Another one bites the dust. Or “Super-B? What Super-B?” November 28, 2012Posted by apetrov in Uncategorized.
Tags: charm physics, particle physics, Super-B experiment
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Studies of New Physics require several independent approaches. In the language of experimental physics it means several different experiments. Better yet, several accelerators that have detectors that study similar things, but produce results with different systematic and statistical uncertainties. For a number of years that was how things were: physicists searched for New Physics in high-energy experiments where new particles could be produced directly (think TeVatron or LHC experiments), or low-energy, extremely clean measurements that explored quantum effects of heavy new physics particles. In other words, New Physics could also be searched for indirectly.
As a prominent example of the later approach, detectors BaBar at SLAC (USA) and Belle at KEK (Japan) studied decays of copiously produced B-mesons in hopes to find glimpses of New Physics in quantum loops. These experiments measured many Standard Model-related parameters (in particular, confirming the mechanism of CP-violation in the Standard Model) and discovered many unexpected effects (like new mesons containing charmed quarks, as well as oscillations of charm mesons). But they did not see any effects that could not be explained by the Standard Model. A way to go in this case was to significantly increase luminosity of the machine, thereby allowing for very rare processes to be observed. Two super-flavor factories (those machines are really like factories, churning out millions of B-mesons) were proposed, the Belle-II experiment at KEK and a new Super-B factory at the newly-created Cabibbo Lab in Frascatti, Italy. I have already written about the Cabibbo Lab.
It appears, however, that Italian government decided today that it cannot fund the Super-B flavor factory. Tommaso Dorigo reported it in his blog this morning. Here is more hard data: there is a press release (in Italian) from the INFN that basically tells you that “economic conditions… were incompatible with the costs of the project evaluated.” Which is another way of saying that Italian government is not going to fund it. This follows by the news from the PhysicsWorld saying the same thing.
Many physicists have been expressing doubts that the original Super-B plan, which was, in my opinion, very bold, could be executed within the proposed time frame. Yet, physicists pressed on… that is until this morning’s announcement. Reality of our world sets in — there is not enough money for basic research…
So, is Higgs finally here? July 4, 2012Posted by apetrov in Uncategorized.
Today is a big day at CERN. There are two collaborations that presented their latest results on the search of the Higgs boson. Did they finally discover the Higgs boson?
Let’s first figure out what it all means? What two collaborations? What is Higgs boson? And, most importantly, what do we mean by “discovered”?
First things first. The two collaborations that I’m talking about are CMS and ATLAS, two huge detectors and hundreds of professors, postdocs and graduate students working to get and work the data that come out of it. The collaborations looked at almost three years (although 2010 does not really count and 2012 is still going — but with fantastic pace) and found signals of the Higgs boson, a particle that was predicted to be there in the minimal Standard Model.
Why is that we need Higgs boson and why is it there? The Standard Model of particle physics is described by its symmetries — or the symmetry group (SU(2)xU(1)) under which matter contents transform. This symmetry tells us how particles interact — and in fact, that makes Standard Model quite a constrained system. So the introduction of this symmetry is very important. However, this symmetry also tells us that all particles that are described there should be massless! What should one do? The idea is to break that symmetry, of course. The problem is how to break that symmetry. One cannot simply add symmetry-breaking terms (that would wreck the whole original setup), one has to do it indirectly. So the idea was proposed to introduce a field that interacts with all fields that are present in the Standard Model. That field also interacts with itself and forms a condensate (i.e. provides non-zero value for energy density of the vacuum) once, roughly speaking, the temperature of the Universe after the Big Bang drops below certain value. This mechanism gives mass to both electroweak gauge bosons (particles that represent weak force) and quark and leptons. The mechanism itself was first proposed in 1962 by Philip Warren Anderson. The model of spontaneous symmetry breaking was independently developed in 1964 by three groups, Robert Brout and Francois Englert; Peter Higgs; and Gerald Guralnik, C. R. Hagen, and Tom Kibble. The particle that manifests this effect is the famous Higgs boson. Read more about it here. I’ll talk more about it in my later posts.
