Primary Knock-on Atom Spectra 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.

Evaluation of tungsten as divertor plasma-facing material: results from ion irradiation experiments and computer simulations

The effect of the primary knock-on atom (PKA) spectrum in radiation damage and the subsequent defect structure formation and their impact in deuterium (D) trapping has been investigated using computer simulations and surrogate ion irradiation experiments. The neutron spectrum for an ‘ITER-like’ divertor shape and parameters has been generated using ATTILA and SPECTER codes to identify the relevant PKA energies. It has been observed that 10 MeV boron (B) produces a PKA spectrum similar to that obtained from a reactor-like neutron spectrum. Experiments have been carried out with ions of gold (Au), B, helium (He) and D with energies ranging from 0.1 MeV–80 MeV for a fluence range of ions m−2– ions m−2, and distinctly different PKA spectra have been produced. While 80 MeV Au ions produced dense and small clusters of interstitial defects (<10 nm), B produced large dislocation loops up to 60 nm in size. At room temperature, the imprint of the cascade is well captured by the vacancies due to their low mobility, and the vacancy defects observed in Au and B irradiation showed significant differences. Molecular dynamics simulations show that at PKA energies exceeding 150 keV, the fragmentation of the cascades takes place, which tends to limit the size of individual defects in the case of 80 MeV Au irradiation. A mechanism based on the competitive capture of mobile interstitials has been proposed to explain the observed large dislocation loops as well as dislocation lines in different irradiation experiments.

Relativistic effect on two-body reaction inducing atomic displacement

The effect of the relativistic treatment of two-body reaction kinematics on atomic recoil energy and subsequent primary damage parameters, such as damage energy and number of atomic displacements, has been studied. If the emitted particle has a mass larger than the incident particle, the ratios of relativistic quantities in comparison to the classic mechanical ones can be null and infinite due to the null recoil energies. However, the relativistic corrections on recoil energies are limited in [-0.6%, 0.5%] and [-6%, 5%] for 20 MeV and 200 MeV neutron discrete (n,α) reactions of 56Fe. For (n,n’) and (n,p) reactions of any nucleus, the relativistic corrections are about 1% on recoil energy for a 20 MeV incident neutron, while the corrections can be more than 30% (about 10% on average) for 200 MeV incident energy. The relativistic effect is less than 5% on (α,n) reaction inducing recoil energy of any nucleus (and within 0.6% on atomic displacement damage for 56Fe) for secondary energy lower than the incident energy of 200 MeV. On the other hand, about 10 keV and 1500 keV broader Primary Knock-on Atom (PKA) spectra are respectively found for 20 MeV and 200 MeV neutron-induced (n,n’), (n,p), and (n,α) reactions of 56Fe. Therefore, the relativistic treatment of two-body reactions should be applied for computing PKA spectra and subsequent radiation damage for high energy neutrons.

Application of special relativity in damage calculations

The Displacement per Atom (DPA) is one of the most important factors that influence the mechanical behaviors of materials during and after irradiation. However, the relativistic effect is not accounted for in typical calculations of DPA cross sections and Primary Knock-on Atom (PKA) spectra. The present work shows the computation of relativistic recoil energy. For incident energy lower than 20 MeV, the relativistic corrections are within 1% for neutron scattering reactions of 56Fe and nuclei with mass larger than 56Fe. The corresponding corrections on damage energy (equivalent to DPA with standard DPA metrics) is smaller than 0.5%. However, the relativistic effect is of importance for high incident energies, such as 200 MeV. For example, the relativistic recoil energy of continuum inelastic scattering is 10% higher than the classical one for all nuclei. The consideration of relativistic effect increases the recoil energy for the elastic scattering. Both positive and negative relativistic corrections are possible for both discrete and the inelastic scatterings. A consequent result is the broadening of PKA spectra.

Simulation of neutron-induced damage in tungsten by irradiation with energetic self-ions

