There was a good bit of feedback from last week’s installment, and I want to point out that I am always glad when people point out flaws in my treatment. I emphasized a particular sort of reactor, and neglected a couple of other ones. I intend to set this right tonight.
The concern that seems to be in the forefront at present is the radiation leakage from the stricken plants. As I write this (20110326), it is still not clear whence it comes, but I suspect that fuel rods are compromised and that nuclear fuel rod material is becoming commingled with the water that is supposed to cool the systems.
I say that because it is unlikely that if the spent fuel rod ponds were the source that the high levels of radioactive materials would have found their way into the turbine rooms, where the subcontractors were exposed to extremely high levels of radiation.
The primary thrust of this piece is to go through some of the fission products in the spent (and in use) fuel rods. This will give us a basic understanding as to why used nuclear fuel is so much more dangerous than new fuel.
First, a little background. There are a few different legacy nuclear reactor types, the most important being the Pressurized Water Reactor (PWR), and the other being the Boiling Water Reactor (BWR). There are more, such as the Advanced Boiling Water Reactor (ABWR), an improvement of the older style Boiling Water Reactor, Pressurized Heavy Water Reactors (PHWR), and the CANDU, which is really just a variant of the design of the PHWR. There are several other designs, but these are the most common for production of electricity.
Last week, I described in some detail how a PWR works, without bothering to look up that design the stricken reactors were in Japan. It turns out those are BWRs, and they differ from PWRs in that there is no heat exchanger, the water to spin the turbine being the same water sent through the reactor core. In this case, that water is boilt within the containment shell of the reactor, and the high pressure steam resulting passed directly to the turbines. This steam is condensed and returned to the loop as water, to go through the cycle again. I do not much like that design because it eliminates a safety stage in opposition to the PWR one that I described last week.
I speculate that since the reactor in question are BWRs, the compromised fuel rods allowed the steam to become contaminated with relatively volatile radionucleides and thus enter the turbine room(s). With a PWR, there is another level of protection, since the water exposed to the core never reaches the turbines themselves, but is recycled back the the core through the closed loop on the “hot” side of the heat exchanger.
I was incorrect when I implied that the reactors in Japan were PWR ones. They are, or now, were, BWR ones. This is a significant difference, but insofar as the problems there, pretty much a difference without a distinction. With either reactor design, loss of water circulation would have had the same end result except for the contamination of the turbine rooms.
Here is how two different readers corrected me. Please do not get me wrong, I absolutely and positively want to be corrected when I need to be. Remember, the difference is betwixt PWR and BWR reactors. Here is the first one. My comment precedes the answer. I have wiped out the identities of the commentors, and have streamlined the comments so as for them to make more sense. If you want to see the whole thing, just click back to last week’s installment.
Me: These pressurized water ones need to be phased out, and rapidly.
Commentor: Interesting thought, but why?
All of the reactors at Fukushima-1 are boiling water reactors, not pressurized water reactors.
Here’s a tip: if you’re going to make grand pronouncements, you should learn the basics … at the very least you should use the right lingo.
Now, for someone asking for clarification, that was a bit harsh.
To make the exact same point, another reader said this:
The fuel rods begin to get hot, heating the water, which in turn is circulated under high pressure (to keep it from boiling) to a heat exchanger, which is used to heat other water. Thus, the water in the reactor core is circulated continuously through the heat exchanger and core, and unless there is an upset, never is released to the environment. The water in the heat exchanger that the core water is boiled and the steam used to spin the turbine, and thus the generator.
That’s correct for a pressurized water reactor (PWR) which is in fact the most common design, but incorrect for a boiling water reactor (BWR), which make up 7 out of the 11 reactors here in Illinois (1) and all of the reactors at Fukushima. In a BWR, the water in the reactor vessel boils and the resulting steam goes directly into the turbines. Thus the turbine steam contains relatively small levels of radionuclides, whereas in a PWR the steam is “clean”.
1) Four of them are the same design (Mark I) as the Fukushima units; the other three are a design about 15 years newer (Mark III). The oldest Mark I (Dresden 2) is 40 years old.
If you Google “headache brain tumor”, you will come away convinced that your headache is actually cancer-Seth Mnookin
You can see that I was corrected by both parties. The latter one did it with courtesy and respect, but the former one with less than that. I prefer the latter.
I never mind being called out for being incorrect, and for those of you that read this series on a regular basis, you know that I actually ask for it. However, I prefer, like most people do, the velvet glove approach rather than the iron gauntlet one. I appreciate the kind correction that the latter commentor gave, and respect the expertise.
