, pnl.media.relations@pnl.gov,(509) 375-3776.
Technical contact: Steve Bruemmer, PNNL Technical Group Leader, Engineering/Materials Resources, (509) 376-0636, stephen.bruemmer@pnl.gov
Related Web Link
"Clearing Up Core Corrosion Cracking," Breakthroughs, Spring/Summer 2001.
Funding: The characterization techniques were developed with support from DOE's Division of Materials Sciences, Office of Basic Energy Sciences. An international cooperative program coordinated by EPRI (formerly the Electric Power Research Institute) and including support from the Nuclear Regulatory Commission plus European and Japanese agencies, has enabled this first-of-a-kind research on highly irradiated components. PNNL was able to apply the knowledge and characterization techniques developed through DOE programs to work with international partners on a problem that plagues the industry worldwide.
Pacific Northwest National Laboratory is a DOE research facility and delivers breakthrough science and technology in the areas of environment, energy, health, fundamental sciences and national security. Battelle, based in Columbus, Ohio, has operated the laboratory for DOE since 1965.
Author: Mike Berriochoa is the Communications Manager for Pacific Northwest National Laboratory's Fundamental Science Division. He works closely with scientists to help them communicate their accomplishments to a broad audience. He has an extensive background in media relations and broadcast journalism. For more science news, see PNNL's News & Publications.