Nuclear Reactions Revision Notes

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20.4Nuclear Reactions

In these revision notes for Nuclear Reactions, we cover the following key points:

  • What are nuclear reactions? How do they occur?
  • What is energy of reaction?
  • What does the total mass of system before and after nuclear reaction indicate regarding the energy produced during the process?
  • What is the difference between nuclear reactions and radioactive decay processes?
  • What is nuclear fission? In which conditions does it occur?
  • How nuclear fission was discovered?
  • What is nuclear fusion? How it is different from nuclear fission?
  • What is the main example of nuclear fusion occurring naturally? What are the stages of this process?
  • What is neutron capture? Where does it differ from nuclear fusion?
  • What are radioactive families?
  • How can nuclear reactions occur spontaneously?
  • What are nuclear power plants? How do they work?
  • What is nuclear bomb? What is its principle of operation?
  • What are some practical applications of nuclear fusion reactions?
  • What does the penetration of nuclear radiation in matter causes in living and non-living things?
  • What are the advantages and disadvantages of nuclear energy use?

Nuclear Reactions Revision Notes

The term "nuclear reactions" refers to a special category of matter-energy interaction that includes all processes in which two or more objects interact with each other through nuclear forces.

The particles involved in nuclear reactions include atomic nuclei and other particles either involved in the process or produced during them, as well as the corresponding radiation emitted. Schematically, a nuclear reaction is written as:

a + X → Y + b

where X is the target nucleus, a is the hitting particle, Y is the daughter (hit) nucleus and b is the particle/s produced during the reaction.

Another important quantity to be considered during a nuclear reaction is the energy of reaction, Q. It represents the energy released or absorbed during a nuclear reaction. Thus, based on mass-energy equivalence, when Q > 0, then the total mass of reaction products is smaller than the mass of target nuclei plus that of hitting particle. Likewise, when the total mass of system increases after the reaction, the energy is absorbed by the system (Q < 0).

Nuclear fission represents a process in which a parent nucleus splits in two daughter nuclei when it "catches" a neutron. The most typical nuclear reaction of fission is when a Uranium-235 nucleus splits into two daughter nuclei (Barium-141 and Krypton-92) after catching a slow (thermal) neutron - a process in which three new neutrons are released according the following scheme:

10n + 92235U → 14156Ba + 9236Kr + 3 ∙ 10n

A large amount of energy is released in fission reactions as there is a large difference between binding energy of parent nucleus (U-235) and the sum of binding energies of daughter nuclei (Ba-141 and Kr-92).

Nuclear fusion is a process in which two parent nuclei merge to form a daughter nucleus. This is a typical case where the "target" and the "shell" are both atomic nuclei. A sufficient amount of kinetic energy in needed in such reactions to overcome the electric force caused by the unbalanced charge in the two parent nuclei (both of them are positively charged). The most typical nuclear fusion reaction is symbolically written as:

21H + 31H → 42He + 10n

where (21H) is the deuterium nucleus and (31H) is the tritium one.

Nuclear fusion occurs very often in nature. An example in this regard is the Sun, in which a large number of proton-proton fusion-type reactions take place.

Neutron capture is another type of nuclear reaction, during which radioactive nuclei are produced. These radioactive nuclei meet three conditions:

  1. Atomic Number Z of nuclei involved does not change
  2. Atomic Mass A of nuclei involved increases by 1 unit
  3. Number of neutrons in the nuclei involved increases by 1 unit

In nuclear reactions occurring spontaneously, heavy nuclei throw spontaneously one or more particles and as a result, they lose the excess of energy by transforming into lighter nuclei. A typical example of this category of nuclear reactions include situations related to radioactive decay. Thus, the natural uranium U-238 converts to actinium Ac-234 by means of an alpha decay, according the scheme:

23892U → 23490Ac + 42He + energy

On the other hand, the actinium (daughter) nucleus is excited as well, so it releases another alpha particle to get rid of this excess of energy according the nuclear reaction

23490Ac → 23088Ra + 42He + energy

and so on. This process continues until a stable nucleus is obtained. The nuclei that participate in a certain chain of nuclear reaction as the one shown above form a radioactive family.

The avalanche effect occurring in nuclear reactions when one neutron gives two new neutrons, two neutrons give four other neutrons and so on, must be controllable to avoid troubles. The following conditions must meet in order to make this process take place safely:

  1. The fast neutrons must not go far from the region in which there are the divisible uranium nuclei;
  2. The fast neutrons must slow down as much as to be easy capturable by uranium nuclei; and
  3. Even when they are in the region containing uranium nuclei, neutrons must avoid being captured by nuclei other than uranium.

If the number N2 of new neutrons produced during the nuclear fission is smaller than the number N1 of shell neutrons coming from the source to hit the uranium nuclei, then the process is not sustainable and it fades with time. If the number N2 of new neutrons produced during the nuclear fission is equal the number N1 of shell neutrons coming from the source to hit the uranium nuclei, the system is known as critical and the process continues at a constant and controllable rate. If the number N2 of new neutrons produced during the nuclear fission is greater than the number N1 of shell neutrons coming from the source to hit the uranium nuclei, the system is known as supercritical and the process continues at an increasing and not controllable rate.

This minimum mass of nuclear material required for a nuclear reaction to occur is known as the critical mass of reaction. Operating with values very close to critical mass is very important when nuclear reaction is involved, as this makes possible stopping the process very easily in case of anomalies.

The main mechanism of nuclear power plants is nuclear reactor, formerly known as an atomic pile, which is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions.

  1. Relying on the low fissile ability of fast neutrons in order to produce only a small number of neutrons (in fast reactors)
  2. Making them slower by using a moderator to increase the probability of neutrons capture (in thermal reactors).

In thermal reactors, we must add substances that slow down quickly the neutrons produced during fission. Low atomic number elements are perfect for this goal.Thermal energy produced during this process heats some (uncontaminated) water producing steam, which is then used to rotate any turbine after flowing very fast through narrow tubes in order to generate electricity.

In nuclear bombs, we again have a fission process but unlike in nuclear reactors, the process here is in supercritical state, i.e. the nuclear reactions are quite non-controllable. This means the chain process rate is in continuous acceleration and the energy delivered remains practically in the region in which the fission takes place. This brings a very high increase in temperature that eventually causes a powerful explosion.

Nuclear reaction that require (and emit) large amounts of energy to occur thermonuclear reactions, as they cannot occur in low or normal temperature. Fusion reaction as examples of thermonuclear reactions.

Some examples of application in practice of nuclear fusion reactions include:

  1. Hydrogen bomb
  2. Neutron bomb
  3. Neutron generators
  4. Energy production

Penetration of nuclear radiation in matter is a normal phenomenon because such radiation is produced in closed environments. Sometimes (in ionization radiation), a ionization process takes place in matter. This is the process in which a neutral atom initially neutral, splits (experiences fission) in two oppositely charged ions. This process may be very powerful, resulting in the generation of a large number of such pairs of ions. They participate actively in chemical reactions, bringing changes in the structure of matter.

The effects of radiation in matter have their advantages and disadvantages. When the effects produced by nuclear radiation are undesirable, this process causes a lot of harm (for example during the non-controllable explosions). This represents a disadvantage of such reactions. On the other hand, the high amount of energy obtained through nuclear processes represents an advantage of radiation interaction with matter.

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