CMS went first (which is a bit unusual, as Atlas, for some alphabetical reason, would always be first to deliver their talk). The talk was delivered by Joe Incandela, a CMS spokesperson. A gist of their talk is that they looked at several possible decay channels of the Higgs boson. First, Higgs can decay to two photons (H → γγ). They see a significant bump at mH = 125 GeV, but only in the combination if different reconstruction techniques. The overall significance is over 4 sigma. Next, they talked about H → ZZ channel. This channel is tougher, as they need to reconstruct Z’s that decay in different decay channels. Now, if they combine their data in H → γγ and in H → ZZ they find that statistical significance for signal that the Higgs is there at 5 sigma. However, once they combine all data, especially the H → ττ channel, their combined statistical significance goes slightly down to 4.9 sigma. This is just below discovery by the standards of Physical Review Letters, a very influential physics journal. But this is in very significant.
The next talk was by ATLAS. Fabiolla Gianotti, ATLAS spokesperson, gave that talk. They also see excess in H → γγ channel, but their statistical significance in that channel is lower, 4.5 sigma. Also, they include the so-called look-elsewhere effect — and then their statistical significance goes down to 3.5 sigma. Then, she discussed the H → ZZ channel. They see the excess with 3.4 sigma significance at mH about 125 GeV. Now, the combined results have excess at mH = 126.5 GeV with (local) statistical significance of 5.0 sigma. This is a discovery.
In passing, ATLAS also see a bump in their 4-lepton channel at approximately 90 GeV. Still not clear what it is….
This discovery is very significant. It tells us that our ideas on how electroweak symmetry is broken are at least partially correct. This is also the first truly elementary particle discovered since the Z-boson. There are still many questions, both experimental and theoretical, about the analyses presented today at CERN. What is going on with the H → ττ channel? Is it really a Standard Model Higgs boson? Or some other scalar particle. We’ll sure to study those things indeed.
P.S. The theorists who described the effects were there and not only were acknowledged by the experimental speakers, but also got to say a couple of words at the end.
Why advertisers should know physics June 12, 2012Posted by apetrov in Uncategorized.
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Some years ago, when I decided to start blogging, I wrote about an interesting advertisement that Ford Motor Company put up in all major magazines (it was even placed on the side of their headquarters in Dearborn, MI):
Here is what I said in that post of mine from several years ago: “What is interesting about this ad is the equations that this lady is writing — they look like the equations from the famous Peskin and Schroeder’s book on Quantum Field Theory (QFT), equations describing renormalization of phi^4 theory! How did Ford get a hold of them?
As it turns out, I happen know the answer. This ad was made by a company that is headquartered in Detroit — I have a business card of one of the authors of this ad!
What happened is that a couple of months ago I was sitting in my office at Wayne State University, looking over my QFT notes that I’m supposed to teach next Fall. A guy showed up at my door and asked to “write down a complicated-looking equation.” Now, that’s not a usual question that I get when I sit in my office during the lunchtime! He quickly explained that he works for this advertisement company (called JWT) and they were contracted by Ford to produce a series of ads that should highlight the talent of Ford engineers and at the same time appeal to young people. (He showed me a prototype of an ad with that girl sitting next to the blackboard.) So his boss sent him to the closest university (which happen to be WSU, we are located 5 min down Woodward Avenue from their office) to fish out a “complicated equation.” The rest is simple — I use Peskin and Schroeder as a main text for my graduate QFT course, so that list of equations was indeed about renormalization of phi^4 theory… I must add that I received no monetary (or any other) compensation…
Amazing, isn’t it?”
The reason I re-post part of that old post is the following. I recently went to Florida to participate in CIPANP-2012 conference (I’ll post my impressions of this conference later this week). Now, Kennedy Space Center is on Cape Canaveral in Florida, so I rented a car and went to visit that marvelous place. The place is truly amazing! Lots of things to see. The place is still making history: I visited it just a couple of days after the historic launch of the Space X‘s Dragon capsule.
