CHAPTER 2: SEVERE ACCIDENT PHENOMENA AND MITIGATION STRATEGIES

Hydrogen production and combustion

Challenges

Hydrogen is generated by the steam oxidation of metals, notably of the zirconium fuel cladding. The reaction is strongly exothermic, and the rate is temperature dependent, so that oxidation of cladding strongly increases the core temperatures and heatup rates. In addition, CO is generated by the core-concrete interaction, after vessel melt-through.

Hydrogen is highly flammable and it may cause large explosions. Its ignition concentration is about 4% by volume in dry air. At or above this concentration deflagrations may occur, which cause moderate pressure loads. At higher concentrations (~>8 %), flames may accelerate and cause higher pressure loads. At even higher concentrations (> 14%) flames may accelerate to detonation ('Deflagration to Detonation Transition'), causing very high pressure loads. At very high hydrogen concentrations (~ 20 % and above) spontaneous detonations may occur. The presence of steam reduces the flammability of hydrogen in air. If the steam concentration is > 55%, then the atmosphere is inert and no combustion is possible. Below this concentration, combustion is possible, but the large amount of steam decreases the combustion loads.

Note that hydrogen is generated both in-vessel and ex-vessel, and later by radiolysis, and flooding an overheated core may generate a substantial amount of additional hydrogen. Note also that the partial volume of the generated hydrogen is quite large and can be on the order of the containment free volume itself, and thereby significantly increase containment pressure as a non-condensable gas component of the atmosphere.

Hydrogen burns may also cause excessive temperature in local areas, e.g. the containment hatch seals.

Hydrogen is extremely volatile, and will homogenize in the containment rapidly. In a compartmentalised containment, however, higher local hydrogen concentrations may exist. Ignition in one compartment may cause jet ignition in an adjacent compartment, which increases pressure loads.

The processes of hydrogen and CO generation, combustion and the associated loads are described in EPRI TBR, Vol. 2, App. D and S. Read more →

More information is present in OECD/NEA/CSNI/ R(2000)7, Read more →
and in OECD/NEA/CSNI/R(1999)16. Read more →

Recent information on the basics physics of hydrogen, hydrogen generation in severe accidents, hydrogen distribution and combustion in containment is presented in the following IAEA TECDOC.

Strategies

The following strategies can be considered:

(1) No strategies needed (e.g. in some large dry containments the pressure rise due to hydrogen combustion is well within the pressure bearing capability of the containment).

(2) Mixing the containment atmosphere to prevent high local concentrations of hydrogen:
  2.1 Promoting passive mixing through containment design (e.g. by opening flaps/ doors that separate compartments, may need additional pneumatic force, i.e. may need a plant modification);
  2.2 Actively mixing atmosphere through the use of safety or non-safety related systems.

(3) Inerting the containment atmosphere, that is, remove or dilute the oxygen.

Note 1: Inerting is employed in all existing BWR containments (nitrogen). Some PWRs can add steam from an auxiliary boiler to the containment.
Note 2: Dilution of the containment atmosphere will also diminish the otherwise potentially large flame acceleration and thereby result in more modest hydrogen combustion loads.
Detonability of hydrogen-air mixtures diluted by CO2, H2 an N2 is described in details in NEA/CSNI/R(2000)7 State-of-the Art Report.

(4) Purging the containment by venting and injection of gases (air, nitrogen, steam from auxiliary boiler).

(5) Consuming hydrogen by recombination or deliberate ignition;
  5.1 Use passive autocatalytic recombiners PARs;
  5.2 Use hydrogen igniters;
  5.3 Use a combination of igniters and passive autocatalytic recombiners.

Note 1: Two types of igniters are mostly used: glow plug and spark plug igniters. Glow plug igniters usually use AC source, spark plug igniters a battery.
Note 2: Passive autocatalytic recombiners may become igniters if the hydrogen concentration is high (~ 10% or above).
Note 3: The exhaust of passive autocatalytic recombiners can be very hot and may cause damage to nearby components (e.g. cables).
Note 4: PAR's require time to start up and become effective in recombining hydrogen, so if the source of hydrogen appears as a sudden RCS depressurization of accumulated hydrogen, the recombiners may not have time to become effective before flammable or detonable hydrogen concentrations locally in the region of the RCS break or release point are reached.

To estimate the content of hydrogen in the containment various measurements are available:
- Hydrogen monitor;
- Atmosphere sampling;
- Computational aid.

Note that the concentration of hydrogen may be inhomogeneous, hence, multiple (usually at least two) monitors are recommended. See IAEA TECDOC 1661, chapter 7. Read more →

A computational aid is sometimes used which gives the amount of hydrogen generated as a function of the amount of fuel cladding reacted (which can be estimated from the evolution of the accident; e.g. 50% reacted if no cooling has occurred, 75% reacted if limited injection took place after core damage).

Figure 2-2: Hydrogen production and combustion Westinghouse Owners Group Comp. Aid #3.