Sunday, January 01, 2012
Saturday, March 12, 2011
Fukushima: A nuclear physics primer
As I am sure everyone has heard by now, there has been an enormous earthquake off the coast of North-East Japan, which (together with the tsunami caused by the quake) has lead to widespread distruction and loss of life. At the moment, however, there is concern over trouble at two nuclear power station. The amount of information coming out through the media is quite limited, and of course isn't easy to decrypt what experts are saying if you don't know what the terminology means.
Caveat: Although I am a physicist, I am the wrong type of physicist to be paricularly expert on nuclear safety. I am writing this simply so this to try to help non-physicists decipher what is going on at the moment in Fukushima.
The first thing you need to know is what a nuclear power plant actually does, and what radiation is. All atoms are composed of a cloud of electrons which surround a nucleous composed of neutrons and protons. The stability of the nucleus is what determines radioactivity, and is determined by the number of protons and neutrons contained in the nucleus. The interaction between these particles determine how tightly they are bound together.
The above diagram shows the relative stability of different atomic elements, running from lighter to heavier elements. As you can see, iron (Fe) has the highest binding energy (meaning that its nucleons are the most tightly bound). Nuclear reactions in which larger nuclei split apart are known as fission reactions (moving towards iron from the right), while reactions where smaller nuclei are combined to form a larger nucleus are called fusion reactions (moving towards iron from the left). All commercial reactors are at present fission reactors. These work by capturing the energy released by large nuclei splitting, usually using uranium as fuel. The reactors at Fukushima are boiling water reactors, which capture this energy by using it to boil water to produce steam used to drive a turbine, which in turn generates an electric current (producing electricity).
Different nuclei come apart in different ways, depending on their composition. Which element a nucleus is is governed by the total number of protons only, and this determines it's chemical properties. The nuclei of the same element can have different numbers of neutrons, and these different varieties are known as isotopes. The chart I showed above is of the most stable isotope for each element. How a particular radioactive element comes apart depends not on only on which element it is, but rather which isotope of that element. Below is a diagram showing how different nuclei decay.
Here the important unexplained types of decay are
- β- corresponding to the emission of an electron (this is a neutron changing into a proton in the nucleus),
- β+ corresponding to the emission of an positron, which is a positively charged version of an electron (this is a proton changing into a neutron in the nucleus)
- α which corresponds to the emission of a group of helium nucleus (2 protons and 2 neutrons tightly bound together), and happens because of the huge spike in binding energy for helium nuclei which can be seen in the first diagram.
- Fission, where the nucleus splits into several large parts
As I mentioned earlier, both Fukushima I and II user boiling water reactors. They use what is called 'light water' which simply means they use purified water to cool the fission reactions. The word 'light' is used to distinguish them from 'heavy water' reactors which use water where the hydrogen is replaced by a heavier isotope called deuterium (which can be used to regulate reactions in some reactor designs).
At Fukushima I, it appears that when the earthquake struck some problem occured with the cooling system for one of the reactors failed. The nuclear reactions produce a lot of heat, and needs to be kept cool by adding water. Unfortunately, even though you can slow down the fission reaction, some byproducts of the decay of uranium are themselves radioactive, and (according to one expert who just appeared on the BBC) can contribute as much as 10% of the energy produced in the reactor, which is essentially impossible to stop. Without the cooling system working, there is a build-up of steam. It is also possible for the fuel to react with the water to produce hydrogen. These gases have to be vented in order for the pressure inside the reactor to be kept within safe limits (so that the reactor doesn't come apart). As long as the reactor is intack, the amount of radiation released should be low. Hydrogen is not radioactive, even if it absorbs a neutron (producing deuterium), the nucleus is stable. Tritium would be bad, but I can't see anyway for that to have been produced. However, Nitrogen 16 will also have been produced, which is extremely radioactive. This may sound bad, but it is infact so radioactive that it decays very quickly, meaning that it doesn't pose a danger any distance from the plant. The half-life is 7 seconds, meaning that 7 seconds after production half of the radioactive nitrogen has converted to safe oxygen. So after after 11 minutes there is on millionth the radiation. Give that wind speeds are relatively slow, the nitrogen will decay before it can cause much trouble (except perhaps for people actually in the plant). This is why we are hearing about high radiation levels in the control room, but little additional radiation right outside the perimeter.
There has also been an explosion at the plant which is causing significant, since it is not entirely clear what has happened. It sounds like this was probably caused by the hydrogen igniting, which has damaged the building, but it seems like the reactor core is still intact. If this is true, and they can keep the reactor cool enough that the fuel does not all melt (a meltdown, which makes it much harder to cool, and which would likely result in the release of much more nasty isotopes) then the amount of radiation released shouldn't pose to much of a health risk. Fortunately the wind seems to be blowing out to see, which also improves the situation.
The latest I have heard is that the authorities are considering using sea water to cool the reactor core, which they will be reluctant to do since it will make the reactor unusable in future, but which should cool the reactor core. Apparently the incident is currently rated as 4 ("Accident with local consequences") on the International Nuclear Event Scale, which is still one level below the Three Mile Island incident in the US, and 3 levels below the maximum level which corresponds to a Chernobyl-like event.
If anyone has any further information they would like to contribute, or any corrections to what I have written above (as I say, I am the wrong type of physicist), then please let me know, wither in the comments or by email.
