Summary:
Most likely the situation at the Fukushima nuclear power plants is as follows:
The reactors at the Fukushima I and II nuclear power plants are all shut down. I.e., there is no nuclear fission (except for naturally occurring spontaneous fission) occurring in the reactors there. There is no nuclear chain reaction in any of those reactors.
Plant operators have been unable to restore cooling flow to Reactors One and Three at the Fukushima, causing overheating of the reactor cores.
There was a hydrogen explosion in the containment building for Reactor One, causing the building's upper panels to be blown off. This is not the same as an explosion in the reactor itself.
The reactor is not going to explode like the reactor at Chernobyl did.
Some core damage has occurred to Reactor One at Fukushima I nuclear power plant, as evidenced by the release of a small amount of fission products (I-131 and Ce-137).
A partial core meltdown may well have occurred at two of the three operating reactors at Fukushima I may have occurred.
The current threat to personnel in the area is limited, but that may change in the future as more products are released.
The situation is still evolving.
Nuclear Basics:
Nuclear reactors are not constructed in such a way as to allow them to undergo a nuclear explosion. Worrying about this is much like worrying that the engine in one's car will suddenly become a fuel-air explosive (FAE) bomb.
Atomic (fission) bombs work by bringing fissile material together in sufficient quantity and with sufficient rapidity that the material will go "prompt critical," a special case of super-criticality in which the number of fissions occurring, and thus the amount of energy being released, increases exponentially, in an uncontrollable fashion. In other words, a bomb.
Nuclear reactors are constructed quite differently. A nuclear reactor is constructed so a self-sustaining controllable nuclear chain reaction occurs. This chain reaction can be controlled by limiting the amount of material exposed to other material at any moment. The physical construction of a nuclear reactor generally makes prompt criticality physically impossible, even during a meltdown. Nuclear reactors are heat sources, not bombs.
A nuclear reactor uses fuel, typically U-238 (uranium with an atomic weight of 238) enriched with U-235 (with an atomic weight of 235), contained in individual reactor components called fuel elements. The physical arrangement of fuel elements can take many forms, but one common form is uranium oxide (UOX) pellets contained within a cylinder to form fuel rods. The fuel elements are then held in place by the reactor structure to allow coolant to flow around the fuel elements, and thus to remove heat from them. There also are channels for the control rods, which are long rods of materials which absorb neutrons, and thus can remove the neutrons which cause the chain reaction. Inserting the control rods controls or, when fully inserted, stops the nuclear fission process and thus shuts down the reactor.
The reactor core (fuel elements plus control rods, plus various other structural elements) is contained within a reactor pressure vessel, a strong steel vessel capable of containing the high pressures at which the power plant operates. The reactor vessel is connected to similarly strong piping which contains the steam and the coolant. The entire coolant system is a sealed, high-pressure system. All connections to the reactor pressure vessel are high up on the pressure vessel, thus leaving the reactor core in a well in the pressure vessel. This provides a last-ditch means of keeping the core covered with water.
The coolant in a boiling water reactor (BWR) is boiled by the heat from the reactor core, passes through piping to the turbogenerators, then to a condenser, a cooling device which uses water to cool the steam, causing it to condense back to water. High-pressure pumps then force the coolant back into the pressure vessel and the cycle begins anew.
The reactor system itself is housed in a containment building, a steel and reinforced concrete structure which is designed to contain radioactive materials in case of a nuclear accident releasing coolant. Typically, for BWRs, the containment facility looks like a square building rather than the domed structures surrounding a pressurized water reactor (PWR). The Fukushima plants have such square containment buildings.
Criticality: This term refers to the ability of a mass of fissile material to sustain a nuclear chain reaction. If the mass is:
Ø Subcritical, the nuclear chain reaction cannot be sustained and will die off.
Ø Critical, the nuclear chain reaction is sustained, with power neither increasing nor decreasing.
Ø Supercritical, the nuclear chain reaction is increasing, with power increasing.
Ø Prompt critical, the nuclear chain reaction is increasing out of control.
The critical mass of U-235 is about 52 kilograms, or a sphere about 17 centimeters (6.7 inches in diameter. For reactor fuel at 20% enrichment (20% U-235 and 80% U-238), the critical mass is closer to 400 kg.
