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And the 2013 Nobel Prize in Physics goes to… October 8, 2013

Posted by apetrov in Particle Physics, Physics, Science, Uncategorized.
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Today the 2013 Nobel Prize in Physics was awarded to François Englert (Université Libre de Bruxelles, Belgium) and Peter W. Higgs (University of Edinburgh, UK). The official citation is “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider.” What did they do almost 50 years ago that warranted their Nobel Prize today? Let’s see (for the simple analogy see my previous post from yesterday).

The overriding principle of building a theory of elementary particle interactions is symmetry. A theory must be invariant under a set of space-time symmetries (such as rotations, boosts), as well as under a set of “internal” symmetries, the ones that are specified by the model builder. This set of symmetries restrict how particles interact and also puts constraints on the properties of those particles. In particular, the symmetries of the Standard Model of particle physics require that W and Z bosons (particles that mediate weak interactions) must be massless. Since we know they must be massive, a new mechanism that generates those masses (i.e. breaks the symmetry) must be put in place. Note that a theory with massive W’s and Z that are “put in theory by hand” is not consistent (renormalizable).

The appropriate mechanism was known in the beginning of the 1960′s. It goes under the name of spontaneous symmetry breaking. In one variant it involves a spin-zero field whose self-interactions are governed by a “Mexican hat”-shaped potential

MexicanHatIt is postulated that the theory ends up in vacuum state that “breaks” the original symmetries of the model (like the valley in the picture above). One problem with this idea was that a theorem by G. Goldstone required a presence of a massless spin-zero particle, which was not experimentally observed. It was Robert Brout, François Englert, Peter Higgs, and somewhat later (but independently), by Gerry Guralnik, C. R. Hagen, Tom Kibble who showed a loophole in a version of Goldstone theorem when it is applied to relativistic gauge theories. In the proposed mechanism massless spin-zero particle does not show up, but gets “eaten” by the massless vector bosons giving them a mass. Precisely as needed for the electroweak bosons W and Z to get their masses!  A massive particle, the Higgs boson, is a consequence of this (BEH or Englert-Brout-Higgs-Guralnik-Hagen-Kibble) mechanism and represents excitation of the Higgs field about its new vacuum state.

It took about 50 years to experimentally confirm the idea by finding the Higgs boson! Tracking the historic timeline, the first paper by Englert and Brout, was sent to Physical Review Letter on 26 June 1964 and published in the issue dated 31 August 1964. Higgs’ paper, received by Physical Review Letters on 31 August 1964 (on the same day Englert and Brout’s paper was published)  and published in the issue dated 19 October 1964. What is interesting is that the original version of the paper by Higgs, submitted to the journal Physics Letters, was rejected (on the grounds that it did not warrant rapid publication). Higgs revised the paper and resubmitted it to Physical Review Letters, where it was published after another revision in which he actually pointed out the possibility of the spin-zero particle — the one that now carries his name. CERN’s announcement of Higgs boson discovery came 4 July 2012.

Is this the last Nobel Prize for particle physics? I think not. There are still many unanswered questions — and the answers would warrant Nobel Prizes. Theory of strong interactions (which ARE responsible for masses of all luminous matter in the Universe) is not yet solved analytically, the nature of dark matter is not known, the picture of how the Universe came to have baryon asymmetry is not cleared. Is there new physics beyond what we already know? And if yes, what is it? These are very interesting questions that need answers.

Higgs mechanism for electrical engineers October 7, 2013

Posted by apetrov in Particle Physics, Physics, Science, Uncategorized.
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Since the Higgs boson’s discovery a little over a year ago at CERN I have been getting a lot of questions from my friends to explain to them “what this Higgs thing does.” So I often tried to use the crowd analogy that is ascribed to Prof. David Miller, to describe the Higgs (or Englert-Brout-Higgs-Guralnik-Hagen-Kibble) mechanism. Interestingly enough, it did not work well for most of my old school friends, majority of whom happen to pursue careers in engineering. So I thought that perhaps another analogy would be more appropriate. Here it is, please let me know what you think!

