Answer to Question #10176 Submitted to "Ask the Experts"

Have there been any studies done about submicroscopic particles at the atomic level being knocked out of the shielding surrounding nuclear reactors by gamma or other forms of radiation? The theory is that differences in temperature between the inside and outside of a reactor's concrete shell will create microfractures and cracks in the shell. This will allow radiation to penetrate the shell. When a gamma ray near the perimeter of the shell hits an atom of concrete, it could act like a pool ball knocking the particle out of the shell like a mortar round at the submicroscopic level. These would travel at high velocities and pass through solid objects in their parabolic paths. They would act somewhat like a gamma ray, photon, or wave. Has any research been done in detecting this theoretical phenomena? It would be a form of radiation fallout, but would not appear in radiation testing with a Geiger counter or other radiation tests, as the particles would be atomic particles that are not radioactive, but could still be very harmful.

There have been many studies of the effects of radiation on materials, including reactor materials. The major penetrating radiations produced by the operations of nuclear reactors are gamma rays and neutrons. Gamma rays are massless photons of electromagnetic energy. They interact primarily with the electrons surrounding atoms of the media through which they travel and produce ionization and atomic excitation in the process. Some high-energy gamma rays (>1.022 MeV) interact by the pair-production process in which the photon disappears in the field of the nucleus with which it interacts and produces a conventional electron and a positively charged electron in the process of energy-to-mass conversion. In general, the energy transfer from gamma photons to atomic nuclei is very small, and translocations of atoms as a result of this transfer is minimal, usually producing little in the way of material damage. The kind of effect you envision would then not be significant with gamma radiation. Some gamma radiation effects on materials may result from chemical changes associated with possible breakage of chemical bonds and possible induction of new bonds.

More significant damage of reactor materials is often associated with energetic neutrons. Neutrons produced in the fission process extend in energies beyond 10 MeV. These neutrons have individual masses a bit greater than a proton and, since they carry no electrical charge, they are able to penetrate the electron cloud around an atom and reach the nucleus. They are then capable of inducing a variety of nuclear reactions in addition to elastic and inelastic scattering events with nuclei of materials through which they pass. These nuclear interactions may yield recoil nuclei that possess sufficient energy to be displaced from their original positions in the lattice of the material in which they reside. It generally takes about 25 to 30 eV of kinetic energy to displace a nucleus from its usual lattice position, and many residual nuclei from neutron interactions will have much more energy than this, often being sufficiently energetic to promote additional secondary interactions that produce more nuclei displacements so that a series of "knock-on" atoms may be produced, a phenomenon similar to what you envision. The total number of displacements that occur are a major determinant of the degree of radiation damage that will result. There have been quantitative analyses of this behavior, and you can find descriptions in many nuclear engineering textbooks (e.g., Glasstone and Sesonske, Nuclear Reactor Engineering, 3^rd ed., 1981, or 4^th ed., Vol. 2, 1994). There are also many texts and articles on radiation damage in material that can be accessed on the Internet.

The radiation-induced neutron damage to some reactor components can be a concern. In particular, metallic components that are subject to the highest neutron (and gamma) fluences, such as the reactor pressure vessel, internal gamma shield, fuel cladding, and certain reactor plumbing. These may experience long-term changes in structural properties that include reductions in ductility, increases in brittleness and increases in tensile strength, and swelling.

The radiation loads on the concrete biological shield that surrounds the reactor vessel are less than those on internal metallic components. Some damage can occur, but I have not heard of any serious deleterious effects associated with radiation damage. In general, radiation damage to concrete, gamma and/or neutron induced, may result in a reduction in compressive strength and a decrease in the modulus of elasticity.

Regarding your concern that knock-on nuclei might be a health concern, keep in mind that the atoms (actually ions when they are produced) displaced from their original locations in solids that are highly irradiated are incapable of penetrating any significant distance. In solids they would likely be stopped within a few to tens of micrometers so that the likelihood of them escaping from the material is nil unless the effect was occurring at a free surface. Even then, the distance traversed in air by released ions would likely not exceed a few millimeters, thus making them of little concern. If they did impinge on an individual's skin, the dead-skin layer would be sufficient to stop them, thus preventing any effects on live tissue.

George Chabot, PhD