Wednesday, November 24, 2010
Theoretical Physics Q and A site progress
The proposed Theoretical Physics stack exchange site has finished its definition phase and has now entered a commitment phase. What this means is that we need as many physicists as possible to agree (by signing up here) to participate in the beta and hopefully ask or answer 10 questions in the first 3 months. This shouldn't be a huge time commitment, but it is really important to get as many research physicists as possible early on. The reason for this is that the democratic nature of Stack Exchange sites means that the direction of the site is set by the participants. Obviously there are a lot more people interested in some aspect of theoretical physics than there are people who have some level of expertise in the field, and so it is important to get as many TP grad students, postdocs and faculty involved at an early stage to insure that the site becomes a TP version of MathOverflow and CSTheory, rather than becoming a site for people to post their pet crackpot theories or discuss pop science topics.
To help convince you to sign up, below are a list of the top 5 examples of both on-topic and off-topic questions chosen during the definition phase.
- Are there entangled states which do not violate any Bell inequality?
- Are XXZ spinchains with uniform couplings exactly solvable?
- Has [specific toy model] been studied in the literature?
- What are the justifying foundations of statistical mechanics without appealing to the ergodic hypothesis?
- Within Twistor String Theory (ala Within), what is currently seen as the significance of the superconformal algebra realized on supertwistor space?
- Why is the sky blue during the day, red during sunrise and sunset and black at night?
- I have found an error in general relativity/quantum mechanics/the second law of thermodynamics. Can someone help me work out the maths?
- Is Dr. Quantum's Double Slit Experiment video scientifically accurate?
- Why do hot things glow?
- What is the current thinking about Lisi's "Exceptionally Simple Theory of Everything"?
Saturday, November 13, 2010
A theoretical physics Q and A site
I've become totally enamored with the CSTheory stack exchange site. It's proving very addictive, but in exchange for the time I spend on the site I am finding that I am learning a lot of new things, and a few new tricks.
I had hoped that the Physics stack exchange site might become a similar resource for physicists (and I still very much self-identify as a physicist). Unfortunately this has turned out not to be the case. There was never a policy dictating that questions should be research level (as there was both for CSTheory and MathOverflow), and this has led to the majority of questions either being basic undergrad type questions or pop-sci question. As a result, there are also very many poor answers which contain common misconceptions, but which get up-voted. Consequently, I don't think the site is likely to be an attractive propositions for active researchers.However, I don't want to just complain and not offer a solution, so I have set up a proposal for a Theoretical Physics stack exchange site. The aim of the site would be to provide a question and answers site aimed at research level questions only, akin to the CSTheory stack exchange site and MathOverflow. Personally I find both sites to be phenomenal resources, and I think it's high time we physicists had something similar.
I proposed the site as being for theoretical and mathematical physics and not physics in general only because I think that spanning both theory and experiment might make the scope of the site too broad, making it harder to get good answers to specific questions in any one area. Experimentalists with theory questions (or better yet, theory answers) are of course encouraged to participate.
The process of defining the site is entirely democratic, so you don't need to worry about whether you trust my judgement or not. If the site reaches beta temporary moderators are elected by the community. This is also why it is important to have a solid group of physicists early on, to set the level and tone of the site.
So, if you are a physicist and this sounds like something that might interest you, why not visit and help shape the scope and level of the site by submitting sample questions or voting on whether you think the questions submitted by others would be consistent with such a site?
Monday, August 23, 2010
CS Theory Q&A site in beta
The mathoverflow-like site for theoretical computer science which I blogged about earlier is about to go into public beta. The site aims to allow users to ask and answer research level questions in TCS, and I must say that my experience with the private beta has been excellent. I have received excellent answers to the 2 questions I asked, and have been able to give reasonable answers to 8 questions so far. It really is an excellent resource. Anyway, if you have an interest in TCS it is well worth participating in the site. It needs as many users as possible to make it a success.
To join the public beta, simply visit http://cstheory.stackexchange.com
Monday, August 09, 2010
P vs NP finally resolved?
A new paper has appeared from Vinay Deolalikar in HP Labs claiming to prove P is not equal to NP (in fact P≠NP is the title). Normally I ignore the constant stream of papers on the subject, but this work looks like a serious attempt and is being taken seriously by a number of people I respect, so it has jumped straight to the top of my reading list.
For more details see posts by the Pontiff, Greg Baker, and R J Lipton.
I really hadn't expected to see this proved within my life time, so I am of course skeptical. Still, even if there is an error, perhaps it opens up a new approach.
Friday, July 02, 2010
If you're a physicist, mathematician or computer scientist and haven't already seen MathOverflow then stop reading this post and go check it out. It is one of the best resources for research level math I have ever seen. It is absolutely shocking the speed and quality of responses to potentially very technical questions.
So, if you are still reading, I will presume you actually know the site. So why do I bring it up? Well, there is currently a proposal for a TheoryOverflow, aimed at Theoretical Computer Science, on the StackExchange site. Basically the way this works is that the people that produce the engine that powers the MathOverflow and StackOverflow websites are looking to produce similar sites in different areas. First an idea is proposed and fleshed out (Stage 1), once this is done it moves to a commitment stage where potential users are asked to commit to using the new site initially if it were made (Stage 2), either posting questions or answers. Finally it moves to a Beta (Stage 3). TheoryOverflow is currently in stage 2, and so needs support.
Rep, if you don't know, is basically reputation, and so people who have contributed to other such sites carry more weight than users who haven't. Different users have different amounts of rep depending on their level of contribution to StackOverflow and sister sites. Unfortunately more users seem to be necessary than usual, because most potential users committing to it seem to either be new or are coming from MathOverflow, and the StackExchange site doesn't care about your MathOverflow rep (though it does about StackOverflow rep).
So basically, TheoryOverflow needs more support. If it turns out even a tiny fraction as useful as MathOverflow, then it will be a fantastic resource. So please help make it happen. You can sign up here.