Beyond Design Basis Thermo-Mechanical Approaches to Improving Safety Performance

Abstract of the technical paper/presentation presented at SMiRT27
March 3-8, 2024

Prepared by:
Thambiayah Nitheanandan
Canadian Nuclear Safety Commission

Abstract:

The conventional design envisages the mechanical structure to be adequately resilient to avoid failure under thermo-mechanical loads anticipated from Design Basis Events (DBE). A resilient material absorbs energy as it elastically deforms until the yield point and, upon unloading, releases the energy. A structure subjected to a Beyond Design Basis (BDB) load may, however, remain within the resilience regime, having only exceeded the design stress but below the yield stress. As the load exerts stress to the material beyond the yield point, the energy absorption shifts from resilience to toughness. Toughness equals much of the total area under the engineering stress-strain curve up to fracture, while resilience makes up only a small portion of the total energy the material absorbs before failure.

Despite the advantage of improved material capacity to absorb energy in the toughness regime, one drawback of this regime is the cliff edge effect associated with plastic deformation. The yield strength is the stress a material experiences while deforming plastically without increased stress. Beyond this strength, the material fails suddenly and uncontrollably, rendering toughness in design less reliable unless adequate research and development data can confidently back up such designs. Recent advancements in instrumentation, material science, and artificial intelligence-driven automated controls can avert catastrophic failures of structures, systems, and components (SSC) in nuclear reactor applications.

The objective of the paper is to explore and capture the new structural thermo-mechanical approaches emerging to prevent and mitigate BDB events. These events are difficult to predict either because they have never occurred or have a low probability of occurring such that they escape the attention of the designers and are closely related to the idea of the black swan theory. Events that occur due to human error, poor design and negligence in construction are not considered BDB events since these are predictable and preventable. Considerable research is being undertaken to enhance system designs. Although BDB accident sequences are low in probability, they are nevertheless a possible source of remaining risk. These risks merit further attention and need to be identified and considered through specific features during the plant design. Examples in three critical areas of development are discussed in the paper to illustrate the modern features available for preventing and mitigating BDB events.

Significant research and development efforts are ongoing in developing accident-tolerant claddings and tolerant fuels, enhancing heat rejection capabilities in reactor vessel outside surface with surface modifications and glass-peening the outside surface of the calandria tube, developing novel means to radiatively reject heat during loss coolant accident with loss of emergency core cooling. All these developments require research, strengthening of codes and regulations, along with continuous dialogue between regulators, designers, operators, and standards organizations. The Canadian Nuclear Safety Commission is keenly following these developments to ensure that nuclear safety is ensured in deploying advanced and conventional nuclear reactor systems in Canada.

To obtain a copy of the abstract’s document, please contact us at cnsc.info.ccsn@cnsc-ccsn.gc.ca or call 613-995-5894 or 1-800-668-5284 (in Canada). When contacting us, please provide the title and date of the abstract.