No abstract

Detecting helium leakage is important in many applications, such as in dry cask nuclear waste storage systems. This work develops a helium detection system based on the relative permittivity (dielectric constant) difference between air and helium. This difference changes the status of an electrostatic microelectromechanical system (MEMS) switch. The switch is a capacitive-based device and requires a very negligible amount of power. Exciting the switch’s electrical resonance enhances the MEMS switch sensitivity to detect low helium concentration. This work simulates two different MEMS switch configurations: a cantilever-based MEMS modeled as a single-degree-freedom model and a clamped-clamped beam MEMS molded using the COMSOL Multiphysics finite-element software. While both configurations demonstrate the switch’s simple operation concept, the clamped-clamped beam was selected for detailed parametric characterization due to its comprehensive modeling approach. The beam detects at least 5% helium concentration levels when excited at 3.8 MHz, near electrical resonance. The switch performance decreases at lower excitation frequencies or increases the circuit resistance. The MEMS sensor detection level was relatively immune to beam thickness and parasitic capacitance changes. However, higher parasitic capacitance increases the switch’s susceptibility to errors, fluctuations, and uncertainties.

Dry cask storage (DCS) is increasingly used for extended long-term storage of spent nuclear fuel. The canister is vacuum dried and then pressurized with helium to ensure an inert atmosphere and efficient heat transfer. Thus, helium leakage detection plays an important part in ensuring the safety of nuclear waste storage facilities. However, as it is a noble light gas with no order or color, using conventional gas sensing techniques for detecting Helium is a challenge. To overcome this challenge, in this paper we present the working principle of a simple electrostatic MEMS structure to detect Helium in ambient air. The working principle of this novel sensor is based on the decrease in the air die-electric constant due to the presence of Helium. While this change is small, we show that activating the MEMS RLC circuit can significantly amplify the sensor response and hence increase its sensitivity. The sensor response is simulated at different Helium concentration levels using a finite element model. The simulations showed that an electrostatic MEMS sensor operating close to the electrical resonance frequency of an RLC circuit showed different deflection and as such a different capacitance at different levels of helium. A decrease up to 20% of the MEMS deflection was observed at 20% Helium concentration compared to near 0% concentration.