Neutron Radioprotection

A rare radiation, dangerous, penetrating, difficult to absorb

The neutron radiation is more penetrating than alpha and beta . It is more dangerous than gamma rays. It is fortunately short-lived and rarely encountered. Its exposure is exceptional: intervention in the core of a reactor, critical accident, and at a different scale, exploding atomic bomb or hydrogen bomb (*).

Under normal circumstances, exposure is reduced to a few neutrons produced by cosmic radiation. It is very low.

In an atomic explosion, the neutron radiation is particularly harmful. But the neutronic flash does not last. Free neutrons have an half-life of 12 minutes. They are generally absorbed by matters before decaying.

In the 80s, low-power atomic bombs were developed, without blast - so not destructing - but releasing an instantaneous neutron flux of fatal intensity. These "neutron bombs" were intended to annihilate enemy combatants, while allowing to occupy the ground shortly after. After widespread protests in public opinion at the time, these weapons were in principle abandoned.

In practice, it is in the vicinity of reactors and accelerators in certain research laboratories that protection against neutrons needs to be implemented because of high neutron fluxes.

Neutrons, slowed down by multiple collisions with the encountered material nuclei, are quickly captured. Generally capture is followed by a de-excitation gamma radiation of which we must be protected. It should also be remembered that part of the captures produced radioactive nuclei. The effects of this radioactivity diluted over time, continue to occur after neutron absorption.

The boron neutron trap
In order to protect against neutron produced in reactors, engineers add boron to the water entering the concrete composition of the surrounding walls. The curve shows that the probability of neutron capture by a nucleus of boron-10 (a naturally occurring isotope of boron) exceeds more than 10 000 times that of a hydrogen nucleus, whatever the neutron energy. But if ratios barely vary, the probabilities themselves greatly increase when the neutron slows down, even so much that a boron-10 nucleus seems huge to a slow neutron. There is sharing of roles: the protons of hydrogen from water slow the neutrons by collisions until they are captured by boron. To make them visible on the diagram, the recoil protons have been magnified relatively to the boron nucleus.
IN2P3 (Source Janis data base)

At last neutrons are captured by the nuclei. To protect oneself, the most effective way is to make such catches, by incorporating in the shielding material nuclei greedy of neutrons. The probabilities of capture of slow neutrons become very important for nuclei such as boron-10 and cadmium. The nucleus becomes very large, as a goalkeeper with sprawling arms.

One protects oneself from neutrons with concrete walls incorporating boron. Concrete contains water, therefore hydrogen that effectively slows down the neutrons. Boron incorporated in the concrete contains 20% boron-10 that is very effective in capturing neutrons. For slow neutrons, this nucleus looks 60 times bigger than it actually is (*).

In the case of fissile material, one should avoid the risk of criticality, that is to say, the unexpected development of chain reactions. For example in the storage pools, the spent fuel assemblies that contain a further 1% fissionable uranium are separated and placed in baskets of steels containing boron.

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