By Béla Lipták, PE, Columnist
A few months ago, I described the safety controls that could have saved the 11 lives lost in the BP accident. In this series I will first describe the process used at the Fukushima plant; next I will show the safety controls that could have prevented this tragedy; finally, I will describe the steps that American nuclear power plants should take to protect against the repetition of such accidents, which be triggered by earthquakes along active faults, hurricanes, terrorism, cyber terrorism or other unexpected events.
The regular nuclear power plants are not potential atomic bombs because the fuel is not concentrated sufficiently to explode like a bomb. The main difference between fission plants and fission bombs is that the plant releases the energy continuously, while the bomb releases it all at once. As of today, some 10,000 fission type nuclear weapons are in storage, and plans are to convert their plutonium into nuclear fuel. Some 440 nuclear power plants are in operation around the world (104 in the United States) generating some 7% of the global energy consumption and about 13% of the global electricity consumption.
Currently there are two breeder reactors in operation, one in Beloyarsk, Russia, and the other in Tsuruga, Japan. If in the future, breeder reactors are built, the risks will increase, because their product (plutonium with a half-life of 24,100 years) can be used directly to build bombs. Research is also in progress to build fusion plants, which operate at millions of degrees temperature and continuously release the same energy that hydrogen bombs release all at once.
The main concern with today's nuclear power plants is that in case of a meltdown they release radioactive isotopes (See table below). The safety record of the nuclear industry is good (about a dozen meltdowns occurred during it's 50 years of existence). Based on that record, the probability of meltdowns globally is one per every two years.
With the exception of two small breeder reactors, one in Beloyarsk, Russia, and the other in Tsuruga, Japan, today only fission plants are in operation which cannot explode like atomic bombs, but they are still dangerous because they can release radioactive iodine, cesium or plutonium, which cause cancer if inhaled or ingested.
In case of a partial or complete meltdown, the produced plutonium can make the region uninhabitable for thousands of years. At Fukushima, the meltdown amounted to 75% of the core at one, 33% at another reactor and plutonium was found in the soil, but as of this writing, its source was not clearly established. (Ed. note: For current information on the status of the Fukushima reactors, go to the IAEA website at www.iaea.org/newscenter/focus/fukushima/index.html.)
The Fission Process
The heart of a nuclear power plant is a high-pressure boiler similar to one burning coal, oil or gas. Yet there are major differences between them. One difference is that the fuel is located inside the reactors. The second difference is that this heat source cannot be turned off completely (by inserting the control rods and by stopping the recirculation pumps), but continues to release heat at a 5% rate for a long time. Therefore, continued cooling is required, even after the plant is shut down.
The third difference is that in a nuclear power plant, a serious accident will result if cooling is lost. Finally, the most important difference is that the waste produced in a nuclear reactor still contains some fuel (uranium in five of the six blocks and MOX in Block 3, which is uranium mixed with plutonium), which continues to generate heat practically forever and, therefore, without cooling, it could melt down. For this reason, nuclear waste would require safe and permanent storage, which was expected to be built a half century ago, but still does not exist. Consequently, the waste just accumulates and is overloading the temporary storage pools everywhere.
Although some argue that this is no worse than what the burning of fossil fuels cause because that waste also accumulates in the water and the air, causing more and more cancer, asthma or global warming. This is not so, because nuclear waste will still be with us even after we run out of uranium, while the consequences of fossil waste will slowly disappear after we run out of fossil fuels.
In a fission reaction under normal operation, a slow-moving neutron is absorbed by the nucleus of an uranium atom, which in turn splits into fast-moving lighter elements:
23592U + n = 23692U = 14456Ba + 8936Kr + 3n + 177 MeV.
and releases three free neutrons and a steady supply of useful energy. This is different from a nuclear bomb, because that is designed to release all its energy at once. During an accident, as the temperature rises, the zirconium cladding (the material that covers the fuel rod) melts at 1200 °C and reacts with the water in the reactor:
Zr + 2H2O = ZrO2 + 2H2.
If this hydrogen comes in contact with oxygen, it can explode. This is what occurred in the Fukushima plant where due to the meltdown of fuel rods (both in the reactor core and in the spent fuel rod pools) hydrogen was generated. The hydrogen from the core accumulated in the primary and from the spent fuel pools in the secondary containments and since both had air in them (not inert gas), exploded. As the temperature increased further, at 2800 °C, 2,800 °C the uranium in the fuel rods also melted releasing radioactive isotopes.