This is a neutral, scientific and thorough technical analysis which tells the truth about the condition of the nuclear reactors in . What you hear in the many other news/rumor sources are mostly fear based and exaggerated.

EagleEyes

 

The original post of Josef Oehmen appeared on Morgsatlarge. Due to the large and unexpected popularity of the original post, Dr.Oehmen handed the blog to the Nuclear Science And Engineering Department in an effort to correct the presented information and provide a starting point. It has since been migrated to this and edited considerably by members of the NSE Community. It should be noted that Josef Oehmen is not affiliated to the MIT NSE department. *** Please note that the original post in no way reflects the views of the authors of the site. The authors have been monitoring the situation, and are presenting facts on the situation as they develop. The original article was adopted as the authors believed it provided a good starting point to provide a summary background on the events at the Fukushima . The authors do not intend to speculate about the safety of the public and the environment, and would like to express their sympathies and condolences to the people of Japan following the recent natural disaster.***

The original post written by Dr Josef Oehmen “Why I am not worried about Japan’s nuclear reactors.” are being reposted in different languages. They have not been checked / verified.

•Japanese
•German
•Spanish

We will have to cover some fundamentals, before we get into what is going on.

Construction of the Fukushima nuclear power plants

The plants at Fukushima are Boiling Water Reactors (BWR for short). A BWR produces electricity by boiling water, and spinning a a turbine with that steam. The nuclear fuel heats water, the water boils and creates steam, the steam then drives turbines that create the electricity, and the steam is then cooled and condensed back to water, and the water returns to be heated by the nuclear fuel. The operates at about 285 °C.

The nuclear fuel is uranium oxide. Uranium oxide is a ceramic with a very high melting point of about 2800 °C. The fuel is manufactured in pellets (cylinders that are about 1 cm tall and 1 cm in diameter). These pellets are then put into a long tube made of Zircaloy (an alloy of zirconium) with a failure temperature of 1200 °C (caused by the auto-catalytic oxidation of water), and sealed tight. This tube is called a fuel rod. These fuel rods are then put together to form assemblies, of which several hundred make up the reactor core.

The solid fuel pellet (a ceramic oxide matrix) is the first barrier that retains many of the radioactive fission products produced by the fission process.  The Zircaloy casing is the second barrier to release that separates the radioactive fuel from the rest of the reactor.

The core is then placed in the pressure vessel. The pressure vessel is a thick vessel that operates at a pressure of about 7 MPa (~1000 psi), and is designed to withstand the high pressures that may occur during an accident. The pressure vessel is the third barrier to radioactive material release.

The entire primary loop of the nuclear reactor – the pressure vessel, pipes, and pumps that contain the coolant (water) – are housed in the containment structure.  This structure is the fourth barrier to radioactive material release. The containment structure is a hermetically (air tight) sealed, very thick structure made of steel and concrete. This structure is designed, built and tested for one single purpose: To contain, indefinitely, a complete core meltdown. To aid in this purpose, a large, thick concrete structure is poured around the containment structure and is referred to as the secondary containment.

Both the main containment structure and the secondary containment structure are housed in the reactor building. The reactor building is an outer shell that is supposed to keep the weather out, but nothing in. (this is the part that was damaged in the explosions, but more to that later).

Fundamentals of nuclear reactions

The uranium fuel generates heat by neutron-induced nuclear fission. Uranium atoms are split into lighter atoms (aka fission products). This process generates heat and more neutrons (one of the particles that forms an atom). When one of these neutrons hits another uranium atom, that atom can split, generating more neutrons and so on. That is called the nuclear chain reaction. During normal, full-power operation, the neutron population in a core is stable (remains the same) and the reactor is in a critical state.

It is worth mentioning at this point that the nuclear fuel in a reactor can never cause a nuclear explosion like a nuclear bomb. At , the explosion was caused by excessive pressure buildup, hydrogen explosion and rupture of all structures, propelling molten core material into the environment.  Note that did not have a containment structure as a barrier to the environment. Why that did not and will not happen in Japan, is discussed further below.

In order to control the nuclear chain reaction, the reactor operators use control rods. The control rods are made of boron which absorbs neutrons.  During normal operation in a BWR, the control rods are used to maintain the chain reaction at a critical state. The control rods are also used to shut the reactor down from 100% power to about 7% power (residual or decay heat).

The residual heat is caused from the radioactive decay of fission products.  Radioactive decay is the process by which the fission products  stabilize themselves by emitting in the form of small particles (, beta, gamma, neutron, etc.).  There is a multitude of fission products that are produced in a reactor, including cesium and iodine.  This residual heat decreases over time after the reactor is shutdown, and must be removed by cooling systems to prevent the fuel rod from overheating and failing as a barrier to radioactive release. Maintaining enough cooling to remove the decay heat in the reactor is the main challenge in the affected reactors in Japan right now.

It is important to note that many of these fission products decay (produce heat) extremely quickly, and become harmless by the time you spell “R-A-D-I-O-N-U-C-L-I-D-E.”  Others decay more slowly, like some cesium, iodine, strontium, and argon.

