Research on the non-destructive testing technique of helium pressure in fuel rods based on the heat transfer method

【Background】:  The helium pressure within a nuclear fuel rod constitutes one of the most critical parameters directly influencing its mechanical performance, dimensional stability, and ultimately, the safe and reliable operation of a nuclear reactor. Traditional measurement techniques, primarily the destructive puncture method, involve physically breaching the rod's cladding to access the internal gas. This approach is not only inherently destructive, rendering the fuel rod unusable, but also inefficient, time-consuming, and costly. Consequently, it prevents comprehensive, 100% inspection of fuel rods during manufacturing or periodic in-service checks, creating a significant gap in quality assurance, batch consistency verification, and safety protocols for the nuclear industry. The pressing need for a reliable, accurate, and non-destructive testing (NDT) method has become increasingly prominent to enhance production quality control, reduce waste, and strengthen operational safety margins. The development of such a technique represents a significant step forward in nuclear fuel technology and reactor safety management.【Purpose】: The primary objective of this research is to develop, simulate, and experimentally validate a novel non-destructive testing device capable of accurately and efficiently determining the internal helium pressure of sealed fuel rods. This study aims to establish a robust and practical alternative to conventional destructive puncture methods, thereby enabling efficient, economical, and comprehensive pressure verification suitable for integration into industrial production lines and potential future in-service inspection routines.【Methods】: The research was designed and executed based on the fundamental principle of heat convection, which is influenced by the density and pressure of the surrounding gas medium. A dedicated non-destructive testing device was conceptually designed and subsequently developed. This device primarily consists of a controlled heating element, an array of high-precision temperature sensors, a data acquisition system, and a thermal isolation chamber. The investigation proceeded in two main phases. First, Fluent finite element simulation software was employed to model and analyze the steady-state and transient heat transfer characteristics from the rod's surface into the surrounding environment under a range of internal helium pressures (specifically targeting 0.98 MPa, 1.76 MPa, and 2.45 MPa). This comprehensive simulation process aimed to establish a precise mathematical relationship, or characteristic function, between the internal helium pressure (P) and a derived temperature-based characteristic value (V). Second, to experimentally validate the simulation model and rigorously assess the physical device's performance, extensive repeatability tests were conducted on three distinct fuel rods with pre-measured internal pressures (0.98 MPa, 1.76 MPa, and 2.45 MPa) using the standard puncture method. Each test involved placing the rod in the device, applying a stable and consistent heat flux, recording the resultant temperature distribution over time until thermal equilibrium was approached, and calculating the characteristic value V for subsequent correlation.【Results】: The finite element simulation yielded an exceptionally accurate and robust correlation between the derived characteristic function V and the actual helium pressure P. This correlation was demonstrated by a coefficient of determination (R²) value as high as 0.99935, indicating a near-perfect fit and a highly predictable relationship. Experimental results obtained from the prototype non-destructive testing device showed excellent agreement with the standard pressure values previously established by the destructive puncture method. The mean deviation between the device's pressure readings and the accepted standard values was calculated to be less than 0.05 MPa across all tested samples and multiple measurement cycles. Furthermore, every single individual measurement deviation consistently remained within the tight ±0.05 MPa threshold, highlighting the method's precision. The repeatability tests further confirmed the device's remarkable stability and reliability, showing consistently low standard deviations (e.g., below 0.02 MPa for repeated measurements on the same rod under identical conditions) across multiple measurements on each individual rod, thereby conclusively indicating high measurement repeatability and operational consistency.【Conclusion】: This study comprehensively confirms the technical feasibility and practical viability of utilizing the heat transfer principle, specifically heat convection, for the non-destructive evaluation of helium pressure inside sealed nuclear fuel rods. The developed NDT device, supported by rigorous simulation and experimental validation, demonstrates significant reliability, accuracy, and repeatability. Its measurement performance, characterized by deviations under 0.05 MPa and high repeatability, coupled with its operational efficiency, meets and potentially exceeds the stringent requirements for rapid quality control in modern nuclear fuel rod production lines. This method provides a viable, non-destructive, and economically attractive alternative to traditional destructive techniques, promising enhanced inspection coverage, improved product quality assurance, and a tangible contribution to elevated safety standards within the nuclear energy industry. Future work will focus on device miniaturization, automation for faster cycle times, and testing under a wider range of conditions.