Why Fukushima No.1 Reactor 1 building exploded.
Fukushima No.1 reactor 1 is a BWR (boiling water reactor). They act in a similar way to pressure cookers in that they have nuclear fuel that heats water to produce steam, under pressure. This steam is then used to drive turbines before being cooled and returned to the reactor as water. The reactor typically operates at around seventy five times atmospheric pressure and around 285°C.
The fuel is Uranium Oxide in pellet form encased in a rod made of Zirconium alloy; this rod has a melting point in the region of 1850°C. Groups of these rods form the core of a nuclear reactor and are situated in the reactor vessel. This pressure vessel contains the water and core and houses the nuclear reaction, it can withstand pressures in excess of 200 times atmospheric pressure and temperatures of over 700°C. This is over double the operating pressure and temperature. The previously mentioned pressure vessel, the primary coolant loop (the circuit that the water/steam takes) pipes, reserves & pumps are all then hermetically sealed inside the reactor casing. This is incredibly thick steel and concrete container on a thick solid concrete raft. This other shell is built to withstand a full meltdown and breach of pressure vessel to prevent radioactive material from reaching the outside world. This entire assembly is kept inside a building, that is there to protect the reactor from the weather leading to any corrosion etc.
Reactors are said to have three levels of containment, the first is the Zirconium alloy (Zircaloy) rods that prevent the fuel directly contacting the water, this is primary containment. Secondary Containment consists of the pressure vessel that houses the core; should any rods melt / any coolant system failure occur, residual heat and pressure can be contained until adequate cooling can be applied. Tertiary containment is the reactor casing or outer shell.
To control the rate of the nuclear reaction control rods are used inside the core groups of fuel rods to partially inhibit it. This stops the reactor going supercritical and melting itself. Control rods work by absorbing neutrons that propagate individual nuclear reactions at an atomic level. When the control rods are fully inserted into the core, the nuclear reaction in the fuel becomes unsustainable and stops. This stops the production of heat by the Uranium fuel. However when Uranium decays in a nuclear reaction, it is not a one step process. Caesium and Iodine isotopes are produced that then decay to harmless final products. These decays are different from those of Uranium and are not stopped by the insertion of the control rods. These isotopes take a few days to decay fully, and during these few days the reactor must constantly be cooled in the same manner as when the Uranium is reacting (critical). Some of the neutrons leave the fuel rods and enter the water inside the pressure vessel. These neutrons react with the water and any dissolved gases to form radioactive material. However, this material decays to harmless products in under a minute.
Now, when the earthquake struck on March 11th, reactor 1 went into automatic shut-down. The control rods were fully inserted before the ground had stopped shaking and the Uranium stopped reacting within seconds. The Caesium & Iodine continued to react producing around 5% of the heat generated when the reactor was operating normally. The earthquake also destroyed the external power supply to the station. Now this means that the coolant pumps have to be powered from elsewhere, to prevent overheating in the reactor. While only 5% of heat generation remains, the temperature of the vessel interior would have not started to drop yet. There are however on-site diesel generators that take up the slack and power the coolant pumps in the event of such failure. These were designed to be safe against any earthquake up to 7.9 and associated tsunami, but the earthquake was 8.9, this is 32 times stronger than was designed for; and so around an hour later the tsunami struck land and disabled these generators. Following this there are batteries designed to last for eight hours while repairs are made. These worked perfectly for eight hours, however sufficient repairs could not be made to restore power to the coolant pumps after this time.
Now the tertiary containment is designed to protect the outside world against the worst case scenario, but everyone would prefer it if the primary and secondary containment were not breached either. So our reactor is hot (9 hours of cooling would not have reduced the temperature significantly), and has more heat being applied to it, and is no longer being cooled. Their aim was to keep the temperature inside the vessel below 700°C, the pressure below 200 atmospheres and the temperature inside the Zircaloy coated fuel rods below 1850°C. As the pressure built to over 150 atmospheres and the vessel temperature hit 550°C, it was decided that venting steam from the pressure vessel was the only option to maintain the integrity of the secondary containment. Steam containing some radioactive material passed through filters to remove any potential Caesium & Iodine particles (should the rods have already been compromised without the engineers knowledge) and then released into the air inside the reactor building and out into the atmosphere. The wind was offshore and the steam was harmless within moments of release.
