Tag Archives: Understanding Nuclear Power

Understanding the Nuclear Disaster in Japan: An Interview with a Nuclear Engineer

Here at Harris Educational our thoughts, hopes, and prayers go out to the people of Japan and to heroic engineers and other staff that are fighting to improve the odds in a very desperate situation caused by the recent 9.0 magnitude earthquake and tsunami.

I’m a fairly technologically literate person. I spend all my spare time reading about science and math and technology. I’ve done this all my life. Even so the Nuclear crisis that currently faces Japan has really driven home to me how little I know about Nuclear Energy and the practical technology that makes it a reality. Sure I understand that Uranium is an unstable atom that can be made to split in a process called fission that produces neutrons, radiation, and lots of heat that can then boil water, make steam, and rotate a turbine to power an electrical generator. But there are lots of questions that I don’t know the answers to. Luckily for me I have an old college friend who spent his time in college studying to become a Nuclear Engineer. His name is Garrett Gilley and he has most graciously volunteered to answer some of my questions and to share his knowledge with Harris Educational’s audience.

H.E.: Thank you for your time and for helping me better understand some of the things I’m hearing in the news about the Nuclear disaster facing Japan. Here are my questions:

H.E.: Its my understanding that “melt down” is not a technical term used by Nuclear Engineers but rather a catch phrase started by the media during the 3 Mile Island incident in the 1970’s. Can you explain what a “Melt Down” means and how that relates to what is going on in Japan right now? Are the reactors there melting down or are there other problems also going on? i.e. what is the biggest threat… release of radiation, release of radioactive materials into the environment, explosion?

G.G.: A “melt down” basically means that a part of the fuel core becomes hot enough to melt. The fuel is made up of cylindrical pellets, typically made with Uranium or Plutonium, inserted into a Zirconium rod. Several of these rods are put together with spacers to form a fuel assembly and the core of a reactor is made up of lots of these assemblies depending on the design and size. Since water is the main form of cooling when water can no longer be circulated or the fuel core becomes uncovered the heat produced from the fission reaction can no longer be removed and the fuel core will begin to reach the melting point of the various materials involved. The 1,2, and 3 reactors at the Fukushima Dai-ichi (“ichi” means one) lost the ability to circulate water because of the earthquake and the tsunami so the water already in the system continued to boil off until the cores have become uncovered and it is believed that the cores have begun to melt. As a last resort they have tried to pump sea water into the system, but my understanding is that it was only partially successful because of other things happening. A melt down does create an increased amount of radiation and radioactive particles, but as long as the containment holds the most of it should stay localized to the site. I should point out that a “china syndrome” has never happened, which is where the core melts down into a slag and burns through the containment vessel and the floor of the containment building and into the earth itself which theoretically would release a large amount of radiation and deadly particles into the water table. The fact is that the potential melt downs of these reactors is probably the greatest test of the old containment units that has ever occurred. In my opinion the fire in the spent fuel pool at reactor 4 is a much bigger problem because that is an immediate release of radiation and particles into the immediate surroundings of the plant. There have also been several explosions as seen and reported. So far they have all been related to either a pressure build up or a hydrogen bubble that became heated and exploded. I won’t say that the chance of a nuclear explosion is 0% but its about as close as you can get. Nuclear plants and the fuel they use are not designed the same way as a nuclear bomb.

H.E.: My understanding is that there are five reactors at the effected plant. At the time of the earthquake three were in operation. Automatic shutdown was triggered by the earthquake but it seems that any system as potentially dangerous as a nuclear reactor would have many layers of protections built in. What kinds of things likely happened to make these layers fail? What can you tell us about the design and operation of a nuclear reactor and its safety mechanisms?

