How does a nuclear reaction take place?
Reactions involving a neutron that impacts against a nucleus are, by far, the most important ones in applied nuclear physics. The operation of nuclear reactors is based on these reactions.
By being bombarded with neutrons, nuclei with heavy atoms can be divided into several fragments formed by nuclei with lighter atoms, with neutron emission and a large release of energy. This type of nuclear reaction is called “nuclear fission reaction”. The fission reactions that take place in nuclear reactors are produced with heavy atom nuclei (U, Pu, Th…).
The operation of nuclear reactors is based on the reactions involving a neutron that impacts against a nucleus
Other neutron-induced nuclear reactions are:
- Elastic dispersion, where the neutron collides elastically with a nucleus, losing part of its energy. The higher the mass number of the nucleus it collides with, the lower its energy loss.
- Inelastic dispersion, produced by high energy neutrons with heavy element nuclei, radiating capture (when the neutron is absorbed by the nucleus it collides with). The probability of this happening is indirectly proportional to the neutron's energy.
Elastic dispersion plays a fundamental role in moderating the energy in neutrons necessary to increase the number of fissions in certain reactors. Similarly, the radiating capture allows the generation of new fissionable isotopes.
Fission products are radioactive, and give way to radioactive series formed by several nucleids. The Immediate neutrons that appear at the moment of nuclear fission are called fast neutrons, and are emitted with high energy and very high velocity. These neutrons cause a series of nuclear reactions, fission being the most important of these, since it will give way to chain reactions. Usually, the number of neutrons that appear per fission is two or three, depending on the nucleus being fissioned.
From the energy point of view, the total energy released in neutron-induced nuclear fission comes from the kinetic energy of fission products, which is approximately 80%. The rest is basically due to neutrons. On average, the fission of a heavy atom nucleus (U, Th, Pu…) produces very high energy. As a reference, if all the nuclei contained within one gram of U-235 were to go into fission, they would produce a constant power of 1 MW (1,000 KW) in one day.
Nuclear fission reactions with neutrons are not produced in the same manner in all nuclei. There are two different types:
- Fissionable nuclei, which are able to undergo fission reactions with neutrons of any energy.
- Fertile nuclei, which, as their name indicates, are able to produce fissionable nuclei through neutronic capture reactions; they also go into fission, but only with very high energy neutrons.
Of the eligible nuclei for being fissioned in reactors (U, Th and Pu), fertile isotopes are those with an even number of nucleons, whereas fissionable ones are those with an odd number. The most important fertile isotopes are: U-238, Pu-240 and Th-232, and the most important fissionable isotopes are U-233, U-235, Pu-239 and Pu-241.
The total energy released in neutron-induced nuclear fission comes from the kinetic energy of fission products
The fission capacity of nuclei is measured through the value of the efficient section that they present for fission (the greater the efficient section, the greater the possibility of fiction), which depends on the energy of neutrons that interact with these nuclei. As the energy lowers, the efficient section increases and so does the fission capacity. For this reason, fission is most likely to happen with thermal (slow) neutrons than with fast ones. Thus, fissionable nuclei, in spite of suffering these reactions with any neutron, will fission in a greater quantity when neutrons are thermal, whereas fertile neutrons, having high fission thresholds, will only fission with the fast ones.
Uranium is used as fuel in a nuclear reactor, being in its natural form in isotopes U-235 and U-238. The first one is a fissionable element, and the second one is fertile. While 97% of all fissions in a reactor are produced by thermal neutrons, U-238 can produce Pu-239 through capture reactions. This element goes into fission in a similar way to that of U-235, increasing the proportion of fissions. Thorium, an element that naturally abounds in Nature, is presented as Thorium-232, which – through capture reactions – produces uranium-233, used as a fissionable element in reactors.
Just as was indicated, in neutron-induced fission reactions new neutrons appear in a number between two and three depending on the nucleus undergoing fission. These neutrons can then cause new fissions, which gives way to the possibility of a chain reaction being produced.
The fission capacity of nuclei is measured through the value of the efficient section that they present for fission
From this concept, a nuclear reactor is defined as a site that can initiate, maintain and control chain fission reactions. These reactions take place inside the reactor nucleus, which is composed by fuel containing fertile and fissionable nuclei, coolant, control elements, structural elements and a moderator in thermal nuclear reactor.
As they move inside the reactor’s nucleus, the produced neutrons can cause new fissions or be captured in the constituent materials. Thus, if at a given instant there are (n) neutrons inside the reactor, as a consequence of the indicated processes, after a certain time these have completely disappeared giving way to a whole new generation of neutrons (n’) that appear by fissions. The relationship between neutrons from two successive generations is called multiplying constant.
K = n' / n
This relationship can take different forms:
- K = 1, in which case the number of electrons produced is the same as the neutrons that disappear. This reactor is called critical.
- K < 1, in which case the chain reaction cannot be maintained. Since the number of neutrons produced is less than the number of neutrons that disappear, after some time the total number of neutrons will be cancelled. This state of the reactor is called subcritical.
- K > 1, in this situation, known as supercritical, the number of neutrons produced is greater than the number of neutrons that disappear. This will cause a diverging state.
A reactor’s normal operation must always take place in critical conditions, that is, with K = 1, except during start-up and shutdown, at which times it must be subcritical.