In the recently released trade paperback, Havok & Wolverine: Meltdown, compiling a 1988 Marvel miniseries, the titular characters travel through India and the former Soviet Union on their way to the Ukraine, and the site of the former Chernobyl Nuclear Plant. Over the course of several pages, antagonists Dr. Neutron and General Meltdown discuss the nuclear power production process. AiPT! Science reached out to nuclear engineer and comics geek Josh Worley to see how right the creative team got it.
Overall, this series of panels is surprisingly accurate — it’s clear Walter and Louise Simonson did some serious research prior to putting ink to paper, although I’ve never heard of neutrons being referred to as “bullets” before. There are only a couple discrepancies with real-world nuclear power production, but there are other key parts of the power production process that have been omitted from these panels that should be mentioned, as well.
Chernobyl’s four nuclear reactors were all of the same design, called the RBMK-1000 (“RBMK” = Reaktor Bolshoy Moshchnosti Kanalny, “-1000” = rated electrical output of 1000 megawatts), which use enriched Uranium-235 (U-235) as the primary fuel, as shown in the comic panel. This is the same Uranium isotope used in conventional water reactors in the United States and throughout much of the world, even today.
In conventional water reactors, the core (where the fuel rods and control rods are located) is housed in a large, cylindrical, steel pressure vessel and filled with deionized water, which serves two purposes. The water acts as a sort of shield (called a moderator) in the neutron fission process, to keep the fuel cool and to help control reactor power levels by slowing neutrons down. Secondly, it’s the process medium that is transported throughout the plant’s piping systems, with the resultant steam directed to the turbine generator in order to generate actual power. This water is in a closed loop system and is constantly being circulated due to large pumps, so additional water is rarely added to it.
The RBMK design utilized a modular construction that was powerful, quick to build, and easy to maintain. The design is unique in that it does not utilize a traditional reactor core or vessel — to build an RBMK reactor using similar safety systems found in conventional water reactors, the cost of each plant would have more than doubled. Each fuel rod is enclosed within an individual water-filled pipe, referred to as a channel (kanalny in the above translation), which allows the water to flow around the fuel. For the moderator, graphite blocks are located around each channel throughout the core.
Despite being somewhat “bare bones” in terms of redundant safety systems with backups (including but not limited to secondary, tertiary, and quaternary backups found in conventional, domestic water reactors), the RBMK design has worked quite well, as there have been no serious incidents at any of the stations over the past thirty or so years (the Chernobyl accident itself notwithstanding). At peak numbers, there were 16 separate reactors operating with this design (all located somewhere in the former Soviet Union). There are currently 10 RBMK reactors still in operation, and the operating licenses for these plants expire every year or two beginning in 2021 and continuing until 2034.
Surprisingly, the remaining three reactors at the Chernobyl site itself continued to operate until the year 2000, roughly 14 years after the accident at reactor #4. There were eight additional RBMK units planned throughout the region, but after the accident, construction was stopped at all locations until each plant was slowly cancelled (the last of which occurred in 2012).
As stated in the comic panel, nuclear fission itself releases a great deal of energy — approximately 900,000 eV (electron volts) is generated per nucleon that fissions. Since there are 235 nucleons in U-235, this equates to roughly 200 million eV produced per uranium atom. Burning coal only produces a few electron volts per carbon atom, so it’s easy to see why nuclear fission is used for electricity generation. As a frame of reference, this ultimately equates to a single, 800 megawatt nuclear reactor producing sufficient electricity to supply power to roughly one million homes.
That being said, water (or graphite blocks) alone isn’t sufficient to control the energy generated from nuclear fission. Control rods made up of boron-carbide, reinforced with a steel alloy, are placed throughout the core in order to fine tune reactor power levels. Boron is highly effective at naturally absorbing stray neutrons that might be forced loose by the energy of the fission reaction. When the plant is completely shut down, all control rods are fully inserted into the core, and at full power, almost all of the rods will have been completely withdrawn from the core.
When starting up a reactor for the very first time, a “neutron source” is placed somewhere near the core. This is essentially just a piece of radioactive material that is made from a particular element/isotope known to produce a multitude of stray neutrons via spontaneously fissioning. Americium is a very common material; the physical item is usually a mix of Am-241 and beryllium layered on top of each other.
When a stray neutron interacts with U-235, there are three main fission product combinations that can occur, and the fragments will always have an atomic weight of around 90-95 and 140-145. It will either split into Krypton (K-94) and Barium (B-139), Xenon (Xe-144) and Strontium (Sr-90), or Cesium (Cs-144) and Rubidium (Rb-90). No matter which of these sets of fission products are made, a few stray neutrons themselves are also produced.
Those stray neutrons interact with other U-235 atoms, which fission and produce even more neutrons, which interact with other U-235 atoms, and so on and so forth. Now that I think about it, I suppose referring to them as neutron bullets isn’t that far from the truth given the energy and force with which the chain reaction propagates.
After a reactor has been in operation, there are enough stray neutrons remaining from the partially burned fuel so that when it is being started up again, you no longer need a separate neutron source. The fuel alone is radioactive enough that the plant can start up on its own, something under close monitoring and control, of course — something the reactor at the Chernobyl plant could have used more of.