Commercial off the shelf (COTS) devices with embedded software offer flexible and wide-ranging benefits recognised from technological advancements. Their use in nuclear safety systems has become prevalent but this has come with a difficult challenge for safety assurance. These new devices are complex and restricted access to evidence from the product developer to support afunctional safety audit can make their justification in safety-critical systems difficult. This paper presents a novel nuclear safety justification strategy termed ‘Model Based Safety Assurance’ (MBSA), which requires less invasive questioning and is thus less resource intensive for the developer. It uses concepts from Model Based Systems Engineering and applies them in the context of safety assurance, to achieve qualification of COTS devices for use in safety systems. The strategy utilises established techniques for software development (e.g. Model Based Testing) but extends their scope to support safety assessments. The paper also discusses the advantages and limitations of MBSA compared with the traditional safety demonstration approach currently used by the civil nuclear industry. Finally, with the help of a case study (based on a real system), it seeks to demonstrate the strength of the approach when combined with software safety assurance techniques such as Statistical Testing and Goal Structuring Notation.