This is the radioactive contamination we first started hearing about.Now when steam under high pressure is exposed to neutrons in the presence of Zirconium, the hydrogen and oxygen atoms can disassociate. So gaseous hydrogen and oxygen are also produced. As the venting continued, the air temperature inside the reactor building would have risen, as would the levels of oxygen and hydrogen. once the hydrogen/oxygen mix loses pressure it begins to recombine into steam. this recombination reaction is highly exothermic (produces heat). So during the venting at some point this excess heat lead to a runaway chain reaction in the hydrogen as the deflagration (recombination) accelerated to become a detonation (explosion). It was this DDT (deflagration to detonation transition) that caused the explosion at reactor 1.
The explosion occurred in the building outside of the tertiary containment and did not significantly damage the outer shell. Simultaneously as the pressure was dropped, the temperature decreased some. However the boiling point of water dropped faster than the ambient temperature change and so the percentage of steam to water inside the vessel increased, exposing the tops of the fuel rods. Without any water cooling exposed rods will begin heating up more rapidly. within about 30-35 minutes the Zircaloy primary containment would have begun to fail. While the fuel pellets themselves do not melt at anything less than 3000°C, Caesium and Iodine would have then begun mixing with the water/steam mix. As venting continued post blast, it was these isotopes that were detected in the vented steam. This meant that the explosion must have damaged the filters.
At this point plan B was enacted. It was decided that seawater would be used, mixed with Boric acid (a liquid form of the control rod material), to flood the reactor vessel. It took ten hours to fill the vessel and the cooling will take around ten days. From this point, pending any extremely unforeseeable circumstances, that this reactor is now safe in that it poses no further threat of total containment failure and radioactive release, beyond that which had already occurred.
Thanks
John Bottomley.
The fuel is Uranium Oxide in pellet form encased in a rod made of Zirconium alloy; this rod has a melting point in the region of 1850°C. Groups of these rods form the core of a nuclear reactor and are situated in the reactor vessel. This pressure vessel contains the water and core and houses the nuclear reaction, it can withstand pressures in excess of 200 times atmospheric pressure and temperatures of over 700°C. This is over double the operating pressure and temperature. The previously mentioned pressure vessel, the primary coolant loop (the circuit that the water/steam takes) pipes, reserves & pumps are all then hermetically sealed inside the reactor casing. This is incredibly thick steel and concrete container on a thick solid concrete raft. This other shell is built to withstand a full meltdown and breach of pressure vessel to prevent radioactive material from reaching the outside world. This entire assembly is kept inside a building, that is there to protect the reactor from the weather leading to any corrosion etc.
Reactors are said to have three levels of containment, the first is the Zirconium alloy (Zircaloy) rods that prevent the fuel directly contacting the water, this is primary containment. Secondary Containment consists of the pressure vessel that houses the core; should any rods melt / any coolant system failure occur, residual heat and pressure can be contained until adequate cooling can be applied. Tertiary containment is the reactor casing or outer shell.
To control the rate of the nuclear reaction control rods are used inside the core groups of fuel rods to partially inhibit it. This stops the reactor going supercritical and melting itself. Control rods work by absorbing neutrons that propagate individual nuclear reactions at an atomic level. When the control rods are fully inserted into the core, the nuclear reaction in the fuel becomes unsustainable and stops. This stops the production of heat by the Uranium fuel. However when Uranium decays in a nuclear reaction, it is not a one step process. Caesium and Iodine isotopes are produced that then decay to harmless final products. These decays are different from those of Uranium and are not stopped by the insertion of the control rods. These isotopes take a few days to decay fully, and during these few days the reactor must constantly be cooled in the same manner as when the Uranium is reacting (critical). Some of the neutrons leave the fuel rods and enter the water inside the pressure vessel. These neutrons react with the water and any dissolved gases to form radioactive material. However, this material decays to harmless products in under a minute.
