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During the subsequent disaster, For these reasons, SiC reactor technicians could not remove fuel cladding is an impor- the decay heat from the reactor cores tant strategic technology quickly enough. Eventually, the heated for advanced nuclear fuels zirconium alloy fuel cladding reacted programs. with high-temperature steam to form However, a key SiC- hydrogen. That hydrogen built up cladding-related prob- inside the reactor containment build- lem—one that heretofore ing and led to explosions that damaged has eluded a satisfactory multiple reactor buildings. solution—is the final seal- The fuel cladding used in the ing of rods composed of the (Credit: EWI.) Japanese reactors is a reliable veteran material. Although hollow technology and current zirconium SiC rods can be formed with Figure 2. SEM image of the bond line between the two SiC blocks. In this area, the joint is 100-percent silicon alloy cladding technology benefits a closed end, one end must and uniformly less than 10 micrometers. from nearly 50 years of development remain open for fuel pellet and commercial operation. Its use has insertion. The challenge is proved to be economical, and it meets to join a SiC end plug to the all safety standards of utilities and regu- SiC cladding tube.6 lators. However, as evidenced by the Fukushima Daiichi disaster, it can fail Background on joining of catastrophically under extremely rare SiC for accident-tolerant beyond-design-basis accident condi- nuclear fuel and EWI tions. work Developing a way to seal SiC in nuclear applications a SiC tube is an inherently The disaster at the Fukushima difficult problem to solve, at Daiichi nuclear plant has fueled interest in SiC-based materials as a gy standpoint, and a solution (Credit: EWI.)-least from a joining technolo meltdown-resistant barrier. This is must be engineered with in- driven by SiC’s excellent thermal and service requirements foremost Figure 3. High-magnification SEM image of aluminum- environmental stability, resistance to in mind. These requirements rich phases interspersed in the silicon bond line. The radiation, resistance to thermal shock include that the joint must lighter-colored areas are the aluminum-rich phase, and and high strength and toughness (espe- be radiation tolerant, able to the darker area is silicon. cially when incorporated into a ceramic withstand temperature tran- matrix composite). sients well in excess of 1,000°C dur- rigorous approach to design a SiC The preference for SiC mainly derives ing LOCA scenarios, be stable under joining solution. from its stability at temperatures in excess flowing water with moderators, and be Many approaches have been devel- of 2,000°C. This implies that it will not able to retain hermeticity. In addition, oped for joining SiC in nuclear envi- melt under what engineers call “loss-of- it must be tough enough to withstand ronments, including glass–ceramic coolant accident” (LOCA) conditions, volumetric swelling of the SiC (on the bonding,6 displacement-reaction bond- and, thus, its use could lead to a large order of 2 volume percent) and vibra- ing using Ti SiC ,8 diffusion bonding increase in existing reactor safety. tion from the flowing water system. with metallic foil inserts,9 and braz-23 SiC has several other properties to Finally, the joining technology must be ing using silicon-containing materi- recommend it. It does not suffer from amenable to manufacturing consider- als.10 Unfortunately, none of these fretting wear, nor does it react to form ations, such as high throughput, while approaches has survived irradiation hydrogen. And, in addition to the safety retaining its thin-gauge fuel-cladding and flowing-water tests mimicking in- improvements, SiC has a lower neutron geometry. service reactor conditions. Furthermore, penalty than zirconium alloys, a prop- The first set of conditions, above, these approaches require the use of high erty that could allow for improved eco- tends to steer the materials selection pressures or extensive heating times nomics if the same thickness of material search toward brittle, high-melting- to form a satisfactory joint—consider- can be used in the new cladding.1 The point materials. These materials, how- ations which would make them difficult material also may allow for higher fuel ever, may not withstand the mechani- to manufacture. burnups (a measure of how much energy cal requirements, and any candidate EWI has taken a different tack on is extracted from a primary nuclear fuel technology must ultimately stand the the SiC joining problem. The goal in source) and reduce the amount of accu- test of manufacturing requirements. this work has been to develop a high- mulated used nuclear fuel. Therefore, engineers must undertake a temperature-tolerant and irradiation- American Ceramic Society Bulletin, Vol. 92, No. 1 | www.ceramics.org 33


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