March 17, 2011

The event is currently ongoing.  Up-to-date information on its progression and management can be found using the links on the right.  What follows is background information to help the understanding of events.

The reactors

Reactor # 1 of the Fukushima Daiichi nuclear power plant was put into operation on March 26, 1971.  It is a General Electric BWR-3 (boiling-water reactor) with an electric power output of 460 megawatts and it has operated without incident to date.  The reactor was initially licensed to operate for forty years and therefore scheduled for permanent shutdown and decommissioning on March 26, 2011.  It appears it may have subsequently been granted a license to operate for ten additional years.  Reactors #2 and #3 are also BWRs but each has an electrical power output of 768MW.

The reactor core is housed in a pressure vessel and cooled during normal operation by water driven by recirculation pumps located outside the vessel.  The pressure inside the vessel is approximately 70 times atmospheric pressure.  The core consists of an array of fuel rods, each encased in a zirconium-alloy clad to prevent release of radioactive fission products into the coolant.  As water heats up from passing through the core, some turns into steam which exits the pressure vessel through a steam line and subsequently drives the turbine electric generator.  The condensate from the turbine is pumped back to the pressure vessel by the feedwater pumps through the feedwater inlet.  

The entire pressure vessel is housed in an airtight (primary) containment vessel which is designed to withstand pressures of several times the atmospheric pressure.  Finally, in the case of unit 1 at Fukushima Daiichi, the containment vessel is located in a larger, cube-like, reactor building (secondary containment).

Simplified plant diagram

Cutaway view of reactor building

The main source of heat generated in the fuel is nuclear fission.  However, after a long period of operation, only approximately 93% of heat is produced directly by fission.  The remaining 7% of the generated heat is produced by radioactive decay of fission products.  This is known as decay heat.  Immediately after a reactor is shut down (the fission reaction is stopped) the core still needs cooling at a level of approximately 7% of normal operation to remove the decay heat.  As the fission products decay, over a period of many days, the cooling requirements are reduced.  After one day, the decay heat generation rate drops approximately ten times, to 0.6% of full power.  The subsequent decrease is slower.  After 10 days the decay heat is approximately 0.3% of full power and after 100 days it is approximately 0.1% of full power.  

Impaired core cooling after shutdown

If an accident occurs and water is not circulated through the shut-down reactor core, the water in the pressure vessel begins to boil.  If the core is uncovered as a consequence of boiling, fuel temperature increases to the point where fuel clad may fail and release some gaseous fission products from the fuel.  If the core remains uncovered for a longer time, the fuel clad will become very hot and react with steam to form zirconium oxide and hydrogen. 

If cooling is not restored, the pressure in the pressure vessel will rise and steam, possibly mixed with some radioactive fission products and hydrogen, may escape into the containment vessel.  For a while the steam will be condensed in the suppression pool of water below the reactor.  However, if cooling is not restored, the pressure in the containment vessel may build up to beyond its design value.  Then it may become necessary to vent some of the steam to the outside, usually through devices which minimize the release of radioactive fission products in vapour form.  Such intermittent venting will have to continue for days until stable core cooling is restored.  Small amounts of fission products, particularly iodine and cesium may be released to the environment when such venting occurs.  Depending on the accident progression, the amounts may be small enough as to not pose any health risks.  Nonetheless, as a precautionary measure, the population in the vicinity of the reactor can be advised to evacuate and is given potassium-iodide pills.  

The non-radioactive iodine in the potassium-iodide pills saturates a person’s thyroid and prevents it from absorbing the radioactive iodine in the air.  Radioactive iodine has a half-life of approximately 8 days, which means that, even if not removed by wind, its quantity will be halved every eight days and hence become insignificant in a few weeks.  Cesium isotopes, on the other hand, have a longer half life and tend to be present, in small quantities, for a few tens of years.  

To maintain cooling even under accident conditions, plants are equipped with emergency core cooling systems (ECCS) which use auxiliary pumps to maintain water flow to the core.  Electric power for ECCS is provided by the electrical grid, backup diesel generators and, in case those fail, batteries.  Batteries, however, can last for only a few hours.

Spent fuel bays

The spent fuel bays are designed to hold spent fuel removed from the reactor core.  In the case of the Fukushima plant spent fuel bays are located in the reactor building but outside the containment vessel.  Fuel removed from the core behaves very similarly to the fuel in the core after shutdown.  Each spent fuel rod starts out generating heat at a level of approximately 7% of the power it used to generate while in the reactor, which drops to 0.3% after one day and to 0.1% after 100 days.  To remove this heat, fuel is stored on racks under water.  Water in the bay is circulated and cooled using pumps.

