Experimental system for studying temperature gradient-driven fracture of oxide nuclear fuel out of reactor

• Published in: Review of scientific instruments 2020-03, Vol.91 (3)
• Main Authors: Patnaik, S., Lopes, D. A., Besmann, T. M., Spencer, B. W., Knight, T. W.
• Format: Article
• Language: English
• Subjects: bandpass filter; beam splitter; boron-nitride test stand; DC power supply; dual-imaging system; induction heater; INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; LabVIEW; NUCLEAR FUEL CYCLE AND FUEL MATERIALS; vacuum chamber
• ISSN: 0034-6748; 1089-7623

Temperature gradients in ceramic light water reactor (LWR) uranium dioxide (UO2) nuclear fuel pellets generate thermal stresses that cause fractures in the fuel beginning early in the life of fresh fuel. The combination of heating due to fission and forced convective cooling on the exterior of LWR fuel rods generates a temperature profile that is difficult to replicate outside the reactor environment. In the present study, a state-of-the-art experimental set-up using electrical heating to replicate fission heating was built and surrogate fuel materials such as ceria (CeO2) were used to validate the system. Cracking experiments were conducted on these surrogates by inducing reactivity-initiated-accident (RIA) like temperature gradients in the pellets via induction and direct resistance heating. Induction heating was done using copper coils and molybdenum susceptors which heated the surrogates to a threshold temperature that is sufficiently high for the fuel material to conduct current. Thereafter, direct resistance heating was used by a D.C. power supply to introduce volumetric heating to replicate LWR operating conditions analogous to fission heating. The pellets were held against nickel electrodes and mounted on a boron nitride test-stand. All the tests were carried out in a stainless-steel vacuum chamber. Simultaneous real-time dual imaging of the surrogate pellet surface has been implemented using an optical and infrared camera system which will be mounted along axial and perpendicular directions to the pellet surface respectively. A beam-splitter was used to split the incoming radiation from the sample into two halves. While one of the beams is transmitted from the splitter through a bandpass filter to obtain optical images, the other beam is reflected from the splitter to the thermal camera to capture full field temperature gradients of the as fabricated pellet surface during crack initiation and propagation. In the current series of tests, a 2-color pyrometer was used for recording and comparing the surface and centerline temperatures of the surrogate pellets in lieu of the thermal camera. A LabVIEW data acquisition system has been set up for collecting useful data during experiments.