What happened at Fukushima (as of March 12, 2011)

The following is a summary of the main facts. The earthquake that hit Japan was several times more powerful than the worst earthquake the nuclear power plant was built for (the Richter scale works logarithmically; for example the difference between an 8.2 and the 8.9 that happened is 5 times, not 0.7).

When the earthquake hit, the nuclear reactors all automatically shutdown. Within seconds after the earthquake started, the control rods had been inserted into the core and the nuclear chain reaction stopped. At this point, the cooling system has to carry away the residual heat, about 7% of the full power heat load under normal operating conditions.

The earthquake destroyed the external power supply of the nuclear reactor. This is a challenging accident for a nuclear power plant, and is referred to as a “loss of offsite power.” The reactor and its backup systems are designed to handle this type of accident by including backup power systems to keep the coolant pumps working. Furthermore, since the power plant had been shut down, it cannot produce any electricity by itself.

For the first hour, the first set of multiple emergency diesel power generators started and provided the electricity that was needed. However, when the tsunami arrived (a very rare and larger than anticipated tsunami) it flooded the diesel generators, causing them to fail.

One of the fundamental tenets of nuclear power plant design is “Defense in Depth.” This approach leads engineers to design a plant that can withstand severe catastrophes, even when several systems fail. A large tsunami that disables all the diesel generators at once is such a scenario, but the tsunami of March 11th was beyond all expectations. To mitigate such an event, engineers designed an extra line of defense by putting everything into the containment structure (see above), that is designed to contain everything inside the structure.

When the diesel generators failed after the tsunami, the reactor operators switched to emergency battery power. The batteries were designed as one of the backup systems to provide power for cooling the core for 8 hours. And they did.

After 8 hours, the batteries ran out, and the residual heat could not be carried away any more.  At this point the plant operators begin to follow emergency procedures that are in place for a “loss of cooling event.” These are procedural steps following the “Depth in Defense” approach. All of this, however shocking it seems to us, is part of the day-to-day training you go through as an operator.

At this time people started talking about the possibility of core meltdown, because if cooling cannot be restored, the core will eventually melt (after several days), and will likely be contained in the containment. Note that the term “meltdown” has a vague definition. “Fuel failure” is a better term to describe the failure of the fuel rod barrier (Zircaloy).  This will occur before the fuel melts, and results from mechanical, chemical, or thermal failures (too much pressure, too much oxidation, or too hot).

However, melting was a long ways from happening and at this time, the primary goal was to manage the core while it was heating up, while ensuring that the fuel cladding remain intact and operational for as long as possible.

Because cooling the core is a priority, the reactor has a number of independent and diverse cooling systems (the reactor water cleanup system, the decay heat removal, the reactor core isolating cooling, the standby liquid cooling system, and others that make up the emergency core cooling system). Which one(s) failed when or did not fail is not clear at this point in time.

Since the operators lost most of their cooling capabilities due to the loss of power, they had to use whatever cooling system capacity they had to get rid of as much heat as possible. But as long as the heat production exceeds the heat removal capacity, the pressure starts increasing as more water boils into steam. The priority now is to maintain the integrity of the fuel rods by keeping the temperature below 1200°C, as well as keeping the pressure at a manageable level. In order to maintain the pressure of the system at a manageable level, steam (and other gases present in the reactor) have to be released from time to time. This process is important during an accident so the pressure does not exceed what the components can handle, so the reactor pressure vessel and the containment structure are designed with several pressure relief valves. So to protect the integrity of the vessel and containment, the operators started venting steam from time to time to control the pressure.

As mentioned previously, steam and other gases are vented.  Some of these gases are radioactive fission products, but they exist in small quantities. Therefore, when the operators started venting the system, some radioactive gases were released to the environment in a controlled manner (ie in small quantities through filters and scrubbers). While some of these gases are radioactive, they did not pose a significant risk to public safety to even the workers on site. This procedure is justified as its consequences are very low, especially when compared to the potential consequences of not venting and risking the containment structures’ integrity.

During this time, mobile generators were transported to the site and some power was restored.  However, more water was boiling off and being vented than was being added to the reactor, thus decreasing the cooling ability of the remaining cooling systems. At some stage during this venting process, the water level may have dropped below the top of the fuel rods.  Regardless, the temperature of some of the fuel rod cladding exceeded 1200 °C, initiating a reaction between the Zircaloy and water. This oxidizing reaction produces , which mixes with the gas-steam mixture being vented.  This is a known and anticipated process, but the amount of produced was unknown because the operators didn’t know the exact temperature of the fuel rods or the water level. Since is extremely combustible, when enough is mixed with air, it reacts with oxygen. If there is enough , it will react rapidly, producing an explosion. At some point during the venting process enough built up inside the containment (there is no air in the containment), so when it was vented to the air an explosion occurred. The explosion took place outside of the containment, but inside and around the reactor building (which has no safety function).  Note that a subsequent and similar explosion occurred at the Unit 3 reactor. This explosion destroyed the top and some of the sides of the reactor building, but did not damage the containment structure or the pressure vessel. While this was not an anticipated event, it happened outside the containment and did not pose a risk to the plant’s safety structures.

