Self-powered Neutron Detectors Research Articles

CONDITION MONITORING OF SELF-POWERED NEUTRON DETECTORS IN THE INCORE NUCLEAR INSTRUMENTATION SYSTEM UNDER TRANSIENTS

The paper describes a new method for condition monitoring of self-powered neutron detectors (SPND) used to monitor neutron flux in the reactor core. The method measures currents in the SPND measuring circuit under static and dynamic conditions. A new diagnostic indicator of SPND condition is introduced allowing for significant enlargement of the detector operating range and change in the method that is used to monitor the value of SPND emitter burnout. The introduction of the additional diagnostic indicator, namely the measuring circuit capacitance, can be used for assessing the neutron flux in the reactor core under transient conditions.

Sorted compressive sensing for reconstruction of failed in-core detector signals

Self-Powered Neutron Detectors(SPNDs) are used to calculate core power distributions, an essential factor in the safe operation of nuclear power plants. Some detectors may fail during normal operation, and signals from failed detectors are isolated from intact signals. The calculated detailed power distribution accuracy depends on the number of available detector signals. Failed detectors decrease the operating margin by enlarging the power distribution measurement error. Therefore, a thorough reconstruction of the failed detector signals is critical. This note suggests a compressive sensing based methodology that rationally reconstructs the readings of failed detectors. The methodology significantly improves reconstruction accuracy by sorting signals and removing high-frequency components from conventional compressive sensing methodology.

A Coupled Deterministic and Monte-Carlo Method for Modeling and Simulation of Self-Powered Neutron Detector

To improve the efficiency of numerical calculation of self-powered neutron detector (SPND) current in pressurized water reactors (PWRs), a coupling method based on high-fidelity neutronics code and an SPND direct response model is proposed. On the detector scale, a Monte-Carlo analysis toolkit, GEANT4-SPND, is developed and verified. The toolkit, which calculates the effective contribution of insulator materials to current, has been proven to be conducive to the calculation of gamma sensitivity. On the reactor scale, detectors modeled using the deterministic method provide information of the nuclear density for the depletion calculation. The particle flux information based on the energy and spatial angular distribution is regarded as the medium of coupling strategy, thus enabling an efficient simulation of SPND current for PWR problems. This work has confirmed the necessity of particle anisotropy for current calculation in real reactor environments. The numerical code and its coupling method are verified based on the experimental values of rhodium, vanadium, platinum, and hafnium SPNDs and the K1 assembly benchmark as well. Moreover, the method is used to calculate a real VVER-1000 reactor operating in China. The numerical results are in good agreement with the values measured in realistic circumstances. What is more, the spatial distribution of the neutron sensitivity of PWR is characterized.

Simulating self-powered neutron detector responses to infer burnup-induced power distribution perturbations in next-generation light water reactors

Understanding how 3D power distribution will be monitored throughout reactor core volumetric space in next-generation nuclear power reactors is crucial to the design, deployment, and licensing of these reactors. Although numerous techniques exist for 3D power distribution monitoring based on the response of both in situ and ex situ sensors currently implemented or proposed for use in the US reactor fleet, crucial details about these techniques are often unclear. The publicly available documentation does not include information such as how well these techniques are characterized and optimized in their implementations and the levels of uncertainty in the inferred 3D power distribution. The work described herein investigated a recently developed 3D power distribution inferencing method as applied to two next-generation reactor simulations: (1) the NuScale small modular reactor design and (2) the Westinghouse AP1000 design, both of which contain in-core strings of vanadium self-powered neutron detectors (SPNDs). This investigation considered a range of SPND string sensor densities, as well as a range of 3D power distribution axial segment sizes. SPND response simulation is informed by neutron flux calculations in representative homogenized cores. For the different sensor densities and power distribution axial segment sizes in these simulations, the average solution error, solver iterations, and run time were tracked to parameterize the sensor-core configuration.

Numerical simulation of Delayed-SPND response current in NECP-Bamboo

Self-Powered Neutron Detectors (SPNDs) are widely used in Gen-III commercial PWR cores to simultaneously and continuously measure the in-core neutron flux density. A response current simulation module has been designed, developed and validated in the NECP-Bamboo code for delayed-SPNDs. First, neutron transport and nuclide depletion simulation are carried out in Bamboo-Lattice to obtain the variation of nuclide composition and the few-group constants. Next, a Monte-Carlo code is employed to calculate the electron escape probability and gamma sensitivity of SPND for each depleted lattice. Finally, the electron escape probability, gamma sensitivity and the few-group constants are regarded as input parameters for the core simulation. The response current is calculated in the Bamboo-Core code during the simulation of core operation. The methodology and the code were verified against measured values on Vanadium and Rhodium SPND. In addition, the response current of Vanadium SPND in the AP1000 core was simulated and validated.