Imagine Higgs field as represented by some quantity of slightly magnetized iron filings, i.e. small pieces of iron that look like powder, spread over a table or other surface to represent Higgs field that permeates the Universe. Iron filings are common not only as dirt in metal shops, they are often used in school experiments and other science demonstrations to visualize the magnetic field.  It is important for them to be slightly magnetized, as this represents self-interaction of the Higgs field. Here they are pictured in a somewhat cartoonish way:

ImageHow can Higgs field generate mass? Moreover, how can one field generate different masses for different types of particles? Let us first make an analogue of fermion mass generation. If we take a small magnet and put it in the filings, the magnet would pick up a bunch of filings, right? How much would it pick up? It depends on the “strength” of that magnet. It could be a little:

Image

…or it could be a lot, depending on what kind of magnet we use — or how strong it is:

Image

If we neglect the masses of our magnets, as we assumed they are small, the mass of the picked up mess with the magnets inside is totally determined by the mass of the picked filings, which in turn is determined by the interaction strength between the magnets and the filings. This is precisely how fermion mass generation works in the Standard Model!

In the Standard Model the massless fermions are coupled to the Higgs field via so-called Yukawa interactions, whose strength is parametrized by a number, the Yukawa coupling constant. For different fermion types (or flavors) the couplings would be numerically different, ranging from one to one part in a million. As a result of interaction with the Higgs field (NOT the boson!) in the form of its vacuum expectation value, all fermions acquire masses (ok, maybe not all — neutrinos could be different). And those masses would depend on the strength of the interaction of fermions with Higgs field, just like in our example with magnets and iron filings!

Now imagine that we simply kicked the table! No magnets. The filings would clamp together to form lumps of filings. Each lump would have a mass, which would only depend on how strong the filings attract to each other (remember that they are slightly magnetized?). If we don’t know how strong they are magnetized, we cannot tell how massive each lamp will be, so we would have to measure their masses.

ImageThis gives a good analogy of the fact that Higgs boson is an excitation of the Higgs field (the fact that was pointed out by Higgs), and why we cannot predict its mass from the first principles, but need a direct observation at the LHC!

Notice that this picture (so far) does not provide direct analogy to how gauge bosons (W’s and Z bosons) receive their masses. W’s and Z are also initially massless because of the gauge (internal) symmetries required by the construction of the Standard Model. We did know their mass from earlier CERN and SLAC experiments — and even prior to those, we knew that W’s were massive from the fact that weak interactions are of the finite range.

To extend our analogy, let’s clean up the mess — literally! Let’s throw a bucket of water over the table covered with those iron filings and see what happens. Streams of water would pick up iron filings and flow from the table. Assuming that that water’s mass is negligible, the total mass of those water streams (aka dirty water) would be completely determined by the mass of picked iron filings, just like masses of W’s and Z are determined by the Higgs field.

This explanation seemed to work better for my engineering friends! What do you think?

Inverse superconductivity in iron telluride April 1, 2012

Posted 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.

LHCb reports observation of CP-violation in charm. Welcome New Physics? Or not? November 14, 2011

Posted by apetrov in Particle Physics, Physics, Science.
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One of CERN collaborations, the LHCb, has reported observation of direct CP-violation in the decays of charmed mesons at the Hadronic Collider Physics Symposium 2011 (HCP 2011) in Paris today. This is a fantastic news! While I am not at HCP 2011, kind folks at LHCb let me know about this fantastic measurement — since charm physics is my specialty.

So, what are we talking about here?

First things first. CP (or Charge Parity) is a set of (discrete) transformations performed on a theory’s Lagrangian — a function that describes what particles we have in a theory and how they interact. If your Lagrangian is symmetric  under this transformation, then particles and antiparticles — matter and antimatter — have the same properties. If not — interactions of matter particles are different from interactions of antimatter particles.  This possible difference is a crucial property of a theory because, according to three Sakharov criteria, the Universe could evolve in what we see around us only if matter and antimatter have different interaction properties. Otherwise, at best, we’d have big chanks of antimatter floating around — or at worst would not not exist at all.

