Fukushima and the Future of Nuclear Power
In this article I will describe the design changes needed to protect against repetition of the Fukushima disaster in the old nuclear power plants when exposed to earthquakes along geological fault lines, 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 some 440 nuclear power plants are in operation around the world (104 in the USA) 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 in Russia, the other in Tsuruga in Japan. If in the future breeder reactors are built, the risks will increase, because their product (plutonium 239) can be used to build bombs without further concentration. Research is also in progress to build fusion plants, which operate at millions of degrees temperature and continuously release the same energy, which hydrogen bombs release all at once.
Today, with the exception of the two small plants mentioned above, only the fission plants are in operation which can not explode like an atomic bomb, but are still dangerous, because it can releases radioactive Iodine or Cesium 137 which emits gamma rays and accumulates in the food chain for decades. In case of a partial or complete meltdown, the produced plutonium (half-life of 24,100 years) can make the region uninhabitable for thousands of years.
The Fission Process
The heart of a nuclear power plant is a high pressure boiler which is similar to a regular 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 can not 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 a serious accident will result, if cooling is lost. Finally, the most important difference is in the waste that is produced, still contains some fuel (uranium 235) 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. Although some argue that this is not worst 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 + 89 36Kr + 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 1,200 ˚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 the primary containment (see figure) was filled with air and therefore, as hydrogen accumulated in it, it exploded. If the containment was filled inert gas (nitrogen), this should not have occurred.
If the fuel rod temperature rises further, the uranium melts at 2,800 ˚C (~ 5,000 ˚F) and radioactive isotopes (iodine-131 and cesium-137) and plutonium are produced. The half lives of meltdown products range from 8 days for iodine, 30 years for cesium and over 2,000 years for plutonium. At the Fukushima plant, where partial meltdown occurred (75% in one and 33% in an other reactor) plutonium was released into the air and was discovered in the soil.
The Faulty Design at Fukushima
Figure 1 shows the design of the Fukusima plant’s main components (to the best of my understanding). These components are numbered. The red numbers identify areas where the design was unsafe. Other design errors are not noted, because they were common to all old boiling water reactors (BWR) and were not unique to the Fukushima plant. One of the worst errors is that the radioactive steam was allowed to leave the first containment, instead of condensing and returning it into the reactor as feed water inside the confinement. Therefore, if a condenser was provided within the confinement and only the heat content of the radioactive steam was sent (by another circulating loop) to drive the turbine generators, no radiation could have gotten out into the reactor building as long as the confinement was undamaged.
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 water 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 umps were in storage at the plant. This made it necessary to dump sea water from helicopters and fire trucks.
Figure 1: The main components of the Fukushima nuclear power plant (The components numbered in black were designed and operated correctly, the ones in red did not). 1-solids filter, 2-vent valve, 3-primary container or drywell, 4-steam separator & dryer, 5-turbines, 6-generators, 7-spent fuel rod pool, 8-reactor core, 9-condenser, 10-down-comer region, 11- reactor pressure vessel (RPV), 12-control rods, 13-recirculation pumps, 14-cooling feed-water pumps, 15-back-up selector switch, 16-torus or wet well, 17-core catcher, 18-diesel generator back-up, 19-battery back-up, 20- the height of the tsunami waves reaching the plant
The 140 tons of fuel rods (8) were in the reactors. The fuel rods were provided with 4 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 4th 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 either the earthquake nor by the 45’ high waves of the tsunami which was still about 18’ high (10) when it 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’ tall waves of the tsunami reached and damaged them. The reason for their low elevation was (probably) both convenience and concern for their stability. The destruction could have occurred, because water entered the diesel fuel tanks which sunk to the bottom because water is heavier than the diesel fuel. As the engine takes it’s 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 had to stop.
The secondary battery backup (19) did work, but it was drastically undersized. It provided only about 8 hours worth of electricity, while about ten times would have been needed for a safe shut down. (It should be noted here that of the 104 American reactors 93 are provided with only 4 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.
Only these two layers of backup and only for the electric power supply were provided at the Fukushima plant. This is unfortunate, because electricity itself is not essential to cool the rectors. For example if emergency cooling water tanks were provided on the roof, which can charge water only 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 it’s cooling systems. For example in this case the Fukushima plant was still generating 5% of its full capacity of high pressure steam and therefore, if some of the turbine generators were still operational (I do not know if they were) the “internal electricity” produced by the generator would have been sufficient to run the plant, even after the grid was destroyed. In addition some less conventional options were also available for backup. For example some of the pumps could have been operated by the reactor’s own heat directly by using Stirling heat engines or could have been operated by steam engines, using the steam produced by the reactors. The design of the Fukushima plant did not provide for any of these options.
Other Design Defects
The plant was not provided with a core catcher (17) either. Therefore, in case of a full meltdown, the lava of the liquefied core would have burned through and sink into the soil, which was prevented even at Chernobil.
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 devised, but that never occurred. Therefore, at the Fukushima plant 1,760 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. Over the decades, the Fukushima plant’s temporary storage pools (and all others everywhere) run out of space as no permanent, earthquake proof storage facilities were built anywhere. (Some improved storage technology evolved, such as storing the fuel rods in dry casks and/or underground, but only temporarily). In President Obama’s 2011 budget proposal, all funding for nuclear waste disposal was eliminated. So as of today, nearly 500 nuclear power plants operate without permanent means of storing the waste they produce.