Secondary Knock-on Atoms Research Articles

Evaluation of neutron radiation damage in zircaloy fuel clad of nuclear power plants: a study based on PKA and dpa calculations

During the lifetime of nuclear power plants, irradiation and mechanical interactions can cause permanent damage to their structural materials. In nuclear reactors, one of the most important of these materials is fuel clad that, in addition to its main role in transferring heat from fuel to coolant, restrains most of the radioactive fission products within its volume. In this article, neutron-induced radiation damage based on Primary Knock-on Atom (PKA) and displacement per atom (dpa) calculations are evaluated on fuel clad in Hot Fuel Assembly (HFA) in a WWER-1000 reactor core as a case study. New couplings of existing nuclear and radiation damage codes and methods are developed for dpa calculations and results are compared together and against experimental results. In the first step, the full reactor core is simulated with MCNPX code (v2.7) to determine the HFA and its relevant neutron-energy spectrum. SPECTER, SPECTRA-PKA and PTRAC card of MCNPX are then employed to evaluate the PKA spectrum under neutron bombardment. Finally, dpa calculated with the Binary Collision Approximation approach is achieved through the use of SRIM-2013 code. A MATLAB interchange program is developed to prepare and transfer the data between the mentioned codes and SRIM. Comprehensive comparisons are performed between results in terms of dpa obtained by (i) NRT calculation (NRT-dpa) (SPECTER and SPECTRA-PKA results), (ii) arc-dpa calculation (MCNPX+arc-dpa), (iii) BCA calculation (PTRAC+SRIM) and (iv) coupled calculation (SPECTRA-PKA+SRIM) (all generations of knock-on atoms and clusters are tracked in the last two methods). These comparisons reveal the portions of damage in Zircaloy due to PKAs, secondary knock-on atoms and cascade atoms, the effect of position-depended PKA and the fraction of displaced atoms that come back to their lattice positions as a result of thermal effects and annealing. Effects of elemental PKA are obtained by means of SPECTRA-PKA code and a visualization map of radiation damage in the clad is presented as an outcome of the SRIM code simulation. Results confirm that value of dpa calculated by MCNPX+arc-dpa is closer to experimental ones as the recombination effect in radiation damage is considered in this method.

A combination method for simulation of secondary knock-on atoms of boron carbide induced by neutron irradiation in SPRR-300

A multiscale sequence of simulation should be used to predict properties of materials under irradiation. Binary collision theory and molecular dynamics (MDs) method are commonly used to characterize the displacement cascades induced by neutrons in a material. In order to reduce the clock time spent for the MD simulation of damages induced by high-energy primary knock-on atoms (PKAs), the damage zones were split into sub-cascade according to the sub-cascade formation criteria. Two well-known codes, Geant4 and TRIM, were used to simulate high-energy PKA-induced cascades in B4C and then produce the secondary knock-on atom (SKA) energy spectrum. It has been found that both high-energy primary knock-on B and C atoms move a long range in the boron carbide. These atoms produce sub-cascades at the tip of trajectory. The energy received by most of the SKAs is <10keV, which can be used as input to reduce the clock time spent for MD simulation.

Analysis of neutron spectrum effects on primary damage in tritium breeding blankets

The effect of neutron spectrum on primary damages in a structural material of a tritium breeding blanket is investigated with a newly established recoil spectrum estimation system. First, a recoil spectrum generation code is developed to obtain the energy spectrum of primary knock-on atoms (PKAs) for a given neutron spectrum utilizing the latest ENDF/B data. Secondly, a method for approximating the high energy tail of the recoil spectrum is introduced to avoid expensive molecular dynamics calculations for high energy PKAs using the concept of recoil energy of the secondary knock-on atoms originated by the INtegration of CAScades (INCAS) model. Thirdly, the modified spectrum is combined with a set of molecular dynamics calculation results to estimate the primary damage parameters such as the number of surviving point defects. Finally, the neutron spectrum is varied by changing the material of the spectral shifter and the result in primary damage parameters is examined.

