A new tool to compare neutron and ion irradiation in materials

The slowing down of neutrons produced in nuclear plants or in spallation sources generates many defects in a material, driving this material far from its equilibrium state. This concentration of defects leads to important structural modifications of solids. To model the impact of radiation in solids, particle accelerators are useful tools. The nature and the energy of ions in accelerators must be chosen to produce similar effects to those occurring in nuclear plants. A new program, DART, based on the binary collision approximation, has been developed to optimize the choice of incident particles. In this program, the anisotropy of the neutron atom interaction is treated with a new formalism. Such a formalism allows us to compute accurate displacement cross sections and recoil spectra for all kinds of particles (ions, neutrons, or electrons). The comparison of these displacement cross sections as well as recoil spectra due to ions, electrons and neutrons permits to define the nature and the energy of ions able to simulate damages in reactors.     

6H-SiC JFET 输出特性及其中子辐照模型

基于沟道调效应、串联电阻效应的考虑,首先建立了一个和实验室符合很好的６Ｈ－ＳｉＣＪＦＥＴ的模型,在该模型中采用了两种电离杂质模型和Ｃａｕｇｈｅｙ－Ｔｈｏｍａｓ方程,接着在分析中子辐照对ＳｉＣ ＪＦＥＴ电参数如电子浓度、迁移率、电阻率和空间电荷区密度影响的基础上,对ＳｉＣ ＪＦＥＴ在室温和３００℃时的辐照响应进行了模拟。模拟结果和实验相符。     

A model for the detector response function in neutron depth profiling

Present practice in thermal neutron depth profiling (TNDP) is to use small fractional detector solid angles of the order of 0.1% or less. This allows one to assume that the distortion in the charged particle spectra caused by the solid angle subtended by the detector can be approximated by a Gaussian distribution. While this simplifies the mathematical analysis greatly, it requires very long nuclear reactor irradiation times for many applications of interest.The present paper derives and verifies a model for spectral distortion caused by detecting with circular detectors subtending any solid angle, which allows the use of much larger detector solid angles and correspondingly shorter irradiations.Experimental results with a circular silicon surface barrier detector in conjunction first with a circular coaxially mounted ²⁴¹Am source of alpha particles passing through various thickness of mylar and then with a boron doped silicon wafer mounted in a thermal neutron reactor beam at a 30 ° angle so as to give an elliptically shaped target of alpha particles both gave excellent agreement with the derived model. The latter experiment is a typical TNDP application.     

A swelling model for stoichiometric SiC at temperatures below 1000°C under neutron irradiation

A simple phenomenological model for the saturation swelling below 1000°C of neutron-irradiated silicon carbide (SiC) is presented in this paper. Under fast neutron irradiation, SiC is known to undergo volumetric expansion (swelling) which quickly saturates at a fast fluence of approximately 10²⁵ n/m² for irradiation temperatures below 1000°C. A previous model due to Balarin attributes swelling to lattice dilation as a result of single point defects. We show in this paper that the experimentally observed linear temperature dependence of saturation swelling can be explained in terms of the formation and growth of small interstitial clusters, resulting directly from collision cascades initiated by energetic neutrons. These loops grow by absorption of mobile carbon interstitials and their composition is subject to stoichiometry constraints, requiring absorption of slower silicon interstitials. Because of cascade re-solution events, the density of loops decreases sharply with temperature as a result of overlap of cascades with larger size loops at higher temperatures. The average radius of these loops increases with temperature. Volumetric swelling is shown to obey a linear temperature dependence as a consequence of the strong decrease in density and the simultaneous increase in average radius, and to saturate with fluence. The model is shown to be consistent with experimental observations. In the temperature range below 500–600°C, swelling seems to be dominated by single point defects, or defect clusters containing only a few atoms, in accordance with the explanation offered by Balarin.     