Fukushima I & II Nuclear Power Plants:
The reactors at Fukushima Dai-ichi Fukushima I Nuclear Power Plant) plant consist of six (6) GE boiling water reactors (BWRs) with two (2) GE advanced boiling water reactors (AWBRs) under construction (scheduled to be on line around 2016 and 2017). The former are conventional BWRs, and require pumps to force cooling water through the reactor cores in a closed, pressurized system. The heat from the reactor core causes the light-water (deuterium-free) coolant to boil, which produces steam to drive the steam turbogenerators and thus to produce electric power. The ABWRs (which are still being constructed) will use natural circulation to circulate coolant. Natural circulation uses thermal differences to force coolant flow, and thus no electric power is needed to circulate the coolant. Natural circulation provides distinct advantages in this sort of situation.
Reactors one through six have a pressurized system which acts as the primary boundary for the reactor coolant, plus a secondary containment building, which is designed to act as an emergency boundary in case of what is called the design leakage accident, involving failure of the primary pressure boundary. As currently planned, the two (2) ABWRs will have three boundaries, and not just two.
In addition to the normal method for transferring heat from the reactor core, there are a number of possible emergency systems. It is not yet clear exactly which emergency systems are incorporated in the Fukushima I and II nuclear power plants.
The Fukushima II, or Dai-ni Fukushima II Nuclear Power Plant), nuclear power plant uses four (4) BWR-5 reactors with Mark II containment buildings. Fukushima II is located 11.5 kilometers (7.1 miles) south of the Fukushima I power plant
The Earthquake and the Reactors:
Fukushima I (Dai-ichi): It was reported that Reactors 1, 2, and 3 were shut down automatically during and following the earthquake. Reactors 4, 5, and 6 were undergoing maintenance, and thus were already shut down and thus should need no or at most minimal forced cooling (depending of course, on how recently the reactors had been shut down for maintenance). This means that the control rods were inserted, and the nuclear fission process stopped. The reactors stopped producing heat from nuclear fission at that point.
Fukushima II (Dai-ni): it was reported the four BWR-5 reactors at the Fukushima II nuclear power plant had been operating but were shut down automatically during the earthquake.
However, the nuclear fission products continue to generate heat, even though the reactor has been shut down. The heat is generated by nuclear decay, and can amount to as much as one percent of the reactor's average power output over the last several days. The amount of decay heat being released decreases over time, but still is significant. Typically, when a reactor is scrammed (shut down) after a long and steady power history, the decay heat released might be:
Immediately after shutdown: 6 or 7% of reactor power at which the plant was operating
One hour after shutdown: About 1.5% of average reactor power when operating
One day after shutdown: About 0.4% of average reactor power when operating
One week after shutdown: About 0.2% of average reactor power when operating
It is this decay heat which must be removed.
Loss of Coolant / Loss of Coolant Circulation:
Fukushima I: Reactor Number One (460 MW) lost coolant circulation (cause not yet published), and it appears that Reactor Number 3 (784 MW) also has lost coolant circulation. With no coolant being circulated, the decay heat cannot be removed from these reactors.. Initially, the decay heat will cause the water to boil, which provides some cooling, but water must be added to keep the core covered, and thus limit core temperatures. If coolant circulation can be restored, then the reactors can be cooled. The primary coolant would pass through the core, be boiled off, and then pass through the turbines and to condensers, where the coolant steam would condense back to water and be cooled further before being circulated back through the core.
Fukushima II: Reactors One, Two, and Four are reported to have compromised cooling systems, with rising temperatures (above 100 degrees C, 212 degrees F) noted.
Until coolant circulation can be restored, there are a number of other possible solutions. One of these, venting, allows more water to boil off, thus removing more heat energy. (It requires significant amounts of energy to It appears this approach has been used at Fukushima I Reactors 1 and 3. There is a disadvantage with venting in that it allows primary coolant to escape. Lost coolant must be replaced in order to keep the reactor core covered. Typically the coolant is replaced by pumping water back into the plant, which requires electric power to run the pumps. Electrical power is provided by emergency diesel generators if other sources of electrical power are unavailable.
Replacing coolant requires forcing coolant back into the reactor pressure vessel against the pressure in the vessel. If the pumps being used cannot provide sufficient pressure, then the pressure in the system must be reduced, usually by venting, to a pressure low enough that the pumps can move water into the pressure vessel.
The possibility of having to vent the reactors at Fukushima II also has been announced, but so far that has not occurred.