I also visited a gift shop and bought the following souvenir there:
See how many mistakes they got in there? And it’s not “rocket science”, it’s freshman physics! Quite embarrassing… Clearly, people from that JWT advertising agency in the example above take their job responsibilities much more seriously.
See that NASA seal in the upper left corner? Since I am sure that NASA scientists know physics, I take it as indication that they never visit their gift shop.
CHARM of Hawaii May 14, 2012Posted by apetrov in Uncategorized.
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I’m blogging from the site of CHARM-2012 conference, which has just started in Honolulu, Hawaii. This is a fantastic conference at a fantastic place! The conference will have four full-packed days filled with many aspects of physics related to charmed quark. As I reported earlier, many exciting recent results are associated with charm quark.
Why is the conference taking part in Hawaii? Besides being a nice place in general, it is almost exactly half way between Japan and the US. This meeting alternates between Asian, US and European locations, and last meeting, in 2009, was in Beijing — so it is US’ turn. There will be many talks from KEK‘s Belle collaboration (which University of Hawaii is a member of), LHC experiments, as well as from Tevatron experiments. Besides, world’s only operating charm experiment (BES 3) is located in Beijing, China. Indeed, there would be many theory talks as well. It shapes to be a very nice conference — and I’ll be reporting about exciting results to be discussed here.
Inverse superconductivity in iron telluride April 1, 2012Posted by apetrov in Funny, Near Physics, Physics, Science, Uncategorized.
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One of the most significant advances of science in the 21st century so far is the 2008 discovery of iron-based high temperature superconductors such as LaFeAsO1-xFx. Previously, all high-temperature superconducting compounds, there so-called cuprates, were based on copper and consisted of copper oxide layers sandwiched between other substances. Much of the interest in those materials has arisen because the new compounds are very different from the cuprates and may help lead to a theory that is different from the conventional BCS theory of superconductivity, where electrons pair up in such a way that so coupled they can then move without resistance through the atomic lattice.
Among those new materials is the iron telluride, FeTe. This compound has the simplest crystal structure and exhibits antiferromagnetic ordering around 70 K and does not show superconductivity. It is now known that substitution of S for Te sites suppresses the antiferromagnetic order and induces superconductivity. Quite amazingly, this is not the most surprising property of those compounds. In a quite remarkable study performed by a group of Japanese physicists, it was shown that the iron-based compound FeTe0.8S0.2 exhibit superconductivity if soaked in red wine. They also performed a study of the effect with different types of wine and other alcoholic beverages, finding that a particular type of wine, 2009 Beajoulais from the French winery of Paul Beaudet, has the most profound effect.
A recent follow-up analysis, however, showed that subsequent and repeated applications of red wine and hard alcoholic beverages, such as cognac or vodka, can induce a new state in the study samples, dubbed the inverse superconductivity. The results, reported in the recent issue of Wine Spectator, clearly show steep increase of the samples’ resistivity after only five consequent applications of the liquid substance. As explained by the lead author of the study, John Piannicca, the results follow the simple model of the electron crowd. Interestingly enough, as reported by Dr. Piannicca, this model was developed by observing the change in the mean free path of a group of students visiting bars near the campus of his University.
Moreover, as was shown in a recent work of a group of scientists at the Siberian institute of Advanced Kevlar Engineering, it is also the quantity of alcohol that was responsible for the onset of inverse superconductivity. While this is also consistent with the already mentioned model of electron crowd, the samples obtained in the Siberian lab required much larger quantities of alcohol to achieve the same effect than those obtained in the American or Japanese labs, which could probably be explained by the specifics of liquid utilization. As was shown, the best effect was achieved with a brand of vodka “Imperia” commonly “recognized for it superbly smooth spirit and pure taste,” as advocated by its producers. It would be interesting to see how other brands would fare in such a study, which is on-going.
Last chance for a Higgs prediction December 13, 2011Posted by apetrov in Uncategorized.