A numerical method for quantitatively analyzing the space charge effect of Self-Powered Neutron Detector

In this study, a numerical method that can quantitatively analyze the space charge effect of Self-Powered Neutron Detector (SPND) was proposed and successfully applied to SPNDSignal toolkit. By simulating the charge accumulation in SPND’s insulator, the space charge distribution in the insulator under the specified radiation field and detector parameters were obtained. The obtained electric field can be imported into SPNDSignal to construct a real electric field in the insulator, which makes the numerical response calculation model for SPND more real. The feasibility of this method was demonstrated by a calculation for Ag SPND’s space charge distribution and detector response. Moreover, the effect of this method on response calculation of SPND utilized in reactor core was verified by performing the space charge distribution calculation and response calculation for Rh SPND used in VVER assembly, and the results show a good agreement with reference data.

즉발응답형 자발 중성자 검출기용 고이득 신호 증폭회로 설계

Self-Powered neutron detectors (SPND) detect current generated from interaction of neutron flux with emitter materials. They are inside a reactor in NPP (Nuclear Power Plant), and currently used to monitor the output and control the operation. Since the signal level of prompt-response SPND is very small (-nA), an analog circuit is necessary to amplify the current signal, and to convert it into voltage. This circuit needs to perform an anti-aliasing function to reduce distortion at the stage of digital conversion. In this paper, a systematic design process for high-gain amplification circuit is presented. Based on error analysis of the circuit, parameters are selected to satisfy design requirements. A third-order anti-aliasing filter is designed. A prototype circuit is built. Measured performance of the circuit confirms that the circuit satisfies all design requirements and validates the efficacy of the design to be used in practical environments.

Sensitivity analysis of AHWR flux mapping system for unavailability of SPNDs

In Advanced Heavy Water Reactor, a set of 168 Self-Powered Neutron Detectors (SPNDs) distributed uniformly at 6 axial locations in 28 vertical In-core Detector Housing (ICDH) units each, has been proposed to be used for monitoring and control of neutron flux distribution in the reactor core. These 168 SPNDs arrived at by applying the K-means clustering approach on the simulated in-core detector measurements, proved to be effective in flux reconstruction during normal reactor operational scenarios such as the spatial flux variations caused by movement of regulating rods, xenon spatial oscillations, and on-power refuelling. However, the accuracy of flux mapping may be affected in case of the unavailability of one or multiple SPNDs. Hence the effect of the unavailability of SPNDs on accuracy characteristics of flux mapping needs to be studied. Motivated by this, the performance of the flux mapping system in case of unavailability of all 6 SPNDs of an ICDH unit under various nominal reactor operational scenarios has been studied. It is evident from the analysis results presented in the paper that though the RMS error is poorer compared to the case when all SPNDs are available, the overall RMS error is better than 7%. The studies also brought out that the ICDH units located in the outer region of the core have more bearing in the flux reconstruction.

Performance evaluation of AHWR flux mapping system during normal operational scenarios

In large nuclear reactors, a number of appropriately distributed in-core neutron detectors are used for monitoring and control of neutron flux distribution in the core. In Advanced Heavy Water Reactor (AHWR) too, a set of 168 self-powered neutron detectors installed vertically at 6 locations in 28 in-core detector housing units has been proposed to be used. This configuration arrived at using the K-means clustering approach, has been found to reconstruct the core neutron flux distribution for a number of steady-state flux shapes. For practical application, in addition to steady-state, the optimized set of in-core detectors should provide reasonably accurate core monitoring for regular reactor operations as well. This paper investigates the accuracy characteristics of flux mapping using the proposed set of 168 in-core self-powered neutron detectors in the reconstruction of time-varying flux shapes encountered during normal reactor operations. Time-dependent flux shapes arising due to perturbations such as the movement of reactivity devices, refuelling and xenon spatial oscillations are considered for this study. Flux reconstruction is performed using the well-known flux synthesis method. From the analysis, it can be observed that the proposed set of in-core detectors reliably reconstructs the flux distribution in the reactor even during the normal operational scenarios. The maximum RMS error in mesh flux using 168 SPNDs and 35 eigenmodes is found to be 4.45% across all the operational scenarios considered.

