Japanese Reactor Explanation

Fukushima Nuclear Power Plants I and II

As a former nuclear engineer with the United States Navy, I am very concerned that the recent reporting regarding the Fukushima reactors has been unnecessarily alarmist and often inaccurate. Yes, there is cause for alarm, but not of the sort being bandied about – at least not yet. As an attempt to counter the current spate of circular, uninformed, and sensationalist journalism, I offer the following. Because this treatise is based entirely on publicly available information, it is acknowledged that there may be some technical inaccuracies due to misinformation or lack of information. However, I hope to provide a good overview of events to date from the perspective of one trained to operate nuclear reactors under a variety of conditions and in various situations. More importantly, I hope to provide a better understanding of the events at Fukushima, as we know them so far.

This essay not intended to minimize the hazards of nuclear power, only to keep the concerns grounded in fact rather than fiction.

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 (福島第一原子力発電所 – 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 (福島第二原子力発電所 – 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 a nuclear 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.

It is time to change things-for the better

Thinking people, from all persuasions, have commented that it does not really matter who we elect as the outcome is always the same, one party does it a little faster or slower than the other. Buckets of ink have been used to write about this question; how come someone goes to Washington with fixed determination to make changes in the system only to become part of the very system and a defender of the status quo.? Just look at our current budget crisis or the Obamacare debacle. Even though much of this is fresh legislative ink, the champions of lower taxes, less government and reduced regulatory intrusion have balked at doing what they said they would do when they were swept into office in November.

I do not fault the newly elected Republicans but rather, their long time leadership. They have grown fond of the benefits of their offices and coyly play both sides and change very little. They catch media heat regardless of their activities but feel politically safer simply massaging the newly passed legislation with minor adjustments that are more compatible to their expressed views. So like others I wonder why this occurs. It certainly seems that they assume that they know better than the electorate and are much like the "Reality TV" shows that portray drama for the viewer because the producers have said it makes "good TV". I am tired of "good TV" and think we need see some changes in all parties and remove all long time officials.

Pundits might have you believe that the elected officials are corrupted by the special interest money that flows into their re-elction coffers. It think that is a secondary influence. Our elected officials have been feather-bedding their positions for years. How many employers provide full, life-time benefits and income for a working tenure of as little as 2 years and a a work year that is only 6-8 months in actual length? How many jobs permit you to be in charge of a prominent committee and then not hold hearings or choose not to show up during votes? Welcome to the Congress of the United States, as it has been crafted by the incumbents of both parties. It is time that "We the People" take this bicameral house back and limit the enrichment these folks receive. When a CEO of a public company leaves and is awarded a "golden parachute", elected folks talk about how the financial rewards are not fair and out of proportion to the benefits awarded "regular employees" in the firm. The suffer from the "pot-kettle" syndrome when they bluster and fume.

Let's make it a rule that these folks serve us and when they are removed from office, because they did not please us, they have to get a real job and their income and benefit package ends.

You might imagine that they would stay very attuned to the needs and wants of the people and less prone to think they are "smarter" and "more informed" than "We the People". When they cavalierly spend our money in their budgetary sessions and then shrug their shoulders because they voted for unpopular policies that lead to their removal, they do not care and remain out of touch even after the election has unseated them, because they are still having us pay their salaries, healthcare and many of their travel perks. They then have the very corporations they condemned hire them to sit on boards and serve in other capacities so they continue to out earn the electorate...so may be they have a point when they look at us and figure they are "smarter" than we are because they keep living off of our industry and incomes.

Columbia Basin Herald Sept. 21, 1989

An article that indicates how far we have wandered in the wilderness in these few short years.

"It is ironic-and somewhat fitting-that we in the United Sates celebrate the anniversary of our Constitution at a time when refugees are streaming out of he Communist bloc, seeking freedom.

Because of Hungary's action in tearing down its border posts, thousands from East Germany and Poland have escaped economic and political oppression.

Their future is uncertain, but their hope for a better life is clear.

It is hard to imagine what life is like in a nation that does not guarantee rights to its people. Perhaps in the last 200 years we may have begun to take these rights for granted.

The U.S. Constitution was forged in 1787 by 55 men working behind closed doors to produce the words we live by.

As Warren Burger, chairman of the Bicentennial of the Constitution, says, they rejected the divine right of kings. For the Constitution places power in the hands of the people.

Through the skillful work of those men in Philadelphia, we have a document that defines a limited government with separated and divided powers, providing checks and balances on the exercise of authority by those who govern.

It spelled out the powers of Congress, the President, and the Supreme Court, defining the federal role in the shaping of the nation but leaving important powers to the states.

It established a republic governed by the rule of law which has survived and prospered.

So secure were our Eighteenth century forefathers in their wisdom that the document they produced has only been amended 26 times in those 202 years.

Ten of those amendments, including free speech and religion, were added five years after ratification and form the Bill of Rights.

It is a blue print for government which other nation's citizens do not share.

The Bill of Rights covers areas which are fundamental to our system of justice; a speedy trial, protection against unreasonable searches and seizures, cruel and unusual punishments, self incrimination and double jeopardy.

The Union victory in the Civil War in 1865 brought the 13 amendment, which abolished slavery. Blacks got the vote five years later.

The most recent change, in 1971, reduced the voting age from 21 to 18 in line with many modern democracies' views about the maturity of youth.

It all adds up to a blueprint for survival, something incredibly dreamed up in the horse and buggy era which survives intact in our space-age high technology world.

As we celebrate its existence this week, let's also take the opportunity to consider what hope it offers the rest of the world. Our Constitution has survived natural calamities, war, economic depressions and political crises.

Presidents, judges, congressmen, and activists come and go, making their mark on history and changing the times they live in.

But the words we live by don't change.

The separation of powers is unique. It allows us to function with a Republican President, Democrat House and Senate, and a Supreme Court split 5-4 between conservatives and liberals. Its checks and balances encourage diversity but result in long-term harmony.

Consider East Germany. A one-party state, a totalitarian militaristic regime where dissent is stamped out, no viable opposition parties survive and the people's role in their government is minimal.

It's no wonder that 10,000 have packed their meager belongings n suitcases, taken advantage of Hungary's unexpected actions, and headed west.

Their rights-few as the are-don't come with a guarantee.

ours do. The Constitution"