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.
The Faulty Design at Fukushima
Figure 1 shows the design of the Fukushima plant’s main components. The red numbers identify equipment and areas where the design was unsafe. One of the worst errors in all BWR designs around the world, including the American ones, is that the cooling water pumps could operate only at low pressures. Therefore, as the reactor temperature and the steam pressure increased, they could no longer pump the cooling water and first required the venting of the radioactive steam (“feed and bleed”). Also, in a properly designed plant, means would have been provided to lower the steam pressure by condensing the high pressure steam and return it with the feedwater.
Another major design deficiency common to most early reactors was that no piping was provided to pump water from the outside into the reactors or into the spent fuel rod ponds. This and the lack of elevated water storage provided with separate diesel generator operated pumps made it impossible to use mobile portable pumps, which should have been stored at the plant. Actually, neither stored fresh water, nor diesel fuel or portable pumps were in storage at the plant. This made it necessary to dump sea water from helicopters and fire trucks.
The 140 tons of fuel rods (8) were in the reactors. The fuel rods were provided with four levels of protection: The first was the zirconium cladding on the fuel rods. The second was the wall of the reactor vessel (11). The third was the primary containment (3), and the fourth, the secondary containment, the reactor building itself. In case of the Fukushima plant, both the building and the primary containment were well-designed as (to my knowledge) they were not damaged by neither the earthquake nor by the 45-ft high waves of the tsunami, which were still about 18 ft high (20) when they reached the plant.
Power Supply Backup
The earthquake destroyed the electric power supply of the plant (the connection to the grid) which by itself should not have been a serious problem, because backup diesel generators (18) were provided. It seems they failed because they were not elevated and the 18-ft waves of the tsunami reached and damaged them. The reason for their being installed at low elevation was probably both convenience and concern for their stability. The destruction of these generators could have occurred because water entered the diesel fuel tanks and sank to the bottom because water is heavier than the diesel fuel. As the engine takes its fuel supply from the bottom of the tanks, water instead of oil reached it. It is also possible that the air intakes of the engines were not elevated and ended up under water. If either or both of these conditions existed, the engine could not operate.
The secondary battery backup (19) was of no use either because it was drastically undersized. It provided only about eight hours worth of electricity, while about ten times that would have been needed to supply the electricity needed for a safe shutdown. (It should be noted here that of the 104 American reactors, 93 are provided with only four-hour battery backups). Another problem in the Fukushima plant was the lack of automatic battery recharging. This could have been provided because the plant was still generating steam at a rate of about 5% of full capacity and, therefore, some of the turbine-generators could have been kept in operation.
No other backup was provided at the Fukushima plant. This is unfortunate, because electricity itself is not essential to cool the reactors. For example, if emergency cooling water tanks were provided on the roof, would have made it possible to charge water just by gravity, and if those tanks were properly sized, the accident could have been prevented.
Similarly, in any plant where excess energy is present, that excess energy can be used directly to run the plant and its cooling systems. This could have been done by providing backup pumps with steam or Stirling type heat drives. The design of the Fukushima plant did not provide for any of these options.
Other Design Defects
Probably the worst design defect was the under-sizing of the spent fuel rod storage pool. This was a universal practice 40 years ago, because everybody assumed that means for permanent storage would shortly be available, but that never occurred. Therefore, at the Fukushima plant 1760 tons of spent fuel rods were in the temporary storage pool (10 times the amount the pools were designed for), requiring continuous cooling to protect against a meltdown. The melting of these spent fuel rods outside the primary containment (3) also caused hydrogen explosions and release of radioactivity. The running out of space in the temporary storage pools is a common problem all over the world because permanent and earthquake proof storage facilities are still not available anywhere.
Some improved storage technology did evolve over the years, such as storing the spent fuel rods in dry casks and/or underground, but these storages are also only temporarily. What is even worse is that, while the temporary storage facilities are getting full, governments are not concentrating on building permanent ones. For example, in President Obama's 2011 budget proposal, all funding for nuclear waste disposal was eliminated. So as of today, nearly 500 nuclear power plants around the world operate without permanent means of storing the waste they produce.
Part 2 of this series (July, 2011) will describe the changes needed in the design of American plants to make them safer and focus on the needed additions of automatic safety controls.