Algorithms for processing self-powered neutron detector signals important for determination of local parameters in each part of the VVER core

These investigation findings prove the possibility of a engineering solution for VVER core automatic protection during operation in both nominal and transient conditions within ICIS using local parameters (i.e. linear heat power of the most stressed fuel rod, departure from nucleate boiling ratio). Such engineering solution will be implemented by safety system software-hardware (PTK-Z) on the basis of signals coming from in-core neutron flux detectors, temperature sensors, primary coolant flow and coolant pressure transducers. The article presents the following: a list of in-core neutron flux detectors, a list of transducers of primary coolant monitoring for thermal engineering conditions (temperature, flow rate, pressure), whose signals are delivered to the terminals of instrumentation gauges (PTK-Z) for the purpose of actuating safety actions in accordance with local parameters. The paper shows arrangement of in-core neutron flux detector equipment, above view of self-powered neutron detector (SPND) location in safety channels, axial arrangement of SPND along the core, algorithms for processing SPND signals important for determination of local parameters in each part of the core in both normal and abnormal operation conditions.

Simulating Self-Powered Neutron Detector Responses to Infer Burnup-Induced Power Distribution Perturbations in Next-Generation Light Water Reactors

Simulating Self-Powered Neutron Detector Responses to Infer Burnup-Induced Power Distribution Perturbations in Next-Generation Light Water Reactors

Development of self-powered neutron detectors used in nuclear medicine for the measurement of neutron flows during treatment of boron neutron therapy

Development of self-powered neutron detectors used in nuclear medicine for the measurement of neutron flows during treatment of boron neutron therapy

A Kalman filtering of the Self-Powered Neutron Detector current to determine excess reactivity

Self-powered neutron detectors are common sensory devices for flux mapping purposes in nuclear reactors. A Kalman filtering strategy has been explored in this work to overcome the inherent delay in certain designs thereof enabling their application for the online monitoring and control activities. Determination of the core excess reactivity by way of the current reading in a Rhodium detector has thus been attempted as a generalization to the well-known reactor inverse kinetics problem. Efficient reactivity estimation is reported. Further analysis carried out for a Cobalt SPND excludes the need for additional current compensation as this detector predominantly enjoys a fairly rapid γinduced electric current.

Development of a Fast-Spectrum Self-Powered Neutron Detector for Molten Salt Experiments in the Versatile Test Reactor

The self-powered neutron detector (SPND) is a widely used flux monitor in thermal nuclear reactors. Although this is a mature technology, the current state of the art is tuned for a thermal neutron spectrum, so many of the devices currently in use lack sensitivity to fast neutrons. Because current in SPNDs is produced through nuclear reactions with the neutron flux inside a reactor, sensitivity in SPNDs is determined by the neutron cross section of the neutron-sensitive portion of the detector, termed the emitter. This neutron cross section drops by orders of magnitude between thermal and fast neutron energies for many emitters in currently used SPNDs, with a corresponding drop in current from the detector. This paper discusses efforts to develop a fast-spectrum self-powered neutron detector (FS-SPND) that is sensitive to neutrons with energies ranging from 0.025 eV up to 1 MeV. An in-depth analysis of Evaluated Nuclear Data File (ENDF)/B-VII.1 neutron-capture cross sections was performed, and four new materials were identified that are suitable emitter candidates for use in measuring fast neutrons. All four materials are stable mid-shell nuclei in the region between doubly magic132Sn and208Pb. Each candidate was simulated with the Geant4 Monte Carlo simulation toolkit to optimize overall detector efficiency.

Development and verification of a simulation toolkit for Self-Powered Neutron Detector

A response simulation toolkit ‘SPNDSignal’ based on Geant4 was developed and verified. SPNDSignal adopts Monte-Carlo method to simulate the coupled neutron-gamma-electron transportation in SPND, and the important physical parameters such as neutron sensitivity and current components can be obtained. Compared with present Monte-Carlo simulation tools, SPNDSignal has advantages: (1) current components induced by neutron and external gamma in reactor can be calculated only through one-step simulation; (2) space charge effect can be considered by simulating electron transportation in real electric field; (3) information of particles obtained by fuel assembly calculation, such as vertex position, kinetic energy, etc., can be directly used as particle source, thus particle information loss and coupling work between different programs can be greatly reduced. The verification work is performed for rhodium, vanadium, silver and hafnium detectors. The simulated neutron sensitivities are in good agreement with experimental results. Moreover, background current of Cobalt SPND was also studied.

Simultaneous Estimation of Neutron Flux and Reactivity in Nuclear Reactors

In this article, we propose a methodology based on cubature Kalman filtering (CKF) for simultaneous estimation of two important variables of nuclear reactors, viz. the neutron flux and the total core reactivity, from signals of delayed response self-powered neutron detectors (SPNDs). Moreover, by using the linearity of our model, the Rao-Blackwellized CKF (RBCKF) algorithm is developed for our estimation problem, which offers benefits in terms of smaller computational load in comparison to standard CKF. Furthermore, a scheme which is termed as adaptive-RBCKF (A-RBCKF) has been designed for adaptation of process noise covariance. A comparative study of the two state estimators, that is, RBCKF and A-RBCKF, through simulations under different transient scenarios demonstrates the efficacy of A-RBCKF over RBCKF.

