question: How does a RBMK reactor explode?
comment: Reactors use really hot water that’s maintained at a constant pressure as a coolant. The pressure prevents the water from boiling at its normal temperature. If there is change in this balance and the water turns to steam in the “primary coolant loop” the water becomes ineffective as a coolant. The reactor gets hotter and so does the water. More heat, more steam. This causes a feedback loop and eventually something in the system will break down.
passages: The reactor emergency protection system (EPS) was designed to shut down the reactor when its operational parameters are exceeded. The design accounted for steam collapse in the core when the fuel element temperature falls below 265 °C, coolant vaporization in fuel channels in cold reactor state, and sticking of some emergency protection rods. However, the slow insertion speed of the control rods, together with their design causing localized positive reactivity as the displacer moves through the lower part of the core, created a number of possible situations where initiation of the EPS could itself cause or aggravate a reactor runaway.
   The short-term ECCS subsystem consists of two groups of six accumulator tanks, containing water blanketed with nitrogen under pressure of 10 MPa, connected by fast-acting valves to the reactor. Each group can supply 50% of the maximum coolant flow to the damaged half of the reactor. The third group is a set of electrical pumps drawing water from the deaerators. The short-term pumps can be powered by the spindown of the main turbogenerators.
   Section::::Reactor design and performance.:Gas circuit.
   Section::::Reactor design and performance.:Cooling and steam circuits.
   The total amount of fuel under stationary conditions is 192 tons.
  
  Section::::Reactor design and performance.:Control rods.
   The moderator blocks are made of nuclear graphite the dimensions of which are on the plane perpendicular to the channels and with several longitudinal dimensions of between and depending on the location in the stack. There are holes of diameter through the longitudinal axis of the blocks for the fuel and control channels. The blocks are stacked inside the reactor vessel into a cylindrical core with a diameter and height of . The maximum allowed temperature of the graphite is up to .
   Section::::Release of fission products into coolant water in a Three Mile Island type accident.
   Reactivity is controlled by control rods that can be inserted into the reactor from above. These rods are made from a neutron absorbing material and, depending on depth of insertion, hinder the chain reaction. If there is an emergency, a reactor shutdown can be performed by full insertion of the control rods into the core.
  
  Section::::Design.:Primary cooling circuits.
   Section::::Reactor design and performance.:Reactor control/supervision systems.
   Due to flaws in its original design, including graphite tips on the control rods, scramming an RBMK reactor could raise reactivity to dangerous levels before lowering it. This was noticed when it caused a power surge at the startup of Ignalina Nuclear Power Plant Unit number 1, in 1983. In 1986, activating AZ-5 on a distressed RBMK was the final action that precipitated the reactor explosion in the Chernobyl disaster. Afterward, other RBMKs were shut down or retrofitted.
  
  Section::::Reactor response.:Decay heat.
   BULLET::::- Some reactor designs have passive safety features that prevent meltdowns from occurring in these extreme circumstances. The Pebble Bed Reactor, for instance, can withstand extreme temperature transients in its fuel. Another example is the CANDU reactor, which has two large masses of relatively cool, low-pressure water (first is the heavy-water moderator; second is the light-water-filled shield tank) that act as heat sinks. Another example is the Hydrogen Moderated Self-regulating Nuclear Power Module, in which the chemical decomposition of the uranium hydride fuel halts the fission reaction by removing the hydrogen moderator. The same principle is used in TRIGA research reactors.
   BULLET::::3. The time required for the molten fuel to breach the primary pressure boundary. The time required for the molten metal of the core to breach the primary pressure boundary (in light water reactors this is the pressure vessel; in CANDU and RBMK reactors this is the array of pressurized fuel channels; in PHWR reactors like Atucha I, it will be a double barrier of channels and the pressure vessel) will depend on temperatures and boundary materials. Whether or not the fuel remains critical in the conditions inside the damaged core or beyond will play a significant role.
  
  Section::::See also.
   Section::::Cracking and overheating of the fuel.:Loss of volatile fission products from pellets.
   In the cases of carbon dioxide cooled graphite moderated reactors such as magnox and AGR power reactors an important corrosion reaction is the reaction of a molecule of carbon dioxide with graphite (carbon) to form two molecules of carbon monoxide. This is one of the processes which limits the working life of this type of reactor.
  
  Section::::Hydriding and Waterside Corrosion.:Water-cooled reactors.
  
