4.01.1.1 Damage Creation: Short-Term Evolution

4.01.1.1.1 Neutron-zirconium interaction

Zirconium alloys are used as structural components for light and heavy water nuclear reactor cores because of their low capture cross section to thermal neutrons and their good corrosion resistance. In a nuclear reactor core, zirconium alloys are subjected to a fast neutron flux (E > 1 MeV), which leads to irradiation damage of the material. In the case of metallic alloys, the irradiation damage is mainly due to elastic interaction between fast neutrons and atoms of the alloy that displace atoms from their crystallo­graphic sites (depending on the energy of the incom­ing neutron) and can create point defects without modifications of the target atom, as opposed to inelastic interactions leading to transmutation, for instance. During the collision between the neutron and the atom, part of the kinetic energy can be trans­ferred to the target atom. The interaction probability is given by the elastic collision differential cross sec — tion1, which depends on both the neutron kinetic energy and the transferred energy.3 For a typical fast neutron of 1 MeV, the mean transferred energy (T) of the Zr atom is T « 22keV. For low value of the transferred energy, the target atom cannot leave its position in the crystal, leading only to an increase of the atomic vibrational amplitude resulting in simple heating of the crystal. If the transferred energy is higher than a threshold value, the displacement energy (Ed), the knocked-on atom can escape from its lattice site and is called the primary knocked-on atom (PKA). For high transferred energy, as is the case for fast neutron irradiation, the PKA interacts with the other atoms of the alloy along its track. On average, at each atomic collision, half of its current kinetic energy is transferred to the collided atom, since they have equal masses. The collided atoms can then interact with other atoms, thus creating a displacement cascade within the crystal.

4.01.1.1.2 Displacement energy in zirconium

In the case of zirconium, the displacement energy has been measured experimentally using electron irra­diations performed at low temperatures (<10 K). The irradiation damage was monitored in situ using elec­trical resistivity changes.4,5 The measured minimum displacement threshold energy transferred to the Zr atoms is Ed = 21-24 eV. Measurements of Ed have also been performed using a high-voltage electron microscope (HVEM) to irradiate a Zr thin foil. The values obtained were found to be weakly orientation dependent, between 24 and 27.5 eV, with a mean Td of 24 eV.6

The displacement energy has also been computed by molecular dynamics (MD) simulations based on various interatomic potentials. The most accurate computations have been performed using a many — body (MB) potential based on the Finnis and Sinclair formalism.7 These authors have found that the displacement energy is significantly anisotropic. Dis­placement energy was found to be minimum for knocking out in the basal plane, that is, in the (1120) directions, corresponding to the most favor­able direction for replacement collision sequences, and to the direction of development of the basal crowdion. The corresponding displacement energy obtained (Ed = 27.5 eV) is slightly above the experi­mental values. The value averaged over all the crys­tallographic directions was found to be 55 eV. The value specified in the norm reference test standard (Standard E521-89, Annual Book of ASTM Stan­dards, ASTM, Philadelphia, PA, USA) is Ed = 40 eV.8 This value is close to the spatial means obtained by MD models.