Lead Slowing-down Spectrometer Research Articles

Nuclear isotopic fissile content assay

Isotopic fissile content assay was performed in a designed lead slowing down spectrometer (LSDS) system. In the system, the fission measurement was performed for 11 – 20 wt% of Pu239 with 1 wt% of Pu241 and U235. The self-shielding factor was applied to the measured fission signal. The fissile assay was performed on U235, Pu239, and Pu241. The content assay of an unknown fissile sample was also performed on U235, Pu239, and Pu241. The procedure of an unknown sample assay was setup. In several simulations, the content of Pu239 was accurately obtained with ∼1% error in the rod base. For the unknown fissile assay, the result was very close to the actual content of Pu239, ∼1% error. The LSDS provided very powerful result for the fissile assay in the mixture. Therefore, the LSDS is a very promising technique in isotopic fissile content assays, especially for Pu239 in uranium and plutonium. Precise fissile material content should be known for reactor safety in fuel cycles and international safeguards on nuclear fuel cycle development.

Thermal analysis at target for neutron generation in fissile assay

A target was designed to produce intense neutrons of 1012 neutrons/sec in order to analyze the fissile content by interrogation in the Lead Slowing Down Spectrometer (LSDS) system. A high energy and current electron linear accelerator was used in neutron production by bremsstrahlung radiation. However, the interaction by electron and radiation in the target generates large amounts of heat. Several coolants were simulated in the target and the heat removal capacity was performed. The thermal property of coolant in the target was calculated using the CFX code. The heat distribution by the cooling method was simulated and the forced convection method was used to calculate the temperature distribution on the different coolants and flow rate. Helium shows a good cooling property, but it needs high flow rate with high pressure. Air may be a good candidate coolant if the limit of neutron activation is satisfied in the circulating system. When the coolants were used in the target, each metal plate of the target was kept under the melting temperature. For the estimation of the coolant performance, the neutron influence by the coolant was examined as well. The material property was also evaluated for various target cases. The displacement per atom (DPA) analysis of the target was performed for a life time operation of the LSDS system. For the target container material (HT-9), the optimum thickness was obtained from the sensitivity simulation.

Activation analysis of targets and lead in a lead slowing down spectrometer system

Abstract A neutron generation system was developed to induce fissile fission in a lead slowing down spectrometer (LSDS) system. The source neutron is one of the key factors for LSDS system work. The LSDS was developed to quantify the isotopic contents of fissile materials in spent nuclear fuel and recycled fuel. The source neutron is produced at a multilayered target by the (e,γ)(γ,n) reaction and slowed down at the lead medium. Activation analysis of the target materials is necessary to estimate the lifetime, durability, and safety of the target system. The CINDER90 code was used for the activation analysis, and it can involve three-dimensional geometry, position dependent neutron flux, and multigroup cross-section libraries. Several sensitivity calculations for a metal target with different geometries, materials, and coolants were done to achieve a high neutron generation rate and a low activation characteristic. Based on the results of the activation analysis, tantalum was chosen as a target material due to its better activation characteristics, and helium gas was suggested as a coolant. In addition, activation in a lead medium was performed. After a distance of 55 cm from the lead surface to the neutron incidence, the neutron intensity dramatically decreased; this result indicates very low activation.

Quantitative Fissile Assay In Used Fuel Using LSDS System

A quantitative assay of isotopic fissile materials (U235, Pu239, Pu241) was done at Korea Atomic Energy Research Institute (KAERI), using lead slowing down spectrometer (LSDS). The optimum design of LSDS was performed based on economics, easy maintenance and assay effectiveness. LSDS system consists of spectrometer, neutron source, detection and control. LSDS system induces fissile fission and fast neutrons are collected at fission chamber. The detected signal has a direct relation to the mass of existing fissile isotopes. Many current commercial assay technologies have a limitation in direct application on isotopic fissile assay of spent fuel, except chemical analysis. In the designed system, the fissile assay model was setup and the correction factor for self-shield was obtained. The isotopic fissile content assay was performed by changing the content of Pu239. Based on the fuel rod, the isotopic content was consistent with ~2% uncertainty for Pu239. By applying the covering (neutron absorber), the effective shielding was obtained and the activation was calculated on the target. From the assay evaluation, LSDS technique is very powerful and direct to analyze the isotopic fissile content. LSDS is applicable for nuclear fuel cycle and spent fuel management for safety and economics. Additionally, an accurate fissile content will contribute to the international transparency and credibility on spent fuel.

