Passive Autocatalytic Igniter for Prevention of Hydrogen Detonation

Abstract

During beyond-design basis accidents in water cooled NPP, radiolysis, oxidation of zirconium and concrete-core interactions lead to transient release of large amounts of hydrogen inside the reactor containment. This involves rapid local formation of flammable mixtures of air and hydrogen which can release large amounts of energy during deflagration. Most NPP are equipped with passive autocatalytic recombiners which deplete combustible gases and allow to remain below the lower ignition limit globally. Rapid hydrogen release however may result in local concentration of combustible gases above the flammability limit. Uncontrolled ignition at high concentration may lead to loadings large enough to endanger containment integrity.
This rapid increase in hydrogen concentration can be limited at levels well below the detonation limit by early deliberate ignition at concentration close to lower ignition limit without endangering the containment. Initiating combustion as soon as small volumes of ignitable gas mixtures have formed leads to a basic gain in safety. The loads incurred are then distributed in time and space and do not pose a threat to containment integrity. For this purpose, Framatome has developed a passive operating catalytic igniter which requires no external power supply. The paper will give an rough overview about:

• Safety concept for controlled ignition
• Structure and function of the Framatome passive autocatalytic igniter
• Product history
• Qualification program