Now, when the earthquake struck on March 11th, reactor 1 went into automatic shut-down. The control rods were fully inserted before the ground had stopped shaking and the Uranium stopped reacting within seconds. The Caesium & Iodine continued to react producing around 5% of the heat generated when the reactor was operating normally. The earthquake also destroyed the external power supply to the station. Now this means that the coolant pumps have to be powered from elsewhere, to prevent overheating in the reactor. While only 5% of heat generation remains, the temperature of the vessel interior would have not started to drop yet. There are however on-site diesel generators that take up the slack and power the coolant pumps in the event of such failure. These were designed to be safe against any earthquake up to 7.9 and associated tsunami, but the earthquake was 8.9, this is 32 times stronger than was designed for; and so around an hour later the tsunami struck land and disabled these generators. Following this there are batteries designed to last for eight hours while repairs are made. These worked perfectly for eight hours, however sufficient repairs could not be made to restore power to the coolant pumps after this time.
Now the tertiary containment is designed to protect the outside world against the worst case scenario, but everyone would prefer it if the primary and secondary containment were not breached either. So our reactor is hot (9 hours of cooling would not have reduced the temperature significantly), and has more heat being applied to it, and is no longer being cooled. Their aim was to keep the temperature inside the vessel below 700°C, the pressure below 200 atmospheres and the temperature inside the Zircaloy coated fuel rods below 1850°C. As the pressure built to over 150 atmospheres and the vessel temperature hit 550°C, it was decided that venting steam from the pressure vessel was the only option to maintain the integrity of the secondary containment. Steam containing some radioactive material passed through filters to remove any potential Caesium & Iodine particles (should the rods have already been compromised without the engineers knowledge) and then released into the air inside the reactor building and out into the atmosphere. The wind was offshore and the steam was harmless within moments of release.
This is the radioactive contamination we first started hearing about.Now when steam under high pressure is exposed to neutrons in the presence of Zirconium, the hydrogen and oxygen atoms can disassociate. So gaseous hydrogen and oxygen are also produced. As the venting continued, the air temperature inside the reactor building would have risen, as would the levels of oxygen and hydrogen. once the hydrogen/oxygen mix loses pressure it begins to recombine into steam. this recombination reaction is highly exothermic (produces heat). So during the venting at some point this excess heat lead to a runaway chain reaction in the hydrogen as the deflagration (recombination) accelerated to become a detonation (explosion). It was this DDT (deflagration to detonation transition) that caused the explosion at reactor 1.
The explosion occurred in the building outside of the tertiary containment and did not significantly damage the outer shell. Simultaneously as the pressure was dropped, the temperature decreased some. However the boiling point of water dropped faster than the ambient temperature change and so the percentage of steam to water inside the vessel increased, exposing the tops of the fuel rods. Without any water cooling exposed rods will begin heating up more rapidly. within about 30-35 minutes the Zircaloy primary containment would have begun to fail. While the fuel pellets themselves do not melt at anything less than 3000°C, Caesium and Iodine would have then begun mixing with the water/steam mix. As venting continued post blast, it was these isotopes that were detected in the vented steam. This meant that the explosion must have damaged the filters.
At this point plan B was enacted. It was decided that seawater would be used, mixed with Boric acid (a liquid form of the control rod material), to flood the reactor vessel. It took ten hours to fill the vessel and the cooling will take around ten days. From this point, pending any extremely unforeseeable circumstances, that this reactor is now safe in that it poses no further threat of total containment failure and radioactive release, beyond that which had already occurred.
Thanks
John Bottomley.