CSP power tower receiver systems during rapid transient weather periods can be vulnerable to thermal shock conditions from rain that which can facilitate the onset of leaks and failures that can have catastrophic consequences. Silicon carbide (SiC) materials have attractive receiver application characteristics for being light weight, having high-strength and excellent thermal shock resistance performance which make them a particularly good fit for receiver absorber materials in CSP. In this investigation, the performance characteristics of Ceramic Tubular Products (CTP) SiC ceramic matrix composite (CMC), multilayered tubes were explored with respect to thermal shock performance for solar receiver applications in next generation CSP plants. Here, thermal shock testing was performed at the Sandia National Laboratories (SNL) Solar Furnace facility using a dynamic stage and thermal shock tube test setup. The tubes tested under incident solar heat flux of 100 W/cm(2) were heated with inner tube temperatures reaching approximately 800 degrees C, with outer temperatures exceeding or just reaching 1000 degrees C for the multilayer and monolithic SiC tubes respectively. The tubes were then quenched with simulated rain. The tubes were then cooled and subjected to hoop stress analysis using an Instron device to assess their subsequent mechanical strength. The on-sun study experimental results indicate an average of 24.2% and 97% higher hoop strength for the CMC tubes than those composed of monolithic SiC and aluminum oxide (Al2O3) respectively.

An experiment was conducted in the MIT Research Reactor (MITR) to irradiate triplex silicon carbide fuel cladding tubes under typical Pressurized Water Reactor conditions. Measurements were made to determine the impact of exposure on strength and swelling. The SiC clad tubes were fabricated by Ceramic Tubular Products (CTP) with dimensions typical of 15 x 15 commercial PWR reactor fuel. The triplex tubes contain 3 layers, an inner monolithic SiC layer to maintain hermeticity, a central SiC/SiC composite layer to provide a graceful failure mode in the event of an accident, and an outer SiC environmental barrier layer. Clad tubes were exposed to 300 degrees C pressurized water containing boric acid, lithium hydroxide, and hydrogen overpressure, typical of PWRs. Thirty nine (39) specimens of various types were exposed to coolant, some within the neutron flux region and some outside the neutron flux region. Twenty seven (27) were removed for examination and test after 4 months exposure. Following examination, twenty specimens were reinserted for additional exposure, along with 19 new specimens. The 4 month specimens were weighed and measured at MIT, and some were shipped to Oak Ridge National Laboratory (ORNL) where they were mechanically tested for hoop strength using a polyurethane plug test apparatus. Results were compared with the pre-irradiation strength and dimensions. Some specimens retained their original strength after exposure, others with a less homogeneous monolith, lost strength.

This paper summarizes the work performed to examine the feasibility of manufacturing internally and externally cooled annular fuel for high -power- density pressurized water reactors (PWRs) and to demonstrate commercially viable manufacturing processes at bench scale. Five different manufacturing processes were considered, and two were selected for further development and demonstration. These are (a) the traditional press and sinter technique currently used in solid pellet manufacture and (b) the vibration compaction (VIPAC) technique, in which granulated and sintered urania fuel particles are vibration compacted into a prefabricated annular space. Two separate pellet manufacturing trials were undertaken, one at the Westinghouse, Columbia, South Carolina, plant and one at INVAP facilities in Argentina. At the INVAP plant the pellets were loaded between small and large cladding tubes and seal welded to demonstrate the entire manufacturing steps. At Atomic Energy of Canada Limited, the VIPAC approach was used to perform short test segments as well as 1219-mm (4-ft)-long fuel rods. The overall conclusion of the work is that the press and sinter technique can produce annular pellets and annular fuel elements that meet the density and dimensional needs of the annular fuel design and hence is a viable approach toward fabrication of such high-power-density fuel. This process is most like that used in current commercial fuel production and hence would pose the least disruption in any future annular fuel use in commercial PWRs. This work also demonstrated that the VIPAC approach is capable of making highquality annular fuel elements, but not with the fuel density required for adequate performance. Addition of uranium metal powder to the vibrated compact was found to be necessary to achieve the required uranium fuel loading.

Most Advanced Reactors for Energy and Space Applications require higher temperature materials for fuel cladding and core internal structures. For temperatures above 500 deg. C, metal alloys do not retain sufficient strength or long term corrosion resistance for use in either water, liquid metal or gas cooled systems. In the case of water cooled systems, such metals react exo-thermically with water during core overheating accidents, thus requiring extensive and expensive emergency systems to protect against major releases. Past efforts to apply ceramic composites (oxide, carbide or nitride based) having passive safety characteristics, good strength properties at high temperatures, and reasonable resistance to crack growth, have not been successful, either because of irradiation induced effects, or lack of impermeability to fission gases. Under a Phase 1 SBIR (Small Business Innovative Research) project sponsored by DOE's Office of Nuclear Energy, the authors have developed a new material system that may solve these problems. A hybrid tubular structure (0.6 inches in outside diameter) consisting of an inner layer of monolithic silicon carbide (SiC) and outer layers of SiC-SiC composite, bonded to the inner layer, has been fabricated in small lengths. Room temperature permeability tests demonstrate zero gas leakage at pressures up to 120more » psig internal pressure. Four point flexural bending tests on these hybrid tubular specimens demonstrate a 'graceful' failure mode: i.e. - the outer composite structure sustains a failure mode under stress that is similar to the yield vs. stress characteristics of metal structures. (authors)« less