Nuclear Power in Japan
Edit (3/15/2011 10:09): Updated with information regarding Daiichi Unit's 3,4 and 5. Along with some radiation information.
Edit (3/15/2011 00:30): Updated with information regarding Daiichi Unit 4 and Unit 2. Also regarding the units at Daini.
Edit (3/14/2011 15:09): Updated with more information on Unit 2
Edit (3/14/2011 12:09): Updated wording in some sections and radiation levels seen at Unit 1.
Edit (3/14/2011 11:48): Updated with a link to an interview with one of my professors.
Edit (3/14/2011 11:00): Updated with information regarding Unit 2's situation, and corrected some verbage regarding where sea water is being placed inside the reactors.
It's hard to gauge where I should begin with this. The events in Japan have hit very close to home for me - in more ways than one. I have numerous friends living in Japan (thankfully they all are either in the Tokyo area or further south), so receiving news was very anxiety inducing. However, the events at Japan's nuclear power plants have been equally as important and anxiety inducing. I was traveling all day yesterday between the east and west coast (Hooray spring break?), and literally thought each time I got off a plane "Well, I wonder if I still have a job...."
As most of you know, I am currently in my fourth and final year of studies for my bachelors in nuclear engineering. I am also continuing my studies for another year in order to get a masters of engineering in nuclear engineering. It has been very frustrating over the past three days to see the knee-jerk reactions from the public as they are fed inaccurate information regarding the status of the 10 nuclear power plants that were shut down because of the earthquakes in Miyagi, Niigata, and Nagano. In school, we have the concept of "If you screw up, not only do you put your job in jeopardy but the future of the industry as a whole." pounded into us from day one. I feel that as a nuclear engineer who will be going into the power industry - it is my duty and responsibility to protect the integrity of the industry, while also ensuring that it is being represented fairly and accurately. The American media outlets have done a fantastic job ensuring that this does not happen by misrepresenting the facts, and calling 'experts' in the field who don't have a clue what they're talking about.
While I am no world-known expert in the field of nuclear engineering, I strongly believe that my education up to this point has given me the ability to take the real facts about the situation and explain what happened in a way that even people with a non-technical background can understand.
Essentially...this will be long. tldr; However, I also hope if will be able to at least give people a stronger understanding of what happened so that they can filter the good media reporting from the bad.
Also, I know a lot of my friends are either nuclear engineers or are part of the nuclear engineering industry. I strongly welcome and encourage any corrections or information you have further. I'm still a baby in this industry, and fully realize that I don't know everything.
The Situation
Ten nuclear power plants (I'll call them units) were automatically shutdown due to the earthquakes. 3 units at the Onagawa Nuclear Power Station, 3 at the Fukushima Daiichi Nuclear Power Satiation (they actually have 6 units, however three were already shutdown when the earthquake hit for routine inspections), and 4 units at the Fukushima Daini Nuclear Power Station.
Let me get the Onagawa units out of the way. They automatically shutdown; however, there was a fire reported at one of their units after the earthquake. The fire was in the basement of the unit's turbine building, and not caused by anything happening in the reactor core. Though, at this time the reactors have been completely shut down, and two of the three units have reached cold-shutdown status (the coolant is at atmospheric pressure and at 95C or less.)
Introduction to Nuclear Engineering
Before I discuss what happened at the Fukushima sites, I want to go over a little bit of nuclear physics and general reactor design. In the current fleet of commercial nuclear reactors across the world, they create power by fissioning uranium atoms. The uranium atom splits when hit by a neutron. When the atom splits, it releases a large amount of energy that turns to heat, along with a neutron or two, and what are called fission products. These fission products are isotopes of various elements in excited states. The fission products return to a non-excited (stable) state by undergoing radioactive decay and releasing energy - therefore more heat.
The uranium in nuclear fuel is put into a pellet form (the pellet is actually uranium-dioxide) that are only about 0.20 cm in diameter. The pellets are placed in 4 meter-long zircalloy tubes called cladding. These fuel rods are assembled into bundles and then placed into the reactor. The fuel is housed in a gigantic and thick steel vessel, called the reactor pressure vessel (RPV). Coolant is fed into the bottom of the RPV and run along the fuel to remove heat. The RPV is then housed in a containment dome or building which is typically composed of concrete with a steel lining or something similar.
Nuclear engineers design reactors for failure. Instead of saying "Oh that accident could never happen!" we say "What is the worst case scenario that could happen in the reactor, and how do we keep it from destroying the core?" From that line of questioning, we develop a design basis for the reactor - in which if any of these incidents happen, the reactor will safely shut down, and the integrity of the reactor core is not compromised. We also develop a series of accidents that are outside of the design basis, in which we can still shut down but the reactor core might be lost in the process. A meltdown would be an out of design basis event. The reactors are also designed so that there are numerous redundant safety systems. That is so if one safety system fails, another one can be actuated and continue to safely shut down the reactor.
All seven units are boiling water reactors, or BWR. Now what is a BWR you ask? Essentially BWRs, as the name might imply, are glorified water heaters. Heat is generated by fissioning material, and the heat is removed by circulating water. The water is converted into steam by the heat and sent do a turbine, which generates power. After going through the turbine, the water is condensed back into water and sent back to remove more heat. The coolant is normal, every day water - with maybe one or two processes done to ensure that the largest amount of heat transfer is achievable.
