The project objectives are to determine the pathway of Cs diffusion in the polycrystalline 3C-SiC substrates. The expected results from LEAP will show whether diffusion occurs along grain boundaries or occurs through the polycrystalline bulk material. This result will address a knowledge gap presented by Hobbins et al at the VHTR FY13 meeting. Sample preparation will require the use of a FIB, including an additional Cr protection layer, to produce the sample tips and TEM to verify that the desired region of the sample (recrystallized SiC) is present in the tips prior to analyzing with the LEAP. Samples include two 3 mm diameter x 0.8 mm thick planar TEM disks and two 0.8 mm diameter TRISO SiC hemispheres (produced from ZrO2 surrogate TRISO particles) that have had Cs implanted into the substrate and then recrystallized through a thermal step. The result is a ~250 nm region composed of some ~200 nm grains in a very fine grained SiC matrix. Samples also are subjected to diffusion steps of 10 hours at 1000°C or 1100°C. Understanding how Cs moves through the SiC layer will provide valuable insight into the kinetic mechanism of Cs diffusion in the SiC layer of the Gen IV TRISO fuel particle. If grain boundaries are a significant contributor to the diffusion of Cs it may be necessary to minimize grain boundary area through coarsening. Through discussion with the instrument technicians, the project is estimated to require about 84 hours of instrument time. The samples are presently ready for delivery at the first available opening in instrument booking.

The Advanced Gas Reactor (AGR) Fuel Development Program (part of the Next Generation Nuclear Plant (NGNP) Program) is tasked to define and validate the Generation IV nuclear fuel cycle and the technologies required for its implementation. The expectation is that through this program, sufficient information will be available to drive the qualification process for the reactor and fuel. While significant work has been published on the effects of fission products in the proposed Generation IV clad fuel particle (TRISO), questions still remain including the behavior of cesium (Cs).

Presentations at the recent VHTR FY13 Technical Review Meeting show post irradiation examination (PIE) results of Cs release to be at very low levels. It is suggested that this release may be from failed SiC layers in the fuel particle or significant local corrosion of the SiC due to interaction with certain fission products. However, a presentation by Hobbins et al suggests that a gap exists in the understanding of the kinetic mechanism for select fission product diffusion and “more data is needed at the micro and nano scale to identify transport mechanism and understand how best to describe SiC.”

The objective of the submitted proposal is to advance the understanding of the diffusion behavior of Cs in the SiC containment layer of TRISO. Understanding how Cs moves through the SiC layer can provide insight on the microstructural engineering of this containment layer. If grain boundaries are a significant contributor to the diffusion of Cs it may be necessary to minimize grain boundary area through coarsening.

The proposed study will provide valuable insight into the kinetic mechanism of Cs diffusion in the SiC layer of the Gen IV TRISO fuel particle and is a natural extension to the work already conducted. It will contribute to the NE programmatic goal of the “development and deployment of next-generation advanced reactors and fuel cycles” by addressing some of the knowledge gaps in Cs diffusion for the AGR Fuel Development Program.