III—1. A severe accident is defined as one for which the accident conditions are more severe than those for a design basis accident and involve significant core degradation. Severe accidents begin with loss of cooling for the reactor core and initial heating-up of the fuel, and continue until either:

(a) The degraded core is stabilized and cooled within the reactor pressure vessel, or

(b) The fuel overheats to the point of melting, the reactor pressure vessel is breached and molten core material is released into the containment.

The potential detrimental effects of a severe accident include:

— Overheating and overpressurization of the containment due to molten core material settling into the reactor cavity,

— The generation of significant amounts of hydrogen and other non­condensable gases owing to the interaction between molten core material and concrete,

— Structural damage to metallic components of the containment due to direct contact with molten core material,

— High pressure ejection of molten core material and subsequent rapid direct heating of the containment.

III-2. The phase of progressive in-vessel heating-up and melting establishes the initial conditions for the assessment of the thermal and mechanical loads that may ultimately threaten the integrity of the containment.

III-3. The ex-vessel progression of severe accidents is affected by the mode and timing of the failure of the reactor pressure vessel, the pressure in the reactor coolant system at vessel failure, the composition, amount and nature of the molten core debris expelled, the type of concrete used in the construction of the containment, and the availability of water in the reactor cavity. Some highly energetic phenomena may be caused by severe accidents. Such phenomena could cause the ultimate load bearing capacity of containments constructed by means of existing technologies to be exceeded, and conse­quently lead to a large early release of radionuclides to the environment.

III-4. For some reactor types the risks associated with severe accidents occurring in conjunction with high pressures in the reactor coolant system would, without countermeasures, contribute significantly to the overall risks associated with severe accidents. Severe accidents occurring in conjunction with high pressures in the reactor coolant system could give rise to unacceptable challenges to the containment barrier.

III—5. At high pressures in the reactor coolant system, the molten core material from the reactor vessel could be ejected in jet form, causing fragmen­tation into small particles. It may be possible for the core debris ejected from the vessel to be swept out of the reactor cavity and into the upper containment. Finely fragmented and dispersed core debris could cause the containment atmosphere to heat up, leading to large pressure spikes. In addition, chemical reactions of the particulate core debris with oxygen and steam could add to the pressurization loads. Hydrogen, either pre-existing in the containment or produced during the direct heating of the containment, could ignite, adding to the loads on the containment. This phenomenon is known as high pressure melt ejection with direct containment heating.

III—6. Loads due to a direct containment heating event may be mitigated by using a design of reactor cavity that reduces the amount of ejected core debris that reaches the upper containment, to the extent that the features of any such design do not unduly interfere with plant operations, including refuelling, maintenance or surveillance activities. Examples of design features of the cavity that would reduce the amount of ejected core debris that reaches the upper containment include:

(a) Ledges or walls to deflect core debris,

(b) Indirect paths from the lower reactor cavity to the upper containment. CONTAINMENT BYPASS

III-7. For pressurized water reactors, the likelihood of creep failure of steam generator tubes for some severe accidents at high pressure of the reactor coolant is not negligible, with the possible consequence of a containment bypass.

III—8. To minimize the potential for containment failure or containment bypass in severe accidents at high pressures of the reactor coolant, the plant features may be enhanced, if necessary, to depressurize the reactor coolant system reliably so as to prevent this process from occurring.