Hydrogen mitigation in water cooled power reactors is an important area of study in the field of safety of nuclear reactors. Hydrogen and oxygen can be generated during normal operation of a power reactor primarily as a consequence of the radiolysis of the water in the core.
During accidents hydrogen and oxygen can be generated also as a consequence of:
- Metal–water reactions in the core
- Radiolysis of the water in the sump or the suppression pool
- Degassing of hydrogen dissolved in the primary coolant
- Chemical reactions with materials in the containment ( interactions of molten core debris with concrete)
- Thermolysis of water – in extreme cases
During DBAs (Design Basis Accidents) such as the large break loss of coolant accident, the production of hydrogen (metal–water reactions in the core) is limited at low values by the operation of the emergency core cooling systems. Also the radiolysis of the water in the core is relatively slow process. Therefore, from the DBAs point of view the hydrogen hazards can be eliminated by maintaining the local hydrogen concentration below its flammability limit (4% of volume). This requirement can be ensured by mixing devices or thermal hydrogen recombiners.
In the past two decades, research activities to analyse the threat of hydrogen in a post-accident condition have focused mainly on the mitigation of hydrogen hazards in severe accidents. In severe accidents consider the possibility of the large scale core degradation and even the possibility of molten core concrete interactions.
For these cases the capacity of conventional DBA hydrogen control is insufficient. During severe accidents, local hydrogen concentrations can exceed its flammability limit in a short time. analysis of the hydrogen threat in post-accident containments is complex and highly plant- and scenario-specific.
In order to avoid the threat of hydrogen explosion, mitigation measures for hydrogen in severe accidents must be implemented. The strategy is usually based on following physical principles:
- Avoiding of formation of flammable mixtures by oxygen control.
- Avoiding of formation of flammable mixtures by hydrogen control
- Avoiding of flammable or detonable concentrations of hydrogen by controlled ignition.
The most effective measures are:
- Passive or active air mixing systems.
- Post accident dilution (PAD) of containment atmosphere by inert gas injection.
- Catalytic recombination of hydrogen.
- Controlled ignition by deliberated ignition system (DIS).