EU DEMO safety and balance of plant design and operating requirements. Issues and possible solutions

Highlights: • An overview of few main design issues of DEMO reactor is outlined. • The possible solution for each of those issues has been briefly discussed. • The importance to develop in a homogeneous way the main systems of DEMO has been pointed in order to find an optimal solution to the issue. • The need of integration of design, safety in view of the nuclear license and the operation of the DEMO plant have been outlined. -- Abstract: The EU DEMO preliminary safety and operating design requirements are being defined aiming at obtaining the nuclear license. The DEMO design approach is being organized, by taking into account the Nuclear Power Plant, ITER and Generation IV lessons learnt. Outstanding challenges remain in areas exhibiting large gaps beyond ITER. Those require a pragmatic approach, especially to evaluate and improve the readiness of technical solutions through dedicated physics and technology R&D. Therefore, a system engineering approach is adopted based on an integral plant design analysis which follows clear defined goals as safety, availability and power provision to the grid. This ensures not only the identification of critical interfaces but also the margin of possible solutions and, moreover, the definition of target parameters for technical systems to be met in order to arrive at an optimal DEMO design. The overall DEMO plant design has to be strongly safety and operation-balance of plant (BoP) oriented. The paper describes a set of important aspects of safety and BoP that require early attention and a continuous reanalysis at any significant design change. This includes: (i) plasma instability and mitigation actions; (ii) radioactive inventory minimization and control; (iii) energy inventory control and mitigation; (iv) power conversion system challenges and possible simplification; (v) huge and complex electrical loads; (vi) layout of the tokamak building versus plant and safety criteria. Any effort to reduce the complexity of a Fusion Power Reactor through simplification and rationalization of the design of the main systems translates into a more robust plant configuration enlarging safety margins and operational thresholds.