The automatic shutdown system of these reactors is called a SCRAM. When a scram is initiated, control rods are driven into the reactor core to halt the uranium fission process. (The control rods are laced with poisons such as boron that absorb free neutrons.) That significantly reduces heat emitted by the fuel, but does not stop it completely since fission products are still present in the fuel. Thus, even though the reactor has stopped fissioning the fuel still needs to continue to be cooled in order to keep the fuel from overheating and melting.
As nuclear engineers, it is our responsibility to ensure that the public is exposed to a little radiation as possible. For that reason, within the reactor we have a series of barriers installed to protect the public from radiation: 1) the fuel cladding, 2) the reactor pressure vessel, 3) the containment dome/building. If the fuel rods were to melt, there are still two barriers to contain the fission products, and protect the public from unnecessary radiation exposure. It should be mentioned, that underneath the RPV, there typically is a pool of water underneath to catch the melted fuel if it actually melted through the RPV.
This is only a brief snippet of the key systems and functions of a reactor, and there are a lot of things which I didn't touch on. More importantly, how does this all tie in with everything that happened?
Daiichi Unit 1
It is now being said that the earthquake which hit the Miyagi prefecture was 9.0 on the Richter scale. This magnitude exceeded the reactors at Fukushima Daiichi and Daini (Unit 1 at Daiichi was reportedly designed for an 8.5 magnitude earthquake) and forced the reactors to automatically SCRAM. At Unit 1 the plant's automatic power systems kicked in to power some safety systems; however, these were lost when power lines were knocked down and the plant lost access to the power grid. At this point diesel generators were actuated to provide power to the plant's cooling system, and functioned for one hour - at which point the tsunami hit and flooded the generators. The plant was designed to withstand a 6.5 meter tsunami wave, and was hit by a 7 meter wave.
At this point, another safety system kicked in - called an isolation condenser - to continue to remove decay heat (the heat created by fission products) from the core, but some coolant evaporated and caused the core to lose coolant. This caused a series of pumps - called the reactor core isolation cooling pumps (RCIC) - to begin to add water in the core. However, a series of control valves that are battery operated lost power as the batteries ran out after 8 hours of continuous use. At this point the plant had no electrical power and was in a black out.
Since coolant was no longer being circulated around the core, it sat and began to heat up past water's boiling point. As the water turned to steam, the coolant water level dropped below the top of the fuel rods and allowed for the cladding to become partially uncovered (I've heard about 1.5 out of 4 meters for Daiichi's Unit 1.) Luckily, they were able to get spare diesel generators to site and start up one of the pump systems to raise water levels in the core. While they were trying to restore cooling function to the core, the pressure inside of the RPV reached about 840 kPa (the vessel is designed for a 400kPa during normal operating situations) The operators released air from the RPV into containment in order to relieve pressure and increase the coolant flow rate across the fuel.
As water was injected into the core, the pools which the injection systems pull water from began to steadily increase with temperature and could not cool off fast enough. (The pools are located inside of containment near the RPV.) This caused for pressure inside of containment to further increase. The operators decided to vent air from containment into the surrounding concrete building in order to reduce pressure. Radiation levels slightly increased around the site as there were controlled releases of air even though the air released from containment had been filtered. Because of this, and the fact that trace amounts of fission products had been detected - the evacuation area around the reactor was extended as a preemptive measure.
However, since the fuel had been exposed to air - parts of the cladding had oxidized and created hydrogen. The hydrogen was vented from the RPV to containment to the exterior building, where it collected. Hydrogen is a very flammable gas, and only a little has to collect before it will ignite - which is what cause the explosion at Unit 1. The explosion caused the exterior building - which has no safety related function - to collapse. The explosion did not damage containment or the RPV. At this point operators decided to flood the containment vessel with a mixture of sea water and boron in order to aid in cooling the core to cold shutdown.
Daiichi Unit 2
Unit 2 at Daiichi was in a similar situation as Unit 1. It lost ability to connect to the power grid, and then lost use of its diesel generators when the tsunami hit. Operators were able to restore cooling functions, but had to begin venting air from the RPV into containment. That being said, they were able to establish a stable coolant level quickly and did not uncover the fuel. However, the operators still performed controlled releases of air from containment to relieve pressure. Operators are not flooding the RPV with sea water and boron since the coolant systems seem to have stabilized the reactor. Confirmed by the IAEA, Unit 2 experienced problems with the emergency cooling system and began flooding the containment vessel with sea water.The water injection system failed, and pressure inside the RPV increased to a little over 700 kPa, requiring the operators to vent the RPV and start inserting sea water.
The operators began injecting seawater after the coolant levels had decreased below the top of the fuel, and continued to drop as seawater was injected. The RPV was vented again in order to decrease the pressure and allow for more seawater to be inserted. This cycle was followed one more time, and operators determined that some of the fuel had been damaged by an increase in radiation levels. So as with Unit's 1 and 3, there was a good chance for hydrogen build up if they had been venting the RPV while the fuel was uncovered. I'd actually expect a large amount more since more of the fuel was uncovered. I'm not sure if they had vented containment also, but at some point there was an explosion. It has taken a long time for there to be any credible information on it - though most of my credible sources seem to be saying the same story, and it was also confirmed by an email I was cc'd on from a professor.
