Neutron Collar Research Articles

A new generation of uranium coincidence fast neutron collars for assay of LWR fresh fuel assemblies

The active uranium neutron coincidence collar provides a means of non-destructively assaying the fissile linear density of Light Water Reactor fresh fuel assemblies containing low enriched uranium. These neutron collars can operate in two modes: a thermal and a fast mode. In fast mode, a neutron collar has an added cadmium (Cd) liner in the sample cavity of the detector to reduce the impact of the burnable poison (thermal neutron absorber) on the detector signal (doubles). The main advantage for operating in fast mode is a detected signal that is less dependent of the burnable neutron poison content and thus less dependent on facility operator declarations. The drawback is that operating in fast mode requires a longer measurement time (∼hour vs tens of minutes for thermal mode) to achieve the statistically needed precision in the measurements. The trend in the modern reactor fuel assemblies is moving to higher burnup by using higher initial enrichment and, consequently, a higher number of burnable poison rods to compensate the initial neutron reactivity. The increase of the burnable poison loading has motivated the development of a new generation of high efficiency fast neutron collars to allow practical measurements in-field by nuclear inspectors. This paper describes the development and performance evaluation of a new generation of neutron collars, for both boiling water reactor (BWR) and pressurized water reactor (PWR) fuels, jointly developed between the US Department of Energy, through Los Alamos National Laboratory, and the Euratom Safeguards Directorate of the European Commission. We present here calibrations with reference fuel assemblies at Los Alamos National Laboratory as well as the results of in-field measurement campaigns in fuel fabrication plants with modern commercial fuel assemblies. The experimental results show that a typical PWR verification can be made in a total time of 30 min with an uncertainty in the measured mass of 2% at one standard deviation (1σ). A BWR verification can be made in 47 min with an uncertainty in the measured mass of 1.9% at 1σ, or a total time of 20 min with 1σ uncertainty in the measured mass of 2.5%.

Modernizing the active neutron collar poison rod correction using simulations to enhance PWR fresh fuel verification

The declared inventory of 235U in fresh, low enriched uranium, nuclear fuel assemblies is routinely verified by nuclear safeguards inspectorates using the UNCL (Uranium Neutron Collar – Light Water Reactor Fuel) nondestructive assay instrument. The trend in modern fuels is towards higher initial enrichment, which in turn requires a larger number of burnable poison pins with higher Gadolinia concentration to hold down the initial reactivity. UNCL assay error for modern fuel assemblies typically exceeds 10%. The traditional algorithm used to correct the response for burnable poison content needs revision to achieve accuracy comparable to the σR of 4.5% expected for poison-free assemblies. In this paper we review the problem and use a large set of 287 Monte Carlo simulations based on an experimentally benchmarked model corresponding to the 16×16 PWR array used in Brazilian Angra type II and III fuel. Simulated relative responses are used to evaluate the functional form of the poison correction and determine parameters optimal for 16×16 PWR assemblies. This update is conducted using between 4–24 Gadolinia pins ranging 2–11 wt% Gd2O3 with assemblies having mean enrichments ranging 2.5%–5%. Experimental validation of the updated coefficients using seven measurements shows bias is reduced by an order of magnitude and σR reduced from 10% to less than 2%. Two updated sets of coefficients are provided, the most accurate of which is directly useable in the existing analysis code (INCC) in use by inspectorate at fuel fabrication facilities.

Sensitivity of the active neutron coincidence collar response during simulated and experimental fresh fuel assay

Verification of the fissile (and fertile) content in fresh nuclear fuel assemblies is conducted by the IAEA to enforce the Nuclear Non-Proliferation Treaty using the UNCL (Uranium Neutron Collar — Light Water Reactor Fuel). The UNCL uses an uncorrelated AmLi neutron source to interrogate the fuel, producing a signal of coincident fission neutrons (Doubles rate) used to assay 235U content of the fuel. The cost of producing calibration assemblies and limited availability of diverse commercial assemblies at any one time historically restricted the ability to explore the full parameter space experimentally. Monte Carlo simulations can overcome this, but introduce additional sources of uncertainty. Here the sensitivity of simulations and measurements to various parameters is assessed for a reference 16×16 PWR assembly of uniform 3.2% enrichment. Uncertainty contributions in this evaluation include: simulated AmLi neutron source spectrum, AmLi neutron emission rate, AmLi anisotropicity, high density polyethylene (HDPE) density, the precise position of the fuel assembly within the detector, and experimentally the statistical uncertainty. The overall total systematic uncertainty estimate for the simulation of the absolute/relative Doubles rates responses are estimated to be approximately 2.0%/1.5%, and for experimental measurements systematic uncertainty reduces to 1.1%. This analysis supports further work using the relative Doubles rates in place of measurements for updating and extending the UNCL analysis methodology as systematic uncertainty is reasonably small.

