Effect Of Neutron Fluence Research Articles

Quantifying the effects of neutron fluence on proton signal retention in CR-39.

This paper reports on investigations on the impact of higher neutron fluences on the detection efficiency of protons with CR-39, a charged particle track detector. CR-39 is widely used as a diagnostic for inertial fusion applications and is an integral component of numerous particle diagnostics at the OMEGA laser facility and National Ignition Facility. As experiments continue to produce higher and higher yields, existing diagnostics are impacted by higher particle fluences than they were originally designed for. This paper presents data from experiments measuring proton signal on pieces of CR-39 with different levels of neutron fluence with two different etch times. The experiments show a decrease in signal recovery with increased neutron fluence, which is exacerbated at longer etch times. At 3h etch time, data suggest a 17% ± 7% signal loss at 1.3 × 105 neutron-induced tracks per cm2 and a 67% ± 21% loss at 6h etch time. Careful signal isolation techniques can recover most of the proton tracks even with moderate neutron fluence.

Fabrication and characterization of high-resolution 4H-SiC epitaxial radiation detectors for challenging reactor dosimetry environments

Reactor dosimetry environments require radiation detectors that are capable of operating at high temperatures in extremely high neutron and gamma-ray dose rates. Silicon carbide (SiC) is one of the most promising wide bandgap semiconductors (3.27 eV) for harsh environment applications due to its radiation hardness, high breakdown voltage, high electron saturation velocity, and high thermal conductivity. In this paper, we summarize the prospect of Schottky barrier radiation detectors, fabricated on highly crystalline low-defect detector-grade n-type 4H-SiC epitaxial layers with thickness ranging from 20 to 250 lm, for harsh environment applications. A comprehensive discussion on the characterization of the parameters that influence the energy resolution has been included. The usage of electrical and radiation spectroscopic measurements for characterizing the junction and rectification properties, minority carrier diffusion lengths, and energy resolution has been elaborated. Characterization of crucial factors that limit the energy resolution of the detectors such as charge trap centers using thermally stimulated transient techniques is summarized. Finally, the effect of neutron fluence on the performance of the 4H-SiC detectors is discussed.

Effect of neutron fluence on microstructure and mechanical properties of Al 5052 irradiated in the National Research Universal (NRU) reactor

In support of the fitness service assessment on the National Research Universal (NRU) vessel and full evaluation of its properties, an Al 5052 rod made from the same material and irradiated in the NRU reactor, was retrieved and sectioned for microstructure characterization and mechanical testing. Along its axial position, the rod was exposed to various levels of neutron fluence ranging from 0 to 11.9x1026 n/m2 (E < 0.625 eV) and 8.5x1025 n/m2 (E > 0.1 MeV). The estimated peak fluence in the NRU vessel is within this range, i.e., 2.4x1026 n/m2 (E < 0.625 eV) and 8.0x1022 n/m2 (E > 0.1 MeV) after 44 years of service. The hardness, chemical compositions, microstructure, tensile and fracture toughness properties were examined from different sections of the Al 5052 I-rod. The study shows that the hardness increased with continuous production of transmuted Si until it reached a plateau, when all of the dissolved Mg (2.5 wt%) was consumed to form radiation-induced Mg–Si precipitates. Excessive Si produced by transmutation primarily precipitated on grain boundaries. Increase in thermal fluence caused the changes in sizes and chemical compositions of the precipitates in the grain interior and boundaries. The yield stress and ultimate tensile strength increased with increasing thermal fluence, but uniform elongation and fracture toughness decreased. The current results show that the NRU vessel wall is expected to have sufficient ductility, strength and material integrity until the planned shutdown in March 2018, and improve the understanding the effects of neutron irradiation on Al 5000 series alloys, which are widely used as vessel materials in experimental reactors.

