Reactors and Atom Bombs
A nuclear reactor is a power plant, that uses nuclear fission to eventually generate electricity. An atom bomb also uses nuclear fission to generate energy causing an explosion. However, due to fundamental differences between the two a nuclear reactor cannot explode like an atom bomb. To understand these differences, it is first important to understand the concept of nuclear fission and criticality.
Background
Fission
Nuclear fission results when a neutron collides with an atom, causing it to become unstable and split. Once the atom splits, neutrons are released, along with energy and radiation, this is shown below in figure 1. In a reactor, the energy released is used to heat water. Eventually once the temperature of the water is high enough, the water will change to steam. The steam is then forced through a turbine, which creates electricity. Depending on the speed of the neutrons, a fission can be either a fast fission or a thermal fission. Thermal fissions pertain to slower moving neutrons.
The concept of fission is the heart of the nuclear industry. In a reactor millions of fission reactions are needed to generate the energy required to heat the water. Causing fissions over and over again would not be an effective way to do this, instead a chain reaction is used. A fission chain reaction is a fission reaction that sustains itself. When a neutron collides with an atom, more neutrons are released. If there is an abundance of atoms, and the neutrons are contained, the neutrons that are released from the first reaction, will cause more atoms to fission releasing more energy and more neutrons. The number of neutrons present after each fission is the criticality of the reactor and is denoted by the multiplication factor k.
The Multiplication Factor and the Six-Factor Formula
The multiplication factor, k, is simply the number of neutrons produced per fission, divided by the number of neutrons lost per fission. The six-factor formula is composed of six factors that when multiplied together equal k, k = η f ε p PTNL PFNL . Each term represents a different way a neutron can be lost or gained.
• η – Production Factor. The production factor compares the number of neutrons produced to the number of neutrons being absorbed. It is dependent on the type of fuel being used.
• f – Thermal Utilization Factor. The thermal utilization factor compares the amount of neutrons being absorbed by the fuel to the amount of neutrons being absorbed everywhere else.
• ε – Fast Fission Factor. The fast fission factor compares the total amount of fissions to the amount of thermal fissions.
• p – Resonance Escape Probability. The resonance escape probability is the probability that a neutron survives the resonances as it slows down from a fast neutron to a thermal neutron.
• PTNL – Thermal Non-leakage Probability. The thermal non-leakage probability is the probability that a thermal neutron will not leak outside of the core.
• PFNL – Fast Non-leakage Probability. The fast non-leakage probability is the probability that a fast neutron will not leak outside of the core.
Figure 2 shows the neutron life cycle, which is simply the terms of the six-factor formula. Once the multiplication factor is determined, the criticality of the reactor can be found.
• k > 1 –> supercritical, more neutrons are being produced than lost.
• k = 1 –> critical, the same amount of neutrons are being produced as lost.
• k < 1 --> subcritical, more neutrons are being lost than produced.
The criticality of a reactor is a crucial design element. A subcritical reactor is useless since it will be impossible to start a fission chain reaction. While a critical reactor is the most desirable, reactors are normally designed to be supercritical, and then through the use of moderators, are scaled back to critical. Now that fission and criticality are understood, it is easy to see the differences between an atomic bomb and a nuclear reactor.
The Differences
There are two main fundamental differences between the design of an atomic bomb, and the design of a nuclear reactor. One difference is the way the fission reactions are controlled and the second difference stems from the enrichment of the fuel.
Control of Nuclear Fission
One main difference between the two is that the fission events in a reactor are monitored and controlled closely. An atom bomb is an uncontrolled fission chain reaction, that releases exorbitant amounts of energy quickly. The design of a nuclear reactor includes control rods. The control rods are placed in the core to control the fission reaction. Control rods do this by absorbing neutrons, which decreases the multiplication factor. The more control rods in the core, the lower the criticality, thus there will be less neutrons available for fission. An atom bomb is designed to maintain all the neutrons produced, making it always supercritical.
Enrichment
The second difference between the two is the enrichment of the fuel. Natural uranium, the uranium found in the earth, cannot be used as a fuel because it is not reactive enough to cause a chain reaction. This is because natural uranium is composed almost entirely of U238, which is a relatively stable element. By enriching it with U235, the uranium becomes more reactive, which increases the production factor. While both fuel for a reactor and fuel for an atomic bomb are enriched, a reactor’s fuel is only enriched around 4 to 5 percent. Whereas an atomic bomb is enriched to about 90 percent. This makes the multiplication factor much larger in a bomb than a reactor, which signifies a greater number of available neutrons. The fuel used in a reactor is not capable of causing an explosion.
Explosions
While a nuclear reactor can never explode like an atomic bomb, an explosion can still occur. All power plants are a potential site for an explosion, because the fuel used, whether it is coal, uranium, or natural gas, needs to be energy dense. At coal plants, sparks can set coal dust on fire causing an explosion. Gas leaks can cause explosions at natural gas plants. Typically at a nuclear reactor, the type of explosion seen would be a steam explosion. A steam explosion could only occur if the reactor suffered a meltdown. A meltdown means that due to lack of coolant, or too much fission, the core becomes so hot that it melts. Due to the intense heat produced, water is turned to steam. Also, the fuel rods melt, turning them into a liquid. This allows the metal to react with the steam, causing an explosion.
The destruction at Chernobyl was caused by a steam explosion. Since the turbine feed valves were closed, the steam in the core had nowhere to go. Then, the pressure in the core did not increase, causing even more steam to be created. When the reactor core began to meltdown, liquid metal touched the steam, causing an explosion. The explosion caused the roof of the core to lift off, exposing the core to air. The air reacted with the graphite moderator in the core, resulting in the production of carbon monoxide. Since carbon monoxide is flammable, it caught fire due to the extreme heat in the core. The fire burned for days.