Fast Neutron Hodoscope Research Articles

A Geant4 evaluation of the Hornyak button and two candidate detectors for the TREAT hodoscope

The performance of traditional Hornyak buttons and two proposed variants for fast-neutron hodoscope applications was evaluated using Geant4. The Hornyak button is a ZnS(Ag)-based device previously deployed at the Idaho National Laboratory’s TRansient REActor Test Facility (better known as TREAT) for monitoring fast neutrons emitted during pulsing of fissile fuel samples. Past use of these devices relied on pulse-shape discrimination to reduce the significant levels of background Cherenkov radiation. Proposed are two simple designs that reduce the overall light guide mass (here, polymethyl methacrylate or PMMA), employ silicon photomultipliers (SiPMs), and can be operated using pulse-height discrimination alone to eliminate background noise to acceptable levels. Geant4 was first used to model a traditional Hornyak button, and for assumed, hodoscope-like conditions, an intrinsic efficiency of 0.35% for mono-directional fission neutrons was predicted. The predicted efficiency is in reasonably good agreement with experimental data from the literature and, hence, served to validate the physics models and approximations employed. Geant4 models were then developed to optimize the materials and geometries of two alternatives to the Hornyak button, one based on a homogeneous mixture of ZnS(Ag) and PMMA, and one based on alternating layers of ZnS(Ag) and PMMA oriented perpendicular to the incident neutron beam. For the same radiation environment, optimized, 5-cm long (along the beam path) devices of the homogeneous and layered designs were predicted to have efficiencies of approximately 1.3% and 3.3%, respectively. For longer devices, i.e., lengths larger than 25 cm, these efficiencies were shown to peak at approximately 2.2% and 5.9%, respectively. Moreover, both designs were shown to discriminate Cherenkov noise intrinsically by using an appropriate pulse-height discriminator level, i.e., pulse-shape discrimination is not needed for these devices.

The CABRI fast neutron Hodoscope: Renovation, qualification program and first results following the experimental reactor restart

The CABRI experimental pulse reactor, located at the Cadarache nuclear research center, southern France, is devoted to the study of Reactivity Initiated Accidents (RIA). For the purpose of the CABRI International Program (CIP), managed and funded by IRSN, in the framework of an OECD/NEA agreement, a huge renovation of the facility has been conducted since 2003. The Cabri Water Loop was then installed to ensure prototypical Pressurized Water Reactor (PWR) conditions for testing irradiated fuel rods. The hodoscope installed in the CABRI reactor is a unique online fuel motion monitoring system, operated by IRSN and dedicated to the measurement of the fast neutrons emitted by the tested rod during the power pulse. It is one of the distinctive features of the CABRI reactor facility, which is operated by CEA. The system is able to determine the fuel motion, if any, with a time resolution of 1 ms and a spatial resolution of 3 mm. The hodoscope equipment has been upgraded as well during the CABRI facility renovation. This paper presents the main outcomes achieved with the hodoscope since October 2015, date of the first criticality of the CABRI reactor in its new Cabri Water Loop configuration. Results obtained during reactor commissioning phase functioning, either in steady-state mode (at low and high power, up to 23 MW) or in transient mode (start-up, possibly beyond 20 GW), are discussed.

Qualification and characterization of electronics of the fast neutron Hodoscope detectors using neutrons from CABRI core

The study of Reactivity Initiated Accidents (RIA) is important to determine up to which limits nuclear fuels can withstand such accidents without clad failure. The CABRI International Program (CIP), conducted by IRSN under an OECD/NEA agreement, has been launched to perform representative RIA Integral Effect Tests (IET) on real irradiated fuel rods in prototypical Pressurized Water Reactors (PWR) conditions. For this purpose, the CABRI experimental pulse reactor, operated by CEA in Cadarache, France, has been strongly renovated, and equipped with a pressurized water loop. The behavior of the test rod, located in that loop in the center of the driver core, is followed in real time during the power transients thanks to the hodoscope, a unique online fuel motion monitoring system, and one of the major distinctive features of CABRI. The hodoscope measures the fast neutrons emitted by the tested rod during the power pulse with a complete set of 153 Fission Chambers and 153 Proton Recoil Counters. During the CABRI facility renovation, the electronic chain of these detectors has been upgraded. In this paper, the performance of the new system is presented describing gain calibration methodology in order to get maximal Signal/Noise ratio for amplification modules, threshold tuning methodology for the discrimination modules (old and new ones), and linear detectors response limit versus different reactor powers for the whole electronic chain.