Tags: cern experiments, fundamental particle, higgs boson, precision measurements
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In a couple of Hours we shall know more than we did before — the CERN experiments, ATLAS and CMS are about to announce their findings about the Higgs boson. In particular, its mass. Why is it interesting? The mass of Higgs, a fundamental particle in the Standard Model (SM) and a manifestation of a Higgs mechanism (discovered by at least five people besides Dr. Peter Higgs), cannot be predicted within the Standard Model. Similarly to masses of quarks and leptons, it comes out from the combination of unknown parameters of the SM.
Yet, Higgs boson would affect precision measurements — its effects could be seen via its quantum effects. So physicists would come out with pictures like the one that accompany this post (from which, incidentally, one can learn that the most likely value of the the Higgs mass given by the minimum of that plot, is already excluded by the direct measurements; what can one say — it’s a tough game).
In principle, Higgs boson mass (or mass of a Higgs-like particle) can be predicted in some models beyond the Standard Model. So, if you have any last-minute predictions, please through your hat in the ring! There are still about two hours before it becomes a “postdiction.” My long-time prediction (as of two years ago) was m_H = 125 +- 5 GeV. What is it based on? A principle that life is tough — the Nature is not always kind to us and we have to work hard to measure what we want to measure. It is true for most measurements/parameters that were studied before, including the most recent study of CP-violation in charm. How scientific is this prediction? I’d say almost as scientific as those based on anthropic principle.
Meanwhile, let’s see what CERN physicists have to say. Tune in here and let’s hope that we don’t overwhelm CERN’s servers.
2011 Physics Nobel Prize and related matters October 4, 2011Posted by apetrov in Uncategorized.
4 October 2011 is a day to remember. And I’m not talking about unveiling of the new iPhone, although it is also quite a remarkable event. Today, a 2011 Nobel Prize in Physics was awarded. As expected, in its annual failure, Thompson Reuters got it wrong in predicting 2011 Nobel Prize in Physics (to give them credit, they do put up the names of the right people, but always in the wrong year; this year they were predicting people from quantum entaglement). Anyways, this year’s Nobel Prize is totally deserving. The Nobel Prize in Physics 2011 was awarded jointly to Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess “for the discovery of the accelerating expansion of the Universe through observations of distant supernovae.”
This Nobel Prize is for the 1998 analysis of data from two collaborations, Supernova Cosmology Project (SCP), headed by Perlmutter, and High-z Supernova Search Team, headed by Schmidt and Reiss. The analyses centered on the the so-called Ia-type supernovae that have consistent peak brightness, which makes them “standard candles” of the Universe. This is an important property, which allows unambiguous measurement of distances (via the Hubble relation between the distance and the redshift) to the galaxy hosts of those supernovae. Using this data, they concluded that the Universe is going through the stage of accelerated expansion! This is a very interesting fact, especially taking into account the fact that the gravitational interaction is attractive!
This led to reevaluation of what we know about the Universe. It is widely accepted now that Dark Energy (i.e. something that permeates space and tends to increase the rate of expansion of the universe) accounts for about 74% of the total mass of the universe! Recalling that Dark Matter is responsible for about 22% of total mass gives us a fact that we really know almost next to nothing about the place we live in…
What is Dark Energy? This is a very good question. The simplest possibility is that it is the old good cosmological constant introduced by Einstein in the beginning of the last century. This leads to a particularly simple model of the Universe called Lambda-CDM model. Whether or not it is true remains to be seen. At any rate, Dark Energy/Dark Matter are currently one of the most exciting avenues for research in astrophysics (which is, of course, my subjective opinion!).
Meanwhile, the annual 2011 Ig Nobel Prizes were awarded on September 29, 2011. Among the most remarkable are
“PHYSICS PRIZE: Philippe Perrin, Cyril Perrot, Dominique Deviterne and Bruno Ragaru (of FRANCE), and Herman Kingma (of THE NETHERLANDS), for determining why discus throwers become dizzy, and why hammer throwers don’t.” As expected, for a work of this magnitude, the prize-winning research was published in the widely-read physics journal Acta Oto-laryngologica.