  Section::::Hydriding and Waterside Corrosion.:Water-cooled reactors.:Corrosion.
   BULLET::::- The local increase in pressure due to the increased heating of the water can generate enhanced heat transfer (usually due to rapid fragmentation of the hot fluid within the colder more volatile one) and a greater pressure wave, this process can be self-sustained. (The mechanics of this stage would then be similar to those in a classical ZND detonation wave).
  
  BULLET::::- Expansion
  
  BULLET::::- This process leads to the whole of the water being suddenly heated to boiling. This causes an increase in pressure (in layman's terms, an explosion), which can result in damage to the plant.
   A review of the subject can be read at and work on the subject continues to this day; in Germany at the FZK some work has been done on the effect of thermite on concrete, this is a simulation of the effect of the molten core of a reactor breaking through the bottom of the pressure vessel into the containment building.
  
  Section::::Lava flows from corium.
   There is a new pressure reactor, called Q-Tube. It features a simple and safe pressure release and reseal mechanism which can prevent a glass pressure reactor from explosions and retain the solvent.
  
  Section::::Types of Pressure Reactors.:Metal pressure reactor.
   Section::::Reactor poisons.:Short-lived poisons and controllability.
   During tests on low power, with low coolant flux through the core, the supervisor noticed several control rods being pulled from the core, and found an operator in the basement opening pneumatic valves. Wrongly opened valves were immediately closed, but some of the control rods did not reenter the core and stuck in almost withdrawn positions, but still low enough for their status lights to indicate them as lowered. Due to a miscommunication between the supervisor and the control room operator, wrong buttons were pressed when the supervisor asked for lowering the control rods into the core. Instead of sealing the withdrawn control rods to the pneumatic system, the safeguard bank of four control rods was withdrawn from the core. The operator noticed that the power level was exponentially increasing, doubling each 2 seconds, and tripped the reactor. Three of the safeguard control rods however were not inserted into the core and the fourth took an abnormally long time, about 90 seconds, to slide back, while the power kept rising. After just 10 seconds 17 MW were reached. The cooling water boiled in the tubes connected to the temporary cooling system, and some of them ruptured; the positive void coefficient of the reactor led to yet higher power increase rate. About 14 seconds later valves were opened to drain the heavy water from the calandria. As this took some time, power increased for 5 more seconds, peaked at 80 MW, then went down as the moderator level decreased and was at zero 25 seconds later. Meanwhile, some fuel elements melted and the calandria was pierced at several places; helium leaked and air was aspirated inside. Hydrogen and other gases evolved by high-temperature reaction of metals with cooling water, and 3–4 minutes later oxyhydrogen exploded in the calandria. During the incident, some gaseous fission products were vented to the atmosphere, and heavy water in the calandria was contaminated with the cooling water and the fission products.
   In 1982, amongst a backdrop of ongoing protests directed at the construction of the first commercial scale breeder reactor in France, a later member of the Swiss Green Party fired five RPG-7 rocket-propelled grenades at the still under construction containment building of the Superphenix reactor. Two grenades hit and caused minor damage to the reinforced concrete outer shell. It was the first time protests reached such heights. After examination of the superficial damage, the prototype fast breeder reactor started and operated for over a decade.
   The reactor is equipped with a fuel rod leak detector. A scintillation counter detector, sensitive to energies of short-lived fission products, is mounted on a special dolly and moved over the outlets of the fuel channels, issuing an alert if increased radioactivity is detected in the steam-water flow.
  
  Section::::Reactor design and performance.:Containment.
   The description of this accident is applicable for the BWR/4.
   Extensive work was done from 1970 to 1990 on the possibility of a steam explosion or FCI when molten 'corium' contacted water. Many experiments suggested quite low conversion of thermal to mechanical energy, whereas the theoretical models available appeared to suggest that much higher efficiencies were possible. A NEA/OECD report was written on the subject in 2000 which states that a steam explosion caused by contact of corium with water has four stages.
  
  BULLET::::- Premixing
   Section::::Light water reactors (LWRs).:Speculative failure modes.
  
  One scenario consists of the reactor pressure vessel failing all at once, with the entire mass of corium dropping into a pool of water (for example, coolant or moderator) and causing extremely rapid generation of steam. The pressure rise within the containment could threaten integrity if rupture disks could not relieve the stress. Exposed flammable substances could burn, but there are few, if any, flammable substances within the containment.
   This style of reactor can be designed to be inherently safe. As it heats, the graphite expands, separating the fuel and reducing the reactor's criticality. This property can simplify the operating controls to a single valve throttling the turbine. When closed, the reactor heats, but produces less power. When open, the reactor cools, but becomes more critical and produces more power.
  