Characterization of SiC–SiC composites for accident tolerant fuel cladding

Abstract

Silicon carbide (SiC) is being investigated for accident tolerant fuel cladding applications due to its high temperature strength, exceptional stability under irradiation, and reduced oxidation compared to Zircaloy under accident conditions. An engineered cladding design combining monolithic SiC and SiC–SiC composite layers could offer a tough, hermetic structure to provide improved performance and safety, with a failure rate comparable to current Zircaloy cladding. Modeling and design efforts require a thorough understanding of the properties and structure of SiC-based cladding. Furthermore, both fabrication and characterization of long, thin-walled SiC–SiC tubes to meet application requirements are challenging. In this work, mechanical and thermal properties of unirradiated, as-fabricated SiC-based cladding structures were measured, and permeability and dimensional control were assessed. In order to account for the tubular geometry of the cladding designs, development and modification of several characterization methods were required.

Introduction

Silicon carbide (SiC) has been investigated for use in both fission and fusion applications, and recently has been considered as a candidate material for accident tolerant fuel cladding for light water reactors [1], [2]. High purity, crystalline SiC is a very stable material under neutron irradiation, undergoing only minimal swelling and strength changes to 40 dpa and higher [3], which represents many times the exposure for a typical light water reactor fuel life. In addition, SiC retains its mechanical properties at high temperature and reacts slowly with steam [4] compared to Zircaloy, thus affording improved safety for water cooled reactors in a loss-of-coolant (LOCA) and other potential accident conditions. However, monolithic SiC alone has low fracture toughness [5], making it unsuitable for nuclear cladding applications where fuel containment is essential and a coolable geometry must be maintained, especially under transient or off-normal conditions. The solution is to employ an engineered composite structure to address this brittle behavior, using strong silicon carbide fibers that reinforce a SiC matrix to form a SiC–SiC composite. Compared to monolithic SiC, these composites offer improved fracture toughness [6], pseudo ductility, and undergo a more graceful failure process [7]. High purity, radiation tolerant silicon carbide composites are typically fabricated using chemical vapor infiltration (CVI). While CVI provides the necessary purity for nuclear applications, it is challenging to reach very low porosity levels (<5%). As a consequence, the composite alone may not be sufficient to contain fission gases within the fuel cladding. Ultimately, a SiC-based cladding structure that is optimized to combine a tough SiC–SiC composite with a monolithic SiC layer, where the dense, monolithic SiC serves as an impermeable fission gas barrier and provides improved corrosion resistance, is the most promising design to achieve a completely SiC-based accident tolerant fuel cladding design.

SiC-based fuel cladding must meet a range of material property and performance requirements in addition to providing strength at high temperature, stability under irradiation, and reduced oxidation compared to Zircaloy. These requirements are primarily driven by differences between properties of silicon carbide structures compared to Zircaloy tubes, and the resulting implications of these differences on the performance. Specifically, the properties of SiC-based cladding are highly dependent on the processing route used, particularly for any fiber reinforced composite layers. In addition, while SiC–SiC composites undergo pseudo-ductile fracture rather than brittle failure, extensive micro-cracking occurs during this process which can lead to a loss of hermeticity. This micro-cracking occurs at strains in the range of 0.1% [7] a strain level at which Zircaloy cladding would not yet exhibit any plastic deformation [8], [9]. Accordingly, attention to characterization and careful development of the SiC-based cladding design is needed to mitigate micro-cracking and ensure hermeticity. Another consideration is that while silicon carbide has a lower irradiated thermal conductivity [5] than Zircaloy, it does have the advantage of not undergoing irradiation-induced creep [10] at LWR operating temperatures like Zircaloy, which will delay pellet-cladding mechanical interactions and associated stresses.

Achieving controllable cladding tube circularity, roughness, and straightness therefore is very important for predictable heat transfer through the cladding. The lower thermal conductivity of a SiC-based cladding leads to higher temperature gradients through the cladding for a given linear heat rate. These temperature gradients can lead to significant stresses due to thermal expansion and irradiation-induced, temperature-dependent swelling [11]. These stresses (and corresponding failure probabilities) can be reduced by decreasing the cladding wall thickness, which in turn lowers the temperature gradient. In addition, the cladding architecture (a combination of composite and monolithic SiC layers), can significantly influence the stress distribution though the cladding thickness during normal operating conditions as well as accident scenarios. With careful design, the stresses on critical layers within the cladding structure can be reduced. However, there are fabrication and handling challenges associated with both reductions in the wall thickness for long fuel cladding tubes, and production of specially designed tube structures.

The implementation of SiC-based accident tolerant cladding tubes in light water reactors will not only require design of optimized structures and development of consistent, scalable fabrication methods, this will also require thorough understanding and characterization of the material being produced. Among other performance metrics, the mechanical and thermal properties must be measured and the permeability must be assessed. A limited collection of test standards has been accepted by the community (ASTM C28.07 ceramic matrix composite sub-group), and development of additional characterization tools is necessary.

In this work, we report on the characterization of SiC-based tubes. The tube structures included fully composite tubes as well as tubes containing a monolithic layer on either the inner or outer surface, and were evaluated in the as-fabricated condition, or after additional processing steps had been performed. Mechanical, thermal, dimensional, and permeability measurements were made, and the utility of different characterization methods was evaluated.