Safety regulations and standards imposed by national regulators on nuclear power plant systems provide high-level requirements, recommendations and/or guidance expressed in natural language. In many cases, this leaves a large margin for interpretation, not all of which are acceptable to a given regulator. Currently the elements that lead to the establishment of acceptable/accepted practices are not always documented, nor are these practices formally modeled. When a new standard appears or when Electricité de France (EDF) has to discuss a standard with another regulator, there is no systematic process to build a practice. Domain-specific modeling, traceability and variability modeling are Model-Driven Engineering (MDE) techniques that could address various aspects of practice formalization. This paper precisely defines the modeling issues that are currently faced by EDF when managing regulatory safety requirements, standards and practices. Then we review existing requirements modeling techniques to understand their benefits and limits according to EDF's needs.