Isotopic Fissile Assay of Spent Fuel in a Lead Slowing-Down Spectrometer System

Abstract A lead slowing-down spectrometer (LSDS) system is under development to analyze isotopic fissile content that is applicable to spent fuel and recycled material. The source neutron mechanism for efficient and effective generation was also determined. The source neutron interacts with a lead medium and produces continuous neutron energy, and this energy generates dominant fission at each fissile, below the unresolved resonance region. From the relationship between the induced fissile fission and the fast fission neutron detection, a mathematical assay model for an isotopic fissile material was set up. The assay model can be expanded for all fissile materials. The correction factor for self-shielding was defined in the fuel assay area. The corrected fission signature provides well-defined fission properties with an increase in the fissile content. The assay procedure was also established. The assay energy range is very important to take into account the prominent fission structure of each fissile material. Fission detection occurred according to the change of the Pu239 weight percent (wt%), but the content of U235 and Pu241 was fixed at 1 wt%. The assay result was obtained with 2∼3% uncertainty for Pu239, depending on the amount of Pu239 in the fuel. The results show that LSDS is a very powerful technique to assay the isotopic fissile content in spent fuel and recycled materials for the reuse of fissile materials. Additionally, a LSDS is applicable during the optimum design of spent fuel storage facilities and their management. The isotopic fissile content assay will increase the transparency and credibility of spent fuel storage.

Recent Developments in Nuclear Data Measurement capabilities at the Gaerttner LINAC Center at RPI

The Gaerttner LINAC Center at RPI uses a 60 MeV electron linear accelerator to produce short pulses of neutrons with duration of 5-5000 ns. The main research thrust at the Center is nuclear data for nuclear reactors and criticality safety applications. The Center includes several setups for time-of-flight measurements including neutron transmission, capture and scattering detectors, and a lead slowing-down spectrometer. Experiments were designed to produce neutron interaction cross sections that cover the energy range of 0.01 eV to 20 MeV. Recently added experiments include: setups for keV and fast neutron transmission, a C 6 D 6 detector array for keV neutron capture measurements, and a fast neutron scattering system. Results discussed here include fast neutron scattering and angular distributions for nat Fe, iron capture measurements for incident neutrons from 1 keV to 2 MeV, fast neutron transmission through W and H 2 O samples, and keV transmission through Mo isotopes.

Design optimization of radiation shielding structure for lead slowing-down spectrometer system

A lead slowing-down spectrometer (LSDS) system is a promising nondestructive assay technique that enables a quantitative measurement of the isotopic contents of major fissile isotopes in spent nuclear fuel and its pyroprocessing counterparts, such as 235U, 239Pu, 241Pu, and, potentially, minor actinides. The LSDS system currently under development at the Korea Atomic Energy Research Institute (Daejeon, Korea) is planned to utilize a high-flux (>1012 n/cm2·s) neutron source comprised of a high-energy (30 MeV)/high-current (∼2 A) electron beam and a heavy metal target, which results in a very intense and complex radiation field for the facility, thus demanding structural shielding to guarantee the safety. Optimization of the structural shielding design was conducted using MCNPX for neutron dose rate evaluation of several representative hypothetical designs. In order to satisfy the construction cost and neutron attenuation capability of the facility, while simultaneously achieving the aimed dose rate limit (<0.06 μSv/h), a few shielding materials [high-density polyethylene (HDPE)–Borax, B4C, and Li2CO3] were considered for the main neutron absorber layer, which is encapsulated within the double-sided concrete wall. The MCNP simulation indicated that HDPE-Borax is the most efficient among the aforementioned candidate materials, and the combined thickness of the shielding layers should exceed 100 cm to satisfy the dose limit on the outside surface of the shielding wall of the facility when limiting the thickness of the HDPE-Borax intermediate layer to below 5 cm. However, the shielding wall must include the instrumentation and installation holes for the LSDS system. The radiation leakage through the holes was substantially mitigated by adopting a zigzag-shape with concrete covers on both sides. The suggested optimized design of the shielding structure satisfies the dose rate limit and can be used for the construction of a facility in the near future.