As noted earlier, the reactor is constructed so that the reactor core is situated low in the pressure vessel, and all openings in the pressure vessel are placed near the top, thus making a well in which the reactor core sits and reducing the chances that the core will become uncovered. Uncovering of the core is highly undesirable, as air is not a very good coolant. (The specific heat, a measure of the amount of heat energy a substance can carry, of water is roughly 4000 times that of air.)
If coolant cannot be circulated, the water in the reactor pressure vessel will boil off. Boiling water consumes considerably more energy than simply heating water, and thus provides a greater cooling effect. It appears that the Fukushima I nuclear power plant has run out of distilled and purified light water normally used for cooling. In that event, any available water can be used to keep the core covered.
Sea water reportedly is being pumped into the overheating Reactors One and Three at Fukushima I. If so, this represents a last-ditch attempt to keep the reactors cool. It is by no means unheard of, but it does mean the power company operating the plants has given up any hope of recovering those plants and restoring them to service.
Venting:
It appears from the reporting that coolant venting has been used for Fukushima I reactors 1 and 3. Venting releases coolant (as steam, upon depressurization) into the containment building. Whether this venting was deliberate or resulted from the lifting of pressure relief (safety) valves, or both, is unclear. Safety valves release the pressure in a closed system before the pressure reaches the point at which the sealed system might rupture. When the reactor coolant is vented to the containment building, any radioactive material in the coolant is released as well. If this material gets to the atmosphere, then it can be detected, and people in the area can be exposed to radiation. This is what happened at the Three Mile Island nuclear power plant in the United States on 28 March 1979.
In addition, venting releases gases in the coolant into the atmosphere as the pressure is releases. (Think of uncapping a soda here - the carbon dioxide in the soda is released as bubbles, or more energetically if the soda has been shaken first.) These gases may be radioactive, and may include hydrogen (on which more below).
Poisoning:
Nuclear fission has been stopped by the insertion of the reactor control rods in the plants under discussion. The effectiveness of this method of controlling the chain reaction requires the reactor structure to remain intact. In a partial or complete meltdown, the exact structure of the reactor core cannot be predicted. In order to ensure the nuclear chain reaction remains shut down, boron is introduced into the reactor core. Reportedly this is being done at Fukushima I by injecting boric acid into the primary coolant system. Boron is a strong neutron absorber, and thus acts to shut down the nuclear fission process the same way that a control rod does. Boron is considered anuclear poison, as it poisons the nuclear fission process. The term nuclear poison does not refer to effects on humans.
Evacuation:
Evacuation is a good precaution when venting is occurring. The area to be evacuated generally can be based on current winds, as well as on the amount of radioactivity being released.
Why is this a good approach? Basically, for a given amount of radioactivity being released, the concentration of that material is reduced as the material spreads (roughly spherical spreading). Thus, by clearing people from the immediate vicinity of the reactor venting the exposure to the population as a whole is reduced to acceptable levels.
Iodine Tablets:
It has been reported that iodine tablets have been issued to people in the vicinity of the Fukushima I nuclear power plant. The purpose of taking iodine tablets is to saturate the body with iodine, and thus (one hopes) reducing or eliminating the uptake of radioactive iodine (particularly long-lived radioactive I-129) from the environment. The human body concentrates iodine in the thyroid gland, which is a key organ for controlling metabolic functions.
So far, it appears the iodine tablets have been issued as a precaution.
Explosion:
The next time the explosion at the Fukushima I reactor number one building is shown, look closely at the video. You will see a shock wave traveling upward just before the rest of the smoke and clouds appear. Based on this, it appears that what happened was that hydrogen gas released by the venting process had gathered at the top of the containment building. Since there is air in the containment building, there also is oxygen present. With both hydrogen and oxygen present, one has an explosive mixture which needs only a spark to set it off, and that is what appears to have happened. It looks as though it was a hydrogen explosion. This does normally not happen inside the coolant system because that is a sealed system with no air present, but once that gas is vented to the containment building, it will tend to collect in the upper part of the building, and an explosion can result. (Look at the later pictures of the reactor one containment building and you will see panels form the upper half of the building have been blown off.)
Now there has also been an explosion at Fukushima I Reactor Three, and it appears this, too, was due to hydrogen accumulating in the containment building.
There will likely be subsequent similar explosions as steam and hydrogen continue to be vented from the damaged reactors.
Why this is not Chernobyl: Too many have cited the Chernobyl Reactor Four explosion and have compared the conditions there with those at the Fukushima nuclear power plants. Bad comparison.
