The Canadian Nuclear Society has issued a statement on some of the claims made in the Canadian media by a representative of the anti-nuclear movement:
CNS Response to Gordon Edwards
This is a response from the Alberta Branch of the Canadian Nuclear Society to comments made by Mr. Gordon Edwards concerning the events at the Fukushima-I nuclear power generating station, in Japan. It was written by Dr. Susanna Harding, with the assistance of Duane Pendergast, head of the Alberta Branch, and Jeremy Whitlock, Chair of the Education and Communications Committee
History of the plant: The reactor in question was ordered by Tokyo Electric Power Company (TEPCO) in 1965, with construction beginning in 1966. The plant first went critical in October of 1970 and was formally commissioned and 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. Over the years it has operated quietly and without incident, but due to its age had been scheduled for decommissioning on March 26, 2011. It is a first generation BWR.
Plant characteristics: This type of plant has a steam generator directly over the nuclear core, with that steam then piped directly to the turbine driving the electric generator. This has certain thermodynamic advantages although at the cost of enhanced maintenance issues. The pressure vessel which houses the reactor core is equipped with electrically driven circulation pumps as well as with a steam driven emergency circulation pump. There exists a system for direct injection of water (ECCS, or Emergency Core Cooling System) which does not depend on electrical power. The entire reactor assembly is housed in a concrete containment structure designed to prevent the release of radioactive material in the event of a breach of the pressure vessel or the associated plumbing. This plant has emergency diesel generators for use when other forms of electrical power are not available.
Safety features: The plant is designed using a "defence in depth" concept. The nuclear fuel is encapsulated in zirconium alloy fuel rods (or pins) designed to contain fission products and gasses produced during operation. Even though water is circulating around these fuel rods, the encapsulation prevents the radioactive material from being taken up into the water. The reactor is equipped with several types of cooling system, at least two of which do not require electrical power to operate. The entire assembly is contained in the pressure vessel, which houses the reactor and which can be isolated from the generator turbine in event of emergency. The control rods mechanisms are designed to shut down the reactor in event of loss of power. Finally, the entire system in enclosed in a purpose-built structure, the containment, intended to keep that which is inside, inside.
Cooling and Meltdowns. A nuclear reactor continues to produce heat even after the reactor is shut down. In order to maintain safe conditions this heat must be removed. Normally this is done with electrically driven circulation pumps which drive cooling water through the core and take the heat to a heatsink. If the water level in the core drops below the top of the fuel, the exposed fuel (which is no longer being cooled) will heat up and warp. In extreme cases this could result in rod failure, thus releasing fission products into the cooling water. However in no case can this result in a nuclear explosion or detonation. The amount of heat generated decreases over time as the more volatile radioactive materials within the core decay, following a trend known as a "decaying exponential curve".
Venting and ECCS. If it becomes necessary to use the ECCS then the pressure within the reactor pressure vessel must be decreased to ambient pressure. This is because use of the ECCS assumes no electrical (or steam) power is available to drive pumps that could force water into the pressure vessel against a head of steam. Venting is normally done in two steps, not both of which might be necessary. First, the reactor pressure vessel is vented into the containment. Then, if necessary, the containment is vented through filters to the outside. At this point gravity fed water can be injected or (as is happening at Fukushima) water supplied by a fire truck can be used to cover the reactor core and cool it.
Fukushima and Chernobyl. The Fukushima-I reactor is a water moderated boiling water reactor. The Chernobyl RBMK reactor was a graphite moderated boiling water reactor. In the Chernobyl incident the reactor was driven into an unstable operating regime by operator action. The RBMK reactor then had an uncontrolled power spike of 100 times full power which blew the roof of the reactor building off (there was no containment structure) and exposed a glowing hot mass of graphite (carbon) to the atmosphere. The resulting chemical fire of carbon mixed with radioactive material burned for several days with results that are well known. The ensuing blast destroyed any capability to cool or control the RBMK reactor. At Fukushima there is no carbon, nothing to burn, such a scenario is not possible here. The Fukushima reactor underwent a controlled shutdown and maintains the capability to be controlled and cooled.
Fukushima and Three Mile Island. TMI-I was a pressurized water reactor (PWR) supplied by Babcock and Wilcox. Due to operator error and poor design the reactor was driven into a state where the core was unintentionally exposed. Without cooling the tops of the fuel assemblies warped and some ruptured. While this was fatal to the use of that facility (and expensive to the shareholders) very little radioactive material was released into the environment. This is very similar to what would happen at Fukushima if the core were to be exposed.
Seawater cooling. News reports have that the reactor at Fukushima is being cooled with seawater, pumped in by fire truck. In order to do this, it is necessary to vent the reactor pressure vessel sufficiently that the pump can move the water into the core. Additionally, the seawater which is being used is doped with boric acid. Boron is a neutron absorber, and it is standard operating procedure in circumstances such as this to add boron in order to ensure that the core stays dead, and to aid in suppressing residual heat generation.
Prognosis. News reports have that there has been damage to the containment building, though there have been conflicting news reports indicating that the damage was confined to an outer building and that the containment is intact. Even if the containment integrity has not been compromised, the focus is now, by whatever means, of keeping the core covered. If the core can be kept covered then the fuel rods will not rupture and there will be minimal environmental impact. It will take approximately ten days to two weeks to bring the heat generation rate down to the point where a core melt is no longer an issue.
Dr. Susanna Harding holds a Ph.D. in astrophysics from the University of Calgary, an M.Sc. in nuclear engineering from the University of Virginia, a B.S. in engineering physics from the University of Santa Clara, and additional degrees and certifications. She is a licensed Professional Physicist, a member of the Canadian Nuclear Society and of the American Nuclear Society, and works as a nuclear engineer in the US for a company which designs and manufactures safety and radiation monitoring systems for the nuclear power industry. Her background includes hands-on experience with nuclear reactors.