“MATHEMATICS PRIZE: Dorothy Martin of the USA (who predicted the world would end in 1954), Pat Robertson of the USA (who predicted the world would end in 1982), Elizabeth Clare Prophet of the USA (who predicted the world would end in 1990), Lee Jang Rim of KOREA (who predicted the world would end in 1992), Credonia Mwerinde of UGANDA (who predicted the world would end in 1999), and Harold Camping of the USA (who predicted the world would end on September 6, 1994 and later predicted that the world will end on October 21, 2011), for teaching the world to be careful when making mathematical assumptions and calculations.” This prize is quite timely, as the world once again is predicted to end 21 December 2012, although, frankly, they could have waited one year for this one.
Once again, the biology prize went for sexuality-related research. This time, among certain type of beetles and certain types of beer bottles (which should make a nice commercial of the type “Fosters is Australian for beer” (C)):
“BIOLOGY PRIZE: Darryl Gwynne (of CANADA and AUSTRALIA and the UK and the USA) and David Rentz (of AUSTRALIA and the USA) for discovering that a certain kind of beetle mates with a certain kind of Australian beer bottle.”
And my personal favorite is this year’s literature prize:
“LITERATURE PRIZE: John Perry of Stanford University, USA, for his Theory of Structured Procrastination, which says: To be a high achiever, always work on something important, using it as a way to avoid doing something that’s even more important.”
I would like to remind my readers that so far, there is only one “Grand Slam winner” — a person who got both Ig Nobel and a Nobel prizes: last year’s recipient of the Physics Nobel Prize Andre Geim.
Congratulations Dr. Yeghiyan! July 26, 2011Posted by apetrov in Near Physics, Particle Physics, Physics, Science, Uncategorized.
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Today my third graduate student at WSU, Gagik Yeghiyan, defended his Ph.D. thesis. Congratulations Dr. Yeghiyan! Good luck to you in your new life as an Assistant Professor at Grand Valley State University!
Update on the situation at Japan’s Fukushima nuclear plant March 15, 2011Posted by apetrov in Near Physics, Physics, Science, Uncategorized.
The situation at Japan’s Fukushima nuclear plant remains fluid, but it makes sense to do an update. It turns out that the situation is more challenging then I originally thought. To recreate what is happening (based mainly on TEPCo’s press releases and Japan’s Nuclear and Industrial Safety Agency (NISA) press releases), let us take a look at the Mark 1 BWR reactor (for a short description of physics of the nuclear power generation and schematics, please see my earlier post):
This picture was modified (by me) from the materials provided by Department of Energy’s Nuclear Regulatory Comission’s (NRC) website. It is Mark-1 BWR-type nuclear power reactor supplied by General Electric.
The Fukushima Daiichi plant operates six reactors, Units 1, 2 and 6 are supplied by General Electric (Unit 1 is the oldest, built in the 70′s — I’ve heard it was supposed to be decommissioned this Spring), while Units 3-5 are supplied by Toshiba and Hitachi. So, what is happening there?
As you already know, the magnitude 9.0 on Richter’s scale earthquake hit Japan. The reactors at the Fukushima plant were designed to withstand the 8.2 magnitude quake. Nevertheless, the structures held (note that the Richter scale is logarithmic, meaning that 9.0 earthquake releases 10 times energy than 8.0). Since Japan is located in seismically-active zone, there exist provisions on what to do in case of one, especially for the nuclear power stations.Reactors 1-3 were operational at the time of the earthquake, while reactors 4-6 were in a shutdown mode.
So, first and foremost, control rods (containing boron, neutron-absorbing material) were automatically inserted. According to TEPCo’s press release, this was done successfully at all three units that were in operation. There was an alarm on Unit 1 that one of the rods was not fully inserted. The alarm then went away. It is now believed that all control rods were fully inserted and chain reaction in fuel assemblies was stopped. Even after this, one must keep circulating water in order to continue cooling fuel assemblies due to the heat produced by decays of nuclear reaction products in the fuel rods. It needs to be done for several days.