Bayesian nonparametric analysis of crack growth rates in irradiated austenitic stainless steels in simulated BWR environments

To investigate influential parameters for irradiation-assisted stress corrosion cracking (IASCC) growth behavior, we attempt to analyze statistically existing data on the crack growth rate (CGR) in irradiated austenitic stainless steels (SSs) in boiling water reactor (BWR) environments using the Bayesian nonparametric (BNP) method. From the probability distribution of CGR and some input parameters, such as yield stress of irradiated material (σYS-irr), stress intensity factor (K), electrochemical corrosion potential (ECP), and fast neutron fluence, the mean CGR is estimated and compared with the measured CGR. The analytical results show good reproducibility of the measured CGR. The results also indicate the possible neutron fluence effects on CGR in high CGR region (i.e., high neutron fluence condition) by radiation-induced segregation (RIS), localized deformation, and/or other mechanisms than radiation hardening.

Neutron fluence effects on SLOWPOKE-2 beryllium reflector composition and reactivity

Within the scope of the conversion process from HEU to LEU of the Jamaican SLOWPOKE-2 reactor (JM-1), the effects of the neutron fluence on the beryllium reflector composition, and the corresponding effect on reactivity throughout the life of the reactor core, have been studied. Two different methods have been used and compared involving MCNP5, ORIGEN2.2, ORIGEN-S and COUPLE codes, reaching excellent agreement between them. The neutron flux profile and energy spectrum specific to the beryllium reflectors of this reactor design have been taken into account to analyze several scenarios, comprising both real and hypothetical conditions and involving different initial reflector compositions and reactor burnups. The analysis has been extended to provide estimates for the similar MNSR reactor design and compared with previously published predictions for the Syrian MNSR. The results show small overall reactivity effects in most cases, being dominated by impurity depletion as opposed to poison buildup, contrarily to what generally occurs in beryllium reflected reactors of far higher power and to MNSR predictions. The resulting reactivity increases are typically of less than 0.4mk for usual impurity levels and maximum HEU core burnup achievable.

Re-evaluating 142Nd/ 144Nd in lunar mare basalts with implications for the early evolution and bulk Sm/Nd of the Moon

The Moon likely accreted from melt and vapor ejected during a cataclysmic collision between Proto-Earth and a Mars-sized impactor very early in solar system history. The identical W, O, K, and Cr isotope compositions between materials from the Earth and Moon require that the material from the two bodies were well-homogenized during the collision process. As such, the ancient isotopic signatures preserved in lunar samples provide constraints on the bulk composition of the Earth. Two recent studies to obtain high-precision 142Nd/ 144Nd ratios of lunar mare basalts yielded contrasting results. In one study, after correction of neutron fluence effects imparted to the Nd isotope compositions of the samples, the coupled 142Nd– 143Nd systematics were interpreted to be consistent with a bulk Moon having a chondritic Sm/Nd ratio [Rankenburg K., Brandon A. D. and Neal C. R. (2006) Neodymium isotope evidence for a chondritic composition of the Moon. Science 312, 1369–1372]. The other study found that their data on the same and similar lunar mare basalts were consistent with a bulk Moon having a superchondritic Sm/Nd ratio [Boyet M. and Carlson R. W. (2007) A highly depleted Moon or a non-magma origin for the lunar crust? Earth Planet. Sci. Lett. 262, 505–516]. Delineating between these two potential scenarios has key ramifications for a comprehensive understanding of the formation and early evolution of the Moon and for constraining the types of materials available for accretion into large terrestrial planets such as Earth. To further examine this issue, the same six lunar mare basalt samples measured in Rankenburg et al. [Rankenburg K., Brandon A. D. and Neal C. R. (2006) Neodymium isotope evidence for a chondritic composition of the Moon. Science 312, 1369–1372] were re-measured for high-precision Nd isotopes using a multidynamic routine with reproducible internal and external precisions to better than ±3 ppm (2 σ) for 142Nd/ 144Nd ratios. The measurements were repeated in a distinct second analytical campaign to further test their reproducibility. Evaluation of accuracy and neutron fluence corrections indicates that the multidynamic Nd isotope measurements in this study and the 3 in Boyet and Carlson [Boyet M. and Carlson R. W. (2007) A highly depleted Moon or a non-magma origin for the lunar crust? Earth Planet. Sci. Lett. 262, 505–516] are reproducible, while static measurements in the previous two studies show analytical artifacts and cannot be used at the resolution of 10 ppm to determine a bulk Moon with either chondritic or superchondritic Sm/Nd ratios. The multidynamic data are best explained by a bulk Moon with a superchondritic Sm/Nd ratio that is similar to the present-day average for depleted MORB. Hafnium isotope data were collected on the same aliquots measured for their 142Nd/ 144Nd isotope ratios in order to assess if the correlation line for 142Nd– 143Nd systematics reflect mixing processes or times at which lunar mantle sources formed. Based on the combined 142Nd– 143Nd– 176Hf obtained we conclude that the 142Nd– 143Nd correlation line measured in this study is best interpreted as an isochron with an age of 229 +24 −20Ma after the onset of nebular condensation. The uncertainties in the data permit the sources of these samples to have formed over a 44 Ma time interval. These new results for lunar mare basalts are thus consistent with a later Sm–Nd isotope closure time of their source regions than some recent studies have postulated, and a superchondritic bulk Sm/Nd ratio of the Moon and Earth. The superchondritic Sm/Nd signature was inherited from the materials that accreted to make up the Earth–Moon system. Although collisional erosion of crust from planetesimals is favored here to remove subchondritic Sm/Nd portions and drive the bulk of these bodies to superchondritic in composition, removal of explosive basalt material via gravitational escape from such bodies, or chondrule sorting in the inner solar system, may also explain the compositional features that deviate from average chondrites that make up the Earth–Moon system. This inferred superchondritic nature for the Earth similar to the modern convecting mantle means that there is no reason to invoke a missing, subchondritic reservoir to mass balance the Earth back to chondritic for Sm/Nd ratios. However, to account for the subchondritic Sm/Nd ratios of continental crust, a second superchondritic Sm/Nd mantle reservoir is required.

