Nuclear fission

The Differences Between Nuclear Fission and Fusion

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Setting the record straight on how these two similar sounding energy sources truly differ.

nuclear fusion vs nuclear fissioin

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Clean and sustainable nuclear energy may sound like a washed up, post-war dream, but the modern reality of this power source is much more complicated. For one, nuclear energy in the coming decades will include not only nuclear fission—the type of reaction already driving nuclear power plants—but also the much more elusive nuclear fusion.

If scientists can solve the remaining puzzle pieces behind these technologies, both nuclear fission and fusion are poised to have a big impact on the world’s energy reserves and green energy efforts in the coming decades. But before they do, let’s set the record straight on how these two similar sounding energy sources truly differ

Nuclear Fission

When you’re thinking about nuclear power, odds are you’re probably picturing a process called nuclear fission. To create nuclear fission, atoms of radioactive elements like Uranium are broken apart with neutrons to release an enormous amount of energy. Inside nuclear reactors, this energy is used to create steam, which in turn powers a turbine to produce electricity.

Nuclear Fusion

Unlike its sibling, nuclear fusion has largely been restricted to the realm of science fiction until recently. Instead of breaking something apart, nuclear fusion happens when light atoms are smashed together to create a heavier atom (e.g. two hydrogen atoms combining to form one helium atom). This interaction creates a huge burst of energy that is still burning at the heart of stars all across the universe.

Unlike fission, nuclear fusion also has the added benefit of being self-sustaining without creating harmful waste. However, achieving and controlling fusion has been a lot more difficult for scientists to crack than fission.

One problem facing fusion technology is that in order to create self-sustaining power (a point called “fusion ignition”) it needs to be sparked by a massive amount of energy. In theory, after this initial power push the fusion reactor should then be able to create and sustain even more power than was initially fed into it. However, actually achieving this is easier said than done.

That said, labs like the U.S.’s National Ignition Facility (NIF) and France’s International Thermonuclear Experimental Reactor (ITER) have made progress in recent years with NIF reporting last summer that their reactor was able to generate up to 70 percent of its input energy. Start-ups like Helion Energy are also working toward this goal using magnetic coils to compression the reactor core.

Scientists have claimed to be on the brink of cracking nuclear fusion for decades, but hopefully with any luck that promise may finally be coming true.

After 70 years, physicists have finally figured out how a fundamental aspect of nuclear fission works

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When you shoot neutrons at a big atom, eventually one will collide with it just right, causing it to split into two smaller atoms and some more neutrons, while releasing a whole bunch of energy. This process of splitting big atoms into smaller atoms is known as nuclear fission, and if the neutrons happen to strike more atoms and repeat the process, there’s a cascade of energy that, depending on your inclination, can either power a city or blow it to smithereens.

This we know, but as humans have been investigating ways to get more and more destructive power out of fission since it was first discovered in 1939, we’ve left some surprisingly fundamental questions about the process unanswered. Such as, exactly how long does it takes for the big nucleus to split up into tinier pieces? According to a new study, fission probably takes about 10 times longer than existing models have predicted.

Why did it take so long to figure out fission’s timescale? Because fission is really complicated, both to measure and to predict. To measure something, you either have to wait until it shoots something into your detectors, like they do at theLarge Hadron Collider, or you have to shoot something like an electron at it and see how that electron bounces off.

Shooting electrons at a big, unstable atom would change how long the fission takes – messing up the thing you’re trying to measure. So you have to sit and wait to see what happens. But we’re talking potentially millions of atoms in a single sample, and knowing precisely when one of those millions of atoms went off is tricky – especially when you’re working on a timescale of trillionths of trillionths of seconds.

A more effective way of measuring it is using theoretical models and computer simulations, but they present their own challenges due to how much is actually going on.

There’s the original nucleus with hundreds of nucleons (protons and neutrons) that splits into two unequal nuclei, plus the photons and neutrons that fly off in random directions. Even if you just try to focus on how each nucleon affects every other nucleon, your task tends to lie somewhere between unreasonable and impossible, depending on the size of your blackboard or your confidence in computers.

To deal with this mountain of challenges, physicists have traditionally treated the nucleus as a single object with some collective properties that seem to be confirmed by experiments. But treating a collection of things as a single entity is a very slippery slope.

Into the fray comes a team led by physicist Aurel Bulgac from the University of Washington. They decided to simulate the nucleus with a sort of combined approach that, in a sense, treated the nucleus as a single object while also tracking the way the individual nucleons that made it up interacted with each other. This wasn’t easy.

All told, the researchers used about 1,760 computers (GPUs, specifically) to solve 56,000 individual equations at each of 120,000 instants of time – all to simulate the 5 billionths of a billionth of a second immediately after an atom of plutonium begins fission. They found that in their simulations, the fission took about 10 times as long as previous models had predicted it would take.

Yeah, but what if their model is wrong? After all, the researchers admit that it has a bit too much energy and there are slightly too few neutrons thrown out by the reaction. Plus, the model wasn’t designed to match any experimental results; it was just a kind of proof-of-concept.

Remarkably, though, the model has been shown to match the results of experiments anyway. In a beautiful piece of scientific candor, the team writes: “The quality of the agreement with experimental observations surprised us in its accuracy since we have made no effort to reproduce any measured data.”

The unexpected match gives them good confidence in the overall accuracy of their results, including the fission’s duration.

Learning that fission takes 10 times longer than we thought might not revolutionise the way we do things, but it remains amazing that there are still unanswered questions about a piece of physics that dominated world politics for most of the eighty years since its discovery.

Fission Power: The Pros, the Cons, and the Math.