G.G.: Actually the Fukushima Dai-ichi plant has 6 reactors on site and they were actually planning to begin building 2 more in the next 5yrs. There are 4 reactors right next to each other that we are seeing on tv and there are 2 more about a mile up from reactor 1. All 6 of these reactors are what we call BWR or Boiling Water Reactors and they were designed by G.E. back in the 60’s and all of these reactors went online between 1970 and 1978. As you have probably guessed these plants are old and were built at the beginning of the nuclear energy age and they did not benefit from the lessons we learned with TMI (Three Mile Island) or Chernobyl. A BWR is basically what it implies, the fission reaction in the reactor vessel boils water just like a pot on the stove, except that the source of heat is inside the pot instead of underneath. The water boils and creates steam which rises through a series of baffles before leaving the reactor vessel. The baffles are used to help remove as much excess water particles from the steam as possible, you want the steam to be dry and not wet. If you put your hand over a boiling pot the steam will feel wet and will burn you, this is because there are still some liquid water atoms in with the steam. I know the concept of dry and wet steam being different things boggled our little engineering minds as well but there is a reason for it. The steam goes from the reactor vessel to the turbine and is used to spin the turbine blades. Turbine blades are very delicate, and pressure is increased on the steam before it gets to the turbine in order to increase its velocity so that it can spin the turbine blade. If your steam is wet its the same as firing a Gatling gun off a Blackhawk at those blades and it will do just as much damage. Of course the turbine(s) spins a generator(s) and electricity is produced and sold. After the steam passes through the turbine it passes into a condenser where it is cooled back into water and is then pumped using recirculating pumps back into the reactor vessel. This entire loop is obviously irradiated and in a BWR is housed inside of containment buildings in case there is an accident.

There are lots of safety systems and mechanisms in a nuclear plant and in most cases they are not only redundant, but sometimes triple and quadruple redundant. But the reactors at Fukushima did not benefit from safety design changes that occurred after they were built since you can’t exactly retrofit the key parts of the loop. However, those plants still had the main safety systems that are required above all else to provide for containment and cooling. A fission reaction is created when a fast neutron splits a Uranium atom which produces a couple of elements, heat energy, and more fast neutrons. (I won’t go any further on that cause just remembering those calculations makes my head hurt). The first safety step is to insert control rods in between groups of fuel assemblies in order to stop or slow down the fission process. Control rods are made from materials that absorb fast neutrons. But you can’t completely stop the reaction right away, most reactors can still produce 1-7% of their power capacity even after the control rods are inserted. The reactor vessel itself that house the fuel is also made of materials that can absorb, shield, and reflect fast neutrons and other radioactive particles. The vessel is usually surrounded by a shielding material then a containment housing (usually concrete mixed with absorption materials). All of that plus other things sit inside an outer containment building. The outer containment building is usually slightly pressurized in order to keep any leaks or wandering particles in the inner containment housing. The other main safety concern is keeping the fuel inside the reactor cool which is done with water. The water that is used to generate steam is also used for cooling especially when the reactor is shut down so that means the key safety point here is the recirculation pump. Now in most designs there are 2 recirculating pumps and a 3rd one kept as redundancy in case one of the others breaks. Any one pump is supposed to be enough to maintain water flow to cool the core when the reactor is shut down. I’m not sure how many pumps each reactor at Fukushima has but it wouldn’t surprise me if the answer was one. These pumps as well as anything else at the plant that requires electricity depends on power to run so in the event of a shutdown everything switches over to off site power. If offsite power is lost then there are diesel generators and if those are lost then there are battery backups to be used until options one and two are available again, preferably within 8-12 hrs. Most power plants also have a tank of boric acid that can be fed into the system as well in order to get the fission reaction under control. Water used in the main reactor loop is called light water and is very very clean. Any introduction of external ‘dirty’ water or even sea water basically means that reactor can never be used for power production again.

HE: I’ve seen in the news that each reactor building has somewhere between 70 and 90 tons of spent fuel rods stored in water tanks above the reactor vessel itself. Why store the spent fuel at the power station (is it performing some function even though its spent?) and what is the danger if this spent fuel looses its cooling water? I’ve heard some news outlets using words like “fears of the stored fuel going critical” is there a possibility of a nuclear explosion? Or is the fear just a breakdown of the storage system and fires or conventional explosions from heat that might spread radioactive materials into the surrounding environment?