It appears that over the course of three days reactor cooling systems kept failing, which resulted in increasing steam pressure in the reactor pressure vessel (see the picture above). In this case you really don’t want to keep the pressure rising, as it eventually would simply blow up the containment vessel and you’d get pretty much what happened in Chernobyl. So, the idea is to gradually release pressure by disposing the (slightly radioactive) steam through the vent line (see above picture). The steam is only slightly radioactive because one is using purified water, which does not get activated by the radiation from the fuel assemblies. This was done at all three units. You have to still keep cooling the core, which was done at Units 1 and 3 with injection of seawater into the Primary Containment Vessel and at Unit 2 with seawater injection into the Reactor Pressure Vessel. Injecting seawater is a desperate move, as it contains salt and other staff that can get activated. Which means that the reactor will be decommissioned regardless of whether there is a meltdown or not. Along with seawater, they injected boric acid to capture neutrons.
Now, if the cooling is ineffective (as it appears is at Fukushima) and you keep disposing steam, you lose the amount of water you have in your reactor (think of a boiling teapot). This leads to water levels in the reactor dropping to the point that the fuel assemblies get exposed to steam. This is what happened at Fukushima. This is bad, because this drops cooling efficiency and fuel rods start to heat up (recall the decays of radioactive decay products that are still going on). At some point, zirconium in the ziralloy (the alloy of zirconium and tin that makes up the fuel rod casings) starts react with water vapor. Here is the chemical reaction:
2 H20 + Zr = 2 H2 + Zr O2 + energy
which means that you start producing hydrogen (H2), some of which will escape into the reactor building. Most likely, escaped hydrogen exploded in units 1-3, blowing off the roofs of the reactor building hosting Unit 1, 2, and 3, like this:
This picture is done by the local TV station and posted on Wikipedia. According to the power station owners, the containment vessels are still intact, which is precisely what they are designed to do. Let’s hope that this is an accurate assessment.
Now, if there is a meltdown (fuel rods are damaged), some of the reaction products might get into the atmosphere (the troubling news is that the monitoring stations did detect small amounts of iodine nearby the reactor). The most immediate concern are radioactive Iodine (half-life of 8 days) and Cesium (half-life is 30 years). Iodine can accumulate in human’s thyroid gland – so the first line of defense is to saturate the gland with non-radioactive Iodine. This is why the population around the station is given iodine tablets as a precaution. The detected amounts of iodine are not of a concern for the US West Coast (too far).
In the case of a serious meltdown, the melted fuel will likely remain in the reactor containment below the rector pressure vessel. This would be bad, but still nowhere near Chernobyl’s explosion. BTW, I was on a school trip in Kiev when the Chernobyl power station blew up. I had to bury my shoes because the radioactivity levels on them were too high (dust)…
To add to the problem, rector unit 4 (which was not operational at the time of an earthquake) developed problems of its own. In particular, it appears that the personnel missed that the water level in the spent fuel pool came down. This exposed spent fuel rods that contain more long-lived radioactive isotopes. You want to keep spent fuel rods in the water to cool them, as the decays still produce heat. In this case, usual convection cooling (warm water is rising and is replaced by cooler water) is sufficient to keep them cool. That is, if there is water! There was report of a fire at the spent fuel pond. This might indicated that the water level in the pool went down and spent fuel caught on fire. This might be bad, as this would release radioactive material in the air. Japanese scientists monitor the situation.
I’ll try to keep you posted as well.
What is happening at Japan’s Fukushima Daiichi power plant? March 12, 2011Posted by apetrov in Near Physics, Physics, Science, Uncategorized.
This is a good question to ask — especially amid speculations about “possible Chernobyl-like nuclear meltdown” and pictures of explosions at the plant. Knowing a little bit of physics (and reading press-releases from TEPCo — Tokyo Electric Power Company), one can make some initial analysis. Clearly, a complete picture will follow in the near future.