Initial indications after the blast point to that there might be a breach in containment - specifically the suppression chamber (the gigantic pool of water that sits under the RPV.) This was indicated by a drop in pressure in the chamber, and also by a brief spike in radiation near the reactor. The radiation jumped to about 8. mrem/hour, but has since decreased to about 3 mrem/hr. Even with the possible breach, operators are still pumping water into the RPV - though they have removed non-essential staff from the unit.
Daiichi Unit 3
Unit 3 is a little different from the first two units at Daiichi. Instead of using uranium as its fuel, it uses a mix of plutonium and uranium. After the reactor was SCRAMed and lost connection with the power grid, it switched over to its backup diesel generators. As with the first two units, the diesel generators were knocked out by the tsunami. Operators were able to connect power supplies to the reactor after an hour and restore its cooling systems - though the water level in the core had reduced. With power restored to pumps, and indications that the coolant levels had dropped, operators began pumping in fresh water into the core and also venting the RPV.
However, during injection of fresh water - the coolant levels settled before the fuel could be covered again. Even with the addition of more water, coolant levels within the core did not rise. Operators believed that the injection system had failed, and began pumping borated salt water into the core. It appeared that the water levels began to rise, but then leveled off again - 2 meters below the top of the fuel. It was then believed that the gauge used to read the water level in the core had failed and was not giving accurate readings of the coolant level. The pressure level in the vessel was low, and the operators saw decreased radiation levels as sea water was continuously added to containment. During this, it also became known that the pressure relief valve was beginning to fail. Reactor operators countered this by installing an air compressor to operate the valve.
The plant had notified the Japanese government and International Atomic Energy Agency that they believed there was a possibility that hydrogen had accumulated. Later, a hydrogen explosion occurred at Unit 3 taking out the reactor building. However, like Unit 1, it appears that containment held and was not compromised.
On March 16th there appeared to be smoke coming from Unit 3. The source of the cause is unknown at this time; however, Japanese officials have considered the possibility that something similar to what had occurred at Unit 2 had also taken place at Unit 3. There was a slight increase in radiation levels, but decreased a few hours later. The source of the increased radiation has not been identified at this time.
Operators and workers are verifying that the pumps are still functional and continuing to cool the core at Unit 3.
Unit 4
So above I said that Units 4-6 were in an outage...and they were. However, these reactors also keep their spent fuel on site in what is called a spent fuel storage pool. Like what is being done with Units 1-3, the spent fuel pools have to be continuous cooled to removed residual decay heat from the spent fuel. Since the fuel has been exposed to air while removing it from the reactor, the cladding can begin to oxidize and create hydrogen. There had been an explosion - what they believe was a hydrogen explosion. Prior to the explosion, The temperature of the spent fuel pool had increased from 40C to 85C on 3/15.
After the explosion, the spent fuel pool was reported to be on fire but was put our a few hours later. However, it is known that while the pool was still on fire, the smoke plumes contained elevated levels of radioactive material. Dose rates had jumped to 40 rem/hr (400 milieSv/hr) around the site.
And now we're presented with a fantastic example of how information can get skewed because the facts are not all immediately present. I had written previously the fire that broke out at Unit 4, claiming that it was suspected to be another hydrogen explosion.
My engineering senses apparently didn't tingle enough for me to notice that something seemed off with that report. A hydrogen explosion by itself should not have sustained a long burning fire (140 minutes or something along those lines?) A professor forwarded me an email from the NEI better explaining the events, combined with more accurate informatoin from World Nuclear News.
On the morning of the 15th there was an explosion at unit 4. After the explosion a fire was reported to have started inside the building. TEPCO cites the source of the fire was oil from a leaking coolant pump that is in the vicinity of the unit's spent fuel pool. The fire was said to have extinguished itself.
During the fire, it was reported that readings over 400mSv/hr were being reported. It is now coming out that the site on average only saw rates at about 100mSv/hr during the fire, and that 400mSv/hr reading was isolated to one location for a brief moment of time. Although 100mSv/hr is still not a dose rate to be desired - it is significantly better than a 400mSv/hr reading.
Another fire broke out in the same location on the morning of 3/16. Workers attempted to extinguish the fire, but had to temporarily delay operations because of high radiation levels, but that a half hour after the fire was seen that there were no longer flames visible. There are now plans to pump water into Unit 4, though they are going slowly due to high radiation levels and some concerns that the unit might become critical if water was pumped in too fast.
Daini Units 1-4
All four units at Daini were automatically SCRAMed during the earthquake. Emergency cooling systems kicked in for all four units; however, failed in Units 1, 2, and 4. Off site power is available to all four units, and water levels are stable.
Operators at Unit 1 were able to restore a heat remover system, and are now cooling the reactor. Operators at Units 2 and 4 are working to restore the heat removal systems. Unit 3 has reached cold shut down. All units are in cold-shut down as of 3/15/2011.