DEVELOPMENT OF LEAD SLOWING DOWN SPECTROMETER FOR ISOTOPIC FISSILE ASSAY

A lead slowing down spectrometer (LSDS) is under development for analysis of isotopic fissile material contents in pyro-processed material, or spent fuel. Many current commercial fissile assay technologies have a limitation in accurate and direct assay of fissile content. However, LSDS is very sensitive in distinguishing fissile fission signals from each isotope. A neutron spectrum analysis was conducted in the spectrometer and the energy resolution was investigated from 0.1eV to 100keV. The spectrum was well shaped in the slowing down energy. The resolution was enough to obtain each fissile from 0.2eV to 1keV. The detector existence in the lead will disturb the source neutron spectrum. It causes a change in resolution and peak amplitude. The intense source neutron production was designed for ∼E12 n's/sec to overcome spent fuel background. The detection sensitivity of U238 and Th232 fission chamber was investigated. The first and second layer detectors increase detection efficiency. Thorium also has a threshold property to detect the fast fission neutrons from fissile fission. However, the detection of Th232 is about 76% of that of U238. A linear detection model was set up over the slowing down neutron energy to obtain each fissile material content. The isotopic fissile assay using LSDS is applicable for the optimum design of spent fuel storage to maximize burnup credit and quality assurance of the recycled nuclear material for safety and economics. LSDS technology will contribute to the transparency and credibility of pyro-process using spent fuel, as internationally demanded.

Intermediate structure in the cross section for the subbarrier fission of 241Am

The cross section for the neutron-induced fission of 241Am at neutron energies between 0.2 eV and 10 keV was measured by means of a lead slowing-down spectrometer (LSDS-100). The parameters of the intermediate structure in the behavior of the fission strength function were estimated.

DESIGN OF LSDS FOR ISOTOPIC FISSILE ASSAY IN SPENT FUEL

A future nuclear energy system is being developed at Korea Atomic Energy Research Institute (KAERI), the system involves a Sodium Fast Reactor (SFR) linked with the pyro-process. The pyro-process produces a source material to fabricate a SFR fuel rod. Therefore, an isotopic fissile content assay is very important for fuel rod safety and SFR economics. A new technology for an analysis of isotopic fissile content has been proposed using a lead slowing down spectrometer (LSDS). The new technology has several features for a fissile analysis from spent fuel: direct isotopic fissile assay, no background interference, and no requirement from burnup history information. Several calculations were done on the designed spectrometer geometry: detection sensitivity, neutron energy spectrum analysis, neutron fission characteristics, self shielding analysis, and neutron production mechanism. The spectrum was well organized even at low neutron energy and the threshold fission chamber was a proper choice to get prompt fast fission neutrons. The characteristic fission signature was obtained in slowing down neutron energy from each fissile isotope. Another application of LSDS is for an optimum design of the spent fuel storage, maximization of the burnup credit and provision of the burnup code correction factor. Additionally, an isotopic fissile content assay will contribute to an increase in transparency and credibility for the utilization of spent fuel nuclear material, as internationally demanded.

Nondestructive Assay Measurements Using the RPI Lead Slowing-Down Spectrometer

The use of a lead slowing-down spectrometer (LSDS) is considered as a possible option for nondestructive assay of fissile material of used nuclear fuel. The primary objective is to quantify fissile isotopes, particularly 239Pu and 235U, via a direct measurement distinguishing them through their characteristic fission spectra in the LSDS. In this paper, we present several assay measurements performed at the Rensselaer Polytechnic Institute (RPI) to support ongoing feasibility studies of the method and to provide benchmark experiments for Monte Carlo calculations of the assay system. A fresh uranium oxide fuel rod from the RPI Walthousen Reactor Critical Facility, a 239Pu-Be source, and several highly enriched 235U disks were assayed in the LSDS. The characteristic fission spectra were measured with 238U and 232Th threshold fission chambers, which are primarily sensitive to fission neutrons with energies above the threshold. Despite the constant neutron and gamma background from the Pu-Be source and the intense interrogation neutron flux, the LSDS system was able to measure the characteristic 235U and 239Pu responses. All measurements were compared to Monte Carlo simulations complementing previous modeling-based studies. It is shown that the available simulation tools and models are well suited to simulate the assay. An absolute calibration technique of the LSDS, which is required to perform quantitative measurements of the assayed fissile materials, is presented.