Modeling of cascade and sub-cascade formation at high PKA energies in irradiated fusion structural materials

A new theoretical model is developed for the investigations of cascade and sub-cascade formation in fusion structural materials under fast neutron irradiation at high primary knock-on atom energies. Light fusion structural materials: such as Be, C and SiC under 14 MeV neutron irradiation in fusion reactor will have the primary knock-on atoms with the energies up to 1 MeV. It is very difficult to use at such high energies the Monte-Carlo or molecular dynamic simulations [H.L. Heinisch, B.N. Singh, Philos. Mag. A67 (1993) 407; H.L. Heinisch, B.N. Singh, J. Nucl. Mater. 251 (1997) 77]. The developed model is based on the analytical consideration of elastic collisions between displaced moving atoms produced by primary knock-on atoms with some kinetic energies obtained from fast neutrons and crystal lattice atoms. The Thomas–Fermi interaction potential is used here for the description of these elastic atomic collisions. The suggested model takes into account also the electronic losses for moving atoms between elastic collisions. The self-consistent criterion for sub-cascade formation is suggested here which is based on the comparison of mean distance of primary knock-on atoms between consequent collisions of them with the target atoms and a size of sub-cascade produced by moving secondary knock-on atoms produced in such collisions. The analytical relations for the most important characteristics of cascades and sub-cascades are determined including the average number of sub-cascades per one primary knock-on atom in the dependence on its energy, the distance between sub-cascades and the average cascade and sub-cascade sizes. The developed model allows determining the total numbers, distribution functions of cascades and sub-cascades in dependence on their sizes and generation rate of cascades and sub-cascades for the different fusion neutron energy spectra. On the basis of this developed model the numerical calculations for main characteristics of cascades and sub-cascades in different fusion structural materials: Fe, V, C, Al, Be and W are performed here using the neutron fluxes and primary knock-on atom energy spectra for fusion reactors: ITER and DEMO.

Quantum Mechanical Description of Displacement Damage Formation

Atomic-scale processes during displacement damage formation have been previously studied using molecular dynamics (MD) calculations and empirical potentials. Low-energy displacements (1 keV) are characterized by a high cross-section for producing secondary knock-on atoms and damage clusters, and determine the threshold displacement energy (an important parameter in NIEL calculations). Here we report first-principles, parameter-free quantum mechanical calculations of the dynamics of low-energy displacement damage events. We find that isolated defects formed by direct displacements result from damage events of les100 eV. For higher energy events, the initial defect profile, which subsequently undergoes thermal annealing to give rise to a final stable defect profile, is the result of the relaxation and recrystallization of an appreciable volume of significantly disordered and locally heated crystal surrounding the primary knock-on atom displacement trajectory.

Angular distribution of atoms sputtered from silicon by 1–10 keV Ar ions

The angular distribution of atoms sputtered from silicon under 1–10 keV Ar ion bombardment (normal incidence) has been studied experimentally and using computer simulation. It has been found that the angular distribution is overcosine in the whole energy range studied. This is connected with a high contribution of secondary knock-on atoms to the sputter flux (cascade sputtering). The simulated angular distributions are shown to be quite sensitive to the variation of the interatomic potential. The results obtained are compared with data from the literature.

Analysis of the contributions of PKA and SKA to the isotope enrichment

The contribution of primary knock-on atoms(PKA) or that of secondary knock-on atoms(SKA) to the zero-fluence isotope enrichment, has been analyzed with the help of Monte Carlo (MC) simulations. For low-energy and zero-fluence (CU0.5Cu0.5)-C-63-Cu-65 sputtering, the present simulations show the isotope enrichment to be the contribution of the momentum asymmetry of SKA. For low-energy and zero-fluence Mo-92(0.5) Mo-100(0.5) sputtering, the MC simulations of Ref. [24] showed the isotope enrichment to be the contribution of PKA. However, for this sputtering, the present simulations show that the isotope enrichment is the contribution of the momentum asymmetry of PKA rather than the contribution of PKA.