In-pile testing of ITER first wall mock-ups at relevant thermal loading conditions

The paper describes the experimental technique and preliminary results of thermal fatigue testing of ITER first wall (FW) water-cooled mock-ups inside the core of the RBT-6 experimental fission reactor (RIAR, Dimitrovgrad, Russia). This experiment has provided simultaneous effect of neutron fluence and thermal cycling damages on the mock-ups. A PC-controlled high-temperature graphite ohmic heater was applied to provide cyclic thermal load onto the mock-ups surface. This experiment lasted for 309 effective irradiation days with a final damage level (CuCrZr) of 1 dpa in the mock-ups. About 3700 thermal cycles with a heat flux of 0.4–0.5 MW/m 2 onto the mock-ups were realized before the heater fails. Then, irradiation was continued in a non-cycling mode.

Effects of Neutron Dose, Dose Rate, and Irradiation Temperature on the Irradiation Embrittlement of a Low-Copper Reactor Pressure Vessel Steel

Abstract A series of irradiation experiments to investigate the effects of neutron fluence and flux on reactor pressure vessel (RPV) steels has been performed using the Japan Material Testing Reactor (JMTR). The irradiation temperature has been precisely controlled at 290°C within an error of ±2°C during reactor operation. The neutron fluence and flux ranged from 9.3 × 1017 to 1.1 × 1020 n/cm2 and from 1.4 × 1011 to 6.3 × 1013 n/cm2/s, respectively. The material used in the present study was an A533B RPV steel in which the copper and phosphorous concentrations are 0.03 and 0.002 wt%, respectively. Charpy impact test results with one-third sized specimens showed no dose-rate dependence. Microstructural observation revealed that a higher irradiation temperature at around 340°C resulted in formation of rather large dislocation loops and smaller microvoids.

A varying temperature irradiation experiment for operation in HFIR

An experiment to directly compare the effects of neutron fluence received at steady temperature on the microstructure and mechanical properties of candidate fusion reactor structural materials to the effects of receiving 10% of the fluence at reduced temperature has been designed. The Varying Temperature Experiment is being fabricated for irradiation in a large Removable Beryllium (RB) position of the High Flux Isotope Reactor (HFIR). For this experiment, specimen temperatures are elevated and controlled whenever there is substantial neutron fluence to the specimens (reactor power >10%). Half of the specimens are irradiated at two distinct temperatures, and half are irradiated at constant temperature. The capsule features to meet these requirements are described. The experiment will operate for ten reactor cycles beginning in early 1998. This project is a collaborative effort under the Japan–USA cooperative program (Jupiter Project).

