The present report consists of two parts. The first and larger part (Sections 1–4) provides an overview of reactivity accidents: how they can arise, the basic principles of the defence against them, methods of analysis and acceptance criteria. The second part summarizes more recent work on ‘beyond-design-basis’ reactivity accidents. Contents: 1. Background and overview; 2. Design basis accidents; 3. Experiments on fuel behaviour; 4. Methods for analysing design basis accidents; 5. Beyond-design-basis accidents; 6. Conclusions.