Advanced oxidation-resistant iron-based alloys for LWR fuel cladding

Abstract

Application of advanced oxidation-resistant iron alloys as light water reactor fuel cladding is proposed. The motivations are based on specific limitations associated with zirconium alloys, currently used as fuel cladding, under design-basis and beyond-design-basis accident scenarios. Using a simplified methodology, gains in safety margins under severe accidents upon transition to advanced oxidation-resistant iron alloys as fuel cladding are showcased. Oxidation behavior, mechanical properties, and irradiation effects of advanced iron alloys are briefly reviewed and compared to zirconium alloys as well as historic austenitic stainless steel cladding materials. Neutronic characteristics of iron-alloy-clad fuel bundles are determined and fed into a simple economic model to estimate the impact on nuclear electricity production cost. Prior experience with steel cladding is combined with the current understanding of the mechanical properties and irradiation behavior of advanced iron alloys to identify a combination of cladding thickness reduction and fuel enrichment increase (∼0.5%) as an efficient route to offset any penalties in cycle length, due to higher neutron absorption in the iron alloy cladding, with modest impact on the economics.

Introduction

In 1975 Hyman Rickover summarized the key considerations that led to his decision almost three decades earlier to use zirconium alloys for the fuel cladding in U.S. Navy’s pressurized water nuclear reactor [1].

Almost four decades later, zirconium alloys enjoy a monopoly for uranium oxide fuel cladding material in light water reactors (LWRs) and are used in other fuel bundle structures such as portions of the grid spacers, as the channel box material for boiling water reactor (BWR) fuel assemblies, and elsewhere. Zirconium triumphed over other fielded clad options such as stainless steel, beryllium, and aluminum due to a combination of its small neutron capture cross section, reasonable corrosion resistance, and structural integrity under envisioned operating conditions. Today, zirconium alloy technology benefits from decades of active research and development, enabling tailored alloy chemistries and production techniques for optimized performance under pressurized or boiling water reactor conditions. The ongoing development activities have led to the introduction of new generations of zirconium-based alloys that exhibit enhanced corrosion resistance while limiting the detrimental irradiation effects (i.e., growth and creep) to those that are frequently inconsequential to reactor operation [2]. As summarized by Edsinger [3], these metallurgical enhancements, combined with incremental improvements in management for optimized reliability over the decades, have led to an impressive record of fuel reliability for the current zirconium-alloy cladding. To achieve this milestone, zirconium-alloy-cladded fuel production, fuel bundle handling, core operation, and water chemistry control have all been simultaneously optimized and improved over this time period.

This manuscript reexamines iron-based alloys for their potential application as nuclear fuel cladding to replace zirconium alloys. The motivation behind this effort is twofold. First, specific limitations with zirconium alloys under both design-basis and beyond-design-basis nuclear reactor accident scenarios provide incentive to explore alternative cladding options that may enable improved accident tolerance while maintaining good performance under normal operating conditions. These limitations are perceived to be serious, and therefore a renewed discussion is needed in the technical and policy communities.

Secondly, several new generations of increasingly higher performance steels are now commercially available that may offer significant performance improvements over the relatively simple austenitic steels that were utilized as cladding in some early commercial fission reactors [4]. Both alloy systems were successful in their early application, although zirconium alloys ultimately prevailed largely owing to their reduced thermal neutron cross section—a clear advantage over iron alloys for compact submarine reactor application. Driven by the nuclear navy, the zirconium alloy system became the focus of significant R&D, resulting in alloys with superior combined nuclear and corrosion performance and a ready-made commercial infrastructure. Finally, superior stress corrosion cracking resistance in BWRs drove the complete phase out of stainless steel claddings. However, given the significant progress over the past five decades in advanced steel making, including corrosion-resistant steels (combined with our present understanding of chemistry control and environmental effects), high-strength steels, and the ability to form thin-walled tubing, it appears clear that the commercial steels utilized in the early commercial reactors can be significantly improved upon, much as the zirconium alloys were improved upon since the days of Rickover’s program.

A review of the limitations with the current zirconium alloy system under design-basis and beyond-design-basis accident scenarios is offered. This review also provides simple metrics to guide selection of new clad materials. Based on the findings of a recent experimental survey of the high-temperature steam oxidation kinetics of historic, present-day commercial, and advanced steels [5], the potential to achieve higher margins of safety through replacement of zirconium alloys with advanced iron-based alloys inside LWR cores is showcased with a simplified set of analyses. A review of earlier generation stainless steels as nuclear fuel cladding in LWRs is provided and the anticipated performance of advanced iron-based alloys is briefly evaluated from a materials and environmental performance perspective, focusing on irradiation behavior, corrosion, and mechanical integrity under LWR normal operating conditions. Finally, the reactor physics characteristics and the impact on economics of nuclear electricity production upon adoption of advanced steel nuclear fuel cladding is examined with a specific set of case studies. In this manner, impactful areas for future targeted R&D in this area are identified and a broad basis for consideration of these advanced cladding concepts is provided.