Tritium release kinetics of lithium silicates with irradiation defects

Tritium release kinetics of lithium ortho- and metasilicates with irradiation defects was studied by out-of-pile experiments. Tritium was released mainly as HTO, but the HT yield increased with the increasing partial pressure of H 2 in the sweep gas. Some dependence of the HT yield on neutron fluence was observed. Tritium release kinetics of HTO and HT were well interpreted by taking a combined regime of diffusion and first-order reaction. The dependence of the diffusion constant and reaction rate constant on neutron fluence was determined. In the studied range, the effects of neutron fluence were observed more clearly for Li 2SiO 3 than for Li 4SiO 4.

Initial Nuclear Assessment for a Low Aspect Ratio Power Plant

Interest in the low aspect ratio (LAR) concept led the U.S. ARIES team to examine the credibility of this advanced concept as a future source of fusion energy. The compactness of the LAR machine imposes severe constraints on the Cu center post (CP). The analysis showed that the lifetime of CP is limited by the Class C low level waste disposal requirements. Identification of potential radioactive waste problems for the Cu conductor resulted in either limiting the lifetime of the unshielded CP to 0.12 FPV or shielding the CP with 20-30 cm of shield. The effects of neutron fluence on conductor resistivity, swelling, and atomic displacement have been assessed. 11 refs., 1 fig., 5 tabs.

Irradiation-assisted stress corrosion cracking of austenitic stainless steels: recent progress and new approaches

Irradiation-assisted stress corrosion cracking (IASCC) of several types of BWR field components fabricated from solution-annealed austenitic stainless steels (SSs), including a core internal weld, were investigated by means of slow-strain-rate test (SSRT), scanning electron microscopy (SEM), Auger electron spectroscopy (AES), and field-emission-gun advanced analytical electron microscopy (FEG-AAEM). Based on the results of the tests and analyses, separate effects of neutron fluence, tensile properties, alloying elements and major impurities identified in the American Society for Testing and Materials (ASTM) specifications, minor impurities, water chemistry, and fabrication-related variables were determined. The results indicate strongly that minor impurities not specified by the ASTM-specifications play important roles, probably through a complex synergism with grain-boundary Cr depletion. These impurities, typically associated with steelmaking and component fabrication processes, are very low or negligible in solubility in steels and are the same impurities that have been known to promote intergranular SCC significantly when they are present in water as ions or soluble compounds. It seems obvious that IASCC is a complex integral problem which involves many variables that are influenced strongly by not only irradiation conditions, water chemistry, and stress but also iron and steelmaking processes, fabrication of the component, and joining and welding. Therefore, for high-stress components in particular, it would be difficult to mitigate IASCC problems at high fluence based on the consideration of water chemistry alone, and other considerations based on material composition and fabrication procedure would be necessary as well.

Radiation damage study of a fast bipolar monolithic charge sensitive preamplifier

A fast bipolar monolithic Charge Sensitive Preamplifier (CSP) implemented in the monolithic 2 μm BiCMOS technology (called HF2CMOS), was studied after neutron irradiation at fluences between 1.26 × 10 12 and 1.09 × 10 14 n/cm 2. Neutron fluence effects on the base spreading resistance, r bb′, and the parallel noise of the bipolar npn input device and on the PMOS transistor in the second stage of the preamplifier, are presented. It is shown that, with the HF2CMOS process, a hardened CSP can be realized.

Fracture and Tensile Properties of Irradiated Zircaloy-2 Pressure Tubes

The mechanical properties of nuclear reactor components degrade as a result of long service exposure in high-temperature, irradiation, and corrosive environments. Fracture toughness and tensile testing are conducted on the pressure tubes of Zircaloy-2 to evaluate the effects of neutron fluence, hydrogen content, and temperature on the mechanical properties. Tensile tests are performed on the base metal, and fracture toughness tests are performed on both the base and weld metals. Neutron irradiation increases the strength, reduces ductility, and significantly degrades fracture toughness. The postirradiation fracture toughness increases substantially as the test temperature is increased from room temperature to 250 C. Hydrogen levels up to 250 ppm are found to have little or no effect on the postirradiation fracture toughness. Because of its anisotropic nature, Zircaloy-2 displays different fracture resistances, depending on crack orientation. In the base metal, the fracture toughness in the longitudinal orientation is higher than that for the circumferential orientation, and this difference is enhanced at higher temperatures. The weld exhibits lower toughness in the longitudinal direction.

