Investigation of the influence and mechanism of annealing temperature on the crushing strength of W layer prepared by chemical vapor deposition for nuclear thermal propulsion: Experimental and modeling analysis

Abstract

The design of tungsten (W)-coated fuel particles represents one of the pivotal strategies for enhancing the performance of cermet fuel. An intact W coating not only facilitates effective containment of fission products, enabling uniform dispersion of the fuel, but also effectively hinders the inward diffusion of the operating medium (H₂), thereby reducing fuel loss. However, during the preparation of cermet fuel, external pressure is often employed simultaneously to achieve high densification of the fuel pellets. In such scenarios, high external pressure may lead to damage to the W coating, rendering it ineffective in fulfilling its designed functions. Given this situation, the present study investigates the compressive performance of W layer annealed at different temperatures by experiments and simulations. The experimental results indicate that as the annealing temperature increases from 900 °C to 2300 °C, the maximum loading force value on the coated particles gradually decreases, ranging from 381.58 N to 135.99 N. Concurrently, the average grain size grows from an initial 0.36 μm at 900 °C to 13.30 μm at 2300 °C. Finite element simulations reveals that the fracture of W-coated particles initiates within the W layer itself, indicating that the crushing strength of the coated particles is equivalent to the crushing strength of the W layer. As the annealing temperature rises, the crushing strength of the W layer initially exhibits an increase, from 869.96 MPa at 900 °C to 874.59 MPa at 1500 °C. Subsequently, it declines to 750.10 MPa at 2300 °C. This trend can be attributed to the combined influences of surface densification and W grain size. Furthermore, a comprehensive discussion on how grain size, influenced by annealing temperature, affects the crushing strength of the W layer is also conducted by the analysis of stress distribution. This study explores, for the first time, the variation in crushing strength of W coatings with annealing temperature, thereby offering robust support for the optimized preparation of cermet fuels used in NTP systems.