Hydrogen can be formed at various stages of a severe accident as a result of chemical accidents between steam and various metals. This hydrogen can burn or detonate, hazarding the containment systems.

The most important contributor to the hydrogen formation process is the ox­idation of the zirconium cladding of the fuel:

Zr + 2H20 = Zr 02 + 2H2

The reaction is exothermic adding to the decay heat. The extent of the chemi­cal reaction is determined by a number of factors, including the access of steam to unreacted metal and the geometiy of the core debris. Other materials that react include chromium and iron and even uranium dioxide.

Hydrogen may he formed at various phases of the accident:

1. When the initial heat-up occurs; perhaps 20-40% of the cladding may react in the first 10 or 20 minutes

2. When further water from the ECCS system or reactor coolant pumps contacts the hot debris

3. When molten debris jets or falls into the vessel lower head and vaporizes water to steam, which then has access to relatively undamaged fuel in the core above

4. When the pressure lower head fails and the molten debris attacks the con­crete of the vessel cavity and containment

In the case of a large loss-of-coolant accident (LOCA), the hydrogen may be released to the containment as it is formed. Conversely, where the primary circuit remains intact, the hydrogen release may occur at the time of lower head failure.

Hydrogen can react with the oxygen within the containment in one of two ways. The first way is by deflagration, or a diffusion flame in which the unburned gas is heated by conduction to a temperature sufficiently high for a chemical re­action. Whether a combustion reaction takes place depends on reaching the min­imum concentration of the hydrogen, i. e., 4-9% by volume. While diffusion flames and slow deflagrations add to the heating load and therefore the pressur­ization of the containment, they do not represent a serious threat to the integrity of most designs. Such a deflagration occurred during the TMl-2 accident.

In the second way, in a detonation, the unburned gas is heated by compres­sion in a shock wave. Initiation can come from a spark or other high-energy source. The consequences of a detonation depend on the concentration of the hydrogen (the higher the concentration the higher the combustion pressure) and the geometry of the containment internals.

One means of controlling hydrogen is to have an inert atmosphere (nitro­gen) in the containment. This is used on some reactor designs, particularly those of BWRs, but has operational disadvantages. Other techniques include catalytic recombiners (which react hydrogen and oxygen to form steam) and ig­niters (which deliberately ignite the hydrogen at the lower mixture concentra­tion) installed at various locations within the containment.