Context: Software is an important part in safety- critical system (SCS) development since it is becoming a major source of hazards. Requirements-related hazards have been as- sociated with many accidents and safety incidents. Requirements issues tend to be mitigated in companies with high processes maturity levels since they do their business in a systematic, consistent and proactive approach. However, requirements en- gineers need systematic guidance to consider safety concerns early in the development process. Goal: the paper investigates which safety practices are suitable to be used in the Requirements Engineering (RE) process for SCS and how to design a safety maturity model for this area. Method: we followed the design science methodology to propose Uni-REPM SCS, a safety module for Unified Requirements Engineering Process Maturity Model (Uni-REPM). We also conducted a static validation with two practitioners and nine academic experts to evaluate its coverage, correctness, usefulness and applicability. Results: The module has seven main processes, fourteen sub-processes and 148 practices that form the basis of safety processes maturity. Moreover, we describe its usage through a tool. Conclusions: The validation indicates a good coverage of practices and well receptivity by the experts. Finally, the module can help companies in evaluating their current practices. IEEE