Assaying Used Nuclear Fuel Assemblies Using Lead Slowing-Down Spectroscopy and Singular Value Decomposition

This study investigates the use of a Lead Slowing-Down Spectrometer (LSDS) for the direct and independent measurement of fissile isotopes in light-water nuclear reactor fuel assemblies. The current study applies MCNPX, a Monte Carlo radiation transport code, to simulate the measurement of the assay of the used nuclear fuel assemblies in the LSDS. An empirical model has been developed based on the calibration of the LSDS to responses generated from the simulated assay of six well-characterized fuel assemblies. The effects of self-shielding are taken into account by using empirical basis vectors calculated from the singular value decomposition (SVD) of a matrix containing the self-shielding functions from the assay of assemblies in the calibration set. The performance of the empirical algorithm was tested on version 1 of the Next-Generation Safeguards Initiative (NGSI) used fuel library consisting of 64 assemblies, as well as a set of 27 diversion assemblies, both of which were developed by Los Alamos National Laboratory. The potential for direct and independent assay of the sum of the masses of Pu-239 and Pu-241 to within 2%, on average, has been demonstrated.

Effect of the size of experimental channels of the lead slowing-down spectrometer SVZ-100 (Institute for Nuclear Research, Moscow) on the moderation constant

Lead slowing-down (LSD) spectrometers have a low energy resolution (about 30%), but their luminosity is 103 to 104 times higher than that of time-of-flight (TOF) spectrometers. A high luminosity of LSD spectrometers makes it possible to use them to measure neutron cross section for samples of mass about several micrograms. These features specify a niche for the application of LSD spectrometers in measuring neutron cross sections for elements hardly available in macroscopic amounts—in particular, for actinides. A mathematical simulation of the parameters of SVZ-100 LSD spectrometer of the Institute for Nuclear Research (INR, Moscow) is performed in the present study on the basis of the MCNPX code. It is found that the moderation constant, which is the main parameter of LSD spectrometers, is highly sensitive to the size and shape of detecting volumes in calculations and, hence, to the real size of experimental channels of the LSD spectrometer.

Fission cross-section measurements of 237Np, 242m Am, and 245Cm with lead slowing-down neutron spectrometer

The cross sections for neutron-induced fission of 237Np, Am, and 245Cm have been measured in the energy range from 0.1 eV to 2 keV using the lead slowing-down neutron spectrometer at the Research Reactor Institute, Kyoto University. The fission cross sections were deduced relative to those for 235U(n,f) and 10B(n,a). The cross section for 237Np(n,f) was obtained with the experimental uncertainty of about 20% in the energy region below 25 eV and typically 13% above this energy. The experimental uncertainties for 242m Am and 245Cm were, respectively, of the order of 14% and 9% in the entire energy region.

Measurement of (n,α) reactions on 147Sm and 149Sm using a lead slowing-down spectrometer

The lead slowing-down spectrometer (LSDS) at Rensselaer Polytechnic Institute (RPI) was used to extend previous measurements of the (n,α) cross section on 147Sm and perform measurements on 149Sm over the energy range 0.1eV–10keV. A compensated detector based on passivated implanted planar silicon (PIPS) detectors was constructed. Signals from two detectors were combined in opposite polarity and digitized providing a single detector capable of discriminating against capture γ's and “γ-flash” while preserving the discrete signals produced by α events and providing a simultaneous background measurement. Extrapolating the results predicts a 0.3±0.3mbarn and 11.5±1.1mbarn thermal neutron (n,α) cross section for 147Sm and 149Sm, respectively. This measurement provides new information on (n,α) reaction cross sections. The method of measuring neutron induced charged particle emission cross sections presented here can be expanded to more isotopes of interest to reactor engineers or astrophysicists, as well as to (n,p) reactions.

