Email from March 15th (so some information will have changed)
Hello family and friends,
I have received a few emails asking about what I thought about the nuclear reactors in Japan and if I had more information about the process of what is unfolding, so I thought I would send out an email to give you more information and explain some of the systems and designs used to mitigate what is happening.
Background info:
Immediately after the earthquake, the reactors’ safety systems worked as they were designed. For an external event such as earthquake, hurricane, plane crash, etc., the plant immediately “scrams” (shuts down) the reactor. This was the case for the reactors in Japan, and it shut down the reactors successfully. In a nuclear reactor, there is always residual heat being produced even after reactor shutdown due to the fission product buildup that occurs during power production (these fission products are radioactive and continue to decay over time, which produces heat in the fuel, i.e. core, of the reactor). There are redundant safety cooling systems that keeps water flowing through the core to continue cooling so that a meltdown does not occur (the primary concern after emergency shutdown is to keep the reactor VESSEL intact, to prevent a large radioactive release into the environment), that are in this case, run on electricity. The tsunami waves swept away the power lines to the reactor buildings, so all external electricity was lost. Since the power lines were lost, the diesel generators were run to keep water flowing, but in the case of reactors #1 and #3 at the Fukushima Daiichi power station, their generators were too badly damaged by the earthquake and/or lost their fuel supply from the tsunami to run.
As the fuel temperature increased, the water began to boil off, increasing the pressure in the reactor vessel (which is housed in the containment building that is made of thick concrete). Battery-powered cooling pumps were able to run for about 8 hours until all their power was consumed. At this point, the reactors now had no electricity to run the coolant pumps. As the water continued to boil off, the water level decreased, exposing the fuel rods to air. The emergency water supply then flooded the core to cover the fuel again. But after time, this water was again boiled off. Once the fuel rods (made of Zirconium) are exposed to air, they begin to heat up very quickly and can crack and release fission products into the reactor VESSEL. Exposure of extremely hot Zirconium to steam also causes an oxidation reaction that produces hydrogen gas. As the pressure of the vessel continued to rise, the emergency pressure relief valves functioned as designed to release some of the pressure in the vessel to prevent a vessel breach from occurring. This released the steam and hydrogen gas into the containment building. Filters are attached to these vents that filter out fission products from being released (although minute amounts of radioactivity are released, the levels are generally so extremely low, it is not much different from the extra radiation exposure received from a few airplane trips). Both reactors #1 and #3 have since had explosions occur in the containment buildings due to this buildup of hydrogen gas. Both reactor VESSELS remain intact even though the containment buildings are damaged.
The last emergency cooling safety system was started, which flooded the vessels with sea water. Now it is unsure if the water levels in the vessels are high enough to cover the entirety of the fuel. If any of the fuel is exposed to air, it will continue to heat up until it melts- thus a “meltdown” (or “partial meltdown” if only a portion of the fuel melts).
A meltdown does NOT mean that radioactivity will be released into the environment. There are additional safety systems in the design to prevent this. As the fuel melts and falls to the bottom of the vessel, if they can keep it covered with water, it should not breech the vessel. If it does not remain covered with water, and it reaches the melting point of the materials of the vessel, then it would melt through the bottom of the vessel, and this substance is now called the “corium” (basically a mixture of melted metal, fuel, and fission products). Underneath the vessel is a “corium catcher”. This is a large pool of water that allows the corium to spread out thinly over a large area. This creates a large surface area that allows for faster and easier cooling and can thus contain the corium from moving any further and thus preventing its release into the environment. This is a worst case scenario.
Future:
This situation is vastly impacting the nuclear community throughout the world. We are holding discussion panels here at MIT in the following few days to hear from faculty, students, and nuclear industry employees about the next course of action nuclear will take on the international scale. My Nuclear Science and Engineering department here at MIT is very diverse in the nationalities of its students and faculty, so I am very much looking forward to hearing the different opinions and courses of action different places in the world are taking.
An article that has a good overview is:
E-mail from March 16th
Hello again,
Due to the latest news reports, I have decided to send out another email for an update on the nuclear reactors in Japan. Today we had a briefing at MIT with a panel of 4 Nuclear Science and Engineering Professors, 1 radiation safety officer, and a radiation environmental hazard specialist. I hope this clears some things up because the information being presented by the media is confusing (I had trouble deciphering it this morning sifting through some articles too!).
