Nuclear Security, Part IIFission Basics


By Béla Lipták, PE, control consultant







Atomic Structure



nuclear reactor



Schematic Diagram of a CANDU reactor
Figure 3: Schematic Diagram of a CANDU reactor:
(Courtesy of Wikipedia



Understanding Nuclear Fission


reactor design


Atoms are neutral when their number of electrons equals the number of protons within their nucleus. When the number of electrons differs from the number of the protons, they have an electric charge and are called ions which can “chemically” combine with other ions of opposite charge.

Different elements have different numbers of protons (atomic numbers) in their nuclei and atoms of the same element can have different atomic masses because they may contain different numbers of neutrons. These are called isotopes (Figure 1). The stable isotopes have definite ratios of neutrons to protons in their nuclei (U-238), while unstable isotopes (U-235) do not. 

During fission, a heavier unstable nucleus splits into two or more lighter nuclei, while releasing a substantial amount of energy. Fissionable materials include the naturally occurring isotope 235U4 and the man made isotope 239Pu5. Fission is initiated when a free neutron of the proper energy (thermal neutron) is captured by the nucleus of a fissionable atom. The most common way of generating thermal neutrons is to allow neutrons from a source—reactor, accelerator or spontaneous fission neutron emitter—to diffuse outward through a large block or tank of very weakly absorbing moderator. When the nucleus captures a thermal neutron, it will "split" producing two or more fission products (atoms of different elements formed from the protons, neutrons, and electrons originally comprising the original nucleus before its fission) plus two or three free neutrons and a tremendous amount of energy (Figure 2).

The emission of two or three free neutrons can split other unstable atoms (produce other fission events), which in turn will cause the emission of even more energy and more free neutrons. Within a few generations, the total amount of energy and the number free neutrons can become tremendous, sufficient to cause a nuclear explosion. For sustained chain reaction, it is necessary to moderate the process, to capture the “fast neutrons” while utilizing the energy released by the neutrons in the thermal range.

Reactor Designs

Today, there are basically three more advanced nuclear power plant designs in use. One is the CANDU system developed in Canada using heavy water moderator tubes (Figure 3). This design is similar to the Chernobyl RMBK design (Figure 4) only in that it uses pressure tubes instead of a pressure vessel, which facilitates on-line refueling, but otherwise it is much safer, because it contains much more cold heavy water. The General Electric design is a direct design (the moderator and the source of the steam to the turbine-generators is the same water). The third is the Westinghouse indirect design (Figure 5) (The high-pressure water in the reactor is the moderator and the coolant) in which the heat from the pressurized moderator water is used to boil the secondary water that is used to generate the steam for the turbines. This indirect Westinghouse AP1000 design is also the basis of the French EPR (Figure 6), and the Russian VVER1000 (Figure 7).

AP600/AP1000 passive safety systems
French EPR design WWER-10ff
Figure 5: Diagram of AP600/AP1000 passive safety systems .
(Courtesy: Wikipedia)
Figure 6: The French EPR design (Courtesy of Wikipedia)
Figure 7: WWER-10ff (also VVER-1000 as a direct transliteration from Russian ВВЭР-1000). WWER-1000 (Water-Water Energetic Reactor, 1000 megawatt electric power) is a Russian nuclear power reactor of PWR type. (Courtesy of Wikipedia)


I will continue this series in the March issue.

Béla Lipták, PE, control consultant, is also editor of the Instrument Engineers’ Handbook and is seeking new co-authors for the for coming new edition.

1 - A braintrust of French, German and Hungarian physicists at France's Center for Theoretical Physics, using some of the world's mightiest supercomputers, have set down the calculations for estimating the mass of protons and neutrons, the particles at the nucleus of atoms. According to the conventional model of particle physics, protons and neutrons comprise smaller particles known as quarks, which in turn are bound by gluons. The odd thing is this: The mass of gluons is zero and the mass of quarks is only 5%. Where, therefore, is the missing 95%? In other words, energy and mass are equivalent, as Einstein proposed in his Special Theory of Relativity in 1905. "It has now been corroborated for the first time."
2 - My reason for inserting the ethnic origin of these hyphenated Americans is not only because I myself am a refugee of the Hungarian Fight for Freedom in 1956, but also to emphasize the need to improve American scientific education, because our advance in the sciences can not forever depend on emigrants.
3 - At that time he first asked: “Why should I write about something I know so little about?” I answered, “Because after Hungary, now the Soviets just occupied Czechoslovakia (it was in 1968) and because I will dedicate the volume to the Hungarian and Czech Freedom Fighters and because I will do that, because I am ashamed that we did nothing to defend them and because you and I, we are both Hungarian-Americans and I want to emphasize that this (similarly to John Neumann’s computer and Theodor von Kármán’s rocket science inventions) are Hungarian-American contributions to the advancement of science.” He smiled, went to the library, read Donald Eckman’s Process Control and wrote the preface, which after 40 years is still the best introduction of the 4 editions of my handbook.
4 - This naturally available nuclear fuel is exhaustible. It’s R/P (Resource/Production) ratio at today’s prices ($70/kg) is 70 years (coal is 100, natural gas is 50 and oil is 40). Its use is relatively safe because its fuel concentration is only 3%, while to build “dirty bombs” requires 90%, and this step in increasing the concentration involves the use of high technology centrifuges, which are not easily available.
5 - Plutonium is man-made. Breeder reactors produce it. The reason why breeder reactors are so dangerous is because their plutonium fuel is already concentrated such that it can directly used to build bombs.
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