This is why many huge experiments built to study CP violation. Big national labs’ flagship experiments were designed to search and study CP-violation (BaBar at SLAC, Belle at KEK, LHCb at CERN), with hopes to see glimpses of New Physics that could explain matter-antimatter asymmetry in the Universe. This new result from LHCb can in principle provide one.

So, what did LHCb see? The reported analysis looks at the difference of a difference — i.e. a difference of CP-violating asymmetries in kaons and pions. The CP-violating asymmetry is defined as the difference between decay widths (roughly speaking, decay probabilities) of a neutral D-meson to decay into a final state, say positive K-meson and a negative K-meson and the same quantity for the D-anti-particle to decay to the same final state. This quantity is also defined for the final state of two pions — and it is CP-violating!

The structure of this CP-violating asymmetry, aCP, is not that simple. Because D0 is a neutral particle it can, in principle, mix with its antiparticle (see here) — and this antiparticle can also decay into the same final state! This process can be also CP-violating (this type of CP-violation is called indirect CP-violation). So the result would depend on both types of CP-violation!

Moreover, experimentally, the asymmetries like this are not easy to measure — there are experimental systematics associated with D-production asymmetries, difference of interactions of positive and negative kaons with matter, etc. For this reason, experimentalists at LHCb decided to report the difference of CP-violating asymmetries, in which many of those effects, like productions asymmetries, would cancel. So, here is the result:

ΔaCP = -0.82 ± 0.21 (stat) ±0.11 (syst) %

In other words, this quantity is 3.5 sigmas away from being zero. The first question that one should ask is whether this quantity is consistent with previous measurements. The biggest question, however, is whether this quantity is consistent with Standard Model expectations.

There is a bunch of previous measurements available for aCP (KK) and aCP (ππ) separately. The thing is that

aCP (KK) = – aCP (ππ)

or approximately so. So by subtracting those quantities we not only subtract the experimental uncertainties, but also enhance the signal! However, looking at the table on page 6 of the talk, one can immediately realize that this measurement is at least consistent with the previous ones.

Is it a sign of something beyond the Standard Model? This one is hard to answer. I usually put an upper bound on the SM value (that is, absolute value) of asymmetries like aCP (KK) at 0.1% — which would make ΔaCP to be about 0.2%. Is it consistent with LHCb findings? Maybe. The size of this asymmetry is notoriously difficult to estimate due to hadronic effects. Maybe it is a sign of New Physics — this could be an exciting conclusion, as we have never seen CP-violation in up-quark sector.

It is interesting that the first “big” result from LHC comes in the realm of charm physics, not Higgs searches. Moreover, all “big” results in the last decade were from the experiments searching for New Physics indirectly, in the “intensity frontier” (this is lingo of US Department of Energy) — with most of them coming from charm physics. Maybe at the very least LHC-b should be renamed as LHC-c?

We have a job… or two! October 21, 2011

Posted by apetrov in Particle Physics, Physics, Science.
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Depending on how the budget for the new year looks like, we (the high energy particle theory group at WSU) will have two new postdoc positions. Please apply, if you are interested! Here is the ad.

—————————————————————————-

The high energy theory group at Wayne State University ( http://www.physics.wayne.edu/heptheory ) anticipates making TWO postdoctoral research appointments to start September 1, 2012, subject to budgetary approval. The initial appointments will be for one year, and may be extended for one or more years depending on the performance and availability of funding.

The group consists of faculty Gil Paz and Alexy A. Petrov, as well as a postdoc and several students. Research interests of the group include particle phenomenology, physics beyond the Standard Model, effective field theori es, heavy quark physics, CP violation, Dark Matter phenomenology and particle astrophysics. The group has close ties to the nuclear theory group of Sean Gavin and Abhijit Majumder. The WSU Department of Physics and Astronomy offers a unique opportunity of close interaction with experimental high energy particle and nuclear physics groups.