Some more background information:
The major problem that has caused this lack of cooling to occur (and subsequent problems), is because of the tsunami that struck the power plant 1 hour after the earthquake. The plant design had barriers 2m high in case of a tsunami, but they were not prepared for an earthquake/tsunami of this magnitude. Thus the wave swept right over the barriers and into the plant. This not only decimated the fuel supply for the generators that would have kept the water pumping to cool the reactor cores (where the fuel is), but it also disconnected the generators from the cooling system and flooded the electrical switchyard. Therefore, before hooking up new generators that could be brought in by ship to the plant site, they have to build/repair this electrical switchyard.
There was also a steam driven cooling system that was supposed to run off of the steam being produced by the decay heat of the fission products in the core in 2 of the reactors. For unknown reasons, this safety design has not been functioning, likely due to damage caused by the tsunami.
Why Nuclear Plants are clustered on the coast to begin with:
Nuclear power plants require a very large amount of cooling, so they are often built next to a large, cool water supply so that there is an abundant amount of water.
To clear up some of the information I stated in the last email:
The reactor vessel is venting steam into the containment building to keep the pressure low in the vessel itself. To keep the pressure low in the containment building, a “suppression pool” is used. A suppression pool is essentially a very large condenser full of water, that cools the steam in the containment building so that it condenses and keeps the pressure of the containment at an acceptable level. The problem was that these suppression pools ran out of water since it was cooling so much steam (cooling the steam raises the temperature of the water in the pool, eventually it also begins to boil off). To be able to pump the seawater into the pools, the pressure in the containment building itself must be below a certain threshold. This was why they vented steam from the containment buildings into the neighboring ordinary plant buildings (releasing a very small amount of radiation levels in the process) in reactors #1 and #3. They were able to reach the pressure threshold and now the water level in the suppression pools seems to be stable, thus allowing the pressure to remain constant in the containment building (so hopefully no more venting of steam into the neighboring buildings, thus keeping even the volatile fission products contained in the containment building.
Both explosions that occurred at reactors #1 and #3 damaged the reactor building, but NOT the containment where the radioactive particles are kept to prevent them from reaching the environment.
This diagram shows the reactor building (which houses in it the containment building and vessel) and suppression pool (labeled as the wetwell): (The suppression pool is also in the containment)
Seawater is continued to be used to flood the reactor vessel itself to keep the fuel (core) covered in water, thus preventing a fuel meltdown.
Problem is now with reactor #2, where the pressure threshold in the containment building has not yet been reached:
Updates:
As you probably have heard, there was another explosion Tuesday morning (Japan time), this time at reactor #2 on the site. This was again due to hydrogen gas buildup, but this time the hydrogen gas had collected in the suppression pool. Damage was done to the suppression pool, but the amount of damage is still unknown.
A fire was also reported in the spent fuel pool in building 4. This was due to an oil leakage from a coolant pump that then caught on fire. The fire has been extinguished. The spent fuel pools (where old fuel is placed to allow for fission product decay) are much simpler to keep covered in water since there are no pumps involved, and pressure is the same as the atmosphere. So they are able to just pump water straight from the sea and into the pool. These pins full of spent fuel also produce a magnitude less amount of heat than the fission products in the reactor vessels right now are creating. (~10MW(thermal) in reactor vessel vs ~1MW(thermal) in spent fuel pool)
Health Concerns:
Wind is currently blowing towards the sea, so almost all radiation released in atmosphere is not being transported to populated areas.
It was reported that a reading of 800millirem/hr radiation level was detected inside the plant after the explosion in reactor containment building #2. These levels were only this high at the plant site, and only for a few hours. To provide a comparison, a radiation worker has a limit of exposure to radiation of 5000millirem/year (a very conservative safety limit to prevent any adverse health effects). The evacuation of the plant workers was done to prevent them from reaching their yearly exposure limit. (otherwise they would no longer be allowed to work at the plant site at all)
To give an example of the spread of radiation from this explosion:
After the reactor #2 explosion, radiation detectors in Tokyo, registered readings that are 20 times the natural background radiation. Background radiation is due to naturally present radioactive particles in the earth and from cosmic radiation. This radiation level rise was only in Tuesday morning in Tokyo, and has since dropped. To give a comparison: One flight from Boston to Tokyo would provide 4 years of natural background radiation (due to cosmic radiation). Also, some places in the world, due to changes in the isotope (types of atoms) composition, have a background radiation 40x that of Japan’s normal background (so 2x that of the elevated levels seen in Tokyo). These places have no higher rates of cancer or other signs of adverse health effects.
I encourage you all to avoid reading CNN as I've found from the Japan crisis and the North Korean attack they tend to sensationalize their news more than other sources. To give the media some credit in this situation I think it is difficult to assess the seriousness of the situation in regard to both the nuclear reactors and death counts (another figure that changes based on which source you read).