A direct comparison of the deuterium (D) decoration of radiation-induced damage in polycrystalline tungsten irradiated with self-ions [present work] and neutrons in the high-flux isotope reactor (HFIR) (Hatano et al., 2013) shows a reasonably good agreement at least up to 0.3 displacement per atom indicating that MeV heavy ions can be a good proxy to simulate neutron-produced damage at room temperature and low dpa. The coefficient of similarity between two kinds of irradiation was obtained experimentally to be Kexp∼0.65±0.1 in the case of the deuterium decoration of both kinds of radiation-induced defects with low and high de-trapping energies for deuterium. We introduced the theoretical estimation for coefficient of similarity between neutron- and self-ion-irradiations, which is a fraction of common area under the curves of two overlapping damage energy spectra of primary knock-on atom (PKA) produced in tungsten by these two types of irradiation. In other words, Ksim is a part of displaced atoms produced in the similar conditions under two different types of irradiation. The theoretical values of Ksim=0.34 and Ksim=0.29 were obtained for tungsten target irradiated with 20MeV self-ions in comparison to irradiation with neutrons in HFIR reactor (>0.1MeV) and 14MeV neutrons, respectively. The theoretical value of Ksim=0.34 is about two times less than the experimental value of Kexp=0.65. It means that high energy PKAs can play more important role in the production of similar damage structure by irradiation with self-ions and neutrons which is responsible for deuterium retention. The model assuming that all cascades with an energy higher than Tc=150keV split into identical sub-cascades gives the value of Ksim=0.64±0.01 for the coefficient of similarity between HFIR-neutron and 20MeV self-ion irradiations that is in an agreement with experimental value of Kexp=0.65±0.1. Consequently, splitting of high-energy part of cascades might take place in W that results in a reduction of the effective PKA spectrum and a change of types and density of post-radiation defects.

Viability of the ESS-Bilbao neutron source for irradiation of nuclear fusion materials

The ESS-Bilbao neutron source, currently under construction, is conceived as a multipurpose facility. It will offer a fast neutron beam line for materials irradiation. In this paper we discuss the viability of ESS-Bilbao for experimental studies of fusion materials. Making use of the already designed target station we have calculated the neutron spectrum expected in the fast neutron line. Then, we have studied the neutron irradiation effects in two model materials: iron and silica. We have calculated the expected PKA (primary knock-on atom) spectra and light species production as well as the damage production in these materials. Regarding structural materials, we conclude that the ESS-Bilbao neutron irradiation facility will play a minor role due to the resulting low neutron fluxes (about two orders of magnitude lower than in fusion reactors). On the other hand, ESS-Bilbao turns out to be relevant for studies of final lenses in laser fusion power plants. A comparison with the conditions expected for HiPER final lenses shows that the fluxes will be only a factor 5 smaller in ESS-Bilbao and the PKA spectra will be very similar. Taking into account, in addition, that relevant effects on lenses occur from the onset of irradiation, we conclude that an appropriate irradiation cell with in situ characterisation techniques will make ESS-Bilbao very attractive for applied neutron damage studies of laser fusion final lenses. Finally, we compare ESS-Bilbao with other facilities.

Fusion materials irradiations at MaRIE'S fission fusion facility

Los Alamos National Laboratory's proposed signature facility, MaRIE, will provide scientists and engineers with new capabilities for modeling, synthesizing, examining, and testing materials of the future that will enhance the USA's energy security and national security. In the area of fusion power, the development of new structural alloys with better tolerance to the harsh radiation environments expected in fusion reactors will lead to improved safety and lower operating costs. The Fission and Fusion Materials Facility (F 3), one of three pillars of the proposed MaRIE facility, will offer researchers unprecedented access to a neutron radiation environment so that the effects of radiation damage on materials can be measured in situ, during irradiation. The calculated radiation damage conditions within the F 3 match, in many respects, that of a fusion reactor first wall, making it well suited for testing fusion materials. Here we report in particular on two important characteristics of the radiation environment with relevancy to radiation damage: the primary knock-on atom spectrum and the impact of the pulse structure of the proton beam on temporal characteristics of the atomic displacement rate. With respect to both of these, analyses show that F 3 has conditions that are consistent with those of a steady-state fusion reactor first wall.

Analysis of displacement damage in materials in nuclear fusion facilities (DEMO, IFMIF and TechnoFusion)

Present pathway to fusion reactors includes a rigorous material testing program. To reach this objective, irradiation facilities must produce the displacement damage per atom (dpa), primary knock-on atom (PKA) spectrum and gaseous elements by transmutation reactions (He, H) as closely as possible to the ones expected in the future fusion reactors (as DEMO).The irradiation parameters (PKA spectra and damage function) of some candidate materials for fusion reactors (Al 2O 3, SiC and Fe) have been studied and then, the suitability of some proposed experimental facilities, such as IFMIF and TechnoFusion, to perform relevant tests with these materials has been assessed.The following method has been applied: neutron fluxes present in different irradiation modules of IFMIF have been calculated by the neutron transport McDeLicious code. In parallel, the energy differential cross sections of PKA have been calculated by using the NJOY code. After that, the damage generated by the PKA spectra was analyzed using the MARLOWE code (binary collision approximation) and custom analysis codes. Finally, to analyze the ions effects in different irradiation conditions in the TechnoFusion irradiation area, the SRIM and Marlowe codes have been used. The results have been compared with the expected ones for a DEMO HCLL reactor.

