Experimental Study and Computational Simulations of Key Pebble Bed Thermomechanics Issues for Design and Safety, Reactor Concepts R-D.

摘要

An experimental and computational study, consisting of modeling and simulation (M&S), of key thermal-mechanical issues affecting the design and safety of pebble-bed (PB) reactors was conducted. The objective was to broaden understanding and experimentally validate thermal-mechanic phenomena of nuclear grade graphite, specifically, spheres in frictional contact as anticipated in the bed under reactor relevant pressures and temperatures. The contact generates graphite dust particulates that can subsequently be transported into the flowing gaseous coolant. Under postulated depressurization transients and with the potential for leaked fission products to be adsorbed onto graphite 'dust', there is the potential for fission products to escape from the primary volume. This is a design safety concern. Furthermore, earlier safety assessment identified the distinct possibility for the dispersed dust to combust in contact with air if sufficient conditions are met. Both of these phenomena were noted as important to design review and containing uncertainty to warrant study. The team designed and conducted two separate effects tests to study and benchmark the potential dust-generation rate, as well as study the conditions under which a dust explosion may occur in a standardized, instrumented explosion chamber. The project was awarded, August 2009, started several months later and concluded September 2013, with a one-year no cost extension. A separate effects experimental rig to investigate the frictional wear and nuclear grade graphite dust generation rate was specifically designed and constructed to operate up to PBR relevant conditions in temperature (up to 750DGC) and pressure (up to 7 MPa helium). The designed apparatus performed well and generated reliable data. In addition, the estimated experimental measurement uncertainty was considered to be acceptable relative to the measured dust masses generated.