Please continue to keep Japan in your prayers. Even though we have been physically unaffected in Korea there is certainly an emotional effect. My 5th grade co-teacher's sister lives in Tokyo and she JUST got a phone call from her yesterday. Before that she had heard nothing and didn't even know if she was alive. Her sister was just able to return to her apartment from the temporary shelter she'd been staying in. She said that Tokyo feels like a war zone. There is no food and normally polite, calm Japanese have reached the desperation of stealing from stores. I sent a message to my couch surf host who lives in Osaka which was unaffected by the earthquake and tsunami. Here was his response:
what a disaster.
i happened to be right near a tv when it struck.
i felt like it was another country (in many ways it still seems so...).
the last time something like this happened in the world (sumatra) it was very widespread, and many more perished, but this time it's compacted onto just 1 country.
the last time something like this happened in japan (kobe earthquake '95) i was here & more directly impacted, but it doesn't even compare with this.
these folks've been hit by the biggest earthquake in their history, followed by an unimaginable tsunami, on top of a cold winter weekend, snow, rain, no power, water, heat, food, etc etc.
needless to say we've been glued to the set 24/7.
beyond this we're all pretty much speechless. it is surreal.
physically we're okay, of course.
we're completely far enough away from the destruction.
it almost seems like another country, in that regard.
mentally, emotionally, spiritually--it's taxing, you know.
financially i can't imagine or estimate what it's going to do to this country.
ouch.
i happened to be right near a tv when it struck.
i felt like it was another country (in many ways it still seems so...).
the last time something like this happened in the world (sumatra) it was very widespread, and many more perished, but this time it's compacted onto just 1 country.
the last time something like this happened in japan (kobe earthquake '95) i was here & more directly impacted, but it doesn't even compare with this.
these folks've been hit by the biggest earthquake in their history, followed by an unimaginable tsunami, on top of a cold winter weekend, snow, rain, no power, water, heat, food, etc etc.
needless to say we've been glued to the set 24/7.
beyond this we're all pretty much speechless. it is surreal.
physically we're okay, of course.
we're completely far enough away from the destruction.
it almost seems like another country, in that regard.
mentally, emotionally, spiritually--it's taxing, you know.
financially i can't imagine or estimate what it's going to do to this country.
ouch.
I'm not sure what the best way to help in this situation is but I encourage you all to do some research and consider donating to an organization that is helping in Japan. Here are a few that I've heard of:
AMERICAN RED CROSS: Emergency Operation Centers are opened in the affected areas and staffed by the chapters. This disaster is on a scale larger than the Japanese Red Cross can typically manage. Donations to the American Red Cross can be allocated for the International Disaster Relief Fund, which then deploys to the region to help.
GLOBALGIVING: Established a fund to disburse donations to organizations providing relief and emergency services to victims of the earthquake and tsunami.
SAVE THE CHILDREN: Mobilizing to provide immediate humanitarian relief in the shape of emergency health care and provision of non-food items and shelter.
SALVATION ARMY: The Salvation Army has been in Japan since 1895 and is currently providing emergency assistance to those in need.
AMERICARES: Emergency team is on full alert, mobilizing resources and dispatching an emergency response manager to the region.
CONVOY OF HOPE: Disaster Response team established connection with in-country partners who have been impacted by the damage and are identifying the needs and areas where Convoy of Hope may be of the greatest assistance.
INTERNATIONAL MEDICAL CORPS: Putting together relief teams, as well as supplies, and are in contact with partners in Japan and other affected countries to assess needs and coordinate our activities.
SHELTER BOX: The first team is mobilizing to head to Japan and begin the response effort.
GLOBALGIVING: Established a fund to disburse donations to organizations providing relief and emergency services to victims of the earthquake and tsunami.
SAVE THE CHILDREN: Mobilizing to provide immediate humanitarian relief in the shape of emergency health care and provision of non-food items and shelter.
SALVATION ARMY: The Salvation Army has been in Japan since 1895 and is currently providing emergency assistance to those in need.
AMERICARES: Emergency team is on full alert, mobilizing resources and dispatching an emergency response manager to the region.
CONVOY OF HOPE: Disaster Response team established connection with in-country partners who have been impacted by the damage and are identifying the needs and areas where Convoy of Hope may be of the greatest assistance.
INTERNATIONAL MEDICAL CORPS: Putting together relief teams, as well as supplies, and are in contact with partners in Japan and other affected countries to assess needs and coordinate our activities.
SHELTER BOX: The first team is mobilizing to head to Japan and begin the response effort.
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