Quantitative Fuel Motion Determination with the CABRI Fast Neutron Hodoscope: Evaluation Methods and Results

The fast neutron hodoscope installed at the CABRI reactor in Cadarache, France, is employed to provide quantitative fuel motion data during experiments in which single liquid-metal fast breeder reactor test pins are subjected to simulated accident conditions. Instrument design and performance are reviewed, the methods for the quantitative evaluation are presented, and error sources are discussed. The most important findings are the axial expansion as a function of time, phenomena related to pin failure (such as time, location, pin failure mode, and fuel mass ejected after failure), and linear fuel mass distributions with a 2-cm axial resolution. In this paper the hodoscope results of the CABRI-1 program are summarized.

A Proton-Recoil Proportional-Counter Array for Neutron-Image Construction

The fuel-motion measurement capability of the fast-neutron hodoscope has been upgraded by the addition of a 360-detector proton-recoil proportional-counter array, which detects high-energy fission neutrons. The current sensitive amplifier/discriminator module for each detector fits into a 12.7- by 12.7- by 102-mm package and costs less than $100 per module. It has a 50-ns rise time, a noise level of 100 nA, and a deadtime per event of 200 ns. Provision has been provided for the independent adjustment of the input current versus discriminator voltage for each module. The new proportional-counters cost approximately $400 each. Each detector has been tested to have the same gain versus voltage response. A space-charge model relating count-rate changes to space-charge effects has also been developed. The new detector array has been operational for approximately two years and has become the main detector system in fuel-motion analysis. It has significantly improved the linearity, stability, count-rate capability, and setup ease of the hodoscope.

Fast-Neutron Hodoscope at TREAT: Methods for Quantitative Determination of Fuel Dispersal

Fuel-motion surveillance using the fast-neutron hodoscope in TREAT experiments has advanced from an initial role of providing time/location/velocity data to that of offering quantitative mass results. The material and radiation surroundings of tha test section contribute to intrinsic and instrumental effects upon hodoscope detectors that require detailed corrections. Depending upon the experiment, count rate compensation is usually required for deadtime, power level, nonlinear response, efficiency, background, and detector calibration. Depending on their magnitude and amenability to analytical and empirical treatment, systematic corrections may be needed for self-shielding, self-multiplication, self-attenuation, flux depression, and other effects. Current verified hodoscope response (for 1- to 7-pin fuel bundles) may be paramatrically characterized under optimum conditions by 1-ms time resolution; 0.25-mm lateral and 5-mm axial-motion displacement resolution; and 50-mg single-pin mass resolution. The experimental and theoretical foundation for this performance is given, with particular emphasis on the geometrical response function and the statistical limits of fuel-motion resolution. Comparisons are made with alternative diagnostic systems.

Optimization of a Hornyak-Button Detector for Fast-Neutron Detection

As part of an ongoing program to improve the fuel-motion capability of the fast-neutron hodoscope, a response model has been developed for the Hornyakbutton detector. The model provides a description of the efficiency and gamma sensitivity of a Hornyak button in terms of detector parameters such as scintillator concentration, grain size, length, and transparency. It is possible using this model to quantitatively choose detector parameters for the Hornyak button.