G.G.: Nuclear fuel is only effective for about 6yrs, after that its ability to effectively produce the right amounts of heat and neutrons is outweighed by the other elements that a split Uranium atom breaks down into. A buildup of those elements elements across the core work as a negative to power production so you have to remove the spent fuel assemblies that now contain some uranium but also those leftover elements and replace them with newer fuel. The standard is to replace a third of the core every 18-24 months. Those fuel assemblies are still highly radioactive and will continue to be radioactive for many many decades after and you have to put them somewhere but you can’t exactly truck them anywhere when they first come out of the reactor. The spent fuel pool was created for this reason. The spent fuel pool allows for the fuel assemblies to be stored, cooled, and monitored. Even though they are no longer a part of the reactor core, they are still producing some fast neutrons which continue to split uranium atoms, fission is a chain reaction after all. And as long as water is continually circulated in the spent fuel pool to remove the residual heat that is produced from the spent fuel, then everything is fine. The top of the spent fuel pool is also either not covered or may be lightly covered depending on the design. The only reason I can think of for it being that way is because the spent fuel is constantly monitored for various things including radiation and particles and sometimes assemblies need to be shuffled around to prevent hot zones in the pool. The design of the particular reactors at Fukushima had the spent fuel pools above and beside the reactor containment area primarily for easy access when taking out the spent fuel and putting in the new. You really don’t want those hot assemblies to be exposed to the air for to long. It was determined at some point in the 80’s the location of that pool was poor design and most later designs put the pool in a different building with varying ways to transport the spent rods from the reactor to the pool. If the spent fuel pool loses its ability to provide cooling water, there is still enough heat produced to boil off the water in the pool which is storing decades worth of spent fuel. As the water temperature increases in the pool and it boils, the irradiated zirconium housing will basically oxidize and breach. The product of oxidization is H2 which rises and potentially creates a very flammable bubble inside of the structure. With enough heat and pressure that bubble will explode which is what happened at reactor 4. At the same time the fuel rods are breaking down, the water is being boiled off and the assemblies are being uncovered which means they are no longer being cooled so now we have a “melt down” affect on the spent fuel. This would most definitely release an immediate high if not lethal amount of radiation and radioactive particles on site but this release should decay over time. There was a theory when I was in school that if you allowed a “hot zone” to fester in a spent fuel pool and you lost cooling then they could go critical or supercritical meaning they would act like the core of a reactor and the rate of neutron production would sore and could potentially lead to a nuclear explosion. But that was all based off of theoretical calculations. Personally I don’t think that could happen for the express reason that the fuel is spent and is no longer used because of its lack of neutron production rates required to overcome the negatives to power production. But the spent fuel burning and spreading radioactive particles is very real and more dangerous because it does not have the same type of containment as the reactor core.

HE: I’ve heard that there were “hydrogen explosions” in the various reactor buildings. How does hydrogen factor into a nuclear reactor? Is it hydrogen that is being liberated in some way from water or produced by nuclear fission? Or is it something used by the reactor. (I was surprised to hear so much about Hydrogen in the news)

G.G.: I don’t recall hydrogen itself being used in any component of the reactor building itself where these explosions occur except for possibly the pressurizing that is used between the inner and outer containment. I know that the air in there is breathable but it is slightly tweaked in order to maintain the small pressure difference. The most likely occurrence in my mind comes from the oxidization of the zirconium fuel rods. Because of the fission reaction combined with the heat the zirconium will actually remove the oxygen from the water molecules and this breakdown leaves hydrogen bubbles mixed in with the steam which is flowing out of the reactor vessel. In order to relieve pressure on the system some of this steam has to be release into the outer containment area which is a prime opportunity for a hydrogen bubble to be created. The reason for the release of steam is because if you don’t and pressure builds up in the reactor vessel, it’ll pop like a pressure cooker and spew lots of radioactive material much worse than the spent fuel pool. So releasing a small amount of radiated steam outweighs popping that reactor vessel by a long shot. If you go back and look at the video of the explosion at reactor 1, even though there was a hydrogen bubble, that explosion was more a result of increased pressure in the outer containment building and it sorta popped like a balloon which is why 2/3 of the building remained intact. The explosion at reactor 3 was most definitely the ignition of a hydrogen bubble and the damage and video show it. The explosion at reactor 4 was also as a result of ignited hydrogen which was produced by the overheated spent fuel pool. These are just my analysis based on my observation of the videos and my knowledge of how and where the hydrogen was produced, the only people that really know for sure are on site at Fukushima.