First of all, the two reactors are considerably different construction and operation. The Chernobyl plant used four Soviet RBMK-1000 reactors, which are graphite pile reactors rather than the sealed boiling water reactors as used at the Fukushima plants. The Chernobyl reactors had a badly flawed design, which allowed the reactors to reach prompt criticality (uncontrollable criticality) when the control rods were inserted during a scram. Prompt criticality is akin to what happens in a nuclear weapon. Modern Western reactor design, such as used at Fukushima I and II, should not allow prompt criticality.
Secondly, the Chernobyl reactor was in operation at the time of the accident (I am sparing you some very pertinent technical details here), whereas the Fukushima reactors had been shut down automatically. This means that fission was continuing at Chernobyl, whereas the controlled fission reaction had stopped at Fukushima.
Thirdly, when the Chernobyl reactor was scrammed (control rods inserted rapidly), the flawed reactor design caused prompt criticality, with an estimated power surge of at least 1700 percent power (17 times rated power) in the reactor core. This means that in the center of the reactor core at Chernobyl nuclear power had reached levels well exceeding design power levels. No power surge at all has been reported in the Fukushima reactors.
Finally, the initial explosion at Chernobyl was a steam explosion, caused by the power surge. This was like a boiler explosion. It is likely the subsequent explosion at Chernobyl may have been from a hydrogen buildup. In any event, it was not a nuclear explosion. The explosions expelled about half the core material into the surrounding countryside. No such explosion, or sequence of events leading to an explosion, is being contemplated at the Fukushima I and II plants.
Core Meltdown:
This sometimes is called the China Syndrome, from the fear that if the core melts, nuclear fission will continue unchecked and the core will melt its was down into the Earth with catastrophic results. Of course, the core could never actually reach China simply because the gravitational pull from the Earth at the center of the Earth is zero. In addition, as soon as the core reached the molten mantle it would no longer be in one place. And it is not even going to reach the mantle. Great science fiction, though! ;-)
First, what is all this about "core meltdown?"
If the decay heat cannot be removed from the reactor core, the core can overheat, causing damage to the fuel elements in the core. The nuclear fuel in a reactor is contained in fuel elements, which can take any of several forms. If the form is cylindrical, these are often called fuel rods. Basically, the uranium fuel is contained in a metal casing. The purpose of the casing (in some cases called fuel cladding) is to contain the radioactive fission products and decay products and keep them from being released into the coolant.
When the reactor core overheats, the fuel elements can be damaged. The first way this can occur is by warping of the elements, and thus possible opening of the seams in the fuel elements. If the heating continues, localized melting of the fuel elements may occur, along with release of fuel and fission products from the fuel element. Finally, in the final stages complete melting is possible, but not as likely.
So, what is going on at Fukushima? Most likely the fuel elements have been compromised. Such core damage cannot be corrected, only contained. The evidence for this is the detection of small amounts of Iodine (I-129 and I-131) and Cesium 137 (Ce-137). These are fission products, and their detection outside the plant would seem to indicate that fuel element damage has occurred. The amount of damage is unknown yet, but clearly some fission products have been released.
Does this indicate a complete "core meltdown?"
No, it does not. It does indicate that there has been some core damage however. Furthermore, it seems as though, given the low levels being reported, the fission products most likely were released during venting (see section above on venting).
Furthermore, the levels being reported do not yet indicate the primary coolant system has been breached.
The Future:
It is not possible yet to foresee how the nuclear accidents at Fukushima I and II will progress. It seems certain that Reactors One and Three will not be placed back in service very quickly, if at all. At the very least it seems likely the current cores will have to be replaced in Reactor One, and possibly in Reactor Three as well. That will take time. Furthermore, the Reactor One containment building will have to be rebuilt. It is quite possible that the reactor may have been damaged beyond repair.
The status, and thus the future, of Reactor Three is less clear.
In addition, it seems quite likely the existing safety systems will be reviewed extensively. In particular, the reasons for the failure of back-up safety systems need to be determined, the systems need to be redesigned, and possibly upgraded or added to, in order to preclude a recurrence and to enhance reliability.
Clearly natural circulation reactors, plants in which the reactor coolant is circulated as the result of being heated, with no circulation pumps required, offer a good solution to the problem of coolant circulation under emergency conditions.
Finally, a thorough review of nuclear safety studies and accident analyses likely will be undertaken. Do the current analyses adequately and accurately reflect possible accidents, whether man-made or nature-caused? If not, what must be done to make them adequate? And so on.
This was written by Lars Hanson who's credentials appear to be documented at:
