Accident tolerant fuels for LWRs: A perspective

Abstract

The motivation for exploring the potential development of accident tolerant fuels in light water reactors to replace existing Zr alloy clad monolithic (U, Pu) oxide fuel is outlined. The evaluation includes a brief review of core degradation processes under design-basis and beyond-design-basis transient conditions. Three general strategies for accident tolerant fuels are being explored: modification of current state-of-the-art zirconium alloy cladding to further improve oxidation resistance (including use of coatings), replacement of Zr alloy cladding with an alternative oxidation-resistant high-performance cladding, and replacement of the monolithic ceramic oxide fuel with alternative fuel forms.

Introduction

Nuclear power has proven to be a reliable, environmentally sustainable, and cost-effective source of large-scale electricity. In the interest of continued technological improvement, further improvements in operational reliability, economics, and safety under normal and transient conditions are being pursued worldwide. In regard to safety during anticipated transients and postulated accidents, the safety of the fuel can be affected by numerous fuel system phenomena, including cladding oxidation/hydriding, pellet-cladding interaction, pellet relocation and dispersal, and cladding embrittlement and fragmentation [1].

The high power density that makes nuclear power economical also makes the system susceptible to severe accidents. Typical power densities in light water reactor (LWR) cores are ∼50–75 MW_th m⁻³, which is about a factor of 100 greater than the average power density in fossil energy plant boilers [2]. In a typical loss of coolant accident (LOCA) scenario, a reactor scram dramatically reduces the power generation in the core. However, substantial amounts of heat continue to be generated following the scram: ∼7% of full power immediately after the scram, ∼1% of full power 4 h after the scram, and ∼0.2% of full power 10 days after the scram [3]. Considering that the thermal full power levels in commercial LWRs often exceed 3000 MW, the post-scram decay heat deposited in a reactor core without circulating coolant (i.e., approaching adiabatic conditions) can lead to rapid temperature increases and subsequent core degradation.

Also due to the high power density in nuclear reactor cores, the system has relatively limited margin for accommodating additional energy generation under normal operating conditions. Unlike fossil energy electricity generation sources, fuel, operation, and maintenance of nuclear reactors constitute only a minority fraction (∼one quarter) of the electricity generation cost (the rest being the capital costs). Therefore, the economics of nuclear reactor operation improves as the power rating of these units increases, which has led to many of the currently operating units undergoing power uprates. While plant power uprates involve a number of considerations, the thermal limits of the current fuels are one factor that is limiting future power level increases since further local power increases in the core could result in fuel damage under anticipated operational occurrences [1].

In addition to LOCA events as discussed above, another safety design basis is related to the potentially rapid power excursions that can occur from reactivity insertion events, such as control rod ejection in an LWR. Energy deposition resulting from reactivity insertion in the core can be limited by appropriate reactor physics design of the core, balancing the design reactivity worth of the control rods with the fuel temperature feedback. However, the specific fuel system response to any given magnitude of energy deposition varies during its lifetime because of the changing thermal–mechanical properties of the fuel/cladding system.

As discussed in Section 2, numerous operational and retrofit design changes were instituted in LWRs in the 1980s following the LOCA at Three Mile Island, and these changes have substantially reduced the probability of core degradation during a severe accident. These specific design features and upgrades were primarily associated with maintaining adequate core cooling in the event that the primary cooling system is not functional. The recent station blackout (SBO) accidents at three of the Japanese Fukushima Dai-ichi reactors following the devastating 2011 earthquake and tsunami have sparked renewed interest in exploring the possibility of further design and fuel system improvements that could improve the safety of LWRs under design-basis (DB) and beyond-design-basis (BDB) accident scenarios. Section 3 provides a brief overview of core degradation phenomena under DB and BDB accident conditions and possible mitigation by design changes and utilization of new fuel systems. Section 4 reviews the overarching motivation for investigating accident tolerant fuel (ATF) systems, and Section 5 briefly summarizes some of the ATF concepts currently being explored. Several accompanying papers in this special issue provide more detailed information on the current status of some specific ATF systems.