Current counts are 900 dead. But there are a huge number of people missing, and many of them are likely buried in either the rather-light rubble (not much collapsed, thanks to Japanese engineering) or Tsunami (against which there is no defense - other than not being there.) If tens of thousands of people are not dead, most as a result of the Tsunami, I will be shocked.
Japan, of course, is no stranger to earthquakes. Located in a geological subduction zone they're frequent. But this is the strongest one on record in Japan, and something like the 4th strongest earthquake ever recorded.
Everyone's attention is focused on the nuclear power plants in Japan; the nation gets about 30% of its electricity from nuclear power. Originally we were told that there was only one plant with a cooling problem; that has since expanded to most of the plants that were operating at the time.
A primer for those who are not familiar with nuclear power in general may be helpful. The plants in question are "BWR" plants, or "Boiling Water Reactors." Fuel comprising a mixture that contains Uranium-235 and/or Plutonium-239 are placed into rods with a zirconium cladding. These are arranged inside a high-pressure container. Hydraulically-controlled rods that contain material that absorbs neutrons are placed in the core as well. The pressure vessel is filled with ordinary water.
Nuclear reactions rely on the neutrons that are emitted when an atom splits hitting another atom of fissile material (the Uranium-235) and causing it to split. In order to cause an atom to split the neutron has to have a certain energy. Most people think of fission as a literal "splitting" action (much like one splits a log); this is in fact not correct. While it is possible for a very fast neutron impact on a fissile atom to cause it to split it's inefficient and nearly all of those impacts will instead bounce off. The goal is to slow the neutrons into the thermal range where instead of bouncing off they are absorbed by the fissile material; it then becomes unstable at an atomic level and splits. That process is very efficient and is what is relied on for nuclear power generation. The water in the core of a BWR serves as that moderator, or neutron "slowing" mechanism.
A boiling-water reactor has, as the name implies, boiling water in the core itself. That means there's steam in the core. Steam, being water vapor, has much lower density than liquid water, and thus tends to slow the reaction down, since it moderates fewer of the neutrons. But this creates a potential problem when the reactor is shut down. Since there is no heat exchanger the actual core water that boils is used to drive the steam turbines and produce power. This steam is mildly radioactive, largely with radioactive isotopes of hydrogen (tritium) and nitrogen (N-16.) This is not normally a problem as the half-life decay time of the nitrogen is very low (seconds) so people can enter the turbine area a few minutes after the reactor is shut down to do work.
When a reactor is shut down in an emergency the steam outlet to the turbines is closed. This causes the pressure to rise rapidly and the steam to collapse back to liquid water in part of the reactor. That in turn causes a temporary power spike in the output of the reactor until the control rods are fully re-inserted, as the neutron moderation capacity goes up as the steam bubbles collapse.
Once the control rods are in power production does not cease immediately. Reactors produce power by atomic decay - it's just that the "decay" is forced by neutron absorption resulting in controlled fission. But when you shut a reactor off there are a lot of byproducts in the fuel that undergo decay for a significant period of time, and that decay also produces heat. As a result you have to provide cooling to the fuel for a significant period of time once the reactor is shut down. Once the fuel is cool enough to remove from the core it still has to sit for a long period of time before you can handle it outside of continual cooling; this is typically done by big "pools" of water into which the spent fuel is placed and allowed to dissipate the residual decay products for a period of time before being transported for reprocessing or disposal.
Unfortunately that residual capacity to develop heat, if not taken care of, can cause serious problems. The zirconium cladding can heat to the point that it cracks open and, in contact with water vapor, liberate hydrogen (the oxygen combines with the zirconium.) This leads to a build-up of hydrogen gas in the steam, but without oxygen nothing bad happens immediately. Cracked fuel rods also allow reaction products out into the coolant. In the worst case the cladding and fuel itself can ignite and burn.
For this reason it is absolutely necessary to make sure that the core remains submerged after the reactor shuts down until the reaction product decay drops to a low enough point to allow the fuel to be unloaded.
From what we know when the reactors "Scrammed" (shut down on an emergency basis) due to the original earthquake event grid power was immediately lost and the on-site diesel generators started. This provided the power necessary to keep the circulation going and remove the reactor heat. But an hour or so later the diesel generators were lost - possibly due to the Tsunami flooding them.
Now there's a problem, because without power you can't run the pumps. And as the water in the core absorbs the heat the pressure rises. Remember that the original design allows for a significant volume of steam in the reactor vessel, and that it is important that the fuel not become uncovered as it requires liquid cooling (the steam is insufficient.)