Field testing of a fast-neutron coincidence collar for fresh uranium fuel verification

The Fast-Neutron Coincidence Collar (FNCL) is an active non-destructive assay system for the measurement of uranium-235 in fresh nuclear fuel assemblies using the neutron coincidence counting (NCC) approach. Unlike traditional NCC instruments, such as the Uranium Neutron Collar (UNCL), the FNCL contains liquid scintillator to detect fast neutrons (>0.1 MeV) with nanosecond-level time resolution. The subsequent reduction of the NCC coincidence window by a factor of one thousand (from microseconds to nanoseconds) allows the uncertainties associated with accidental neutron coincidences to be virtually eliminated resulting in precise and accurate assays of fresh fuel assemblies (FFAs) in significantly less time than alternatives. The measurement of fast neutrons also minimizes the systematic uncertainties due to burnable neutron poisons commonly present in the fuel matrix.The FNCL was tested at two nuclear fuel fabrication facilities in Brazil and the Republic of Korea with continued support from the two States. 39 measurements of FFAs were obtained with varying geometry, enrichment and burnable neutron poison content. These field test measurements and conclusions drawn pertaining to the FNCL performance are discussed.

Methods for diagnosing and quantifying double pulsing in a Uranium Neutron Collar system using shift register logic

Double pulsing in neutron coincidence counters is the result of a pulse processing chain- 3 He proportional counter timing incompatibility known to nondestructive assay system designers. It is not currently widely acknowledged, or accounted for, in the user community. However, it is gaining attention as list mode data acquisition and analysis becomes more commonly used for system diagnostics, revealing features that have been overlooked by historic timing gate selection using traditional shift register data acquisition methods. Double pulsing increases the apparent number of measured neutron events from a source. Therefore, it may contribute to a falsely increased count rate if present at the operational high voltage used when assaying samples, even with set predelays. The authors have previously studied the effects of double pulsing, using list mode data acquisition and analysis on neutron pulse trains in three common neutron coincidence counting systems used in routine measurements for international safeguards: a Canberra Industries JCC-71 Neutron Coincidence Collar, a variant on the Canberra Industries JCC-51 Active Well Neutron Coincidence Counter (the Large Volume Active Well Neutron Coincidence Counter), and an AnTech Inc. N2071 Neutron Coincidence Collar. This non-ideal behavior was isolated to the Amptek A111 Charge Sensitive Preamplifier & Discriminator chip used in both the Canberra Industries and Antech systems. The double pulsing fraction was calculated in post-analysis for various high voltage settings in these A111-based systems, assuming no predelay settings. In this work, the authors expand upon this identification and analysis to make it more translatable between list mode data acquisition and analysis and shift register-based analysis. By investigating the double pulsing fractions in a JCC-71 Uranium Neutron Collar as a function of predelay settings, as well as describing and performing alternative tests and analyses to diagnose and quantify the double pulsing using shift register logic, this work hopes to complete the picture of double pulsing identification and analysis.

Active fast neutron singles assay of 235U enrichment in small samples of triuranium octoxide

The sensitivity of an active fast neutron assay system for the measurement of 235U enrichment in small samples of low-enriched triuranium octoxide (U3O8) is described. The system comprises four organic liquid scintillation detectors in a polyethylene container in which a stimulating americium-lithium neutron source is placed below the sample to be interrogated. The sensitivities of both the singles and doubles assay have been corrected for the contribution from the stimulating source and for spontaneous fission in 238U and found to be (0.116 ± 0.008) cps per detector per % wt. (7%) and (0.0006 ± 0.0002) cps per detector per % wt. (33%), respectively. The singles approach with the 4-detector arrangement has been compared via Monte Carlo (MCNP-5) simulations with a fast neutron counting system based on 12 detectors in which the source is placed adjacent to the sample, similar to the arrangement referred to as the liquid scintillator uranium neutron collar. This comparison confirms that whilst singles assay with the 4-detector arrangement is feasible, the collar arrangement does not yield a singles yield that can be correlated with enrichment due to the perturbative scattering by the U3O8 sample. The 4-detector arrangement is particularly suitable for small samples of fresh, low-enriched material that might arise in forensic applications and the analysis of un-irradiated, orphan wastes. In these cases the efficiency of coincidence methods may be too low to yield a practical route to enrichment assessment. Conversely, the use of many detectors and/or a high-intensity source by which sensitivity might be increased might restrict the portability and ease of use of the apparatus. A correction for the contribution by (α, n) reactions, the stimulating source, spontaneous fission in 238U and self-multiplication in U3O8 is made via preparatory passive measurements.