Minicomputer-based test system for analyzing the electronic components of a large scale scientific research instrument

Large scale research experiments often require the use of highly specialized and sophisticated electronic equipment including computer-based data acquisition systems capable of gathering large amounts of data over very short time intervals. Equipment malfunctions can mean a loss of experimental data integrity and valuable research time. A diagnostic test system has been developed to monitor the electronic equipment of such an experiment, the Fast Neutron Hodoscope at the Transient Reactor Test Facility in Idaho. The system allows hodoscope electronics designers at Argonne National Laboratory in Illinois, and hodoscope maintenance engineers in Idaho, to observe simultaneously digital and analog signals from the hodoscope electronics while the hodoscope is in operation. The diagnostic test system can be controlled either locally by the engineers at the research site in Idaho or remotely by the designers in Illinois via a computer to computer telephone link between the two sites. As a result potential equipment problems as well as actual failures can often be pinpointed and corrected with minimal lost time and cost to the research program.

Loss-of-Flow Transient Reactor Test Facility Tests L6 and L7 with Irradiated Liquid-Metal Fast Breeder Reactor Type Fuel

This paper describes Transient Reactor Test Facility tests L6 and L7, in which a loss-of-flow accident sequence was simulated using three fuel elements containing (Pu,U)O/sub 2/. Fuel dispersal rates at 10 and 20 times nominal power were measured using the 1.2-m fast neutron hodoscope. The measured axial fuel density variations were weighted with typical liquid-metal fast breeder reactor fuel-worth distributions so that the significance of the fuel motion could be assessed. The experimental fuel-worth changes were also compared with the fuel-worth changes computed by fuel motion models SLUMPY and LEVITATE. 11 refs.

Proton-Recoil Proportional Counter Tests at Treat

A methane filled proton-recoil proportional counter will be used as a fission neutron detector in the fastneutron hodoscope. To provide meaningful fuel-motion information the proportional counter should have: a linear response over a wide range of reactor powers (space charge effects are minimized); a good signal-to-background ratio (the number of high energy neutrons detected must be maximized relative to low energy neutrons, and the gamma ray sensitivity must be kept small); and a detector efficiency for fission neutrons above 1 MeV of approximately 1%. In addition, it is desirable that the detector and the associated amplifier/discriminator be capable of operating at counting rates in excess of 500 kHz. This paper reports on tests that were conducted on several proportional counters at the TREAT reactor. The goal of these tests was the determination of a set of counter parameters that would optimize fuel motion detection. The results show that a proton-recoil proportional counter with a methane gas pressure of 0.5 MPa, a counter diameter of 25.4 mm, an anode wire diameter of 0.0504 mm, and a length of 200 mm will be able to meet most of the hodoscope detector requirements.

TREAT Fast-Neutron Hodoscope: Improvements in Time and Mass Resolution of Fuel Motion

The fast-neutron hodoscope at the Transient Reactor Test Facility detects fuel motion in thick opaque capsules during in-core destructive transient tests. Counts from several hundred detectors, each with rates up to a megahertz, are collected at intervals as short as a millisecond for up to tens of seconds. The large amount of data must be decoded, normalized, represented in suitable forms, and analyzed. A computer-controlled magnetic disk data acquisition system has been installed which provides shorter data collection intervals, simplifies decoding, and permits immediate data analysis. Data normalizations and representations have been developed which significantly increase the dynamic range, yield sensitive quantitative indications of fuel mass motion, and render the data intuitively comprehensible. The improved hodoscope system performance level is demonstrated by results from a recent transient test, Pinex-2, which show the quantitative evolution of fuel mass motion from 1515 MW peak reactor power through a 1 MW post-scram radiation level.

Hodoscope Performance Tests on a 91-Pin Fuel Bundle at PARKA

Joint LASL/ANL studies of the fuel motion performance capability of a fast-neutron hodoscope have been performed on a 91-pin fuel bundle. Optimization of the neutron detectors used in the hodoscope was also investigated. The neutron detectors tested were a Hornyak button and a stilbene detector with pulse shape discrimination. These studies show that a hodoscope can provide useful fuel motion detection capability for large bundles and that the amount of information provided is greatly increased by the use of neutron detectors with high neutron thresholds and high detection efficiency. These measurements indicate that the TREAT hodoscope can be upgraded to meet most of the fuel motion diagnostic requirements of new safety facilities.

