Fission Fragments 101
A crash course on what happens after fission.
Ever wondered what happens to parts of an atom after fission takes place in a reactor? Well, today we put that question to rest. To wrap up our series “Isotopes and Elements,” we @StrategicSwag aim to demystify the topic for all you wonks on what happens after an atom fissions.
Simply, nuclear fission is the act of taking an atom and splitting it into two smaller atoms, releasing a burst of energy. So far so good? Great. But what about the two smaller atoms? Are all these bits and pieces the same shape/size/element? Are fission fragments radioactive? Where do they go?
Turns out that fission fragments are not all the same. After fission takes place, some fragments remain extremely radioactive and many are contained in the fuel assembly (at least in the case of a reactor). So, lets look at this beauty of a graph below, and don’t let the many lines and colors scare you off too quick:
Every line above represents a different fission fragment generated in a nuclear reactor during the lifetime of the fuel. Further, each fragment contributes heat and energy onto spent fuel while the reactor is operating, or even after the reactor shuts down. The ‘slopeness’ of each line signifies the longevity of half-life — steepness goes to show a short-lived half-life whereas a flatline demonstrates long-lived. Note the logarithmic scale and how some isotopes still produce upwards of several megawatts of heat after shutdown.
With that, fission fragments are not a trivial byproduct to be ignored — we can clearly see that some of the fragments are extremely hot and radioactive. Further, there are about 95 different types of fission fragments, and the most energetic are displayed here. As it turns out, U-235 doesn’t always split evenly or nicely, and the distribution of the isotopes varies from reactor to reactor and configuration to configuration. Physics at this level always tends to get a bit wonky (haha, get it?) because it deals with probabilities rather than certainties. No two U-235 will split the same, so the best way forward is to normalize.
A quick analogy: Ever been burned by an oven while baking cookies after thinking it was already turned off and cool? A nuclear reactor is a lot like that (sans cookies), in the way that fuel assemblies require constant cooling even after shut down due to the heat it is still generating. The amount of power (heat) generated is shown here:
We see that immediately after the reactor is shutdown, much like our cookie oven, the fuel assemblies are still hot and produce close to 100 megawatts of heat within the hours following shutdown. This energy is no trivial matter. In fact, it is the reason why spent fuel must be constantly cooled. The graph above shows the energy each isotope emits as a function of time, and we see that some of these are very long lived, adding an extra layer to the problem.
Fission fragments also often produce extremely penetrating gamma rays that are harmful, which leads us to one of the largest challenges when dealing with fission fragments: nuclear waste disposal. Fission fragments are hot both radioactively and thermally, and this presents a problem for storage if you don’t want any of your stored waste to leak and contaminate.
For the more technical oriented folks here, these graphs were generated by StrategicSwagger using the SCALE/ORIGEN-ARP simulation software developed at Oak Ridge National Laboratory using the technical parameters of a 1465 MegaWatt Thermal General Electric Boiling Water Reactor 3 with Mk. II containment and a burnup of 30 megawatt days per metric ton of uranium. In less ‘sciencey’ terms, these graphs tell us what happened at a reactor of almost identical size and build to the Fukushima-Daiichi reactor when it is shut down. If you would like to look at the spreadsheet of the data and graphs, the data can be found here: https://db.tt/KmJpRoGg
We at StrategicSwagger are not here to tell you what to do with your waste. There are many good ideas out there and I’m sure all of you wonks have your favorite solution. We are here instead to demystify the situation and add a little science to the mix.
So it’s up to you wonks to think about these things. What do we do with our nuclear waste? How do we deal with byproduct fission fragments?
Now, thanks to these handy graphs and blurb, next time you go out on a date and the subject of nuclear waste and/or fission fragments comes up, you can rest assured your knowledge on this matter will guarantee that there will be a date #2.
Written by: Cervando A. Bañuelos II
Edited by: Marianne Nari Fisher