Co-60 and neutron irradiation of MOS-controlled thyristors

The authors report changes in MOS-controlled thyristor (MCT) performance caused by irradiation with fast neutrons (E>1 MeV) and with Co-60 gamma rays. The results are significant because the MCT is representative of a growing class of power devices that combine bipolar and field-effect-transistor (FET) technologies and because MCTs are being considered for use in systems that must survive exposure to radiation. The MCTs tested, which were designed to hold off 400 V and carry 50 A, were operable with +or-10-V control voltages after they received a total dose of 1 Mrad(Si), but they could not be turned on after exposure to 1E12 n/cm/sup 2/. This observation is consistent with calculations of the effects of neutron fluence on minority-carrier transport in the MCT. The observed shifts with total dose of V/sub on/ and, at least in part, of V/sub bias/ are consistent with the known behavior of p-channel and n-channel polysilicon gate FETs. >

Further progress towards the construction of a cubical fast-neutron dose equivalent dosimeter based on the CR-39 detector with front radiators

Abstract Following the work reported elsewhere1,2, we have tested the cubical dosimeter described below at some other neutron energies than those reported previously. The dosimeter consists of three orthogonally arranged 500 μm thick CR-39 detectors together with their optimized front-radiator stacks (each consisting of a 150 μm thick sheet of polyimide, followed by 800 μm polystyrene and 3 mm polyethylene sheets). We find that, following chemical + electrochemical etching of the detector, such a dosimeter assembly yields a dose-equivalent response which is fairly energy- and direction-independent. The effect of neutron fluence on the dosimeter response and on the bulk etch rate of the detector has also been studied. The response of the dosimeter is found to be highly dependent upon the neutron fluence beyond that corresponding to a dose of ∼10 mSv. The bulk etch rate of the detector remains, however, unaffected up to ∼1 Sv.

The effect of neutron fluence on the gamma-ray decay behavior of elements in a fusion reactor environment

The effect of neutron fluence on the gamma-ray decay behavior of elements in a fusion reactor environment

Some studies on the neutron-induced tracks in Makrofol-polycarbonate detector

In this work the effect of neutron fluence, energies and angle of incidence on the response of PC Makrofol foils of 300 μm thick had been investigated. The foils were etched electrochemically in alcoholic KOH solution at 20 KV/cm, 500 KHz and 65 °C. The results showed that the track density has a significant effect on the shape of the etched spots due to the mutual influence of the adjacent tracks. Three distinct conditions were observed: a threshold energy between 0.47 and 4.37 MeV, an increasing function between 4.37 and 6.07 MeV and an energy independent region between 6.07 and 15.83 MeV. Moreover, the response of the detector proved to be angular dependent.

Friction and source hardening in irradiated mild steel

The effect of neutron fluence on stress-strain behavior of mild steel at ambient temperature revealed that Luders strain increases with fluence and at the highest fluence employed here namely 1.4x10/sup 19/n/cm/sup 2/ fracture occurred during Luders propagation. The lower yield stress increased as cube root of fluence and friction stress increased as square root of fluence. The interstitial impurity atoms (C and N) combine with irradiation induced defects thereby resulting in reduced net concentration of free C and N in solution and thus the source pinning effect decreases in irradiated steels as observed here. The increase in Luders strain with the corresponding decrease in source hardening resulted in reduced work-hardening in irradiated steels as expected.

Computer Studies of Radiation Effects in Avalanche Diodes

A computer program DIODRAD was written to solve equations similar to Misawals for radiation effects in IMPATT diodes. This program permits the user to select integration accuracy, doping profile, and operating conditions in calculating the effects of neutron fluence and gamma flux on diode performance. Results from this work compare favorably with previous theoretical and experimental data.