Simulation, design optimization, and experimental validation of a silver SPND for neutron flux mapping in the Tehran MTR

This paper deals with the simulation-based design optimization and experimental validation of the characteristics of an in-core silver Self-Powered Neutron Detector (SPND). Optimized dimensions of the SPND are determined by combining Monte Carlo simulations and analytical methods. As a first step, the Monte Carlo transport code MCNPX is used to follow the trajectory and fate of the neutrons emitted from an external source. This simulation is able to seamlessly integrate various phenomena, including neutron slowing-down and shielding effects. Then, the expected number of beta particles and their energy spectrum following a neutron capture reaction in the silver emitter are fetched from the TENDEL database using the JANIS software interface and integrated with the data from the first step to yield the origin and spectrum of the source electrons. Eventually, the MCNPX transport code is used for the Monte Carlo calculation of the ballistic current of beta particles in the various regions of the SPND. Then, the output current and the maximum insulator thickness to avoid breakdown are determined. The optimum design of the SPND is then manufactured and experimental tests are conducted. The calculated design parameters of this detector have been found in good agreement with the obtained experimental results.

Analysis and comparison of direct inversion and Kalman filter methods for self-powered neutron detector compensation

Many light-water reactors use self-powered neutron detectors (SPNDs) to monitor neutron flux. Some SPNDs suffer from delayed response. To compensate for this inherent delay, many methods such as the direct inversion method (DIM) and the Kalman filter method can be used. In the current investigation, these two methods were applied to the most common SPNDs, rhodium SPND (Rh-SPND) and vanadium SPND (V-SPND). The DIM, a leading compensation method, is used for V-SPND and Rh-SPND response compensation. The Kalman filter, which is a closed-loop compensation method, has also been developed for both SPNDs. The performance of the DIM and Kalman method for V-SPND and Rh-SPND was computed and surveyed with various sampling times. To evaluate the sensitivity of the compensator algorithm to miscalculations of the prompt fraction of the detector signal, computations were also conducted with various prompt fractions. Results show that the Kalman method is much more stable with different prompt fractions. Both methods improve the response time of V-SPNDs and Rh-SPNDs, and thus would allow them to be used for reactor protection, reactor control functions, reactor core monitoring, and for fulfilling surveillance requirements.

In-Core Power Measurement Using SiC Semiconductor Detector

A SiC detector was fabricated and tested in harsh radiation environment of the High Flux Advanced Neutron Application Reactor (HANARO) reactor core at Korea Atomic Energy Research Institute (KAERI). A 4H-SiC with 30 µm thick epitaxial layer was used as the radiation sensor, and the detector was designed to be tolerable against thermal and radiation damages. Alpha response and I-V characteristics of fabricated SiC sensor was measured and detection performance of the SiC detector was evaluated in a neutron field of HANARO ex-core neutron irradiation facility. After preliminary tests, reactor power monitoring using the SiC detector was carried out by inserting it into the HANARO irradiation hole, IP-4. Radiation-induced current of the detector was recorded as reactor power increased up to 10 MWth. Maximum thermal and fast neutron fluxes were 9.4×1012 and 2.5×109 neutrons/cm2/sec, respectively, and total neutron fluence irradiated on the detector was 4.7×1016 neutrons/cm2. The detector showed good linearity of response up to the tested fluence, with R2 = 0.9997. Response speed of SiC detector was compared to that of a Rh self-powered neutron detector (SPND) in terms of signal saturation time. Averaged SiC detector saturation time was 12.8 seconds, approximately 11 times faster that of the Rh SPND.

MINERAL INSULATORS FOR KOMPTON DETECTORS OF NEUTRONS WITH A METAL HAFNIUM EMITTER

One of the priority tasks of nuclear power industry is to increase the reliability and safety of nuclear power plants. In this regard, traditional detectors for measuring the neutron flux in the reactor core are being improved and new ones are developed. This work is dedicated to one of the tasks of creating a compton self-powered neutron detector, namely, the formation of an insulator of magnesium oxide on an emitter of metallic hafnium. The effect of three types of magnesium oxide of different purity and structural state, as well as annealing temperature on the electrical resistance of the insulator between the collector and the emitter of the detector, is studied. The detector prototypes were manufactured by filling with magnesium oxidepowders or by applying liquid-phase mixturesthereof.