Displacement damage quantification in future fusion systems

The demanding neutronic environment in a future fusion power plant will require a thorough understanding of the mechanisms at work in producing displacement damage, their magnitudes, and their effects on the materials and their properties. It is a pre-requisite to have a precise understanding of the Primary Knock-on Atom (PKA) energy spectra caused by 14.1 MeV fusion neutrons in order to have a good foundation for a quantitative determination of the damage driving phenomena produced as a result of neutron encounters. At the higher neutron energies involved in fusion compared to fission, inelastic collisions become very much more significant. This adds to the complexity of estimates of the deposited energy. A Monte-Carlo transport code (MCNP5) has been adapted to provide neutron induced PKA energy spectra for both elastic and inelastic neutron collisions. The introduction of PKA calculations of this type to an MCNP code is new. The Norgett–Robinson–Torrens [M. Norgett, M. Robinson, I. Torrens, A proposed method of calculating displacement dose rates, Nucl. Eng. Design 33 (1975) 50–54] modification to the Kinchin–Pease [G. Kinchin, R. Pease, The displacements of atoms in solids by radiation, Rep. Prog. Phys. 23 (1955) 1–51] model of atomic displacements allows a convenient normalisation measure, the displacements per atom (dpa), allowing for both elastic and inelastic neutron collisions. Application of this work to models of the Power Plant Conceptual Study (PPCS) [G. Marbach, I. Cook, D. Maisonnier, The EU power plant conceptual study, Fusion Eng. Design 63–64 (2002) 1–9] very efficiently generates the PKA spectra and dpa damage as a function of location, providing data that can be used in the design of both power plants and materials testing facilities.

Radiation damage conditions for ESS target hull and irradiation rigs

In addition to their original mission, neutron spallation sources were also considered as a possible test bed for fusion material irradiation. Accordingly an irradiation module with 0.83 l subdivided into 10 instrumented horizontal rigs was proposed in the target–reflector interface of the ESS 5 MW long-pulse target. The irradiation conditions of this irradiation module and of the front-end target hull are given on the basis of calculations with a detailed geometry model using the neutron, photon and charged particle transport code MCNPX. Neutron and primary knock-on atom spectra, displacement damage and gas production rates as well as spallation element production rates were derived and compared with those of a fusion demonstration reactor (DEMO) and the International Fusion Materials Irradiation Facility (IFMIF). In particular, it was shown that the He/dpa ratio for the ESS reflector position is between 5 and 6 in Fe-based alloys, that is about a factor of two lower than expected for the DEMO fusion reactor. While the generation of spallation products like P, S and Ca, which are known to induce structural material embrittlement, is between 40 and 90 appm per year in the ESS target hull and is comparable with their initial concentration in the alloy, their production at the reflector position is negligibly small.

Method using the primary knock-on atom spectrum to characterize electrical degradation of monocrystalline silicon solar cells by space protons

In this work, a method to calculate the equivalent dose for orbits in which the main part of the damage is due to protons, is presented. The method compares the primary knock-on atom distribution and the depth dependence of damage for the cases of space-proton irradiation and laboratory irradiation of silicon solar cells, as obtained from simulations using the transport of ions in matter code. It is shown that, in order to simulate the right depth damage distribution in space for a solar cell covered by glass, it is convenient to irradiate the cell from the backside with a proton energy dependent of the glass thickness. The method is applied to predict the electrical degradation of a silicon solar cell at the end of its life in a low altitude orbit and to estimate the damage produced as consequence of the October 19, 1989 solar proton event. The results obtained are compared with predictions of the U.S. Jet Propulsion Laboratory method.

00/01881 Evaluation of neutron energy spectrum effects and RPV thru-wall attenuation based on molecular dynamics cascade simulations

Displacement cascade formation in iron has been investigated by the method of molecular dynamics (MD) for cascade energies up to 40 keV. The results of these simulations have been used in the SPECOMP code to obtain effective, energy-dependent cross sections for two measures of primary damage production: (1) the number of surviving point defects expressed as a fraction of the those predicted by the standard secondary displacement model by Norgett, Robinson, and Torrens (NRT), and (2) the fraction of the surviving interstitials contained in clusters that formed during the cascade event. The primary knockon atom spectra for iron obtained from the SPECTER code have been used to weight these MD-based damage production cross sections in order to obtain spectrally-averaged values for several locations in commercial fission reactors and test reactors. An evaluation of these results indicates that neutron energy spectrum differences between the various environments do not lead to significant differences between the average primary damage formation parameters. In particular, spectrum-averaged defect production cross sections obtained for PWR and BWR neutron spectra were not significantly different. A representative application of the new defect production cross sections is provided by examining how they vary as a function of depth into the reactor pressure vessel wall. A slight difference was noted between the damage attenuation in a PWR vessel and a BWR vessel. This observation could be explained by a subtle difference in the energy dependence of the neutron spectra. Overall, the simulations indicate that spectrum-averaged defect production cross sections do not vary much among the various environments in light-water moderated fission reactors. As such, the results support the use of dpa as a damage correlation parameter and provide guidance for choosing the primary damage source term in kinetic embrittlement models.