Monte Carlo simulation of angular distribution and fractionation in the zero-fluence isotope sputtering

A static Monte Carlo simulation program has been used to study the influence of primary and secondary knock-on atoms on the angular distribution and the fractionation, for zero-fluence sputtering of 92Mo– 100Mo. At 10 keV Xe ion energy, calculations show that significant depth-dependent momentum asymmetry exists in the near-surface region, as in Refs. (L.P. Zheng, R.S. Li, M.Y.Li, Nucl. Instr. and Meth. B 100 (1995) 490; L.P. Zheng, M.Y. Li, Nucl. Instr. and Meth. B 114 (1996) 28). The calculations also show that at the polar emission angle range between 0° and 90° the probabilities of the sputtered primary knock-on atoms for 92Mo and 100Mo are small (6.1% for 92Mo and 6.6% for 100Mo), especially, between 0° and 30° they are very small (2.5% for 92Mo and 2.8% for 100Mo). At 0.1 keV Ar ion energy, the calculations show that partial momentum flux ratios, for the primary and the secondary knock-on atoms, deviate far from one, through marker plane 1 (about an atomic layer distance from the surface).

A Monte Carlo simulation of angular distribution in low energy sputtering

A Monte Carlo simulation is used to study the angular distribution of sputtered atoms from Fe and Lu targets bombarded by Ne, Ar and Kr ions at energies of 0.2 keV and 0.5 keV. The sputtered atoms are distinguished as primary knock-on atoms (PKA) and secondary knock-on atoms (SKA) in the simulation. It is found that the angular distribution of the PKAs tends to an under-cosine shape, while the angular distribution of the SKAs shows either a cosine shape or an over-cosine shape. The calculations also give the contributions of the PKAs and the SKAs to sputtering yields. Based on the computations, the mechanism of the under-cosine angular ejection distribution at low energy sputtering is discussed.

Light Ion Sputtering of Metals and Low Z Compounds as Studied with the Monte-Carlo Code Trim

Sputtering yields for light ions in the energy range of 0.1-10 keV (particles from fusion plasma) or 40-160 keV under oblique angles (from neutral beam injectors) are difficult to predict by analytic theories. In particular, the sputtering of first wall coatings with low Z compound materials, e.g. TiB/sub 2/, TiC, cannot be reliably treated in an analytic theory. For these reasons, a large number of cases were studied with the Monte-Carlo code TRIM over the past years. Numerous results were obtained for H, D, T, and He ions incident at various energies and angles on fusion first wall materials (metals and low Z compounds). In addition the sputtering yields as a function of incident energy and angle, and the angular and energy distributions of the sputtered atoms were investigated. Further studies were performed to gain more information on the mechanisms involved: sputtered atoms resulting from incident versus reflected ions, primary knock-on versus secondary knock-on atoms, atoms from the surface versus deeper layers of origin, etc. Experimental data, as far as available, will be compared with the TRIM results.

Sputtering studies with the Monte Carlo Program TRIM.SP

The Monte Carlo Program TRIM.SP (sputtering version of TRIM) was used to determine sputtering yields and energy and angular distributions of sputtered particles in physical (collisional) sputtering processes. The output is set up to distinguish between the contributions of primary and secondary knock-on atoms as caused by in- and outgoing incident ions, in order to get a better understanding of the sputtering mechanisms and to check on previous theoretical models. The influence of the interatomic potential and the inelastic energy loss model as well as the surface binding energy on the sputtering yield is investigated. Further results are sputtering yields versus incident energy and angle as well as total angular distributions of sputtered particles and energy distributions in specific solid angles for non-normal incidence. The calculated data are compared with experimental results as far as possible. From this comparison it turns out that the TRIM.SP is able to reproduce experimental results even in very special details of angular and energy distributions.

Ejection of fast recoils under ion bombardment of crystals

Abstract Ejection of surface recoils produced by Cu crystal irradiation with 1–40 keV Ar ions has been studied using computer simulation in terms of the atomic chain model. The case where a parallel ion beam bombards a nonvibrating chain at 25° to the chain axis is considered here as example. Only high energy (> 0.2 keV) backsputtered and also backscattered particles are analyzed. It has been found that the main contribution to the flux of backsputtered particles is from the primary and secondary knock-on atoms. The atoms may be ejected in a given direction by various ways whose number is a function of the initial ion energy and ejection angle. The angular and energy distributions of these atoms are pronouncedly anisotropic due, in particular, to the shadowing and focusing effects in scattering of ions and recoils by the chain atoms. The possibility for the particles to be doubly focussed is noted. The results obtained in terms of the two- atomic model are also considered here.