Nuclear fission

Nuclear fission

Nuclear fission is the process where the nucleus of a large atom splits into smaller, approximately same-sized nuclei after the absorption of a free neutron. The resulting products are more free neutrons, two smaller, stable nuclei and energy, in various forms including gamma radiation. Nuclear fission can be spontaneous, occurring as a radioactive decay process, but is more often induced by the bombardment of nuclei by neutrons. The free neutrons generated by a single atom-splitting event may go on to split further atoms, thus generating more free neutrons which go on to split further atoms and so on. This self-sustaining release of neutrons is known as a chain reaction. The energy liberated is as a result of the difference in the energy required to bind together the protons and neutrons in the larger atom, versus the energy required to bind the protons and neutrons together in the two smaller nuclei. It is referred to as the nuclear binding energy and is calculated using Einstein’s famed relationship, E = mc2 . The huge quantity of energy released for what seems like very little effort is what makes nuclear fission an attractive option for power stations and weaponry.

In nature, very few materials exist that undergo nuclear fission. The most common are Uranium-235 and Plutonium-239. Uranium-235 fission occurs when a slow neutron is captured by a U-235 nucleus. A fast neutron will not be captured, so neutrons must be slowed, or moderated, to increase the likelihood of fission. The capture could result in many possible outcomes; one such is two stable nuclei of Caesium-143 and Rubidium-90, three free neutrons and a binding energy yield. Less than 1% of Uranium is the fissionable isotope U-235, the vast majority is non-fissionable U-238. In order to produce a sustainable fission for use in, for instance, civil nuclear power generation, the composition of uranium-235 is altered by a process called isotope separation such that a greater percentage of it becomes the fissionable U-235. Uranium treated in this way is called enriched uranium. Uranium must be enriched to varying degrees depending on the application. For use in light water reactors uranium enrichment to about 2.5% to 3.5% U-235 is required; for breeder reactors, enrichment to about 15% to 30% is required. Heavy water reactors can use natural uranium and no enrichment because deuterium is a better moderator. For weapons use, uranium needs to be enriched to in excess of 90% U-235.


Light water reactors

Perhaps the most common use of controlled nuclear fission is in nuclear reactors. The most common form of reactor used in the United States is the Light Water Reactor, so called because light water is used as a coolant and moderator. Light water is the term used to describe water made using the common isotope protium, which has a single proton in its nucleus (as opposed to heavy water, which uses the heavier hydrogen isotope deuterium, which has a single neutron and a single proton in its nucleus.)

The core of the reactor contains fuel assemblies. Each fuel assembly comprises a hundred or so nuclear fuel rods, each about 12 feet in length and about the cross-sectional area of a dime. Within each fuel rod are stacked many small pellets of uranium-235 or uranium oxide enriched to about 3%. Each pellet is about an inch in length. Similarly, control rods are filled with pellets of substances such as Cadmium or Hafnium that readily absorb neutrons. The control rods are gathered in bundles of about 20 rods and mounted on an assembly and can be raised or lowered in to the reactor core. When fully lowered, they absorb free neutrons and prevent a chain reaction; when fully raised, more free neutrons are available for capture, leading to a greater rate of fission, and hence energy production. The reactors energy production is controlled in this way.

The entire core is assembled within a water-filled steel vessel, called the reactor vessel. Two common variants of the light water reactor exist. In the boiling water reactor, the heat generated by ongoing fission boils the water, creating steam. This steam is then channeled in to turbines linked to a generator to produce electricity. Steam is then returned to the water vessel via a condenser.
In the second variant, the pressurized water reactor, the heat generated by nuclear fission is directed through a heat exchanger, which then boils water to generate steam. The steam is again channelled through turbines, a condenser and back to the water vessel.

Fast breeder reactors

Since very little natural uranium is of the fissionable U-235 form, a means of generating nuclear energy via the relatively abundant U-238 isotope would be desirable. Breeder reactors do this. In fact, they go one better since, as their name implies, they generate energy and breed more fuel than they consume. In breeder reactors, a core is built using fissionable Pu-239. The core is surrounded by a layer of U-238. As the Pu-239 undergoes spontaneous fission, it releases neutrons. These neutrons convert the surrounding jacket of U-238 in to Pu-239. So, the reactor breeds fuel as it operates. When all the U-238 has been converted to Pu-239, the core is reloaded with U-238. The conversion process requires fast moving neutrons for fission to occur, unlike standard U-235 fission which requires slow moving (or thermal) neutrons. As a result, breeder reactors cannot use water as a coolant since it also acts as a moderator, converting fast neutrons in to thermal neutrons. Various coolants have been used in breeder reactors, including mercury and lead; however, liquid sodium has proved to be the desirable choice in power stations.

There are certain inherent risks involved in using breeder reactors. Pu-239 is extremely toxic so any accident at such a reactor will certainly have wide-reaching, large-scale repercussions. In addition, in order to use the fuel created in the breeder reactor, it must be reprocessed. The reprocessing method is controversial in that it can be easily be employed to extract weapons grade plutonium from a reactor.

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