News & Views, Volume 43 | Using Falcon to Develop RIA Pellet-Cladding Mechanical Interaction (PCMI) Failure Criteria
By: John Alvis
Introduction
The goal to achieve higher fuel rod burnup levels has produced considerable interest in the transient response of high burnup nuclear fuel. Several experimental programs have been initiated to generate data on the behavior of high burnup fuel under transient conditions representative of Reactivity Initiated Accidents (RIAs). A RIA is an important postulated accident for the design of Light Water Reactors (LWRs). It is considered the bounding accident for uncontrolled reactivity insertions.
The initial results from RIA-simulation tests on fuel rod segments with burnup levels above 50 GWd/tU, namely CABRI REP Na-1 (conducted in 1993) and NSRR HBO-1 (conducted in 1994), raised concerns that the licensing criteria defined in the Standard Review Plan (NUREG-0800) may be inappropriate beyond a certain level of burnup. Figure 1 is an example of a typical high burnup fuel cladding showing the oxidized and hydrided cladding of higher burnup fuel rods. Figure 2 shows the typical radial crack path in oxidized and hydrided cladding, subjected to RIA simulation tests. As a consequence of these findings, EPRI with the assistance of the Structural Integrity’s Nuclear Fuel Technology Division (formally ANATECH) and other nuclear industry members conducted an extensive review and assessment of the observed behavior of high burnup fuel under RIA conditions. The objective was to conduct a detailed analysis of the data obtained from RIA-simulation experiments and to evaluate the applicability of the data to commercial LWR fuel behavior during a Rod Ejection Accident (REA) or Control Rod Drop Accident (CRDA). The assessment included a review of the fuel segments used in the tests, the test procedures, in-pile instrumentation measurements, post-test examination results, and a detailed analytical evaluation of several key RIA-simulation tests.
As plants age, the need for inspection for service related damage to ensure unit reliability increases. There are several approaches that plants can take to reduce the risk of premature failures and proactively manage their DMWs. First is metallurgical sampling. Based on temperature profiles across the boiler, operating conditions, and operating history, DMWs can be selected for laboratory analysis. This will provide some insight into possible damage accumulation; however, the better approach, if damage is suspected, is to perform an ultrasonic inspection of the DMWs. This allows inspection of all the DMWs, and only requires access and surface preparation. If indications are detected, then tube sampling should be performed. It is critical to perform a metallurgical analysis of several of the DMWs suspected of containing service damage to confirm that the indications are service related and to help establish the extent of the damage compared to ultrasonic testing results. Typical DMW damage is described in the Featured Damage Mechanism article. The importance of the metallurgical analysis is demonstrated in the three following case studies.
As all of us who work with nuclear energy know the US nuclear industry is engaged in a multi-year effort to generate power more efficiently, economically and safely. A key goal includes a significant reduction in operating expenses. This initiative is termed “Delivering the Nuclear Promise” (DNP) and is supported by nuclear utilities, vendors such as Structural Integrity, the Nuclear Energy Institute (NEI), Institute 