Cross-Section Measurements for Neutron-Induced Fission of Minor Actinides with Lead Slowing-Down Spectrometer at KURRI

Cross-Section Measurements for Neutron-Induced Fission of Minor Actinides with Lead Slowing-Down Spectrometer at KURRI

Cross section for the subbarrier fission of 244Cm

The cross section for 244Cm fission induced by neutrons of energy in the range between 0.07 eV and 20 keV was measured by using the lead slowing-down spectrometer (LSDS-100) of the Institute for Nuclear Research (Russian Academy of Sciences, Moscow). The parameters of the resonance areas were determined for the lowest eight s-wave neutron resonances, and the respective fission widths were evaluated. Also, the parameters of the intermediate structure in the cross section for the subbarrier fission of 244Cm nuclei were evaluated. The results were compared with available data and recommendations based on evaluations.

Fission fragment mass and energy distributions as a function of incident neutron energy measured in a lead slowing-down spectrometer

A new method of measuring fission fragment mass and energy distributions as a function of incident neutron energy in the range from below 0.1 eV to 1 keV has been developed. The method involves placing a double-sided Frisch-gridded fission chamber in Rensselaer Polytechnic Institute's lead slowing-down spectrometer (LSDS). The high neutron flux of the LSDS allows for the measurement of the energy-dependent, neutron-induced fission cross sections simultaneously with the mass and kinetic energy of the fission fragments of various small samples. The samples may be isotopes that are not available in large quantities (submicrograms) or with small fission cross sections (microbarns). The fission chamber consists of two anodes shielded by Frisch grids on either side of a single cathode. The sample is located in the center of the cathode and is made by depositing small amounts of actinides on very thin films. The chamber was successfully tested and calibrated using $0.41\ifmmode\pm\else\textpm\fi{}0.04$ ng of $^{252}\mathrm{Cf}$ and the resulting mass distributions were compared to those of previous work. As a proof of concept, the chamber was placed in the LSDS to measure the neutron-induced fission cross section and fragment mass and energy distributions of $25.3\ifmmode\pm\else\textpm\fi{}0.5 \ensuremath{\mu}$g of $^{235}\mathrm{U}$. Changes in the mass distributions as a function of incident neutron energy are evident and are examined using the multimodal fission mode model.

The nuclear data program at rensselaer

The nuclear data program at the Rensselaer Polytechnic Institute (RPI) is centered around a 60 MeV pulsed electron Linear Accelerator (LINAC) configured to deliver pulsed neutron beams. The LINAC electron pulse width can vary between 5 ns and 5 us, a repetition rate of 1–500 pulses per second and neutron yield of up to 1013 n/s. Over the years several experimental setups and techniques were developed to enable a variety of measurement capabilities. The experiments cover the energy range from 0.001 eV to 20 MeV and included neutron transmission, capture, scattering and fission measurements. The facility is also equipped with a Lead Slowing-Down Spectrometer (LSDS) producing a high neutron flux that is used for simultaneous measurements of the fission cross section and fission fragment mass and energy distributions. Detectors for (n, alpha) and (n, p) cross section measurements using the LSDS were also developed and measurements of (n, alpha) cross section were completed. The high neutron flux inside the LSDS enables measurements on small samples (sub micrograms) or samples with small cross section (sub millibarns).In order to fully utilize the capabilities of the facility, several measurement techniques were developed; high accuracy (<1%) total cross sections were measured using iron and uranium filtered neutron beams. A system for fast neutron scattering measurements using an array of liquid scintillators and a digital data acquisition system was recently developed and used for several measurements and is now being adapted for fission neutron studies.Methods for simultaneous measurements of fission and capture cross sections using the RPI multiplicity detector are under development and new modular detectors for both fast and resonance region neutron detection were developed and deployed.These developments result in numerous measurements on different materials and provide input to improved evaluated nuclear data libraries and thus help improve the accuracy of calculations of nuclear reactors and other nuclear systems.

Cross section for the subthreshold fission of 236U

The cross section for 236 U fission in the neutron-energy range En =0 .001−20 keV was measured by using the INR RAS (Institute of Nuclear Research, Russian Academy of Sciences, Moscow) LSDS-100 neutron spectrometer of the lead slowing-down spectrometer type. The resonance fission areas of the resonances at 5.45 eV and 1.28 keV were found, and the fission widths of these resonances were evaluated. The cross section for the 238 U(n, f ) fission process was measured, and the threshold sensitivity of the LSDS-100 to small values of fission cross sections was estimated. The well-known intermediate structure in the cross section for the neutron-induced subbarrier fission of 236 Uw as confirmed.