What to take from this?
With out a doubt this is a lot of information to digest. I'm still unsure if all of this is correct - there has been a lot of combination of information taken from various sources. Additionally, the situation is still very dynamic and changes quickly.
Nevertheless, there are still some conclusions and statements that can be made.
Will this be another Chernobyl?
No. This situation is very different situation from Chernobyl. The reactor is technically very different from the reactor that was at Chernobyl. The reactors at the Fukushima stations are cooled and moderated using light water, the reactors all have a containment building, and the coolant systems are designed to move heat away from the reactor core. At Chernobyl the reactor was moderated with graphite (which is flammable and explosive at high temperatures), had no containment, and the coolant system actually fed hot steam back into the reactor core - causing the temperature to increase even more. The reactor cores at Fukushima CANNOT EXPLODE. I cannot stress that enough. Yes there have been hydrogen explosions outside of containment, but nothing inside containment can cause an explosion like the one seen in Chernobyl.
Even if the reactor core fully melts - similar to what happened at Three Mile Island (TMI) - there are so many extra barriers and protection systems built in, I highly doubt there would be an unexpected release of radiation outside of containment.
It should also be pointed out that these events were triggered by natural disasters that greatly exceeded the design basis of the reactors. At both Chernobyl and TMI, the accidents were caused primarily by operator error. From what we can tell, the reactor operators at the Fukushima plants are doing everything in their power to cool the reactors with the limited systems they have access too. If this doesn't speak to strength and safety designed into these reactors - I don't know what does.
What about these radiation leaks?! (For Reference: 1 microSevert = 0.1 mrem)
Most of these 'radiation leaks' have actually been planned releases of radiation. Since the air is from inside the reactor pressure vessel, the steam has been slightly activated and now we know it contains some fission products. During all the controlled releases, the plants have been closely monitoring radiation levels around the plants and halt venting when levels get too high.
That being said, the limits for allowable radiation exposure are extremely low - and are set this way for a reason. At one point, it was reported that about 155.7 mrem of radiation was detected at Unit 1 on the 13th. That has now dropped down to about 4 mrem.
In the US, the public is allowed to be exposed to 500 mrem per year as long as the exposure is not continuous. On average, I see 333 mrem of non-continuous exposure over the course of year in my normal, every day life. As a radiation worker - that 500 mrem limit increases to 5000 mrem.
So in reality, the amount of radiation that has been released in a controlled fashion has been minimal. People have been evacuated out as far as they have on a precautionary basis, and are being checked for radiation for those reasons too.
However, in light of the issues that have arisen at Fukushima Daiichi Unit 4, the situation has obviously changed While it's far from ideal to have 40 rem/hr of radiation floating around in the air, the level has began to decrease since the fire was extinguished. Radiation levels this high could cause some health problems - but it's hard to say what. You have to have about 100 rem of acute radiation exposure to see fast set radiation poisoning.
That being said, as the radiation disperses through the air, the dose exposure will also decrease since the fission products are not so concentrated together. Essentially, think of a person farting in a room. At first - if you're standing next to the person - it'll smell really bad, but the further you get away from them the less the smell it.
As a friend pointed out - currently Iwaki is seeing an exposure rate of about 4 microseverts/hr. As a radiation worker I am allowed to have 50,000 microseverts of exposure in a year. I am allowed have more exposure in a year than Iwaki will see in a year - assuming that rate stays constant. Crazy right?!
Some news agencies are trying to spin it that workers have abandoned the sight because of the high radiation levels. There have been times when non-essential personnel at individual units have been evacuated to reduce their accumulated dose. The evacuations are on a unit by unit basis, and for the most part only temporary. (This might not be the case for Unit 2 - I have not verified that.) In addition, the Japanese government has increased the allowable dose threshold for RadWorkers by a factor of 2.5 - again attesting to how much safety is built into our designs and limitations.
I am not trying to downplay the situation - it is still very serious, and everything needs to be done to ensure that these reactors are completely shut down and that the public is protected from as much radiation exposure as possible. The reactor operators are really doing everything they can to make this happen - and have been able to prevent the worst case scenarios from becoming a reality. That being said - it is no where near the level of catastrophe that the media is painting it out to be.
I hope that this has provided more clear information on what has happened in Japan regarding the nuclear power plants effected by the earth quake. Please feel free to ask for more information on anything I've mentioned here - I'll try to answer to the best of my ability, or direct your towards an official source of information.
Speaking of sources. All my information has been retrieved from the following, credible websites:
http://www.world-nuclear-news.org/default.aspx
http://www.nisa.meti.go.jp/english/
http://www.iaea.org/newscenter/news/tsunamiupdate01.html
http://www.new.ans.org/pi/resources/dosechart/
http://online.wsj.com/home-page
http://www.nei.org/newsandevents/information-on-the-japanese-earthquake-and-reactors-in-that-region/
Also, one of my professors was just interviewed: http://www.wten.com/Global/story.asp?S=14243346
Another good explanation of the situation: http://mitnse.com/2011/03/13/why-i-am-not-worried-about-japans-nuclear-reactors/
And here: http://morgsatlarge.wordpress.com/2011/03/13/why-i-am-not-worried-about-japans-nuclear-reactors/
Edit (3/15/2011 00:30): Updated with information regarding Daiichi Unit 4 and Unit 2. Also regarding the units at Daini.