Since some of the fuel rod cladding exceeded 1200 °C, some fuel damage occurred. The nuclear material itself was still intact, but the surrounding Zircaloy shell had started failing. At this time, some of the radioactive fission products (cesium, iodine, etc.) started to mix with the water and steam. It was reported that a small amount of cesium and iodine was measured in the steam that was released into the atmosphere.

Since the reactor’s cooling capability was limited, and the water inventory in the reactor was decreasing, engineers decided to inject sea water (mixed with boric acid – a neutron absorber) to ensure the rods remain covered with water.  Although the reactor had been shut down, boric acid is added as a conservative measure to ensure the reactor stays shut down.  Boric acid is also capable of trapping some of the remaining iodine in the water so that it cannot escape, however this trapping is not the primary function of the boric acid.

The water used in the cooling system is purified, demineralized water. The reason to use pure water is to limit the corrosion potential of the coolant water during normal operation. Injecting seawater will require more cleanup after the event, but provided cooling at the time.

This process decreased the temperature of the fuel rods to a non-damaging level. Because the reactor had been shut down a long time ago, the decay heat had decreased to a significantly lower level, so the pressure in the plant stabilized, and venting was no longer required.

***UPDATE – 3/14 8:15 pm EST***

Units 1 and 3 are currently in a stable condition according to TEPCO press releases, but the extent of the fuel damage is unknown.  That said, levels at the Fukushima plant have fallen to 231 micro sieverts (23.1 millirem) as of 2:30 pm March 14th (local time).

***UPDATE – 3/14 10:55 pm EST***

The details about what happened at the Unit 2 reactor are still being determined.  The post on what is happening at the Unit 2 reactor contains more up-to-date information.  Radiation levels have increased, but to what level remains unknown.

 


Status Update – 3/14/11 at 1:00 pm EST (Source – The Federation of Electric Power Companies of Japan (FEPC))

Posted on by mitnse

As of 1:00PM (EST), March 14, 2011

* Radiation Levels
o At 9:37AM (JST) on March 14, a radiation level of 3130 micro
sievert was recorded at the Fukushima Daiichi Nuclear Power
Station.
o At 10:35AM on March 14, a radiation level of 326 micro
sievert was recorded at the Fukushima Daiichi Nuclear Power
Station.
o Most recently, at 2:30PM on March 14, a radiation level of
231 micro sievert was recorded at Fukushima Daiichi Nuclear
Power Station.
* Fukushima Daiichi Unit 1 reactor
o As of 12:00AM on March 15, the injection of seawater
continues into the primary containment vessel.
* Fukushima Daiichi Unit 2 reactor
o At 12:00PM on March 14, in response to lower water levels,
TEPCO began preparations for injecting seawater into the
reactor core.
o At 5:16PM on March 14, the water level in the reactor core
covered the top of the fuel rods.
o At 6:20PM on March 14, TEPCO began to inject seawater into
the reactor core.
o For a short time around 6:22PM on March 14, the water level
inside the reactor core fell below the lower measuring range
of the gauge.  As a result, TEPCO believes that the fuel
rods in the reactor core might have been fully exposed.
o At 7:54PM on March 14, engineers confirmed that the gauge
recorded the injection of seawater into the reactor core.
o At 8:37PM on March 14, in order to alleviate the buildup of
pressure, slightly radioactive vapor, that posed no health
threat, was passed through a filtration system and emitted
outside via a ventilation stack from Fukushima Daiichi Unit
2 reactor vessel.
* Fukushima Daiichi Unit 3 reactor
o At 11:01AM on March 14, an explosion occurred at Fukushima
Daiichi Unit 3 reactor damaging the roof of the secondary
containment building. Caused by the interaction of hydrogen
and oxygen vapor, in a fashion to Unit 1 reactor, the
explosion *did not damage the primary containment vessel* or
the reactor core.
o As of 12:38AM (JST) on March 15, the injection of seawater
has been suspended.
* Fukushima Daini Unit 1 reactor
o As of 1:24AM on March 14, TEPCO commenced the cooling
process after the pumping system was restored.
o At 10:15AM on March 14, TEPCO confirmed that the average
water temperature held constant below 212 degrees Fahrenheit.
* Fukushima Daini Unit 2 reactor
o At 7:13AM on March 14, TEPCO commenced the cooling process.
o As of 3:52PM on March 14, the cooling function was restored
and the core temperature was stabilized below 212 degrees
Fahrenheit.
* Fukushima Daini Unit 3 reactor
o As of 12:15PM on March 13, reactor has been cooled down and
stabilized.
* Fukushima Daini Unit 4 reactor__
o At 3:42PM on March 14, cooling of the reactor commenced,
with TEPCO engineers working to achieve cold shutdown.

To get more actual facts and reliable report on the condition of Japanese nuclear reactors, visit: MIT NSE Nuclear Information Hub