If the fuel is originally enriched in U235 (which BWR fuel is) care must be taken when storing it to avoid “criticality”.  “Criticality” can occur when too many fuel bundles are stacked in close proximity to each other in water and, consequently, start behaving as a reactor, that is additional power begins to be generated by fission.  To avoid criticality, boron (a strong neutron absorber), is used in the form of boric acid dissolved in water or in the form of plates affixed to the fuel racks in the bay.  The boron plates in the spent fuel bay have the same role as the shutoff rods in the reactor: they stop fission.  Draining the water from the spent fuel bay makes criticality impossible, but fuel rods will then overheat and potentially release some fission products directly into the surrounding building since no pressure vessel or containment vessel surrounds the spent fuel.  Spent fuel is not combustible but, if it reaches a high temperature, hydrogen can be produced through zirconium oxidation and can subsequently ignite.

Simplified event outline

In the case of reactor # 1 at Fukushima Daiichi, the reactor shut down as designed when the earthquake hit and, in the absence of grid power, cooling of the core was maintained by the ECCS pumps powered by the backup diesel generators.  Approximately one hour later, the diesel generators went offline due to tsunami damage to their fuel supply and power to the ECCS continued to be provided by batteries.  From then on, the main focus of the incident management team was to restore power to the pumps and adequate cooling to the core.  The efforts were hampered by frequent aftershocks and tsunami alerts.  As adequate cooling was not immediately restored, some steam and fission products built up in the containment vessel raising the pressure and making it necessary to vent some of the steam to the exterior.  As a precautionary measure, the population in the vicinity of the reactor was evacuated and provided with potassium-iodide pills to minimize exposure to radioactive iodine emissions.  

It appears that some of the hydrogen generated by zirconium oxidation accumulated in the outer reactor building (secondary containment) and subsequently ignited leading to the destruction of the upper-level walls of that building.  The reactor (primary) containment vessel itself does not appear to have been damaged.  

In order to speed up cooling, the incident-management team decided to use readily-available sea water to cool the core.  (The Fukushima Daiichi plant is located on the coast.)  The salt in the sea water makes it very costly, if not impossible, to clean up and restart the reactor.  

Reactors # 2 and # 3 shut down as well but they too began suffering impaired core cooling two days after the earthquake, followed by hydrogen explosions.  Some damage to the containment vessel at these two reactors may have occurred. Their cores had to be cooled with sea water as well.   

Reactors # 4 #5 and #6 were not in operation at the time of the earthquake.  However, the spent fuel bay at reactor # 4 lost adequate cooling due to the earthquake and subsequent power outage and some fuel overheated, apparently starting a fire in adjacent rooms in the reactor building.  To restore cooling and avoid “re-criticality” water and boric acid had to be poured in using helicopters.

Up-to-date details of the reactor cores and spent fuel bays are periodically posted by the Japan Atomic Industrial Forum (link on side bar).  Status, water levels and temperatures of the spent fuel bays are updated periodically by the International Atomic Energy Agency (link on side bar).

Core “meltdown”

“Meltdown” refers to the situation where at least some of the fuel in the reactor core reaches a high-enough temperature to melt.  Under meltdown conditions the release of gaseous fission products from the fuel into the pressure vessel is enhanced.  However, even under partial meltdown conditions the reactor pressure vessel and the (primary) containment vessel continue to act as barriers to the free release of radioactive fission products into the atmosphere.  It is only when both these barriers fail that a sizeable release of radioactive fission products can occur.  In that case, submerging the fuel in water is an effective way of trapping the gaseous fission products and reducing the amount released into the atmosphere.  

Radiation dose and dose equivalent

Radiation dose quantifies the energy deposited by radiation in matter.  It is measured in Grays (Gy). Since different types of radiation have different biological effects, another quantity, the “effective dose”, is used to measure the biological effects of radiation.  Effective dose is measured in Sieverts (Sv).  For gamma radiation and most of the beta radiation, one Sievert corresponds to one Gray.  

Dose and dose equivalents are cumulative. Under normal circumstances, each person accumulates approximately 2mSv/year, which is the same as 5µSv/day or 0.2 µSv/hr.  Dose can also be acquired in single exposures during medical procedures.  For example a breast x-ray has an effective dose equivalent of 50 µSv, while an abdominal or pelvic CT has a two-hundred-times higher effective dose equivalent, at 10mSv.  The effective dose equivalent from radiotherapy is higher still and can reach 0.5 Sv.  Mild symptoms of radiation syndrome start developing above 2 Sv.