Neutron radiation

In addition to existing in the nucleus, it is possible to have free neutrons as a form of radiation. Neutrons are unique among the types of radiation, in that they only have interactions with other nuclei (nuclear reactions). These interactions can be:

- Elastic scattering. For example, if the neutron hits a hydrogen atom (even in water), the moving neutron and the proton at rest behave like billiard-balls. As the neutron and the proton have about the same mass, the neutron will be stopped completely in a central collision and the proton will carry away the full kinetic energy of the neutron. In non-central collisions the neutron and the proton share the kinetic energy. The neutron is slowed down.

- Inelastic scattering. A part of the kinetic energy of the neutron is absorbed by the target nucleus that in turn emits γ radiation.

- Neutron capture. The neutron is captured by a nucleus forming a new isotope of the nucleus it reacted with. This is called neutron activation, because the resulting nuclei are radioactive and emit a characteristic γ radiation.

- Other types of nuclear reactions are possible, including fission.

Neutrons are very penetrating and the ease with which they can be shielded and detected depends heavily on their energy. They can cause significant cell damage by indirect ionization and other processes as they pass through the body.

In summary then, neutron radiation:

− is very penetrating, but can be shielded by hydrogenous material for fast neutrons, and by cadmium or boron for slow thermal neutrons,

− is an external and internal hazard, and

− is detected only with special instruments.

2.4. Fission

When certain heavy isotopes such as U-235 or Pu-239 absorb neutrons, they can make the nucleus so unstable, that rather than emit radiation, the nucleus splits into two parts and emits neutrons and gamma radiation. This process is called fission. If some of the neutrons emitted can be used to cause further fission, then a chain reaction is initiated (Figure 2.8).

This is the reaction used in nuclear reactors, where the energy from the fission in the chain reaction is converted to heat in the material that, in turn, heats up a coolant such as water. This is used to make steam, rotate a turbo-generator and generate electricity.

Another phenomenon is called spontaneous fission. Here a heavy nucleus splits spontaneously without being hit by a neutron. In this process neutrons are also released. One example of spontaneous fission is provided by the artificial neutron source californium-252.

Spontaneous fission may also occur with uranium.

FIG. 2.8. The fission reaction 2.4.1. Criticality

When the chain reaction is self-sustaining at a constant rate, it is said to be critical. This means that on average one neutron resulting from each fission gives rise to one new fission.

The rate of reaction depends on a number of conditions.

The main influences are:

(1) The mass of fissile material. The closer the fissile atoms are packed together, the greater the chance that a neutron hits a fissile nucleus on its way through the material.

(2) The shape of the fissile material. The most efficient shape is a sphere where the ratio of volume to surface area is optimal. That means that compared to all other shapes, the lowest number of neutrons can escape the surface to be lost to initiate further fissions.

(3) The surrounding material to the fissile mass. Having escaped the surface of the fissile material, neutrons may be reflected in the surrounding matter and re-enter the surface.

(4) Moderation of neutrons. The probability of fissioning a fissile nucleus depends on the energy of the neutron. The probability of fission increases if the neutrons become slower. Some material is especially efficient in slowing down (moderating) neutrons by scattering reactions with nuclei. Examples are water with its high content of hydrogen, heavy water where hydrogen is replaced by deuterium, or solid carbon.

(5) Neutron capture. Some materials capture free neutrons and form new nuclei. Those neutrons are lost to participation in further fission processes. Cadmium and boron are well known to capture slow neutrons, an important property used in reactor control systems. Another important example is the breeding of plutonium 239 by neutron capture in uranium 238 (see Figure 2.9).

FIG. 2.9. Breeding process by neutron capture in uranium fuel

(Credit: NAGRA Switzerland)

Taking the optimum values for all of these main influences leads to a minimum mass with which a chain reaction can occur. This is called the smallest critical mass. Unintentional criticality is always to be avoided. For this reason, strict controls are established to ensure that reactor fuel elements or other fissile material are not brought together in a way which could allow them to go critical. Sophisticated models are used and calculations are performed to guarantee that fissile material does not go critical during transport. This is also the case when evaluating the possible consequences during and following severe accidents. One decisive step is to limit the amount of fissile material taking into consideration the conditions that are encountered in transport. This is achieved by introducing the concept of a “Criticality Safety Index” (see para 528 TS-R-1).