Saturday, February 23, 2013

Whats So Scary About Nuclear Power?

Warning Diehard movie spoilers ahead:

I watched Diehard last weekend and it was entertaining; huzzah explosions and all that. I only had one issue with the movie and that was the complete misunderstanding of nuclear energy. My instinctual anger at horrible physics was related to one scene in the movie, it wasn't even 30 seconds long, yet there was still soo much misinformation.

The plot of the die hard movie revolved the idea that Chernobyl was a man made accident (it was) which was brought about by illegal weapons merchants getting greedy selling reactor byproducts or fuels (it wasn't). There was a warehouse containing weapons grade uranium 235 which, would simply not happen as a reactor byproduct or a fuel, and even this didn't set off my bad science alert. I'm more than willing to accept some fudging of accuracy for the merits of storytelling. I got pissed off when they entered the warehouse containing said uranium and determined the radiation had been pooling for decades, and they had a spray to neutralize it. Those two statements actually angered me.

Nuclear fuels and waste products are relatively safe. You don't want to put them in your pillow and sleep on them every night but handling them is not some sort of life threatening risk. Diehard even accurately depicted this, however they had to clean up "pooled" radiation. Radiation isn't a gas or a liquid. It simply can't, ever or under any circumstances pool. Radio-nuclides don't become more dangerous over time. Radiation radiates, until it hits something and is absorbed.
Yep not really great against radiation

 It's pretty simple, and yet someone felt they needed to address this issue write a script, develop a technology and CGI the cleaning of the radiation for something that simply wasn't even a major plot point.  They even said the fuel is stable and there are no major short term exposure risks. They took off their protective suits. Maybe I'm perseverating, but to me, Hollywood acts like it's some sort of intellectual mecca, yet no one on a big budget movie said hey this is dumb. The fact that they filmed the clip, which again wasn't a critical plot point at all, and then dumped it on the public is more indicative of a fundamental misunderstanding of nuclear physics in the general populace.

From the above image you can see the four main types of nuclear radiation and the corresponding materials required to stop them.

Alpha Radiation: Alpha radiation is the same thing as a helium nucleus its large and that's why it can be stopped by paper. Not really a threat to anyone.

Neutron Radiation: Neutron radiation is essentially only produced during a nuclear reaction, its barrier requirements are very high and it essentially disappears when a reactor is shut down.

Beta Radiation: Beta radiation comes in two forms beta + and beta-. This is simply an electron or the oppositely charged component called a positron. It is readily stopped by stuff that's thicker than a piece of paper and beta emitting material is usually trapped in the pipes circulating around a reactor.

Gamma radiation: Gamma radiation is essentially light, its light that we can't see but its just light. It's emitted from particles and in the reactor. overwhelmingly gamma radiation is what people are referring to as radiation in a nuclear plant.

From this synopsis you should be able to get that the only two types of radiation we worry about is beta and gamma, the others only exist when the reactor is running or are essentially nonthreatening. Beta emitters are produced as some of the fuel degrades or byproducts escape into the water. In a nuclear plant beta radiation is safely contained in pipes. The only time it becomes a threat is when work has to be completed, assuming proper procedures are followed (they are or you get fired) the exposure and risk is minimal. The only radiation truly worth tracking is gamma exposure.

Both beta and gamma radiation are produced from radio-nuclides breaking down into more stable component's. nuclides that break down are called radioactive. If you just said aha that's why radiation levels couldn't have been worse after fifty years you get a gold star. As more of a nuclide breaks down there is less remaining, so radiation levels drop. The way we measure the activity of decomposing material is through half lives. A half life simply put is the period of time required for you to have half of the starting radioactive material.

Uranium is all natural so it must be safe :-P

So how dangerous is the material, well that depends on the half life and the amount of time elapsed. Because Uranium 235 has a half life of about 703 million years its not all that dangerous. The statement "A candle that burns twice as bright burns half as long" is a solid analogy. Radio-nuclides that have really short half lives are dangerous but not for a very long time. There are some radioactive materials which decay into more radioactive materials before becoming stable but these are uncommon.

So what makes an element radioactive? Well in short it's the number of neutrons an element has (protons and electrons define what the element is).  The total number of protons and neutrons determines the atomic mass of an element. The different atomic masses are what we call isotopes which is is the difference when talking about Uranium 235 vs. say the more common Uranium 238.  The difference between these two isotopes is 3 neutrons and that difference makes uranium 235 more prone to undergo fission in a nuclear reaction. We can say that Uranium 235 is more fissile than Uranium 238.