HE: What can you tell me about the risks the workers at the nuclear plant are taking by working there to fix these problems? I know they’ve evacuated civilians within a 12 mile radius of the plant and the US Government has issued orders that any US citizens within a 50 mile radius of the plant should leave. Tonight the news media is using phrases like “suicide mission” to describe the job these people must perform. Is their life span shortened? Have they already been exposed to enough radiation that they can not survive?

G.G.: Without a doubt anyone that works at a nuclear facility is taking a risk. You are told when you enter any nuclear engineering program how dangerous the things are that you are playing with. You’re not just trying to control things you can see, you’re trying to control things you can’t see. If something major happens you know that you have to do everything you possibly can to bring it under control and to an end in order to prevent something from happening to the environment and to potentially millions of people. Those guys that are staying there to work on this runaway event are doing so knowing full well that their lives will most likely be shortened if not lost. If there is even a 1% chance to get this event under control, those workers will stay there and try it. I knew the first day when they initially vented some of the steam in reactor one and there was a slight increase in radiation that these workers were more concerned with bringing things under control and not with themselves. I can only imagine how they are feeling now in what seems to us to be an impossible situation but yet they are still there and still trying to come up with ways to bring it under control. We have to also remember that on top of the obvious doses of radiation that they have already received, they have also lost their homes and friends, family, and co-workers from the earthquake and tsunami that hit the nearby town where most of them stayed. I read one report on the first night where after the tsunami one worker rushed into the town to find anyone from the next shift who may have been at home sleeping at the time and he was only able to find a few. To me no matter how this turns out those guys are all heroes. And pretty soon there are going to have to be some even tougher decisions. There are some pretty extreme measures that are probably going to have to be taken especially at reactors 3 and 4 that will basically be suicide missions. I just hope that things can somehow be turned around before that point is reached but they truly are running out of time.

I think the Japanese did an excellent job evacuating the civilians to a 12 mile or more radius and it will probably increase one more time. I think the U.S. government making the 50 mile suggestion for its citizens living there is probably just being overly cautious but not a bad idea either.

H.E.: Based on any information you can find can you tell us how this disaster compares to 3 Mile Island and to Chernobyl? What are the best and worst case scenarios going forward.

G.G.: Comparing this to Chernobyl is like apples and oranges. Two completely different types of reactors in every way and at Chernobyl they were attempting to perform a test and they purposely turned off their redundant safety systems. Instead of stopping the experiment when the test conditions were not met, they tried to force the reactor to produce those conditions, which caused the reactor to ramp up and run out of control with no safety mechanisms in place and leading to a hydrogen explosion the exposed the core and spewed radioactive material making thousands of acres uninhabitable for several lifetimes. TMI’s incident is a little closer even though it also is a different type of reactor and design from Fukushima. Its similar in that the coolant pumps failed and the reactor shutdown, but TMI also had a pressure operated relief valve which was stuck closed not allowing coolant from the emergency tank into the reactor vessel which allowed half the core to become uncovered and to melt. The radiation released from that incident however didn’t even get close to the amount you would normally receive annually from nature. So the reality is that the situation at Fukushima Dai-ishi is entirely unique. The best we can hope for is that they are somehow able to put out the fire and be able to get cooling water into the reactor vessels and spent fuel pools and bring the temperatures back under control. Worst case scenario would be a Chernobyl type event where containment is breached and there is a massive release of radioactive particles into the environment for miles and into the natural water table.

H.E.: What is the end game here? If the existing reactors can be kept cool can the other reactors at the facility still be used in the future or is the site too contaminated to be used?