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The process of nuclear fission was first discovered in 1938; however, it wasn’t fully explained until a year later. Today – less than 100 years after its initial discovery – it is the poster child of the ‘green energy’ movement (and not in a good way) that is sweeping across the globe.  Most of what we hear about the pitfalls of using fission technology are sensationalist, but there is no doubt that this process has led to nuclear disasters. Recently, reports have stated that the radioactivity level spiked to a level 6,500 times higher than the legal limit at Fukushima, and issues continue to presist in that area. This process has also been linked to  non-localized devastation. During Chernobyl, the Soviet government evacuated about 115,000 people from the most heavily contaminated areas in 1986; however, another 220,000 people had to be evacuated from surrounding areas in subsequent years.

Credit: U.S. NRC

Now, there is a huge debate amongst people as to whether governments world-wide should pursue the continuation of developing safer nuclear power plants, or if it should be scrapped  all together in place of something that is perceived as “safer.” Given the overall importance of the debate to the environment and to our exponentially growing energy needs, everyone should have a proper understanding of the topic; however, for the most part – very few people have more than a very basic understanding of the science and mathematics behind the process. In this article, I want to attempt a more thorough explanation than you may have read before.

Atomic Fission:

Nuclear Fission (Source)

As most of you will hopefully be aware of, nuclear fission is a chain reaction involving large, unstable nuclei. This chain reaction ignites when a neutron collides with another neutron, resulting in it becoming even more unstable – before one nucleus  divides into two ‘daughter’ nuclei and (on average) 3 more neutrons. After which, the additional neutrons go on to initiate another fission reaction with those they come in contact with. Those neutrons then incite a reaction between other neutrons and so on and so forth (like the domino effect). The most common fuel used for fission is Uranium -235 (that’s 92 protons and 143 neutrons), and the 2 products (plus neutrons) of this reaction could be a range of sized nuclei.

As with any reaction/equation, when broken up, the final number must still sum to what you started with, and this is also true of fission reactions. Ultimately, the total number of nucleons (protons and neutrons) after fission, in whichever new combinations, must still add up to the original number.

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So what good is fission to us? Well it produces energy of course! But where does this energy actually come from? I mentioned that the number of protons and neutrons remains the same, and that they are just rearranged into more stable combinations; this is true. However, when adding up the total masses before and after, you will find that the mass will DECREASE. Said decrease in mass is the answer to our question, as the lost mass is converted into pure energy.

With a little prior knowledge (and a very familiar equation), we can calculate the amount of energy produced. An example goes as follows… (warning, complicated math is contained below)

Let us take this reaction:

1 neutron + Uranium-235 à Strontium – 98 + Xenon – 136 + 3 neutrons (Rounded values in relative atomic mass)
  • Mass before = 236.053u
  • Mass after = 235.840u
  • Mass change = 0.213u

To convert this result into kilograms, we multiply the number by 1.661×10^-27 (the mass in kilograms of a nucleon). So:

0.213 x (1.661×10^-27) = 3.538×10^-26kg

Next, using E=MC^2 we can convert this mass into energy (using the rounded value for C)

(3.538×10^-26) x (3×10^8)^2 = 3.18×10^-11J

This isn’t a very large amount of energy – but remember that this is just for a single atom of Uranium! So suppose we could persuade it to fission completely, how much energy would be produced for one gram of Uranium? Since we know how much energy is produced by one atom of uranium, to find the energy produced by one gram, all we need to do is know how many atoms are in a gram. To figure this out, we use Avogadro’s constant, which is equal to the number of atoms of any element in one mole of that element. That number is 6.022×10^23, and we use it in the following equation (probably more familiar to chemists than physicists)

Number of atoms = (mass x Avogadro’s no.) /molar mass Therefore the number of atoms in a gram of Uranium can be calculated as:

(0.001kg x 6.022×10^23)/0.236053 = 2.55×10^21

Now we can multiply this number by the amount of energy produced by a single fission reaction and we get:

(2.55×10^21)x(3.18×10^-11) = 8.11×10^10J

This is a HUGE amount of energy for just a single gram of fuel. Especially when compared to the amount of energy generated by coal or oil, and remains the reason why Uranium is so widely used (despite the potential dangers). Ultimately, The amount of free energy contained in nuclear fuel is millions of times the amount of free energy contained in a similar mass of chemical fuel, such as gasoline. Moreover, the process of decomposition produces a huge amount of heat, a large volume of heavy element atoms, and a lot of neutrons. In addition to these products, the nuclear fission also produces a big volume of radioactive waste. Obviously, this waste needs to be disposed of, as it could cause serious destruction to the environment, should it leak. Proper storage is extravagantly expensive.

But of course, there are a number of advantages to this kind of power. Getting rid of our dependence on fossil fuels is probably the biggest advantage of nuclear power. Power plants that burn coal are highly destructive to the environment (whereas nuclear fission is really only destructive if there is a leak or meltdown). Moreover, the mining process destroys vast swatches of Earth, including a number of diverse habitats. There is also the issue of oil spills (we all probably remember the infamous BP contamination of the Gulf). More importantly, the nuclear fuel used is much more efficient and found in abundance. Large reserves of uranium are spread in many parts of the world. Scientific estimates suggest that the rate at which the fossil fuel are being used today, their reserves are bound to become empty by the end of this century. Yet, the byproducts of the fission process remain radioactive for thousands of years and can cause serious harm to living beings. Although the chances are rare, a nuclear power disaster can decimate a habitat/ecosystem (depending on the size and nature of the disaster).

In the end, it is each individual’s responsibility to acquire the knowledge necessary to make decisions and be informed. Hopefully, this post helped you start (or continue) this journey of discovery.