Nuclear  Energy: think of the amount of nuclear energy derived being equal to the difference between your staring element and final element

So how do we get Uranium 235, well hydrogen and other elements undergo fusion in stars but that essentially only forms iron. When stars supernova the released energy forces fusion to higher elements.  Then as time goes on the unstable elements decay leaving the more stable ones behind. Uranium 235 may have the impressive half life of 703.8 million years but Uranium 238 has a half life of almost 4.5 billion.  This leaves us with a relative abundance of about .72% Uranium 235.

We like Uranium 235 though, its easier to use so we have to .... enrich the uranium sample. How do you do that? Primarily through diffusion. In short because Uranium 235 is less massive than Uranium 238 it moves faster. So if you turn the uranium into a gas or a liquid through chemical processes then attach this to a really really long tube or series of tubes the Uranium 235 and Uranium 238 will flow at different rates. Considering were talking about a 3 neutron difference this process is needless to say fairly expensive.

Lot's of tubes and a really long time
The higher the purity you need the more expensive, and difficult it is to produce. That's why I said enriched Uranium not weapons grade. See nuclear reactors don't run on weapons grade, they don't even run close to it. Most reactors run on less than 5% Uranium 235. In fact most reactors aren't too picky in terms of how well refined the fuel is. A few reactors particularly molten salt reactors need less than 2% Uranium. There are a few reactors which run with higher enrichment such as research reactors and military nuclear reactors but they are the exception rather than the rule. The rest of the fuel is a ceramic material coupled with  a few other fissile materials. For a comparison weapons grade uranium hovers around 80% Uranium 235.

How do we know the limitations and that rectors wont go boom? Well that has to do with the measurable concept of criticality.  Criticality is a fancy way of saying whether a nuclear reaction is slowing, speeding up or maintaining speed.  The definition of criticality is essentially dividing the current generation of free neutrons by the previous generation. if the value is 1 the reactor is running critical, if the value is greater than 1 the reactor is running super-critical and if the value is less than one it is running sub-critical. Yes reactors run critical or super-critical all the time and no one dies (unlike the movies). When the control rods are withdrawn from the fuel cells the reactor will run super-critical until it becomes critical.

So when we are looking to blow crap up we know that we need the number or unrestricted neutrons to essentially remain critical while the rest of the material explodes. the less refined the material the more of it is needed to maintain this unrestricted fission.The mass of material needed to maintain the number of neutrons generated is known as the... critical mass. At a 6% concentration of Uranium 235 the mass required to sustain a nuclear blast is infinite. This doesn't just mean that reactors won't go boom, it means they physically can not.

It also means the Diehard explanation for Chernobyl is a bit flaky, if the bad guy was buying weapons grade uranium to run as fuel then the operators would have simply adjusted the reactor to run critical, no boom. If he was trying to run the reactor to produce Uranium 235... well he cant.  The original concerns were for reactors producing plutonium which would still require enrichment to be weapons usable. This would have again collapsed the entire story. Like I said it was a story telling device so I tried to merely grimace. It would also mean that in order for the plot to have happened socialist governments have to be the most corrupt bribe taking ...oh wait never mind.

So what did happen at Chernobyl, or Fukushima? and what is a meltdown? Nuclear plants and reactors are inherently safe. In fact you either have to try to cause them to fail or there needs to be an act of god. This actually explains both Chernobyl and Fukushima but only vaguely.  The only weakness to a nuclear reactor is its coolant supply. See nuclear reactions release a lot of energy, literally shit loads of it. Even with the control rods in there is still some latent heat and reactions occurring. As such cooling water must always be supplied to the reactor even during shut down. Even the spent fuel has a cooling system. A lack of cooling over a prolonged period of time will start to raise temperatures significantly. If things get too hot parts fail.

See Chernobyl was a byproduct of Russian engineering which is an oxymoron like military intelligence. It was also a generation 1 reactor built as a publicity stunt by the soviet empire. In short it was almost doomed to fail.  Chernobyl had no form of containment, there was no cement dome or pressure seals or really any safeguards in case of equipment failure.  But even that oversight wasn't as dumb as the reactor operators deciding to run a drill around coolant failure procedures... by actually shutting down the coolant.  Im guessing USSR school fire drills must have consisted of lighting the school on fire.