If you can't run pumps you've got serious trouble. Some plants have the ability to use some of the steam generated to run pumps on at least an emergency basis, thereby being "self-generating." I've been unable to determine if this is the case at these plants; it appears not, and they were reduced to battery power when the diesels failed. It also appears that their instrumentation was either damaged or destroyed, as nobody seemed to know for sure what the actual water level was in those reactors.
If the heat cannot be removed through an emergency heat exchanger your only option is to dissipate the energy by releasing steam. But doing so means the water level in the reactor falls; you then need power to be able to make up the water in the reactor vessel and the pumps have to be able to overcome the pressure in the system to feed the water in. That status of that capability has been unclear since the earthquake.
What we know at this point is that they have detected Cesium in the surrounding area. This is a solid indication that the fuel rod integrity has been compromised, as that's the only way reaction products (of which Cesium is one) can get into the coolant and then outside the plant. This, in turn, strongly implies that the core has become uncovered for at least some period of time, and that, in turn, implies that the operators and physicists at the plant either did not know the water level in the reactor, or they had to release pressure to prevent a loss of integrity in the reactor vessel even though they knew they could not make up the water loss via feed pumps.
The good news is that the explosion destroyed the top of the concrete-wall building surrounding the containment which is said to be intact (high-strength steel) and then the reactor vessel is inside that. The bad news is that if hydrogen has been produced in meaningful quantity (and that's a fair bit of it folks) it is a near-certainty that serious fuel damage has occurred and the water in the plant has been contaminated with high-level reaction products.
Key now is getting sufficient cooling to the core to prevent the remaining water from being boiled off and the containment melted through. Temperatures that can be developed in the fuel, absent cooling, are more than sufficient to violate the containment and expose the fuel to atmospheric oxygen. That in turn will initiate oxidation ("fire", although the heat to activate it is coming from nuclear decay rather than the usual chemical processes) and those sorts of fires are very difficult if not impossible to extinguish until the entirety of the fuel available is burned up.
Latest reports are that they are now attempting to flood the containment building with borated seawater. This is a last-ditch attempt to get temperatures under control, and should work. Doing so economically destroys the plant, and it appears they were attempting to avoid that outcome up until this point.
I know this incident is going to cause every green and every red to come out and scream that we must stop all nuclear power now.
Sorry, nope.
There is risk in all human endeavors. This is no exception. Energy production comes with risk, and it cannot be avoided. Go live in Texas City if you wish; they refine a lot of petroleum there, and there are a lot of unexplained cancers - even though airborne levels of various products are well within "safe" limits.
Fermi I in Monroe was a Fast Breeder reactor that came close to a loss-of-control event. Had it occurred, it would have been catastrophic. While it was technically a power-producing plant it was low in output and considered a proof-of-design plant. We never built another one.
Liquid Salt Thorium Reactors are passively-safe. That is, their working fluid (which also happens to be the reactant mass) is maintained in the reactor by active cooling of a freeze plug. If power is lost then the plug melts and the coolant drains into chambers that are sufficiently distant from one another that criticality is lost at the same time. In addition this design has a negative temperature coefficient and thus cannot thermally run away, and neutron moderation necessary for criticality to be maintained is in the graphite moderators inside the reactor vessel. Since the coolant does not boil at working or emergency-shutdown temperatures and contains the fuel within it an emergency shutdown does not expose the reactor to the risks that are currently being experienced in Japan. Finally, the higher working temperatures of the primary fluid allow for much-higher efficiency combined-cycle turbines to be used as opposed to the Rankine-cycle turbines in current BWR and PWR designs. This latter design is not "Pie in the Sky"; we operated one of these reactors in the 1960s for four years at the Oak Ridge National Laboratory.
The reality of our modern life is that we must have energy production if we intend to have a vibrant economy. All forms of energy production come with risk, whether it's due to the risk of chemical exposure in various forms or radiation. When these systems operate normally they do not harm people, but industrial accidents happen, even without the forcing factor of Mother Nature coming into play.
We must accept these risks if we are to enjoy our modern way of life. Despite the ongoing challenges in Japan today, I would fully support a nuclear power plant being constructed 10 miles upwind of my home, just as I fully supported offshore drilling out my back window even after the BP well disaster. We have no right in this country to demand consumption of resources we are unwilling to develop ourselves, and for which we are unwilling to accept the risks.
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