Edit (3/14/2011 15:09): Updated with more information on Unit 2
Edit (3/14/2011 12:09): Updated wording in some sections and radiation levels seen at Unit 1.
Edit (3/14/2011 11:48): Updated with a link to an interview with one of my professors.
Edit (3/14/2011 11:00): Updated with information regarding Unit 2's situation, and corrected some verbage regarding where sea water is being placed inside the reactors.
It's hard to gauge where I should begin with this. The events in Japan have hit very close to home for me - in more ways than one. I have numerous friends living in Japan (thankfully they all are either in the Tokyo area or further south), so receiving news was very anxiety inducing. However, the events at Japan's nuclear power plants have been equally as important and anxiety inducing. I was traveling all day yesterday between the east and west coast (Hooray spring break?), and literally thought each time I got off a plane "Well, I wonder if I still have a job...."
As most of you know, I am currently in my fourth and final year of studies for my bachelors in nuclear engineering. I am also continuing my studies for another year in order to get a masters of engineering in nuclear engineering. It has been very frustrating over the past three days to see the knee-jerk reactions from the public as they are fed inaccurate information regarding the status of the 10 nuclear power plants that were shut down because of the earthquakes in Miyagi, Niigata, and Nagano. In school, we have the concept of "If you screw up, not only do you put your job in jeopardy but the future of the industry as a whole." pounded into us from day one. I feel that as a nuclear engineer who will be going into the power industry - it is my duty and responsibility to protect the integrity of the industry, while also ensuring that it is being represented fairly and accurately. The American media outlets have done a fantastic job ensuring that this does not happen by misrepresenting the facts, and calling 'experts' in the field who don't have a clue what they're talking about.
While I am no world-known expert in the field of nuclear engineering, I strongly believe that my education up to this point has given me the ability to take the real facts about the situation and explain what happened in a way that even people with a non-technical background can understand.
Essentially...this will be long. tldr; However, I also hope if will be able to at least give people a stronger understanding of what happened so that they can filter the good media reporting from the bad.
Also, I know a lot of my friends are either nuclear engineers or are part of the nuclear engineering industry. I strongly welcome and encourage any corrections or information you have further. I'm still a baby in this industry, and fully realize that I don't know everything.
The Situation
Ten nuclear power plants (I'll call them units) were automatically shutdown due to the earthquakes. 3 units at the Onagawa Nuclear Power Station, 3 at the Fukushima Daiichi Nuclear Power Satiation (they actually have 6 units, however three were already shutdown when the earthquake hit for routine inspections), and 4 units at the Fukushima Daini Nuclear Power Station.
Let me get the Onagawa units out of the way. They automatically shutdown; however, there was a fire reported at one of their units after the earthquake. The fire was in the basement of the unit's turbine building, and not caused by anything happening in the reactor core. Though, at this time the reactors have been completely shut down, and two of the three units have reached cold-shutdown status (the coolant is at atmospheric pressure and at 95C or less.)
Introduction to Nuclear Engineering
Before I discuss what happened at the Fukushima sites, I want to go over a little bit of nuclear physics and general reactor design. In the current fleet of commercial nuclear reactors across the world, they create power by fissioning uranium atoms. The uranium atom splits when hit by a neutron. When the atom splits, it releases a large amount of energy that turns to heat, along with a neutron or two, and what are called fission products. These fission products are isotopes of various elements in excited states. The fission products return to a non-excited (stable) state by undergoing radioactive decay and releasing energy - therefore more heat.
The uranium in nuclear fuel is put into a pellet form (the pellet is actually uranium-dioxide) that are only about 0.20 cm in diameter. The pellets are placed in 4 meter-long zircalloy tubes called cladding. These fuel rods are assembled into bundles and then placed into the reactor. The fuel is housed in a gigantic and thick steel vessel, called the reactor pressure vessel (RPV). Coolant is fed into the bottom of the RPV and run along the fuel to remove heat. The RPV is then housed in a containment dome or building which is typically composed of concrete with a steel lining or something similar.
Nuclear engineers design reactors for failure. Instead of saying "Oh that accident could never happen!" we say "What is the worst case scenario that could happen in the reactor, and how do we keep it from destroying the core?" From that line of questioning, we develop a design basis for the reactor - in which if any of these incidents happen, the reactor will safely shut down, and the integrity of the reactor core is not compromised. We also develop a series of accidents that are outside of the design basis, in which we can still shut down but the reactor core might be lost in the process. A meltdown would be an out of design basis event. The reactors are also designed so that there are numerous redundant safety systems. That is so if one safety system fails, another one can be actuated and continue to safely shut down the reactor.
All seven units are boiling water reactors, or BWR. Now what is a BWR you ask? Essentially BWRs, as the name might imply, are glorified water heaters. Heat is generated by fissioning material, and the heat is removed by circulating water. The water is converted into steam by the heat and sent do a turbine, which generates power. After going through the turbine, the water is condensed back into water and sent back to remove more heat. The coolant is normal, every day water - with maybe one or two processes done to ensure that the largest amount of heat transfer is achievable.