A new facility for investigation on neutron irradiation effect on superconducting properties of Nb3Sn strand for fusion magnet

To perform post irradiation tests of superconducting strands, a 15.5 T superconducting magnet and a variable temperature insert (VTI) were installed at a radiation control area in Oarai center, Institute for Materials Research, Tohoku University. Both of the 15.5 T magnet and the VTI are conductively cooled with GM refrigeration. The commissioning test of the system is still ongoing because unexpected troubles occurred during the commissioning. Also, a SQUID system with the maximum field of 7 T has been installed at another radiation control area to investigate the magnetic property of uranium and its isotopes. These devices are very useful to study the electro-magnetic properties of the neutron irradiated superconducting strands. This paper will introduce the new superconducting test facility, and some data recently obtained will be presented together with the data of magnetization evaluated with the SQUID, and the discussion on the irradiation effect on superconducting properties will be performed.     

A relation between irradiation temperature, flux intensity and start-temperature for stored energy release in irradiated graphite

In graphite which has been subjected to fast-neutron bombardment, the temperature at which the release of stored-energy commences during thermal annealing at a linear rate of rise of temperature with time, the start-temperature is of both theoretical and of practical interest. Consideration of defects which are just stable under the irradiation conditions in terms of Vand annealing kinetics enables a temperature representing the onset of the energy release to be calculated with sufficient precision for use in practical applications.     

A new equation for activity calculation in pulse irradiation:derivation,simulation, and experimental validation

To calculate the radioactivity of product nuclides generated in pulse irradiation, it is generally assumed that the irradiation is approximately continuous in the entire irradiation period(ti) and the flux of the incoming irradiation particle can be obtained by averaging their intensity in each pulse period(T). However, this approximation fails to acknowledge the fact that the product nuclides are not created in each pulse period(T)evenly: They are only produced in a very short pulse width(tp) and then decay in a relatively long rest time(tr = T-tp). Given by the enormous number of pulses, the sum of these decays may not be negligible. To make the activity calculation in accordance with the real situation in pulse irradiation, we scrutinize the details of irradiation and decay processes in each pulse, apply the geometric series to obtain the activity superimposition of millions of pulses,and derive a novel activity equation particularly suitable for pulse irradiation. The experimental results,numerical simulations,and activity measurements from photon activation driven by a pulsed electron LINAC have confirmed the validity of this new equation. The comparison between the new and traditional equations indicates that their discrepancy could be significant under certain conditions. The limitations of the new activity equation for pulse irradiation are discussed as well.     

A new series-expansion approach in Monte Carlo: Application to neutron shielding

A new method for series expansion by means of the Monte Carlo technique is presented. It is applied to neutron shielding problems with very good results.     

A new three-dimensional method of characteristics for the neutron transport calculation

The method of characteristics (MOC) is a very flexible and effective method for the neutron transport calculation in a complex geometry. It has been well developed in two-dimensional geometries but meets serious difficulty in three-dimensional geometries because of the requirements of large computer memory and long computational time. Due to the demand related to the advanced reactor design for complex geometries, an efficient and flexible three-dimensional MOC is needed. This paper presents a modular ray tracing technique to reduce the amount of the ray tracing data and consequently reduce the memory. In this method, the object geometry is dissected into many cuboid cells by a background mesh. Typical geometric cells are picked out and ray traced, and only the ray tracing data in these typical cells is stored. Furthermore, the Coarse Mesh Finite Difference (CMFD) acceleration method is employed to save computing time. Numerical results demonstrate that the modular ray tracing technique can significantly reduce the amount of ray tracing data, and the CMFD acceleration is effective in shorting the computing time.     

A new method for the evaluation of neutron elastic transformation matrices

A new method for the evaluation of the center-of-mass to laboratory co-ordinates system transformation matrix is developed, by which each element of the matrix is represented as a finite sum of terms depending on the mass number of the scattering nucleus. At present, the elastic transformation matrices given in the ENDF/B files are evaluated by the Amster method, by which each element of the transformation matrix is represented as an infinite sum of terms. A comparison is made between the values of the elements of the elastic transformation matrix of ⁶Li obtained by the new method and those given in the ENDF/B-V file, and some discrepancies are revealed.