A review of in-pile fuel safety tests of TRISO fuel forms and future testing opportunities in non-HTGR applications

The tristructural isotropic (TRISO) fuel particle is arguably the most robust nuclear fuel form ever developed. TRISO fuels have been realized in high temperature gas-cooled reactors (HTGRs) and have been proposed for many other nuclear energy applications including light water reactors, microreactors, nuclear thermal propulsion, and salt-cooled reactors. Significant data exist for steady-state irradiation testing and out-of-pile core conduction cooldown testing of TRISO fuel relevant to conventional HTGR applications. However, there is a lack of in-pile transient test data, especially related to fast transients, for advanced applications of TRISO fuel particles. This review article outlines the proposed advanced applications of TRISO particles, the existing in-pile transient and accident testing data, and highlights the potential value of additional in-pile transient test data for these new applications. Specifically, this review identifies the need for in-pile testing data to support licensing of TRISO fuel in light water reactors, microreactors, nuclear thermal propulsion, and salt-cooled reactor applications. In advanced non-HTGR TRISO applications the temperature transients may occur much faster than in HTGR depressurized loss-of-forced cooling (DLOFC) accidents (e.g. 0.05 degrees C/s in a DLOFC versus up to 1000 degrees C/s in an advanced TRISO application). Generally, historical tests from the literature indicate significant fuel particle failure at energy depositions greater than 1400 J/g-fuel, mostly due to kernel melting. Fuel compact graphite matrix failure was observed above 2300 J/g-fuel in historical tests of compacts. Failure thresholds and mechanisms may be different for particles with fuel materials different from uranium dioxide (such as uranium oxycarbide and uranium nitride), particles with different irradiation histories, and particles with different structure (layers and relative thickness). Although kernel melting was the dominant failure mechanism during historical tests, other failure mechanisms may be possible. One example is thermal stresses between coating layers during quick increases in temperature which could result in crack propagation and delamination of coating layers. Another example is matrix failure due to stresses, for example a graphite matrix in conventional HTGR-like fuel or silicon carbide matrix in fully ceramic microencapsulated fuel. The transient reactor test facility (TREAT) at Idaho National Laboratory is an ideal location for these fuel tests to be carried out in the United States. (C) 2020 Elsevier B.V. All rights reserved.