Fast-Neutron Hodoscope at TREAT: Data Processing, Analysis, and Results

The fast-neutron hodoscope at the Transient Reactor Test Facility is designed for the determination of fuel motion during the course of brief (0.1- to 30-sec) power transients. During the course of a transient test, data must be recorded from each of 334 hodoscope channels at count rates up to 2 million/sec each, down to millisecond time intervals. This is accomplished in a relatively reliable and inexpensive manner by displaying counts from each detector sequentially in binary code on a lamp panel, which is photographed by a high-speed framing camera, producing a film record of the transient test. After chemical development, the film is examined by a computer-controlled flying-spot scanner, and the position and density of candidate lamp images are recorded on magnetic tape. Through further computer processing, these images are sorted and decoded, and the count rate is recovered for each detector at each instant of collection time. A cathode-ray tube and a plotter, both computer controlled, are used to recreate and analyze the fuel motion history of the experiment. Analysis is directed toward fuel distortion or expansion prior to clad failure, slumping, dispersion, amount and rates of movement, post-scram relocation, and ultimate disposition of fuel.

Multi-Detector Integrated Fission Counter Array

A multi-chamber fission array has been designed, fabricated, and tested with integrated and condensed electronic circuitry. The intention was to develop an array of over 100 channels of detectors able to cover the high power range of TREAT transients in conjunction with the fast neutron hodoscope. The array has been developed through a prototype stage consisting of 31 possible channels. A wide variety of operating conditions have been tested, including choice of fissile isotope, coating thickness, gas filling, plate spacing, high voltage, and electronic circuitry. The configuration tested at TREAT consisted of ten active channels incorporated in a doubly contained system. Miniaturized electronic circuits are appended almost directly to the parallel plates of the fission counters. Cost and space limitations were major factors in the design.

High-Resolution Fast-Neutron and Gamma Digital Radiography

The fast neutron hodoscope at TREAT can be modified for high resolution radiography. Time-resolved radiography normally performed by the hodoscope with coarse spatial resolution may be traded for good space definition at the expense of time resolution, allowing the hodoscope to become a dual purpose instrument. The modifications require remote motorized scanning of the collimator and simultaneous 334-channel data collection over extended intervals of time. By doing so, it becomes possible to generate digital radiographs without the necessity for mechanical disturbance of the object being viewed. Furthermore, separate and distinct images of fuel and steel may be produced.

Fast-Neutron Hodoscope at Treat: Development and Operation

A description is given of a fast-neutron hodoscope to detect fuel motion within test samples inserted in the center of the Transient Reactor Test Facility (TREAT) reactor core. The fuel motion is induced under simulated hypothetical conditions. The hodoscope system includes components necessary to acquire, store, decode, and process the collected data.

The Fast Neutron Hodoscope at Treat: Operational Experience and Improvementsx

The multichannel hodoscope at TREAT is capable of detecting motion of fuel resulting from intentional meltdown of fuel pins contained within a test capsule. The system has been in operation in its present form since 1969, having participated in over 50 transients. Operational experience and evolutionary improvements are the subject matter of this report. The rather complex hodoscope system, which is supported by an involved series of steps before results can be achieved, has worked well within design prediction. Computer-aided reconstruction of the data has been provided to obtain displays, plots, and movies of fuel motion events.

Extended Capability for Fast Neutron Hodoscope Used at Treat

The fast neutron hodoscope used at TREAT reactor for fuel meltdown studies has been improved to permit count rates above 106/sec in each channel. Also a digital readout system, based on integrated circuit scalers, has been introduced. Peak instantaneous data rate capability, now about 107/sec, can be expanded to 109/sec.