With all of the conflicting information coming from Japan, I shall not even attempt to describe what is going on there. What we will discuss this week is what is in fuel rods in a reactor core, or in the spent fuel rod ponds. Uranium-235 is not actually that dangerous itself, unless it is widely dispersed into the environment, in particular the air and water. Plutonium-239 is more hazardous, since it has a higher specific activity, as measured by the half life of the isotopes. The half life is the amount of time required for half of a given number of nuclei to decay away (often to other radioactive isotopes, by the way). For U-235 it is about 731,000 years, and for Pu-239 it is 24,000 years, so very roughly one can say that the plutonium is around 30 times more radioactive than the uranium. However, these are not the problem isotopes in Japan. The fission products are much more troubling, and so are some of the activation products, other nuclei that have been made extremely radioactive because of neutron absorption. We shall consider the fission products first, and stipulate that these are coming from U-235. Other isotopes have slightly different yields.
The most common fission isotope is cesium-133, which is not radioactive, which accounts to around 6.8% of the total fission products. However, it is converted to the extremely highly radioactive cesium-134 (half life only about 2.1 days) by neutron activation, and this isotope is a major contributor to heat in the core. In the spent fuel pool it is not really much of a factor, since in 10 days only 1/32nd of the original amount is left. The next most common fission product is iodine-135, also intensely radioactive (half life only 6.6 hours), so it is a major contributor of heat if it still being produced in the fuel rods. It decays away rapidly, obviously. Neither cesium-134 nor iodine-134 are significant environmental hazards since they decay away so rapidly.
Also at around 6.3% yield is zirconium-93, with a half life of about 1.5 million years. It is not a significant source of heat in the reactor core because of its relatively long half life. At 6.1% yield is cesium-137 (half life of about 30 years). Cs-137 is often used as heat source industrially because of it high activity and relatively middliing energy gamma emission. It is also one of the nuclei of concern in “dirty bombs” because lots of those heat sources are loosely guarded in the former Soviet Union.
Also at about 6.1% is technicium-99 (half life of about 214,000 years). It is not particularly a problem for heat buildup in the core, but is responsible for long term radioactivity in spent fuel. Coming in at about 5.8% is strontium-90 (half life of about 29 years) which is active enough to cause significant heating. At about 2.8% is iodine-131 (half life of right at 8 days), active enough to be a very significant source of heat. At about 2.3% yield is promethium-147 (half life of about 2.6 years), also a rather significant heat source. Finally, at about is samarium-149, which is stable.
There are many more fission products in a U-235 fuel rod, but for the sake of brevity I shall limit the discussion to those that are produced in more than 1% yield. That is not to say that these fission products are not important, but as the amount produced becomes smaller, their contribution to heating is significantly less. All of the isotopes mentioned above, except for the stable ones, are significant sources of potentially harmful radiation, but the isotopes of three elements are of particular concern insofar as effects on people are concerned.
Probably the most significant one is iodine-131. Since iodine is selectively absorbed by the thyroid gland, it accumulates there and is readily absorbed. With a half life of 8 days, it is active enough to do significant damage to the thyroid. This is why potassium iodide tablets are distributed in areas close to high levels of contamination. The idea is to load up the thyroid gland with nonradioactive iodine before exposure to the radioactive kind is experienced, to make the radioactive isotope less apt to be absorbed. The thyroid essentially quits absorbing iodine if it already has enough. By the way, this is the same isotope of iodine used in medicine to treat certain thyroid conditions.
Another element that is problematic is cesium, and two radioactive cesium isotopes are produced in high yield. Cesium is chemically similar to sodium and potassium both essential minerals in humans, and so is readily absorbed. The third one of concern is the strontium-90, because strontium is a calcium mimic, being absorbed and incorporated into the bone. This is particularly hazardous, since the bone marrow is the formative tissue for all sorts of blood cells, and irradiation of the blood forming cells can lead to leukemias and other blood diseases. As a matter of fact, Marie Curie died of aplastic anemia, almost certainly caused by her work with radioisotopes before it was know that radiation was harmful to the body.
Activation products, those isotopes produced by absorption of neutrons by rather low mass elements, are also of concern (actinides are a special case of activation products and shall be considered in a bit).
A troubling one is tritium, or hydrogen-3. It is formed from boron-10, and boron is a common element used as a control material. With a half life of only 12.3 years, it is intensely radioactive and reacts with atmospheric oxygen to produce water. This is not a good thing. Another one of concern is carbon-14, half life of 5730 years, formed from the nitrogen-14 in the air, and to a much lesser extent naturally occurring carbon-13. While not that big a deal in water moderated reactors, it is a significant problem in graphite moderated ones. By the way, carbon-14 is also formed naturally in the atmosphere by cosmic radiation, and this is the basis for carbon-14 dating. It reacts immediately with atmospheric oxygen to form radioactive carbon dioxide and is taken up by plants during photosynthesis.
Another isotope of concern, at least in the Japanese case, is chlorine-36, with a half life of around 300,000 years. Normally, it is not produced in water moderated reactors because purified fresh water is used. However, with the use of seawater for the past over a week to try to cool the reactors, no doubt significant quantities of this isotope have been formed, since it is formed from the naturally occurring, stable chlorine-35, and seawater is rich in it.
