We address the problem of safety alignment for large-scale language models with a Mixture-of-Experts (MoE) architecture. Specifically, we formalize and analyze the "positional vulnerability" where the safety-related behavior of an MoE model depends on specific expert modules. We present an analytical framework, called SAFEx, to identify, characterize, and validate safety-critical experts, classifying them into Harmful Content Detection Groups (HCDGs) and Harmful Response Control Groups (HRCGs). We investigate causality and test mitigation strategies using expert-level interventions. We demonstrate that blocking SAFEx-selected experts significantly impacts safety behavior for the Qwen3-30B-A3B model. Furthermore, we use LoRA to perform lightweight adaptation targeting HRCGs and improve the rejection rate for adversarial prompts without full model retraining through negative weight merging.