Analysis of calibration data for the uranium active neutron coincidence counting collar with attention to errors in the measured neutron coincidence rate

The declared linear density of 238U and 235U in fresh low enriched uranium light water reactor fuel assemblies can be verified for nuclear safeguards purposes using a neutron coincidence counter collar in passive and active mode, respectively. The active mode calibration of the Uranium Neutron Collar – Light water reactor fuel (UNCL) instrument is normally performed using a non-linear fitting technique. The fitting technique relates the measured neutron coincidence rate (the predictor) to the linear density of 235U (the response) in order to estimate model parameters of the nonlinear Padé equation, which traditionally is used to model the calibration data. Alternatively, following a simple data transformation, the fitting can also be performed using standard linear fitting methods. This paper compares performance of the nonlinear technique to the linear technique, using a range of possible error variance magnitudes in the measured neutron coincidence rate. We develop the required formalism and then apply the traditional (nonlinear) and alternative approaches (linear) to the same experimental and corresponding simulated representative datasets. We find that, in this context, because of the magnitude of the errors in the predictor, it is preferable not to transform to a linear model, and it is preferable not to adjust for the errors in the predictor when inferring the model parameters.

A new fast neutron collar for safeguards inspection measurements of fresh low enriched uranium fuel assemblies containing burnable poison rods

Safeguards inspection measurements must be performed in a timely manner in order to detect the diversion of significant quantities of nuclear material. A shorter measurement time can increase the number of items that a nuclear safeguards inspector can reliably measure during a period of access to a nuclear facility. In turn, this improves the reliability of the acquired statistical sample, which is used to inform decisions regarding compliance. Safeguards inspection measurements should also maintain independence from facility operator declarations. Existing neutron collars employ thermal neutron interrogation for safeguards inspection measurements of fresh fuel assemblies.A new fast neutron collar has been developed for safeguards inspection measurements of fresh low-enriched uranium (LEU) fuel assemblies containing gadolinia (Gd2O3) burnable poison rods. The Euratom Fast Collar (EFC) was designed with high neutron detection efficiency to make a fast (Cd) mode measurement viable whilst meeting the high counting precision and short assay time requirements of the Euratom safeguards inspectorate. A fast mode measurement reduces the instrument sensitivity to burnable poison rod content and therefore reduces the applied poison correction, consequently reducing the dependence on the operator declaration of the poison content within an assembly. The EFC non-destructive assay (NDA) of typical modern European pressurized water reactor (PWR) fresh fuel assembly designs have been simulated using Monte Carlo N-particle extended transport code (MCNPX) simulations. Simulations predict that the EFC can achieve 2% relative statistical uncertainty on the doubles neutron counting rate for a fast mode measurement in an assay time of 600s (10min) with the available 241AmLi (α,n) interrogation source strength of 5.7×104s−1. Furthermore, the calibration range of the new collar has been extended to verify 235U content in variable PWR fuel designs in the presence of up to 32 gadolinia burnable poison rods with Gd concentrations of up to 12wt%. Monte Carlo calculations predict that the EFC has a lower statistical uncertainty for measurements performed in the fast neutron mode than its predecessor neutron collar design. This paper describes the physics design and calculated performance characteristics of the EFC. The Gd response is presented over a realistic range for modern PWR fuel designs.