If steel has been exposed to a high neutron flux, some iron-55 will surely have been formed from iron-54, although the natural abundance of iron-54 is only around 6% of the total. However, it is intensely radioactive with a half life of only 2.7 years. Two others of concern are nickel-59 (half life of around 76,000 years) formed from nickel-58 in alloy and stainless steels. Another one of concern is silver-108m, with a half life of around 418 years. The “m” stands for metastable, because this is a nuclear excited state of silver-108 that is kinetically more stable than silver-108 itself. The metastable state has a half life of 418 years, as I said, emitting a gamma photon (or more than one). When it finally reaches the ground state, this half life is only around 2.4 minutes. This a problem, because as we reported last week, control rods are around 85% silver, and around half of that is silver-107, the parent isotope of silver-108m. Finally, there are cadmium-113m and cadmium-113, produced from natural cadmium-112 in nuclear control rods and discussed last time. Cadmium-113 is not really a problem, because it has a half life in the quadrillions of years, so it is not very active. However, the 113m one has a half life of only around 14 years, and so is quite radioactive.
The special case of neutron activation about which I spoke a few minutes ago involves irradiation of the nuclear fuel itself, or actinide formation. For example, when uranium-238 (the bulk of the uranium in a reactor core) absorbs a neutron, uranium-239 is produced. With a half life of only around 23.5 minutes, it is extremely radioactive. The U-239 ejects an electron to form neptunium-239, also highly radioactive with a half life of around 2.3 days. It also emits an electron, forming plutonium-239 (half life of 24,000 years), which is stuff of which fission bombs are often made. It is also used as a reactor fuel, and at least one of the units in Japan uses plutonium as part of its fuel mix. The Pu-239 can also absorb a neutron, forming Pu-240 (half life of about 6600 years). This is just one set of many, many reactions that can occur with the heavy elements. The end result is that before very long fuel rods contain hundreds of different heavy, some highly radioactive, isotopes. To go into more detail would not be useful, but one good thing is that many of these materials are rather high boiling solids, so are not as easy to disperse as iodine, for example.
However, they are serious hazards when they escape into the environment, especially if for some reason they are dispersed in the air. The graphite fire at Chernobyl dispersed essentially everything, and the situation in Japan is apt not to be nearly as energetic, so large area dispersal of actinides is unlikely by an atmospheric mechanism. For some perspective, even the open air detonations of thermonuclear devices (“hydrogen bombs”) in the Pacific did not produce highly health threatening air dispersions of radioactive elements (although there was worldwide distribution, just at very low levels), and essentially everything was injected high into the atmosphere. One Soviet detonation was so energetic ( the 50 megaton AN602, tested on 19611030) that it actually created a new radiation belt above the earth, but no where were lethal levels of radiation experienced, outside of the offset for the test range.
Therefore, there is no reason to panic over the release in Japan. Certainly, there is cause for concern, partarticulaly in the region in the immediate vicinity around the plants. There is no doubt that some of that population will be affected, most likely with elevated cancer rates in years to come. Also without doubt, the workers at the plant will suffer severe health effects. But the rest of the Japanese population, and certainly just about everyone everywhere else will see no effects.
Please do not get me wrong: this is the second largest nuclear power plant incident ever, and I am not trying to minimize its importance nor the negative effects that certainly result from it. However, it is important to keep things in perspective and not overreact to a situation that the media have sort of overhyped. This is also an opportunity to learn some important lessons so that positive regulatory steps can be taken worldwide to increase the safety level of existing nuclear power plants. Like it or not, they are with us to stay, and I strongly suspect that more will be built in the near future. Next week we shall close this series by looking at the latest generation technology that goes a long way to mitigate the problems with these old design plants.
Well, you have done it again! You have wasted many more einsteins of perfectly good photons reading this hot material. I usually insert a political joke here, but will instead this week make a very rate reference to sport. Most of you know that I am originally an Arkansawyer (only the self important folks say “Arkansan”). Since the Razorbacks did not go anywhere in the NCAA basketball tournament, I have to congratulate the Kentucky Wildcats for making it to the semifinals. The Bluegrass is my adopted home now, and although I will always root for the Razorbacks when they play the Wildcats, I am pleased that Kentucky won today. Those kids played their hearts out and NEVER gave up, even though late in the game it looked like they would snatch defeat from the jaws of victory.
I always learn much more than I could possibly hope to teach by writing this series, so please keep those comments, questions, corrections (as discussed above), and other feedback coming. The comments are always the best part of the post, and remember that no scientific or technology issue is off topic here. I shall hang around as long as comments warrant, and shall return tomorrow evening around 9:00 Eastern for Review Time.
Warmest regards,
Doc
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hot topics?
Warmest regards,
Doc