The automatic shutdown system of these reactors is called a SCRAM. When a scram is initiated, control rods are driven into the reactor core to halt the uranium fission process. (The control rods are laced with poisons such as boron that absorb free neutrons.) That significantly reduces heat emitted by the fuel, but does not stop it completely since fission products are still present in the fuel. Thus, even though the reactor has stopped fissioning the fuel still needs to continue to be cooled in order to keep the fuel from overheating and melting.
As nuclear engineers, it is our responsibility to ensure that the public is exposed to a little radiation as possible. For that reason, within the reactor we have a series of barriers installed to protect the public from radiation: 1) the fuel cladding, 2) the reactor pressure vessel, 3) the containment dome/building. If the fuel rods were to melt, there are still two barriers to contain the fission products, and protect the public from unnecessary radiation exposure. It should be mentioned, that underneath the RPV, there typically is a pool of water underneath to catch the melted fuel if it actually melted through the RPV.
This is only a brief snippet of the key systems and functions of a reactor, and there are a lot of things which I didn't touch on. More importantly, how does this all tie in with everything that happened?
Daiichi Unit 1
It is now being said that the earthquake which hit the Miyagi prefecture was 9.0 on the Richter scale. This magnitude exceeded the reactors at Fukushima Daiichi and Daini (Unit 1 at Daiichi was reportedly designed for an 8.5 magnitude earthquake) and forced the reactors to automatically SCRAM. At Unit 1 the plant's automatic power systems kicked in to power some safety systems; however, these were lost when power lines were knocked down and the plant lost access to the power grid. At this point diesel generators were actuated to provide power to the plant's cooling system, and functioned for one hour - at which point the tsunami hit and flooded the generators. The plant was designed to withstand a 6.5 meter tsunami wave, and was hit by a 7 meter wave.
At this point, another safety system kicked in - called an isolation condenser - to continue to remove decay heat (the heat created by fission products) from the core, but some coolant evaporated and caused the core to lose coolant. This caused a series of pumps - called the reactor core isolation cooling pumps (RCIC) - to begin to add water in the core. However, a series of control valves that are battery operated lost power as the batteries ran out after 8 hours of continuous use. At this point the plant had no electrical power and was in a black out.
Since coolant was no longer being circulated around the core, it sat and began to heat up past water's boiling point. As the water turned to steam, the coolant water level dropped below the top of the fuel rods and allowed for the cladding to become partially uncovered (I've heard about 1.5 out of 4 meters for Daiichi's Unit 1.) Luckily, they were able to get spare diesel generators to site and start up one of the pump systems to raise water levels in the core. While they were trying to restore cooling function to the core, the pressure inside of the RPV reached about 840 kPa (the vessel is designed for a 400kPa during normal operating situations) The operators released air from the RPV into containment in order to relieve pressure and increase the coolant flow rate across the fuel.
As water was injected into the core, the pools which the injection systems pull water from began to steadily increase with temperature and could not cool off fast enough. (The pools are located inside of containment near the RPV.) This caused for pressure inside of containment to further increase. The operators decided to vent air from containment into the surrounding concrete building in order to reduce pressure. Radiation levels slightly increased around the site as there were controlled releases of air even though the air released from containment had been filtered. Because of this, and the fact that trace amounts of fission products had been detected - the evacuation area around the reactor was extended as a preemptive measure.
However, since the fuel had been exposed to air - parts of the cladding had oxidized and created hydrogen. The hydrogen was vented from the RPV to containment to the exterior building, where it collected. Hydrogen is a very flammable gas, and only a little has to collect before it will ignite - which is what cause the explosion at Unit 1. The explosion caused the exterior building - which has no safety related function - to collapse. The explosion did not damage containment or the RPV. At this point operators decided to flood the containment vessel with a mixture of sea water and boron in order to aid in cooling the core to cold shutdown.
Daiichi Unit 2
Unit 2 at Daiichi was in a similar situation as Unit 1. It lost ability to connect to the power grid, and then lost use of its diesel generators when the tsunami hit. Operators were able to restore cooling functions, but had to begin venting air from the RPV into containment. That being said, they were able to establish a stable coolant level quickly and did not uncover the fuel. However, the operators still performed controlled releases of air from containment to relieve pressure. Operators are not flooding the RPV with sea water and boron since the coolant systems seem to have stabilized the reactor. Confirmed by the IAEA, Unit 2 experienced problems with the emergency cooling system and began flooding the containment vessel with sea water.The water injection system failed, and pressure inside the RPV increased to a little over 700 kPa, requiring the operators to vent the RPV and start inserting sea water.
The operators began injecting seawater after the coolant levels had decreased below the top of the fuel, and continued to drop as seawater was injected. The RPV was vented again in order to decrease the pressure and allow for more seawater to be inserted. This cycle was followed one more time, and operators determined that some of the fuel had been damaged by an increase in radiation levels. So as with Unit's 1 and 3, there was a good chance for hydrogen build up if they had been venting the RPV while the fuel was uncovered. I'd actually expect a large amount more since more of the fuel was uncovered. I'm not sure if they had vented containment also, but at some point there was an explosion. It has taken a long time for there to be any credible information on it - though most of my credible sources seem to be saying the same story, and it was also confirmed by an email I was cc'd on from a professor.