G.G.: I don’t think the entire site is too contaminated just yet, at least not around reactors 5 and 6. And as long as there is not a massive release of particles into the air and into the natural surroundings the radiation levels will decay in a relatively short amount of time. But even if we get the best case scenario the functionality of reactors 1-4 are over. Those explosions combined with melted fuel and the addition of sea water have pretty much ended the lives of those reactors. In reality they were all entering an end of life cycle anyway but going out like this is not really good. Whether or not they are able to use 5 and 6 or even build the two new ones that were planned we will just have to wait and see.

H.E.: Given that we’ve seen reports tonight that one of the reactor buildings has lost its roof and is on fire and that helicopters are making water drops on the building is there any hope that things can be contained or is this now a loosing battle?

G.G.: As long as the primary containment and the reactor vessel remain intact its never a losing battle. As I’ve said earlier, it is only assumed that the core can melt through the vessel and containment floor but there has never been a situation where it could really happen like this. TMI was close but they had the luxury of on site power and backup pumps and external water tanks. However dropping water on those buildings runs the risk of jostling the fuel assemblies around inside the pools which are pretty much open to the atmosphere and creating more problems, but given the alternative of doing nothing there’s not really any other choice right now.

H.E.: Have or Will radioactive materials be released into the environment and if so will they get into the food chain? How will it effect the oceans? Will airborne particles reach the U.S.?

G.G.: There has been some release into the area surrounding the site and into the air but dispersion increases greatly the farther away you get from the site. And radiation from a particle decays over time as its energy is absorbed by the atmosphere and whatever else it touches which is why they have seen spikes of radiation on site after an explosion but then it goes back down. So there is not a consistent output of high radiation at this point in time. A massive consistent release would effect the ocean near the plant the most but just like air the father away it gets the more diluted and less effective it becomes. Plus we’re dealing with heavy metals which tend to sink in water. Will airborne particles reach the U.S.? Maybe 1 googlezilianth (like that, I made it up) of a particle sometime down the road. I mean if you made a detector sensitive enough you could probably still detect particles left over from testing in world war 2 but getting a sun tan is more dangerous than that. I actually laughed when they reported on the news last night that people on the west coast were making a run on pharmacies buying up iodine and other things. Its okay if you just wanna be prepared for something to happen here, but going overboard because of something happening 7000 miles away will just make you broke.

H.E.: These plants in Japan were designed and built prior to 3 Mile Island and Chernobyl. How are the nuclear power plants that are in service in the US today different or safer than the plants in Japan, and what other lessons are potentially being built into future plants to keep these kinds of problems from happening. Are plants as safe as they can be and this was just a “perfect storm” of bad conditions, or are we able to reduce risks in current and future nuclear power plants?

G.G.: Well, in reality we have 23 plants here that have the exact same design as the Fukushima plants, they were designed by G.E. after all. Several have already been mothballed after reaching their end of life cycle according to our standards. Personally I always felt that the BWR design was a higher risk than the PWR design mainly because it’s a single loop system which increases the safety concerns and design issues. But the BWR only makes up about 1/3 of our total nuclear plants. We did learn lots of things from TMI and Chernobyl however. When it comes to safety the plant designs were changed accordingly and some older plants were retrofitted with newer safety systems. One of the newer BWR designs actually has water powered recirculation pumps inside the reactor vessel itself which can be used to provided temporary cooling control if needed. So a lot of changes have been made towards containment and cooling over the last 20yrs. Spent fuel is also a major concern so all new plants built will actually have covered containment for the spent fuel pools. There are also several new plant designs utilizing spent fuel as a source for power instead of just storing it, in fact the Chinese are building the first reactor of that type. Current plant safety always comes into question when a big event happens, just like right after 9/11. Whether our plants here are safer or not than Japan’s, I don’t know. I like to think our’s probably are just because our watchdog is a bit more active than theirs. Here in the U.S. there is an Nuclear Regulatory official on site 24/7 at every plant constantly making safety reports and evaluations. But when you have such an unthinkable event like what happened in Japan you just can’t say with 100% certainty that nothing that bad will ever happen. I had a coach in high school that always said “When you least expect it, expect it”. We expected and prepared plans for an earthquake, we’ve never expected that to be followed immediately by a 30ft tall wave of water.