Applications including CV, a list of publications, a brief statement of research interests and three letters of recommendation should be submitted to Academic Jobs Online at http://academicjobsonline.org/ajo/jobs/1128

or by mail to

Prof. Gil Paz
Department of Physics and Astronomy
Wayne State University
Detroit, Michigan, 48201

or

Prof. Alexey A. Petrov
Department of Physics and Astronomy
Wayne State University
Detroit, Michigan, 48201

or electronically to gilpaz@wayne.edu or apetrov@wayne.edu. The deadline for application is January 15, 2012. Later applications will be considered until the positions are filled. Informal inquiries are welcomed.

Wayne State University is an affirmative action/equal opportunity employer. Women and members of minority groups are encouraged to apply.

Why do physicists go to Aspen? September 1, 2011

Posted by apetrov in Near Physics, Particle Physics, Physics, Science.
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While the most obvious answer to this question is “to ski”, it is, nonetheless, not the correct one. Yes, skiing is great here in the winter (and hiking is great in the summer), but most of the time physicists come here to work. The reason is Aspen Center for Physics. I write “here” because I’m currently participating in one of the programs organized by the Center (the program is called “Flavor Origins” — it brought together theorists working on the problems of neutrinos, heavy and light quarks, CP-violation, etc.). The Center, which exists here since 1961, organizes workshops and conferences. But the main reason that theorists (and occasional experimentalists) come here is to talk to other theorists. In short, it is as if you are visiting a huge theory group — you can work individually or with your colleagues, but you can always knock on an office door and bounce your ideas off someone else visiting the Center, etc. It is great to have such a concentration of theorists of different trades. And it leads to breakthroughs and simply good papers. As it is said on the Center’s website:

“Many seminal papers have been written in Aspen, which has grown to be the largest center for theoretical physics in the world during its summer sessions. Among many other subjects, the theories of superstrings, chaos, evolution of stars and galaxies, and high temperature superconductivity have all made large strides in recent Aspen seasons.”

There is almost always someone with an expertise in a subject that you have a question about. And that makes this Center great. And, of course, hiking and skiing is also good. The only “downside” (note the quotes) is that you can meet a real bear (even at the Center) or other wildlife. Today a snake came to check out a lecture on conformal field theories…

P.S. Also check out my blog on Quantum Diaries

Congratulations Dr. Yeghiyan! July 26, 2011

Posted 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!

Site selected for the Italian Super-B factory, the Cabibbo Lab June 13, 2011

Posted by apetrov in Particle Physics, Physics, Science.
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As I blogged some time ago, Italian government decided to fund a new accelerator for precision studies of New Physics in decays of heavy-flavored mesons, the so-called SuperB factory, a high-intensity B-factory, which is designed to look for glimpses of New Physics in rare decays of B- and D-mesons (for professional description of the physics case, see here; for Conceptual design Report (CDR) see here).

Last week a decision was made for a location of the site of the new machine. It will be built on campus of the University of Rome ‘Tor Vergata’. Here is the picture of the proposed site (shamelessly taken from the talk of Roberto Petronzio, President of the Italian National Institute for Nuclear Physics at XVII SuperB Workshop and Kick Off Meeting – La Biodola (Isola d’Elba) Italy):

 

The (“green”) site is located reasonably close (4.5 km) to another well-known Italian National Lab in Frascati, Laboratori Nazionali di Frascati (LNF). The new lab will be a CERN-like consortium. The name for the lab was proposed: Cabibbo Lab, after the great Italian physicist Nicola Cabibbo whose name is associated with some of the most important objects in flavor physics.
The new lab will bring lots of talent from all over the world and, besides experiments in high energy physics, will be used as a light source for other physics experiments. It is great that even at the time when finances are tight, European governments realize that fundamental physics is important for the future of their countries. These are exciting times for the European physics!

Update on the situation at Japan’s Fukushima nuclear plant March 15, 2011

Posted by apetrov in Near Physics, Physics, Science, Uncategorized.
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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, 2011

Posted by apetrov in Near Physics, Physics, Science, Uncategorized.
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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.

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