By the time they decided to restore cooling the plant had failed, the components had gotten hot enough to start producing flammable gasses. This exploded breaching their lack of containment with a variety of radioactive salts and byproducts. So that's kind of a bad problem, whats worse is it happened behind the iron curtain and information and aid was refused by the soviets. All in all the total death count was.. 31 people. That's it the largest nuclear disaster in the history of mankind killed 31 people. There are other estimates which are higher but 31 is the only actual number of solidly recorded deaths.

The next big nuclear disaster is Fukushima, which resulted after a massive earthquake and tsunami basically destroyed all power and utilities transmission to the plant. As a result of not being able to maintain pumping capabilities the reactor overheated and ... oh ya there was containment.  Only one of the reactors breached containment.  and the release was not all that bad. 0 people died in the incident. Most incident reports and even a lawsuit are alleging that the government and a lack of a questioning attitude are to blame for Fukushima. Fukushima was another publicity nuclear powerplant, attempting to show the world the glory of Japan.

So what does that mean for us? Well we learned a lot from these two disasters. modern plants are being designed with automatic redundancies and gravity based cooling systems. Because of current design modification plants are able to be produced almost 80% cheaper than in previous years even with extra redundancy systems. The Westinghouse AP1000 is so well designed that we are actually seeing nuclear plants constructed again after a halt in production for almost 30 years in the US. Like I said nuclear plants are insanely safe and they generate crap load of power. Nuclear power is the safest source in terms of deaths per trillion kilowatt hours with 90, wind comes in second with 150. Its also got the smallest carbon footprint for those who care.

I have explained how radiation works, and how it's not dangerous. I also explained how nuclear plants are inherently safe and how some of the terminology should be applied to them.  What I haven't covered is where the energy comes from. That's important because its the easiest way to explain why nuclear power will always beat out wind and solar. We started walking down the nuclear road after Einstein's famous equation E=MC^2. For those who don't read math it states that  the energy that can be produced from a given mass is the same as that mass multiplied by the square of the speed of light.  The speed of light is a big number so squaring that means that only  a little bit of mass is needed to make that change. you can find estimates around to try to get an idea of how much energy is in uranium but a pound of uranium contains as much energy as about 3 million tons of coal.

I'd rather not show the math for a fission reaction because fusion is simply easier and demonstrates the same principle. First lets look at a hydrogen from a periodic table.

Ok I lied that's deuterium an isotope of hydrogen containing an additional neutron. You can do the same thing with hydrogen but the result is a lot less stable and the math is more complex. So now were gonna grab some helium.

Were going to use  2 kilogram moles of deuterium to produce 1 kilogram mole of Helium. a kilogram weighs about 2.5 pounds so this is about 5 pounds of stuff. A mole basically is the atomic mass of something in the units I want to measure. It ensure a relatively consistent number of atoms for the reaction so I'm starting with 4.028 kg of deuterium and through fusion producing 4.002602 kg of Helium. so the math is 4.028 kg-4.002602 kg=.025398 kilograms of mass lost. not a whole hell of a lot of mass lost but all of it and I mean every bit of it is converted to energy. So pulling out Einsteins formula .025398*8.98755179*10^16 m^2/s^2, or 2.28*10^15 watts.

That's a shitload of energy but really doesn't mean jack to me. That's a number bigger than my comprehension so how long would that amount of energy power a 100 watt light bulb. so we take our value divide it by 100W*24 hours/day* 365days/yr.
(.025398*c^2)/(100*24*365)=2.6*10^9 years so our 5 pounds of stuff is capable of powering a lightbulb for 2.6 billion years. Granted the max efficiency is gonna be somewhere around say 40% so that 5 pounds of stuff will realistically only power that light bulb for around a billion years.  Lets leave it at a shitload of energy.

Of course that's fusion but fission reactor potential energy isn't that much different even if it was off by a factor of 100 its still crap loads of energy. That energy abundance is why we don't realistically have to worry about running out of energy and why we have plenty of time to figure out fusion. Uranium isnt the only fissile element, while its currently used in modern reactors there are other technologies available.

Currently research is resuming on molten salt and thorium reactors as the generation 4 technology. These reactor types were abandoned due to political pressure but they are actually safer and more abundant than current uranium reactors (fun fact uranium is more abundant than silver). Assuming a switch to these technologies it would mean that meltdowns would be even less likely to occur if not impossible.

Fun fact modern nuclear power costs are around 6-7cents per kilowatt hour coal and natural gas is about 3-5cents and wind and solar run around 12-20cents per kilowatt hour depending on the technology. with gen IV reactors looking to be cost comparable to coal i think we can kiss wind and solar goodbye..... without political intervention.

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