Numerical investigation of helium leakage and transport from in-vessel cooling-circuit in fusion reactor using DSMC method

Early detection of micro-leaks is essential for fusion reactors because such leaks precede a large-break loss-of-coolant accident (LOCA), which can seriously threaten vacuum-vessel structural integrity and reactor safety. In high-vacuum vessels, micro-leak identification depends on the transport characteristics of the plume released from the leak, making plume simulation and feature analysis important for leak detection. However, micro-leak plumes evolve across transitional and free-molecular regimes, where conventional continuum methods become limited. The direct simulation Monte Carlo (DSMC) method provides a promising approach for resolving cross-regime rarefied plume transport. In this work, DSMC is used to investigate helium leak plumes in the CFEDR vacuum vessel. Representative leak locations at the inboard blanket, outboard blanket and lower divertor, together with different leak intensities, are analyzed in terms of plume morphology, pressure distribution, affected volume and probe response. The results show that the plume evolves within 1-100 ms from a directional jet into a vessel-scale diffusive field, while pressure decays by approximately two to three orders of magnitude across the chamber. The pressure responses differ among the three representative leak locations, indicating that poloidally distributed probes can provide location-dependent signal features for the future development of leak-localization methods. At a leak intensity of 5 × 10^(-5) kg/s, representative window probes reach the 10^(-4) Pa threshold within 4.35-8.50 ms, whereas lower-intensity leaks remain confined near the source and require more sensitive or cumulative detection. These findings provide preliminary guidance for multi-probe, multi-threshold monitoring strategies for early in-vessel LOCA warning in fusion reactors.