H.E.: Much time has been given in the media to radioactive iodine. What causes Radioactive Iodine (is it a byproduct of fission?) and what other chemicals or contaminants are released by a “melt down”?

G.G.: I-131 is a large byproduct of fission. And if ingested it will do tissue damage especially to your thyroid. The good news is it has a half life of about 8 days so it doesn’t last very long. After that it releases a beta ray and turns into Xenon. Xenon is bad for producing nuclear power cause it sucks up neutrons. I’m not really sure how to answer the melt down question because the particles created depend on how the uranium atom splits and also on the half life of that particle and what it decays into. Iodine is one possibility but it only happens about 5% of the time. There are several different possible combinations. And if they use Plutonium fuel it changes. Plutonium is what is in reactor 3 at Fukushima. I know that the news has mentioned detecting Cesium which is an indicator of a meltdown and I think that is because of what happens when the zirconium breaks down. There may be some documents at the http://www.nrc.gov site that can give some details of this. I know they have a teaching section with documents for would-be engineers and professors.

H.E.: Anything else that you want to say or tell us?

G.G.: I just hope that everyone will try to step back and learn a few things about this event and not be quick to make judgments solely based on what the media is reporting. Having grown up through TMI and Chernobyl I will admit that I was very afraid of anything nuclear until I actually chose Nuclear Engineering as my major in college. I learned a lot about the industry and how the plants are designed and their differences from the bombs. Safety was drilled into us for 4 solid years and using the reactor on campus gave us first hand knowledge and experience. Right about the time that we were all comfortable with the safety and design of nuclear power, they took us to a simulation control room at an actual commercial plant and put us through the smaller version of control room training that employees get. And the night they hit us with a “double whammy” it was a humbling experience and you realize the weight of that job. Those reactors in Japan are old and were nearing the end of life cycle and I would bet had some unreported issues and probably some equipment that was on the decline long before the earthquake and tsunami hit. When that earthquake hit I can just about picture what the inside of that control room was like with the red lights popping across the board and the alarms going off and the reactors hitting SCRAM conditions. After it was over those guys probably checked to make sure the control rods were all in and the turbine and generators had shut down and began checking the hundreds of gauges to look for any indication of damage from the earthquake across the entire facility. When they didn’t have off site power right away the diesel generators would have kicked in and someone would have been checking to make sure the recirc pumps were still working. Then the tsunami hit and wiped out the generators, turned the secondary cooling intakes into a debris field and did so much more damage. Now your generators are junk, the battery backup to the pumps on reactor 1 won’t switch on and everything outside is a mess. The nearby town is gone, the roads are partially washed away, and any hope of getting offsite power right away is gone. You just can’t fully prepare yourself for that kind craziness. As a Nuclear Engineer I am conflicted because I know that this type of thing happening is so rare that its an opportunity to learn something that may have never been tried or considered before, but at the same time I hope they regain control as quickly as possible and end this event. We will just have to wait and watch as this continues to unfold and hope that this will inspire new breakthroughs in the nuclear industry. Who knows, maybe someone will figure out a way to completely neutralize the effects of radiation or a way to produce a viable fission reaction without all the nasty by-products.

I hope I was able to answer your questions make things a little more clear, although I may have been a little long winded. 🙂

H.E.: Not longwinded at all. I thank you so much for sharing your knowledge. You’ve definitely helped me better understand what is going on and how that relates to our safety, the situation in Japan, and the future of Nuclear Power Plants. You’ve also given me a very deep respect for what the workers at these plants are going through. I share your hopes that this situation can be brought under control quickly.

If anyone reading this article would like to learn more, there is an excellent resource at:

http://spectrum.ieee.org/static/japans-earthquake-and-nuclear-emergency