Micro-Pocket Fission Detectors Research Articles

Integrated Circuit Design for Radiation-Hardened Charge-Sensitive Amplifier Survived up to 2 Mrad.

According to the continuous development of metal-oxide semiconductor (MOS) fabrication technology, transistors have naturally become more radiation-tolerant through steadily decreasing gate-oxide thickness, increasing the tunneling probability between gate-oxide and channel. Unfortunately, despite this radiation-hardened property of developed transistors, the field of nuclear power plants (NPPs) requires even higher radiation hardness levels. Particularly, total ionizing dose (TID) of approximately 1 Mrad could be required for readout circuitry under severe accident conditions with 100 Mrad around a reactor in-core required. In harsh radiating environments such as NPPs, sensors such as micro-pocket-fission detectors (MPFD) would be a promising technology to be operated for detecting neutrons in reactor cores. For those sensors, readout circuits should be fundamentally placed close to sensing devices for minimizing signal interferences and white noise. Therefore, radiation hardening ability is necessary for the circuits under high radiation environments. This paper presents various integrated circuit designs for a radiation hardened charge-sensitive amplifier (CSA) by using SiGe 130 nm and Si 180 nm fabrication processes with different channel widths and transistor types of complementary metal-oxide-semiconductor (CMOS) and bipolar CMOS (BiCMOS). These circuits were tested under γ–ray environment with Cobalt-60 of high level activity: 490 kCi. The experiment results indicate amplitude degradation of 2.85%–34.3%, fall time increase of 201–1730 ns, as well as a signal-to-noise ratio (SNR) of 0.07–11.6 dB decrease with irradiation dose increase. These results can provide design guidelines for radiation hardening operational amplifiers in terms of transistor sizes and structures.

Electrodeposition of low-enriched uranium onto small platinum electrodes

An updated method of preparing thin, uranium-coated, 0.25-mm diameter platinum electrodes using the cyclic potential sweep method is described. Sixty-four samples were prepared using a 0.2 M UO 2 (NO 3 ) 2 solution with 19.695% enriched uranium. Improvements were made to the sample preparation, electrodeposition , and analysis methods, producing smoother samples with more uniform surface features than prior work. Alpha-particle spectrometry was used to determine the mass of uranium deposited on each sample. The average mass of uranium deposited on the metallic electrode from 100 potential sweep cycles was 0.987 × 10 −7 g with a standard deviation of 0.860 × 10 −7 g. Removing the outliers reduces the average deposited mass to 0.741 × 10 −7 g with a standard deviation of 0.410 × 10 −7 g. A smooth, uniform deposition was observed over the entire electrode after deposition and no cracking or crystalline formations were observed. This improved method of electrodeposition remains an attractive method of depositing fissile material onto small metallic substrates for the fabrication of Micro-Pocket Fission Detectors.

Fabrication and Testing of a Modular Micro-Pocket Fission Detector Instrumentation System for Test Nuclear Reactors

Advancements in nuclear reactor core modeling and computational capability have encouraged further development of in-core neutron sensors. Measurement of the neutron-flux distribution within the reactor core provides a more complete understanding of the operating conditions in the reactor than typical ex-core sensors. Micro-Pocket Fission Detectors have been developed and tested previously but have been limited to single-node operation and have utilized highly specialized designs. The development of a widely deployable, multi-node Micro-Pocket Fission Detector assembly will enhance nuclear research capabilities. A modular, four-node Micro-Pocket Fission Detector array was designed, fabricated, and tested at Kansas State University. The array was constructed from materials that do not significantly perturb the neutron flux in the reactor core. All four sensor nodes were equally spaced axially in the array to span the fuel-region of the reactor core. The array was filled with neon gas, serving as an ionization medium in the small cavities of the Micro-Pocket Fission Detectors. The modular design of the instrument facilitates the testing and deployment of numerous sensor arrays. The unified design drastically improved device ruggedness and simplified construction from previous designs. Five 8-mm penetrations in the upper grid plate of the Kansas State University TRIGA Mk. II research nuclear reactor were utilized to deploy the array between fuel elements in the core. The Micro-Pocket Fission Detector array was coupled to an electronic support system which has been specially developed to support pulse-mode operation. The Micro-Pocket Fission Detector array composed of four sensors was used to monitor local neutron flux at a constant reactor power of 100 kWth at different axial locations simultaneously. The array was positioned at five different radial locations within the core to emulate the deployment of multiple arrays and develop a 2-dimensional measurement of neutron flux in the reactor core.

Fabrication and testing of a 5-node micro-pocket fission detector array for real-time, spatial, iron-wire port neutron-flux monitoring

Micro-Pocket Fission Detectors (MPFDs) can be used to monitor a neutron flux within the core of a nuclear reactor. When assembled into an array, MPFDs can be used to simultaneously monitor the neutron flux at multiple locations within the reactor. One permutation of the MPFD is designed to be deployed into an iron wire flux port, typical for most research and commercial reactors. A single 5-node MPFD array was fabricated at deployed into a dry flux port within the central thimble of the Kansas State University TRIGA Mk. II research nuclear reactor. The sensitivity of each MPFD node at operating voltages of +50V, +100V, and +200V was observed and an optimal operating voltage of +100V was identified. No cross-talk was observed between instrument channels during real-time neutron flux measurement. Linear power tracking and real-time response to reactor power were both observed between 10kWth and 100kWth reactor power.