Subcascade formation in displacement cascade simulations: Implications for fusion reactor materials

Displacement cascade formation in iron has been investigated by the method of molecular dynamics (MD) for cascade energies up to 40 keV, corresponding to PKA energies up to 61 keV. The results of these simulations have been used in the SPECOMP code to obtain effective, energy-dependent cross sections for two measures of primary damage production: (1) the number of surviving point defects expressed as a fraction of the those predicted by the standard secondary displacement model by Norgett, Robinson, and Torrens (NRT), and (2) the fraction of the surviving interstitials contained in clusters that formed during the cascade event. The primary knockon atom spectra for iron obtained from the SPECTER code have been used to weight these MD-based damage production cross sections in order to obtain spectrally averaged values for several locations in commercial fission reactors, materials test reactors, and a DT fusion reactor (ITER) first wall. An evaluation of these results indicates that neutron energy spectrum differences between the various environments do not lead to significant differences between the average primary damage formation parameters. This conclusion implies that the displacement damage component of radiation damage produced in a DT fusion reactor should be well simulated by irradiation in a fission reactor neutron spectrum, and that differences in nuclear transmutation production may be the primary source of uncertainty in the prediction of material performance at high doses in DT fusion reactors.

New approaches to simulating fusion damage in fission reactors

Some novel approaches to simulating key fusion reactor damage production parameters are outlined. These involve alloy fabrication with pre-irradiated and/or isotopically modified nickel, chromium and iron. Irradiations of these alloys in the High Flux Isotope Reactor can be used to study the effects of the He/dpa and H/dpa ratio, elemental evolution and primary knock-on atom spectrum. A second approach to producing controlled He/dpa involves in-situ implantation with high energy a particles from the triple fission of 233U; the 233U is bred in-situ from 232Th in HFIR. In principle, this technique can be applied to any alloy system.

Neutron cross sections for defect production by high energy displacement cascades in copper

Defect production cross sections for copper have been devised, based on computer simulations of displacement cascades. One thousand cascades ranging in energy from 200 eV to 200 keV were generated with the MARLOWE computer code. The cascades were subjected to a semi-empirical cascade quenching procedure and to short-term annealing with the ALSOME computer code. Functions were fitted to the numbers of defects produced as a function of primary knock-on atom (PKA) damage energy for the following defect types: 1) the total number of point defects after quenching and after short-term annealing, 2) the numbers of free interstitials and free vacancies after shortterm annealing, and 3) the numbers and sizes of vacancy and interstitial clusters after shortterm annealing. In addition, a function describing the number of distinct damage regions (lobes) per cascade was fitted to results of a graphical analysis of the cascade configurations. The defect production functions have been folded into PKA spectra using the NJOY nuclear data processing code system with ENDF/B-V nuclear data to yield neutron cross sections for defect production in copper. The free vacancy cross section displays much less variation with neutron energy than the cross sections for damage energy or total point defects.

Sensitivity of Primary Knock-On Atom Spectra and Displacement per Atom Cross Sections to Different Secondary Neutron Energy and Angular Distributions and “In-Group” Weighting Schemes

Sensitivity of Primary Knock-On Atom Spectra and Displacement per Atom Cross Sections to Different Secondary Neutron Energy and Angular Distributions and “In-Group” Weighting Schemes

Radiation-Damage Calculations: Primary Knock-On Atom Spectra, Displacement Rates, and Gas Production Rates

A data base of primary knock-on atom spectra and an analysis program have been created to assist experimentalists in studying, evaluating, and correlating radiation-damage effects in different neutron environments. Since experimentally obtained typical controlled thermonuclear reactor (CTR) neutron spectra are not presently available, the data base can be extremely useful in relating currently obtainable radiation damage to that anticipated in future fusion devices. However, the usefulness of the data base is not restricted to only CTR needs. Most of the elements of interest to the radiation-damage community and all neutron reactions of any significance for these elements have been processed, using available ENDF/B-IV cross-section data, and are included in the data base. Calculated data such as primary knock-on atom spectra, displacement rates, and gas production rates, obtained with the data base, for different radiation environments are presented and compared with previous calculations.