CHAPTER 1: PROGRESSION OF A SEVERE ACCIDENT

Progression of a Severe Accident 

The following paragraphs provide a general summary of the progression of a severe accident in water-cooled power reactors. First, the sequence of accident progression is summarized for LWRs in general that are common for both PWRs and BWRs. In BWRs and PWRs the reactor fuel is arranged in vertical oriented fuel assemblies within a large pressure vessel where the coolant flows upwards in the core. Then a description of severe accident progression for the most common Pressurized Heavy Water Reactor (PHWR) type - the CANDU - is provided owing to the dramatically different geometry of the CANDU pressure tube reactor design, where the fuel assemblies are situated in horizontally oriented pressure tubes and the coolant flow moves laterally through each separate pressure tube.

LWR Core Damage Progression in Case of Unmitigated Severe Accident

If the reactor core becomes uncovered due to loss of coolant by initiating events (e.g. such as pipe breaks), the reactor fuel can become uncoolable resulting in overheating and severe damage. Upon overheating of the fuel, the Zircaloy cladding may first begin to deform due to local ballooning. As the fuel cladding continues to increase in temperature it may begin to oxidize in the steam environment, producing substantial quantities of hydrogen gas as well as additional chemical heat from the exothermic reaction with steam. The additional chemical oxidation heat, together with the residual decay heat, will ultimately melt the fuel cladding, resulting in loss of rod geometry and downward fuel relocation in the severely damaged core. The collapsing fuel stack will progressively relocate downward towards the core plate in PWRs and BWRs, eventually accumulating in the lower reactor vessel head where residual water may quench the fuel debris or produce energetic fuel/coolant interactions.

Once the remaining water in the lower reactor vessel head has evaporated, the debris will thermally attack the vessel wall and finally melt through it and relocate to the reactor cavity. During this whole process, a large amount of hydrogen is generated by the oxidation of the zirconium cladding and other metals by steam, and transported to the containment, where it can both pressurize the containment as a non-condensable gas as well as form an explosive hydrogen/air mixture. Significant threats to the containment barrier can result from gradual containment over-pressure or transient pressurization events from hydrogen burns or explosions. Molten core material will attack the cavity (BWR: drywell floor) and ablate the concrete and generate additional large quantities of non-condensable or flammable gases (H2, CO and CO2) from concrete decomposition. This can also cause overpressure in the containment, challenging the containment integrity. Additionally, molten material may ultimately melt through the basemat and contaminate the ground water.

A detailed description of the progression of a severe accident of a BWR and a PWR is available in EPRI TBR Vol. 1, sec. 2.5; Read more. A detailed overview of phenomena is given in EPRI TBR Vol. 2, located as an attachment to the Vol. 1, and in the subsequent sections of this Module. State-of-the-Art reports on the various subjects have also been published by the OECD/NEA/CSNI, Paris, France. See Index to NEA/CSNI Documents.

CANDU Core Damage Progression in Case of Unmitigated Severe Accident

In a hypothetical unmitigated CANDU severe accident, a total of five distinguishable Core Damage States (CDSs) can be identified as follows (see Fig. 2-1):

CDS1: After a sustained period of dryout due to the loss of coolant in the fuel channels, the fuel heats-up and the pressure tubes would balloon into contact with the Calandria tubes. The moderator will be the main heat sink.

CDS2: As the moderator heats-up and starts to boil, the moderator pressure increases and burst discs rupture causing partial loss of moderator due to expulsion and with continuous boiling, the moderator level would drop, and the fuel channels would start to be uncovered gradually. As the uncovered fuel channels continue to heat-up in steam environment they disassemble and form debris where part of it drops into the boiling moderator and other parts become suspended by the covered channels. Eventually all the core channels will be disassembled due to the heat-up and under the load of the suspended debris.

CDS3: The Calandria vessel becomes empty from water and the melted corium is accumulated in the bottom of the Calandria. The shield tank becomes the main heat sink. The water in the shield tank will start to boil. The pressure relief devices of the shield tank will open causing the steam to escape and the water level in the shield tank to drop.

CDS4: As the Calandria vessel becomes uncovered (shield tank empty) the corium will melt through the vessel material causing Calandria vessel failure. Corium will be relocated into the shield tank. If the tank is made of concrete as in CANDU-6 (called calandria vault), MCCI starts.

CDS5: As the corium penetrates the shield tank material, it creates a hole and displaces to the containment floor where MCCI takes places (or continues in case of CANDU-6).

 

Figure 2-1: Core damage states in case of unmitigated severe accident in a CANDU reactor.


Severe Accident Phenomena and Challenges to Fission Product Barriers

Due to the extreme environmental conditions associated with core degradation and related phenomena, a number of serious challenges to the fission product barriers, most importantly the containment boundary integrity, can result. These are:

       • Bypass of the containment (e.g. caused by steam generator tube rupture, or an interfacing system LOCA (ISLOCA).

       • Core relocation to the lower plenum (challenges the RPV integrity by potential vessel meltthrough).

       • Hydrogen production and combustion (explosions may challenge containment integrity).

       • High Pressure Melt Ejection with possible Direct Containment Heating (DCH - challenges containment integrity).

       • (Molten) Core Concrete Interaction (MCCI - generates large masses of CO2, may lead to basemat penetration).

       • Containment pressurization (by steam, non-condensables from the MCCI, and H2- and CO- combustion, may lead to slow static containment overpressurization and failure and releases to the environment).

       • Sub-atmospheric Containment Pressure (late phenomenon, after steam condensation, may lead to containment collapse).

Other natural processes and engineered safety features can mitigate releases to the environment and reduce consequences, including:

       • Gravitational settling of airborne radioactivity in the containment;

       • Scrubbing of airborne radioactivity by containment sprays and submergence of ex-vessel debris under water as well as scrubbing effects of suppression pools and containment vent filtering

Finally, some mitigating actions can have unintended negative effects such as:

       • Containment deinerting by action of containment sprays;

       • Additional hydrogen generation caused by water addition to overheated core materials.

A further description of these and other phenomena is presented in next sections: more detail is available in EPRI TBR, sec. 2.2.1. Read more →