Initial indications after the blast point to that there might be a breach in containment - specifically the suppression chamber (the gigantic pool of water that sits under the RPV.) This was indicated by a drop in pressure in the chamber, and also by a brief spike in radiation near the reactor. The radiation jumped to about 8. mrem/hour, but has since decreased to about 3 mrem/hr. Even with the possible breach, operators are still pumping water into the RPV - though they have removed non-essential staff from the unit.
Daiichi Unit 3
Unit 3 is a little different from the first two units at Daiichi. Instead of using uranium as its fuel, it uses a mix of plutonium and uranium. After the reactor was SCRAMed and lost connection with the power grid, it switched over to its backup diesel generators. As with the first two units, the diesel generators were knocked out by the tsunami. Operators were able to connect power supplies to the reactor after an hour and restore its cooling systems - though the water level in the core had reduced. With power restored to pumps, and indications that the coolant levels had dropped, operators began pumping in fresh water into the core and also venting the RPV.
However, during injection of fresh water - the coolant levels settled before the fuel could be covered again. Even with the addition of more water, coolant levels within the core did not rise. Operators believed that the injection system had failed, and began pumping borated salt water into the core. It appeared that the water levels began to rise, but then leveled off again - 2 meters below the top of the fuel. It was then believed that the gauge used to read the water level in the core had failed and was not giving accurate readings of the coolant level. The pressure level in the vessel was low, and the operators saw decreased radiation levels as sea water was continuously added to containment. During this, it also became known that the pressure relief valve was beginning to fail. Reactor operators countered this by installing an air compressor to operate the valve.
The plant had notified the Japanese government and International Atomic Energy Agency that they believed there was a possibility that hydrogen had accumulated. Later, a hydrogen explosion occurred at Unit 3 taking out the reactor building. However, like Unit 1, it appears that containment held and was not compromised.
On March 16th there appeared to be smoke coming from Unit 3. The source of the cause is unknown at this time; however, Japanese officials have considered the possibility that something similar to what had occurred at Unit 2 had also taken place at Unit 3. There was a slight increase in radiation levels, but decreased a few hours later. The source of the increased radiation has not been identified at this time.
Operators and workers are verifying that the pumps are still functional and continuing to cool the core at Unit 3.
Unit 4
So above I said that Units 4-6 were in an outage...and they were. However, these reactors also keep their spent fuel on site in what is called a spent fuel storage pool. Like what is being done with Units 1-3, the spent fuel pools have to be continuous cooled to removed residual decay heat from the spent fuel. Since the fuel has been exposed to air while removing it from the reactor, the cladding can begin to oxidize and create hydrogen. There had been an explosion - what they believe was a hydrogen explosion. Prior to the explosion, The temperature of the spent fuel pool had increased from 40C to 85C on 3/15.
After the explosion, the spent fuel pool was reported to be on fire but was put our a few hours later. However, it is known that while the pool was still on fire, the smoke plumes contained elevated levels of radioactive material. Dose rates had jumped to 40 rem/hr (400 milieSv/hr) around the site.
And now we're presented with a fantastic example of how information can get skewed because the facts are not all immediately present. I had written previously the fire that broke out at Unit 4, claiming that it was suspected to be another hydrogen explosion.
My engineering senses apparently didn't tingle enough for me to notice that something seemed off with that report. A hydrogen explosion by itself should not have sustained a long burning fire (140 minutes or something along those lines?) A professor forwarded me an email from the NEI better explaining the events, combined with more accurate informatoin from World Nuclear News.
On the morning of the 15th there was an explosion at unit 4. After the explosion a fire was reported to have started inside the building. TEPCO cites the source of the fire was oil from a leaking coolant pump that is in the vicinity of the unit's spent fuel pool. The fire was said to have extinguished itself.
During the fire, it was reported that readings over 400mSv/hr were being reported. It is now coming out that the site on average only saw rates at about 100mSv/hr during the fire, and that 400mSv/hr reading was isolated to one location for a brief moment of time. Although 100mSv/hr is still not a dose rate to be desired - it is significantly better than a 400mSv/hr reading.
Another fire broke out in the same location on the morning of 3/16. Workers attempted to extinguish the fire, but had to temporarily delay operations because of high radiation levels, but that a half hour after the fire was seen that there were no longer flames visible. There are now plans to pump water into Unit 4, though they are going slowly due to high radiation levels and some concerns that the unit might become critical if water was pumped in too fast.
Daini Units 1-4
All four units at Daini were automatically SCRAMed during the earthquake. Emergency cooling systems kicked in for all four units; however, failed in Units 1, 2, and 4. Off site power is available to all four units, and water levels are stable.
Operators at Unit 1 were able to restore a heat remover system, and are now cooling the reactor. Operators at Units 2 and 4 are working to restore the heat removal systems. Unit 3 has reached cold shut down. All units are in cold-shut down as of 3/15/2011.
What to take from this?
