Doesn't lowering control rods into the core stop the nuclear reactions? No, about 6% of the nuclear reactions persist after the control rods are fully inserted. Although the control rods stop the fission reaction from one fuel rod to another, they can't stop fuel and by-products of fission inside each fuel rod from continuing to undergo nuclear reactions. That's why the heat from this 6% activity must be continually removed from the core even after the reactor has been shut down.
So just this residual 6% is enough to potentially cause a melt-down? Yes, unless the heat is removed from the core somehow. Usually the heat is carried away by circulating water.
Why isn't the water circulating? Because the tsunami destroyed the primary and backup power systems for the water pumps. The crisis was caused by the tsunami, not the earthquake itself.
Should the reactors have been located oceanside? They must be located near water for cooling. Japan doesn't have many inland lakes or large rivers that provide sufficient water, so these plants were built on the ocean.
Why must nuclear fuel rods be clad? Because the fuel isn't metallic; it's a powder consisting of uranium oxide and, in some cases, plutonium oxide. The powder is compressed into cylindrical pellets. Multiple pellets fit inside a tube called a rod. The material that makes the tube itself is called the cladding.
Why are fuel rods clad in zirconium? Because at the usual temperatures of the core, zirconium doesn't react with the water which surrounds the rods and carries away heat. Also, zirconium doesn't absorb neutrons; therefore zirconium doesn't impede the fission reactions when the reactor is operating normally.
What produces the hydrogen in a nuclear accident? When zirconium reaches 2200 degrees -- far above the normal operating temperature of a reactor -- it reacts chemically with water to produce zirconium oxide and hydrogen gas. This reaction can't be prevented. Worse, the reaction with water is exothermic -- meaning that it generates even more heat.
What happens to the hydrogen gas? It builds up inside the reactor vessel with increasing pressure until it must be released. If it's not released, or vented, the pressure will thwart the reactor's cooling systems by making it too difficult to pump water into the reactor vessel and cool the core.
Are their other gases inside the reactor vessel when it gets so hot? Yes, some of the cooling water will have converted to steam, despite the elevated pressure. Also, if the zirconium deteriorates and the fuel rods leak, gaseous by-products of nuclear fission such as radioactive xenon and radioactive krypton that naturally accumulate inside the fuel rods will enter the vessel.
So the operators must eventually vent the vessel to relieve the pressure? Yes. When they do this, the hydrogen, steam, and radioactive gases escape the vessel into the interior of the surrounding building. The hydrogen is not radioactive, but it's flammable (remember the Hindenberg?). The steam is not radioactive, either. The xenon and other gases are radioactive, but they're heavier than air and tend to sink into the bottom of the reactor building -- assuming the building is still air-tight.
Is that all that comes out when the vessel is vented? No, if the fuel tubes are severely ruptured, some of the uranium and plutonium compounds might be dispersed -- but these compounds are very heavy, so they don't disperse well unless there is an explosion or fire. Other radioactive by-products of nuclear fission like cesium and iodine compounds that are solids might be vented too. These molecules are not so heavy, so they're more likely to spread if they come airborne. Detecting cesium in the surrounding area is a good indication that radioactivity has escaped the plant into the environment. Some of these radioactive by-products of nuclear fission have relatively long half-lives, so they constitute a long-term radiation risk.
So what blew up? The hydrogen gas eventually reached a combustible proportion with oxygen inside the surrounding building, and inevitably a spark of some kind set it off. All the steam, radioactive gases, and airborne radioactive heavy metals that were mixed with the hydrogen then enter the atmosphere.
How much radioactivity was that? When the buildings for reactors 1 and 3 blew up, apparently not much. But there are indications that considerably more radioactivity was released when the building for reactor 2 blew up.
Did the vessel itself blow up? Not for reactors 1 and 3. There are indications, however, that the vessel for reactor 2 might be damaged. No one can get close enough to look. Instruments are not always reliable under these conditions.
Will the other buildings eventually blow up? I haven't heard that their cooling systems are malfunctioning. This is all about keeping the cores cool. If the cooling systems are working for the other reactors, they won't blow. Of course, the explosions from 1, 2, and 3 could have damaged those systems. I believe one or two of the other reactors at the site had been shut down prior to the earthquake for reasons unrelated to safety.
Why are they adding boron to the cooling water? Because boron absorbs neutrons and reduces the remaining nuclear reactions that are underway.
Will the boron help? Over a very long time, it would, but some of the boron tends to escape when the vessel is vented.
Why can't enough water be pumped into the vessel to cool it? There's already a lot of gas pressure inside the vessel, so it's difficult to pump water inside. When the water does enter the vessel, some of it tends to become steam immediately -- so the vessel must be vented again. This leads to a "pump and dump" cycle that is an inefficient way to remove the heat that's constantly being generated in the core. It's also hard on the pumps, valves, and pipes. After a while it becomes difficult or impossible to keep the core immersed in water... and then the core temperatures get even higher.
What happens as the core temperatures increase? The powdery fuel, the by-products trapped within it, the bottoms of the control rods, and whatever remains of the zirconium mix together and enter a very viscous state comparable to lava.
If the fuel melts, what happens? First it comes in direct contact with the vessel body, which has eight inches of steel at the bottom. Steel is a good conductor of heat, so it transports away some of the heat from the fuel. If the vessel is not kept cool the steel will eventually melt, but it absorbs a lot of heat in the process.
What's below the steel? There is a layer of thick concrete under the steel. At first it will resist the melted fuel and steel, but eventually it may melt or deteriorate because of chemical reactions at extreme temperatures. At Three Mile Island, the steel and concrete held.
What's below the concrete? The lava-like mass, called corium, may penetrate the foundation of the plant and go underneath the plant into the ground, where it disperses. Eventually it will cool, and the nuclear reactions will slow down.
And how much radioactivity will be released? No one really knows.
What about the spent fuel rods in cooling pools on the upper levels of the reactor buildings? Basically they pose the same issues as the fuel rods inside the reactor vessel, although the ongoing nuclear reactions in the spent fuel rods are less intense. These spent fuel rods must be kept cool with circulating water.
What happens if the water stops circulating in the cooling pools? It boils off. If the water can't be replenished, the spent fuel rods are exposed to the air.
What happens to exposed spent fuel rods? The rods get so hot that the zirconium cladding catches fire. As the zirconium turns into zirconium oxide -- another exothermic reaction -- all the fission by-products inside the rods as well as any leftover uranium oxide and plutonium oxide will be exposed to the atmosphere. Given that the heat of the fire will cause convection, these by-products easily become an aerosol, enter the atmosphere, and disperse in the prevailing winds.
What's the immediate impact? The spent fuel releases a lot of radiation in the vicinity of the plant.
What's the impact over a broader area? The scenario of a high volume of airborne radioactive compounds entering the atmosphere is daunting. Some emit gamma or beta radiation that the human body receives from outside. Some emit alpha particles, so they are dangerous if inhaled or ingested. Some like iodine are biologically absorbed (this is why potassium iodide tablets saturate your body with iodine and prevent uptake of the radioactive iodine isotope). Some like plutonium are chemically toxic in addition to being highly radioactive.
Is this similar to Chernobyl? It's not, in terms of how it began. But it could be, in terms of impact.
