Types of nuclear accidents

Nuclear accidents can be placed into categories depending on how they occurred. In this post, I will describe the types and also give examples.

Meltdown:

It is often referred to as a nuclear/core meltdown, core melt accident, partial core melt, or just meltdown, and is defined as the accidental melting of a reactor core due to overheating, and can describe a complete or partial collapse. Generally, reactors produce heat as they function, which is why they have cooling systems to pump coolant in the reactor. If said coolant is lost (LOCA - Loss Of Coolant Accident), has very little pressure (loss of pressure control accident) -both of them are described in further detail below-, very little flow rate, or if a criticality excursion (an accidental and uncontrol fission chain reaction) occurs that leads to a power level above the design limits of the reactor, the heat generated by the reactor can surpass the amount of heat that the cooling systems can remove, and thus lead to at least one fuel element to exceed its melting point. Once that happens, the cladding of the fuel has been breeched and the fuel (can be uranium, thorium, or plutonium depending on the reactor type) as well as various fission products within can be released into the coolant. Superheated steam and hot metal inside the reactor can result in steam or hydrogen explosion, and/or steam hammer (a surge/wave of pressure when a fluid/gas is forced to stop moving or change direction way too fast), thus leading to potential containment destruction and release of radioactive material into the environment, which would further lead to contamination of the environment and exposure to people and animals. It is possible for a reactor core to be damaged without being online due to decay heat (heat released due to radioactive decay). 

More about LOCA and loss of pressure control accident: Loss of coolant can mean either the physical loss of coolant or the loss of the means to cool down a reactor sufficiently. Specifically, PWRs can have steam form in the core due to the excessive heating of coolant. During a loss of pressure control accident, the pressure of a confined coolant drops without a way to restore it. Sometimes, that can reduce the efficiency of heat transfer and thus pose a challenge to reactor cooling.

Nowadays, there are many safety systems to prevent meltdowns. 

Light water reactors:

For light water reactors to have a damaged core, failure of heat removal within the core and the failure of the emergency core cooling system are necessary, which can lead to the potential escape of molten corium (lava-like mixture produced during meltdown consisting of nuclear fuel, fission products, control rods and other parts of the reactor, water, steam, products of their reactions with air, and concrete if the corium breached the vessel), leading to a full meltdown. It is created through the following process: uncovering of the core (exposing of the core to a steam environment and it heating up due to the reactor coolant pumps not running, however can be delayed up to 2 hours with a two-phase mix of steam and water), heat up before damage (heating up of fuel rods in a steam environment), ballooning and bursting of the fuel (in thirty minutes, the core temperature would reach 830°C and the zircaloy cladding may blow and burst -the first stage of core damage- and possibly restrict coolant flow to the core), rapid oxidation (production of hydrogen and release of a lot of heat during the oxidation process of zircaloy), debris bed formation (the molten materials will solidify at the lower parts of the fuel rods since there's a lower temperature), and corium relocation (molten materials from the core are released into water in the reactor and cause the production of steam, in case they break a steam explosion is likely, and criticality can start again if the materials are left in the core and break up in unborated water).

Meltdowns in history include but are not limited to: Westinghouse TR-2, EBR-1, Fermi 1, Three Mile Island, SL-1, the Sodium Reactor Experiment in Santa Susana, submarines K-27, K-140 and K-431, Chernobyl, Fukushima, Lucens, Windscale, Chapelcross, Saint-Laurent.

Fun fact! There is a theory that during a meltdown, the corium can hypothetically burn through the crust and body of the Earth and until reaching, presumably, China. For China, the other side of the Earth is the Atucha NPP of Argentina.

CANDU reactors:

CANDU means Canadian Deuterium Uranium reactor. Reactors of that kind have two large water reservoirs around their fuel/coolant channels with low temperature and pressure, which can prevent a meltdown or at worst case scenario to prevent melted fuel from breaching the core vessel.

AGR/Gas-cooled reactors:

AGRs (Advanced Gas-cooled Reactors) can only have a meltdown under the most extreme of circumstances due to their coolant, carbon dioxide, which has a large volume, high pressure, and a good high heat transfer, and thus core damage, if it occurs at all, will do so in days.

RBMK reactors:

RBMK means Reaktor Bolshoy Moshchnosti Kanalyy (high power channel type reactor) and exists only in post-Soviet states and operated only in Russia. They have no containment buildings, are unstable, and their emergency core cooling systems are inadequate. Their core is hard to cool as the coolant water can be lost with the reactor still going as the moderator is graphite, and the rapid shutdown takes 10-15 seconds. That is what makes RBMK reactors susceptible to meltdowns, however the ones operated nowadays are fixed and much safer than the pre-Chernobyl ones.

MKER reactors:

MKER is the fixed version of the RBMK. It has traits of the older reactor type such as its ability to be operated during refueling but also improvements such as improved quality of the reactor parts, advanced computer controls, a very strong containment structure, a fast rapid shutdown system, and others. Light water moderation as well as lower core power density help to regulate heat.

VVER reactor:

VVER means vodo-vodyanoy energetichesky reaktor (water-water -meaning water moderated and water cooled- energetic reactor) and is similar to the PWRs. The VVER-440 V230 has no containment building, and it also has no emergency core coolant system, and the interior of the reactor pressure vessel is prone to rust. However, the VVER-1000 is much safer.