With out a doubt this is a lot of information to digest. I'm still unsure if all of this is correct - there has been a lot of combination of information taken from various sources. Additionally, the situation is still very dynamic and changes quickly.
Nevertheless, there are still some conclusions and statements that can be made.
Will this be another Chernobyl?
No. This situation is very different situation from Chernobyl. The reactor is technically very different from the reactor that was at Chernobyl. The reactors at the Fukushima stations are cooled and moderated using light water, the reactors all have a containment building, and the coolant systems are designed to move heat away from the reactor core. At Chernobyl the reactor was moderated with graphite (which is flammable and explosive at high temperatures), had no containment, and the coolant system actually fed hot steam back into the reactor core - causing the temperature to increase even more. The reactor cores at Fukushima CANNOT EXPLODE. I cannot stress that enough. Yes there have been hydrogen explosions outside of containment, but nothing inside containment can cause an explosion like the one seen in Chernobyl.
Even if the reactor core fully melts - similar to what happened at Three Mile Island (TMI) - there are so many extra barriers and protection systems built in, I highly doubt there would be an unexpected release of radiation outside of containment.
It should also be pointed out that these events were triggered by natural disasters that greatly exceeded the design basis of the reactors. At both Chernobyl and TMI, the accidents were caused primarily by operator error. From what we can tell, the reactor operators at the Fukushima plants are doing everything in their power to cool the reactors with the limited systems they have access too. If this doesn't speak to strength and safety designed into these reactors - I don't know what does.
What about these radiation leaks?! (For Reference: 1 microSevert = 0.1 mrem)
Most of these 'radiation leaks' have actually been planned releases of radiation. Since the air is from inside the reactor pressure vessel, the steam has been slightly activated and now we know it contains some fission products. During all the controlled releases, the plants have been closely monitoring radiation levels around the plants and halt venting when levels get too high.
That being said, the limits for allowable radiation exposure are extremely low - and are set this way for a reason. At one point, it was reported that about 155.7 mrem of radiation was detected at Unit 1 on the 13th. That has now dropped down to about 4 mrem.
In the US, the public is allowed to be exposed to 500 mrem per year as long as the exposure is not continuous. On average, I see 333 mrem of non-continuous exposure over the course of year in my normal, every day life. As a radiation worker - that 500 mrem limit increases to 5000 mrem.
So in reality, the amount of radiation that has been released in a controlled fashion has been minimal. People have been evacuated out as far as they have on a precautionary basis, and are being checked for radiation for those reasons too.
However, in light of the issues that have arisen at Fukushima Daiichi Unit 4, the situation has obviously changed While it's far from ideal to have 40 rem/hr of radiation floating around in the air, the level has began to decrease since the fire was extinguished. Radiation levels this high could cause some health problems - but it's hard to say what. You have to have about 100 rem of acute radiation exposure to see fast set radiation poisoning.
That being said, as the radiation disperses through the air, the dose exposure will also decrease since the fission products are not so concentrated together. Essentially, think of a person farting in a room. At first - if you're standing next to the person - it'll smell really bad, but the further you get away from them the less the smell it.
As a friend pointed out - currently Iwaki is seeing an exposure rate of about 4 microseverts/hr. As a radiation worker I am allowed to have 50,000 microseverts of exposure in a year. I am allowed have more exposure in a year than Iwaki will see in a year - assuming that rate stays constant. Crazy right?!
Some news agencies are trying to spin it that workers have abandoned the sight because of the high radiation levels. There have been times when non-essential personnel at individual units have been evacuated to reduce their accumulated dose. The evacuations are on a unit by unit basis, and for the most part only temporary. (This might not be the case for Unit 2 - I have not verified that.) In addition, the Japanese government has increased the allowable dose threshold for RadWorkers by a factor of 2.5 - again attesting to how much safety is built into our designs and limitations.
I am not trying to downplay the situation - it is still very serious, and everything needs to be done to ensure that these reactors are completely shut down and that the public is protected from as much radiation exposure as possible. The reactor operators are really doing everything they can to make this happen - and have been able to prevent the worst case scenarios from becoming a reality. That being said - it is no where near the level of catastrophe that the media is painting it out to be.
I hope that this has provided more clear information on what has happened in Japan regarding the nuclear power plants effected by the earth quake. Please feel free to ask for more information on anything I've mentioned here - I'll try to answer to the best of my ability, or direct your towards an official source of information.
Speaking of sources. All my information has been retrieved from the following, credible websites:
http://www.world-nuclear-news.org/default.aspx
http://www.nisa.meti.go.jp/english/
http://www.iaea.org/newscenter/news/tsunamiupdate01.html
http://www.new.ans.org/pi/resources/dosechart/
http://online.wsj.com/home-page
http://www.nei.org/newsandevents/information-on-the-japanese-earthquake-and-reactors-in-that-region/
Also, one of my professors was just interviewed: http://www.wten.com/Global/story.asp?S=14243346
Another good explanation of the situation: http://mitnse.com/2011/03/13/why-i-am-not-worried-about-japans-nuclear-reactors/
And here: http://morgsatlarge.wordpress.com/2011/03/13/why-i-am-not-worried-about-japans-nuclear-reactors/