In accident situations, reactor materials can undergo chemical reactions that release stored energy in addition to the generation of decay heat. The primary reactions which occur at operating or modestly low temperatures are listed in Table 1.6 and elaborated below:

• For the sodium reactor, oxidation of the sodium coolant released by steam generator tubing failure by contact with secondary system water which also produces hydrogen; of less concern is the sodium reaction with air which causes relatively low heat release but vigorous emission of oxide fumes. The sodium leak in the Monju reactor to air from failure of an instrument penetration in December, 1995, caused only modest sodium leakage to the piping compartment. The event forced the shutdown of the reactor for 14 years, even though the overwhelming portion of this period was due to loss of public confidence versus the need for repairs and refurbishment. The EBR-II had to deal with numerous sodium leaks during its 30-year operating lifetime. These leaks were safely managed and the reactor operated as both a research reactor and a small power demonstrator.

• For graphite-moderated reactors, graphite oxidation from inadvertent air ingress; release of stored energy due to atom displacements in graphite (Wigner energy) can also occur as happened in the UK Windscale reactor, but the elevated operating temperature of modern SMR gas-cooled reactors eliminates this energy storage mechanism.

All other chemical reactions of interest occur at very high temperatures which would be encountered if the reactors suffered conditions of core degradation. These include the following:

• For water-cooled reactors, oxidation of the zircaloy and steel core cladding and structures by the primary water coolant; this reaction is not only strongly exothermic but also produces hydrogen. The hydrogen when mixed with dry air is flammable in a composition range between 4% and 75% H2. Typically containments are sized in PWRs to maintain hydrogen content below 4% by volume; BWRs employ smaller containments by virtue of their pressure suppression design which then requires either inerting (Mark I and II designs) or employment of hydrogen recombiners and igniters (Mark III design) to prevent hydrogen burning or explosions.

• For all liquid-cooled reactors, oxidation of metals that may exist in molten core material (called corium) by water and carbon dioxide released from thermal decomposition of the concrete containment basemat upon contact with corium; corium contact with the basemat could only occur if the reactor vessel failed.

A major positive characteristic of lead and lead-bismuth coolants is that their reactions with water/steam and air are slight and hence of no reactor safety consequence.