So, first of all, what is the problem? To understand this, let me note that nuclear reactors at the Fukushima Daiichi plant are of the Boiling Water Reactor (BWR) type — quite different in design from Three Mile Island’s PWR-type reactor and Chernobyl’s RBMK reactor. In order to see the logic of what Japanese engineers are doing, it is useful to see how the BWR reactor works.
Here is the scheme of BWR-type reactor, taken from the Wikipedia page on BWR. The physics here is very simple. Fission reaction in the uranium fuel assemblies (2) heat water (blue stuff, 7), which turns into steam (red stuff, 6) in the reactor vessel (1). The steam exits the vessel and spins the turbine (8 and 9) that generates electricity. That steam is cooled down and returned into the reactor vessel (as water) and the process begins again.
Simply speaking, fission reaction happens when a slow (thermal) neutron is absorbed by a uranium (U-235) nucleus, which then splits into several (two) lighter daughter nuclei, and several fast neutrons (about 3), releasing energy that is converted into heat. In order to have sustained nuclear reaction one needs to slow down those produced neutrons so that they could be absorbed by other U235 nuclei to initiate chain fission reaction. Different reactor designs use different moderators to do that: water (BWR, PWR), graphite (RBMK), etc.
This simple excursion into nuclear physics tells us that the rate of power generation can regulated by controlling the flux of thermal neutrons. This is indeed what is done by the control rods (3) that are usually made of a material (boron) that absorbs neutrons.
What happens in case of an earthquake? Well, the automatic control systems first and foremost would kill the sustained fission reaction that is going in the fuel elements. This was done at the Fukushima plant immediately by inserting the control rods (notice that the control rods are inserted from below). So, what’s the problem then? Why is the water vapor’s pressure rising?
The problem is that during the fission reaction one also produces a lot of short-lived nuclear isotopes. Normally, if you would like to shut down a reactor (say, to refuel), you need to allow for some time (several days) for those isotopes to decay. During that time, water is still being circulated through the reactor core in order to take away the heat produced in the decays of those short-lived isotopes. This is done via pumps that are operated via electricity from (a) power grid or (b) diesel generators or (c) batteries. After the earthquake, the grid was knocked out and the diesel generators got damaged. The pumps are now running on the batteries and the water vapor pressure inside the reactor vessel is rising — by the way, the normal operating pressure there is about 75 atmospheres!!! TEPCo reports that the pressure there rose twice that, so the plant operators decided to release steam from the vessel. Now, to cool down the reactor (until those short-lived isotopes decay) they decided to flood the containment vessel with sea water.
So, as you see, the Chernobyl-type of explosion is highly unlikely at the Fukushima plant. I think the reactor will cool down in a couple of days. BTW, it appears that the reported explosion happened at the pumping system…
The only troubling news is that the monitoring stations appear to detect small amounts of iodine and cesium isotopes (to quote TEPCo’s press release “The value of radioactive material (iodine, etc) is increasing according to the monitoring car at the site (outside of the site). One of the monitoring posts is also indicating higher than normal level.”). Those isotopes are normally produced in the nuclear fuel rods. This might indicate that one or more rods are damaged.
Update (3/14/2011): It appears that water circulation systems in reactors 1 and 3 of the power station failed on 3/12-13. The reactor containment is now cooled by sea water (with added boric acid to further capture neutrons). Sea water is not an ideal coolant — purified fresh water is — sea water contains salt and other things that can become radioactive in the core of a reactor. Thus, the spent water will most likely be transferred to the spent fuel pools (place on the power station’s campus where spent fuel rods spend some time before being transferred to the permanent storage facility). It also appears that there were two hydrogen explosions in Units 1 and, recently, 3. Where did the hydrogen come from? It most likely came from the chemical reaction on the zircalloy’s casings of the fuel assemblies. Zircalloy, an alloy of zirconium, tin and sometimes other things, contains zirconium. That zirconium reacts with oxygen in water and releases hydrogen. It is, however, believed that in both cases the containment vessels held up. Those containment vessels did not exist at the Chernobyl’s power station.
Update (3/15/2011): I decided to put updates in the separate post.