Fabrication and testing of a 4-node micro-pocket fission detector array for the Kansas State University TRIGA Mk. II research nuclear reactor

Advancements in nuclear reactor core modeling and computational capability have encouraged further development of in-core neutron sensors. Micro-Pocket Fission Detectors (MPFDs) have been fabricated and tested previously, but successful testing of these prior detectors was limited to single-node operation with specialized designs. Described in this work is a modular, four-node MPFD array fabricated and tested at Kansas State University (KSU). The four sensor nodes were equally spaced to span the length of the fuel-region of the KSU TRIGA Mk. II research nuclear reactor core. The encapsulated array was filled with argon gas, serving as an ionization medium in the small cavities of the MPFDs. The unified design improved device ruggedness and simplified construction over previous designs. A 0.315-in. (8-mm) penetration in the upper grid plate of the KSU TRIGA Mk. II research nuclear reactor was used to deploy the array between fuel elements in the core. The MPFD array was coupled to an electronic support system which has been developed to support pulse-mode operation. Neutron-induced pulses were observed on all four sensor channels. Stable device operation was confirmed by testing under steady-state reactor conditions. Each of the four sensors in the array responded to changes in reactor power between 10 kWth and full power (750 kWth). Reactor power transients were observed in real-time including positive transients with periods of 5, 15, and 30s. Finally, manual reactor power oscillations were observed in real-time.

MCNP6 simulated performance of Micro-Pocket Fission Detectors (MPFDs) in the Transient REActor Test (TREAT) facility

Micro-Pocket Fission Detectors (MPFDs) are under development for in-core neutron flux measurements at the Transient REActor Test facility (TREAT) and in other experiments at Idaho National Laboratory (INL). The sensitivity of MPFDs to the energy dependent neutron flux at TREAT has been determined for 0.0300-μm thick active material coatings of 242Pu, 232Th, natural uranium, and 93% enriched 235U. Self-shielding effects in the active material of the MPFD was also confirmed to be negligible. Finally, fission fragment energy deposition was found to be in conformance with previously reported results.

Micro-Pocket Fission Detectors (MPFDs) for in-core neutron detection

Neutron sensors capable of real-time measurement of neutrons in high-flux environments are necessary for tests aimed at demonstrating the performance of experimental nuclear reactor fuels and materials in material test reactors (MTRs). In-core Micro-Pocket Fission Detectors (MPFDs) have been studied at Kansas State University for many years. Previous MPFD prototypes were successfully built and tested with promising results. Efforts are now underway to develop advanced MPFDs with radiation-resistant, high-temperature materials capable of withstanding irradiation test conditions in high performance material and test reactors. Stackable MPFDs have been designed, built, and successfully demonstrated as in-core neutron sensors. Advances in the electrodeposition and measurement of neutron reactive material, along with refinements to composition optimization simulations, have enhanced the capabilities of contemporary MPFDs.

Response functions for calculating axial power-density profiles in fuel rods using in-core neutron detectors

Micro-pocket fission detectors (MPFDs) have recently been fabricated and successfully tested as in-core flux monitoring devices in the nuclear reactor in Kansas State University. These detectors are sufficiently small to be inserted between fuel elements. Data from an MPFD array can be converted into a 3D power-density map of the reactor core for real-time flux monitoring. In the present study, the necessary mathematical models are developed that relate the power-density profiles in fuel rods to the MPFDs' response. The applicability of an inversion algorithm proposed for obtaining power-density profiles is also verified in this study.

Micro-pocket fission detectors (MPFD) for in-core neutron flux monitoring

Micro-pocket fission detectors (MPFD) have been fabricated and tested as in-core flux monitors in the 250 kW TRIGA nuclear reactor at Kansas State University. The prototype devices have been coated with a natural uranyl-nitrate to provide a neutron reactive coating. The devices are composed of alumina substrates sealed together to form a miniature gas pocket 3 mm in diameter and 1 mm wide. The devices are radiation hard and can operate in pulse mode in a neutron flux exceeding 10 12 cm −2 s −1. Placed in the central thimble of the reactor core, the MPFDs have shown count rate linearity from low to high power. Dead time losses become apparent at power levels exceeding 100 kW, yet are still low enough to allow for pulse mode operation.

Determining axial fuel-rod power-density profiles from in-core neutron flux measurements

A method is proposed for determining power-density profiles in nuclear reactor fuel rods from neutron flux measurements obtained by multiple in-core micropocket fission detectors (MPFD). A general formulation is presented that relates the power-density profiles in a reactor's fuel rods to the flux density at detector locations. Key to this formulation is the construction of an appropriate response function that relates the flux at any position in the core to the rate at which neutrons are born at different depths in the fuel rods. In this preliminary study simple, although not unrealistic, response functions for thermal neutrons and for fast neutrons are derived and used to illustrate the analysis methods. To find the unknown power-density profiles in the fuel rods, given the measured fluxes at various locations through the core, requires inversion of an ill-posed Fredholm integral equation. With numerical quadrature, this integral equation can be converted into a (generally, underdetermined) set of algebraic equations for the power densities at various axial positions in the fuel rods. To solve these underdetermined equations, the linear regularization method is employed. Results are presented for simulated data for a single fuel rod and for multiple rods in various configurations. This preliminary study indicates that in-core flux measurements can indeed be used to infer power-density profiles in reactor fuel rods.