Small-sized Reactors of Different Types: Regulatory Framework to Be Re-Thought? 

Wolfgang Kröger 

Swiss Federal Institute of Technology Zurich (ETH) Zurich, Switzerland 

Abstract: All types of large reactors, subject of intensive development, are represented in SMR lines. A study of evolutionary (mostly water cooled), revolutionary (sodium or gas cooled), and exotic (salt or lead cooled) designs is performed, focusing on safety characteristics and assessment against tightened-up requirements; notably robustness against malicious interventions and instability of societies. In general, lower power and operating pressure reduce the potential of catastrophic releases; increased safety margins and special design characteristics almost eliminate risk of severe core damage, triggered by Reactivity Induced Accidents or Station Blackout. Active systems and early operator actions are avoided; the need for a tight containment, and emergency planning is often negated. 

  However, concept-specific accident scenarios such as fierce chemical reactions, flawed fuel addition, overcooling/freezing or air/water ingress deserve attention. Most developers claim that classical regulatory approaches to safety are inappropriate. However, relying on “one line of defense” and replacing active systems by passive, inherent mechanisms result in a shift of safety proofs to material properties, validity of experiments and computer codes, completeness of scenarios — under constraints of increased uncertainties. Furthermore, some reactor concepts are closely linked to elements of the fuel cycle, introducing new challenges. It seems evident that new regulatory concepts need to be developed — aiming to avoid unnecessary safety measures, while ensuring exceedingly high standards - and regulators to be educated, both in parallel with technological developments. 

Key words: novel reactor concepts, tightened safety requirements, regulatory framework