Calibration, characterisation and Monte Carlo modelling of a fast-UNCL

This paper describes the calibration, characterisation and Monte Carlo modelling of a new IAEA Uranium Neutron Collar (UNCL) for LWR fuel, which can be operated in both passive and active modes. It can employ either 35 3He tubes (in active configuration) or 44 tubes at 10atm pressure (in its passive configuration) and thus can be operated in fast mode (with Cd liner) as its efficiency is higher than that of the standard UNCL. Furthermore, it has an adjustable internal cavity which allows the measurement of varying sizes of fuel assemblies such as WWER, PWR and BWR. It is intended to be used with Cd liners in active mode (with an AmLi interrogation source in place) by the inspectorate for the determination of the 235U content in fresh fuel assemblies, especially in cases where high concentrations of burnable poisons cause problems with accurate assays.A campaign of measurements has been carried out at the JRC Performance Laboratories (PERLA) in Ispra (Italy) using various radionuclide neutron sources (252Cf, 241AmLi and PuGa) and our BWR and PWR reference assemblies, in order to calibrate and characterise the counter as well as assess its performance and determine its optimum operational parameters.Furthermore, the fast-UNCL has been extensively modelled at JRC using the Monte Carlo code, MCNP-PTA, which simulates both the neutron transport and the coincidence electronics. The model has been validated using our measurements which agreed well with calculations. The WWER1000 fuel assembly for which there are no representative reference materials for an adequate calibration of the counter, has also been modelled and the response of the counter to this fuel assembly has been simulated. Subsequently numerical calibrations curves have been obtained for the above fuel assemblies in various modes (fast and thermal). The sensitivity of the counter to fuel rods substitution as well as other important aspects and the parameters of the fast-UNCL performance have been investigated and assessed.

Neutron interrogation of shielded/unshielded uranium by a 4MeV linac

A non-destructive active assay method was developed for revealing illicit trafficking of uranium. Photoneutrons produced in beryllium or heavy water by bremsstrahlung from a linac induced fission in the samples. Delayed fission neutrons were detected by a neutron collar built up of (3)He counters embedded in polyethylene moderator. High-enriched uranium samples shielded and unshielded by lead up to 14 mm thickness were detected, with a performance practically unaltered. 25 mg (235)U can be revealed in a 1 min interrogation time.

Neutron interrogation of high-enriched uranium by a 4 MeV linac

For revealing unauthorized transport (illicit trafficking) of nuclear materials, a non-destructive method reported earlier, utilizing a 4 MeV linear accelerator for photoneutron interrogation, was further developed. The linac served as a pulsed neutron source for assay of highly enriched uranium. Produced in beryllium or heavy water by bremsstrahlung, neutrons subsequently induced fission in the samples. Delayed neutrons were detected by a newly designed neutron collar built up of 14 3He counters embedded in a polyethylene moderator. A PC controlled multiscaler served as a time analyzer, triggering the detector startup by the beam pulse. Significant progress was achieved in enhancing the detector response, hence the sensitivity for revealing illicit material. A lower sensitivity limit of the order of 10 mg 235U was determined in a 20 s measurement time with a reasonable amount of beryllium (170 g) or of heavy water (100 g) and a mean electron current of 10 μA. Sensitivity can be further enhanced by increasing the measurement time.

Photoneutron interrogation of low-enriched uranium induced by bremsstrahlung from a 4 MeV linac

Revealing smuggled nuclear material by passive γ-detection is hindered, because the weak radiation can easily be shielded. Neutrons, as penetrate shielding, represent a detection potential, by inducing fission in the nuclear material. A 4MeV linear accelerator was used as a pulsed neutron source for active interrogation of U-bearing material. Produced in heavy water by bremsstrahlung, neutrons subsequently induced fissions in UO2 samples. Delayed fission neutrons were detected in a neutron collar built up by 3He counters in a polyamide container. The counters were gated to be detached from high voltage during the electron pulse. Irradiation-measurement cycles were carried out with a 25Hz pulse repetition rate as optimum setting. The time analyser start-up was externally triggered and synchronised by the electron beam pulse. The response of the system was studied as a function of the intensity of the electron current, the amount of heavy water, U enrichment, and total U content. Sensitivity limit was achieved as 0.5g 235U and/or 30g 238U in a 20s measurement time (500 cycles) with the amount of heavy water of 100g and a mean electron current of 2μA. Because of the long decay time of the prompt (interrogating and fission) neutron pulse, about a half of the time interval (40ms) between pulses is only available for counting delayed neutrons.

Analysis of Time Evolution of Neutron Intensity Measured with a High-sensitive Neutron Collar during the JCO Criticality Accident

Analysis of Time Evolution of Neutron Intensity Measured with a High-sensitive Neutron Collar during the JCO Criticality Accident

Analysis of Time Evolution of Neutron Intensity Measured with a High-sensitive Neutron Collar during the JCO Criticality Accident

Click to increase image sizeClick to decrease image sizeKEYWORDS: JCO criticality accidentfission neutron measurementneutron coincidence collarUNCLhelium 3 detectordose rate evolutiondose estimation