A Primer on Fission Reactions - Part II
Public apprehension of nuclear power can be ameliorated by explaining how this process operates along with its attendant advantages over available alternatives. Part II discusses fission reactions.
Generation of electrical power from nuclear reactions has costly and controversial. Much of the public reluctance to embrace nuclear power originates from lack of understanding of the underlying physics. The public is not to blame for this deficiency. National security concerns regarding exploitation of strong and weak nuclear forces from the early Atomic Age sought to reduce proliferation of explosives that also employ these forces, albeit under enormously different conditions.
Moreover, these nuclear forces operate at distances that extend to only one million millionth of a millimeter or less. Their actions lie well outside our sensation. The descriptions provided here merely aid in comprehending the phenomena surrounding atomic nuclei. They are intended to quell collective anxiety by presenting what nuclear fission does. Part I of this Primer sought to describe the materials involved in controlling nuclear fission. Part II presents the reactions.
As explained in Part I, fission can be induced with uranium U-235 as nuclear fuel after mining and enrichment. Neutrons bombard U-235 nuclei to initiate this exothermic process. Upon capture of a neutron, the U-235 nucleus divides into two separate nuclei plus an average of about 2 ½ neutrons and gamma rays (high energy photons) releasing heat used to drive turbines to generate electric power. Each occurrence of U-235 fission produces 0.032 nano-joules. Scaled to an ounce (equivalent to six teaspoons of water), this yields 2.35 tera-joules (653 mega-watt-hours) – almost fifteen million times as much energy as the combustion of methane (the primary constituent in natural gas) on a per weight basis.
For fission to commence, the fuel nucleus must capture a neutron, followed by release of two or three neutrons in addition to two smaller nuclei. The neutrons expelled by fission can strike other U-235 nuclei and thereby sustain continuous reaction. However, these escaping neutrons have too much energy for uranium capture, traveling at almost seven percent the speed of light. Hence, the neutrons must be scattered off other nuclei to reduce this particle energy by eight orders of magnitude (thereby decelerating by its square root or four orders of magnitude). Such materials are called “moderators” despite their purpose being to accelerate nuclear reactions. Hydrogen (H2), carbon (C) and oxygen (O2), in green outline in the annotated Periodic Table (repeated from Part I), are frequently used as moderators in the forms of water and graphite.
The thermal energy produced by reactions must be transferred to the generator via a heat exchanger medium or coolant, such as water, although some designs employ sodium (Na) and helium (He) in purple outline, as well as carbon dioxide. Absorption of neutrons quenches a reaction, and boron (B) and cadmium (Cd), in cyan outline, possess properties to aid control of the reaction, especially during transient operations, such as shutdown.
Fission subdivides a massive nucleus into lighter and heavier counterpart elements. For U-235, such division produces a distribution of initial nuclei products: lighter and heavier components, concentrated respectively around element-38 strontium (Sr) and element-54 Xenon (Xe), which are the most frequent results. Closer nuclei separations between the lighter and heavier products extend to element-42 molybdenium (Mo) and element-50 tin (Sn). Farther nuclei separations between lighter and heavier products extend to element-34 selenium (Se) and element 58 cerium (Ce), as shown in the example distribution plot.
Upon absorption of a neutron, the U-235 nucleus separates into two or three high energy neutrons together with a pair of product nuclei. Example products with the release of two neutrons include strontium Sr-94 and xenon Xe-140 for example, although the elements range across four atomic numbers in both directions as shown in yellow outline and the above plot. These products possess various radioactive half-lives and neutron absorption characteristics, and thereby influence reaction continuance.
Alpha and beta decay expands this menagerie, which accumulate in the fuel pellets. Alpha decay ejects a helium (He) nucleus. Beta decay denotes escape of an electron from neutron disintegration. The reaction and decay elements differ in physical and chemical properties, which can induce corrosion and stress damage. The plutonium-239 nucleus also divides similarly, for example xenon Xe-134 and zirconium Zr-103.
Many of the product elements resulting from U-235 fission rapidly decay by beta radiation into isotopes of elements with much longer half-lives or into stable isotopes. For instance, a U-235 nucleus absorbing a neutron can divide into fragments with 143 nucleons and 90 nucleons, while releasing three neutrons. As an example of the former, the half-life of lanthanum (La) of a quarter-hour decays into cerium (Ce) and then to praseodymium (Pr) with respective half-lives of a day and two weeks, finally becoming neodymium (Nd) as a stable element. The latter as bromine (Br) decays to krypton (Kr) and to rubidium (Rb), all with half-lives of seconds or minutes, and then to strontium (Sr) with its 29-year half-life, eventually decaying to yttrium (Y) of a few days finally becoming zirconium (Zr) as a stable element.
After the short-lived isotopes have decayed, the bulk of radioactivity (beta and gamma radiation) originates from cesium (Cs) and strontium (Sr). Of the isotopes resulting from U-235 fission, those that receive the most attention are cesium Cs-137 and strontium Sr-90 and iodine I-131, due to their half-lives extending into decades and disconcerting radioactivity levels. Reprocessing can chemically isolate these materials to reduce disposal mass, as well as recycle the unspent uranium, but financial and political resistance has cast this option in limbo over nearly half a century.
Plentiful designs exist that exploit this highly concentrated form of energy. The most common employ water as a moderator and coolant. Uranium is about forty times as plentiful in the earth’s crust than silver (Ag), and thorium is about three times more plentiful still. The world’s highest grade deposits are located in Saskatchewan, Canada. The largest deposits of thorium are in eastern India.
In short, shortage of fissile or fertile materials does not limit expansion of nuclear power. Facility accidents have been investigated, leading to safer designs and protocols, although high capital costs remain daunting.[4] Industry develops newer modular designs for improved standardization and financial flexibility. The political hobgoblins that plague adoption of nuclear energy are founded on popular apprehension from lack of familiarity. Contemporary Luddites who never cracked open a physics textbook extol these fears into near panic while they concurrently announce our doom from anthropogenic global warming.
All forms of energy release involve tradeoffs. Nuclear fission enables continuous energy production with minimal footprint due to its high concentration of binding forces. Its complexities arising from managing radioactivity, neutron collision response, chemical volatility, phase states and mechanical properties represent challenges that have been exhaustively analyzed and empirically tested over the past seven decades.
Reason and logic won’t suffice to ensure public acceptance of nuclear power based on comparative cost analysis and unbiased risk assessment. Emotional comfort must play a part to persuade voting citizens, policy-makers and economic shareholders towards expanded contribution of splitting atoms in satisfying our energy needs. Perhaps hugs and kisses will lead us down that yellowcake road. But absent objective description of its energy release, appeals can be expected to fall on deaf ears due to ignorance.[5] So this modest attempt to explain power from subatomic particles hopefully offers a flicker of illumination to dispel that darkness into something we can at least dimly perceive.
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[4] Real Engineering, “The Economics of Nuclear Energy” video 2020.
[5] R. Rhodes “Why Nuclear Power must be Part of the Solution” Yale Environment, 2019. https://e360.yale.edu/features/why-nuclear-power-must-be-part-of-the-energy-solution-environmentalists-climate
Thank you for a much needed and clearly written treatise. Educating the voters and politicians is the only path to the energy independence possible by the utilization of nuclear power. This is a n admirable effort toward that goal.