Design-basis and beyond-design-basis event:

The structures, systems, and components of NPPs need to be acceptable in order to ensure that the NPP can withstand a certain even without endangering the health or safety of personnel and the public. The types of the design-basis event include criticality, earthquakes, explosions, fires, floods, and tornadoes. Poor design, lack of safety procedures, and human error are included in design-basis events however if they are poorly handled, they can lead to beyond-design-basis events (ex. Chernobyl, Three Mile Island). Beyond-design-basis events can bypass any safety systems of an NPP as they cannot be taken into account during design (ex Fukushima, the combination of tsunami and earthquake).

Criticality:

Criticality accident/critical excursion/critical power excursion/divergent chain reaction means an accidental uncontrolled nuclear fission chain reaction, and is defined as an event that involves the unintended arrangement or accumulation of a critical mass or fissile material, and can result in radiation poisoning. Criticality occurring in a reactor core is how reactors work and so it is only dangerous outside one. The fission chain reaction is influenced by mass, enrichment, reflection, geometry, interaction, concentration, temperature, moderation, volume, absorption, density, and shape. Two types of criticality accidents exist: process (breach of criticality safety controls) and reactor (operator error, ex. Chernobyl). Excursions are separated into prompt criticality (ex. Chernobyl, SL-1), transient criticality, exponential, and steady-state.

Criticality accidents from history include but are not limited to: Oakridge, Vinča, SL-1, Wood River Junction, Kyshtym, Fukushima (1978), Sarov, Shika, Tokaimura, five in Los Alamos, four in Severodvinsk.

Many criticalities emit blue light, Cherenkov radiation, resulting from the emission of charged particles passing through dialectric mediums such as water at a higher speed than the phase velocity of light in that medium. Some people have reported a wave of heat during criticalities however it is not yet known if its psychosomatic or physical, and heat and the glow are always observed together.

Decay heat:

Around 6.5% of the normal heat output of the reactor is produced by the delayed beta decay of fission products, and even after the reactor is shut down, decay heat is still produced. A LOCA can lead to dangerous levels of temperature due to the lack of coolant and result in dangerous circumstances (ex. Three Mile Island, Fukushima, Blayais). Decay heat can be removed through the essential service water system which releases heat into a body of water or through cooling towers.

Transport:

During the transport of radioactive material, it can be lost or the shielding can be damaged, ex. Cochabamba.

Equipment failure:

Failure of equipment such as electronics of a particle accelerator treating cancer (ex. Białystok) or control software related to radiotherapy (ex. Therac-25)  can result in radiation exposure to patients.

Human error:

Human error is defined as an action performed by a person that did not intend it or was not desirable that led the task or system outside its acceptable limits, and has been responsible for multiple nuclear accidents. It can result from errors in problem detection and diagnosis, failure to recall or remember information, lack of vigilance by a single person or a group etc.

Orphan/lost source:

It is defined as a sealed radioactive source no longer under regulatory control that can pose danger to the public. Usually, orphan sources were originally made for nuclear medicine and other legitimate reasons that were lost, stolen, misplaced or abandoned, then used for scrap metal and causing great contamination and potentially exposure to the ones working on them (ex. Goiânia). To prevent such accidents, the IAEA provides guides to scrap metal collectors in order for them to be aware on how a source might look like, as scrap metal is usually where a lost one can be found.

Honorary mentions - USA military terminology:

The Department of Defence of the USA has its own terminology for some kinds of nuclear incidents.

Pinnacle:

It's a flagword when reporting nuclear incidents than denotes that an event is of interest to the Department of Defence, National Command Authority, and Major Commands, and thus is not a kind of accident by itself, simply a way to describe the importance of one. Pinnacle events can include anything generating a higher level of military action, causing reactions by the public, affecting relationships with other countries, causing immediate coverage in news media, is against the country's interest, and affecting its current policy.

Broken Arrow:

It is defined as unexpected events involving nuclear weapons or components without the risk of nuclear war, such as accidental explosions, destruction of the weapon, contamination, lost during transport, or a public hazard. It is a term in USA, and other countries may have another name for it. Examples of Broken Arrow incidents include but are not limited to: the collision between a B-52G and a KC-135 Stratotanker in Palomares, disappearance of a B-47, and the Damascus Titan missile explosion).

Nucflash:

Detonation/possible detonation of a nuclear weapon creating a risk of nuclear war, such as the detection of unidentified objects by a system that appear as a threat, the unauthorised flight or deviation of one by aircraft either nuclear or carrying nuclear weapons penetrating the space of a nuclear-armed country, and the accidental or unauthorised launch of nuclear weapons in the direction or having the ability to reach the space of a nuclear-armed country, or detonation/possible detonation of nuclear weapons. It has the highest precedence when reporting a nuclear incident.

Emergency Disablement:

Emergency destruction of nuclear weapons.

Emergency Evacuation:

Emergency evacuation of nuclear weapons.

Empty Quiver:

Theft, loss, or seizure of a fully functioning nuclear weapon.

Non-Pinnacle:
Bent Spear:

Incidents regarding nuclear weapons, components, vehicles transporting nuclear material, and/or violations of handling and safety regulations, that even though are of interest, are not serious enough to be categorised under Pinnacle (ex. flight of a B-52 carrying live nuclear warheads by mistake).

Faded Giant:

Any accidents either radiological or about military nuclear reactors (not weapons in any case), for example SL-1.

Dull Sword:

Minor incidents regarding nuclear weapons, components, or systems, or which can affect their deployments, such as problems with vehicles able to carry nuclear weapons but not having any during the time of the incident, damage to equipment, tools, nuclear weapon diagnostic testers, or to release systems of aircraft capable of carrying nuclear weapons.

Nuclear accidents are very rare nowadays, which is why one must not fear radiation.