Search

Given the variety of ways that nuclear reactor core power may be perturbed, reactor operators and developers are keen on understanding the accuracy and convergence time during which perturbations in reactor power distribution may be synthesized (i.e., inferred) from an array of in-core radiation detectors. A simulation study was conducted as described herein using a highly detailed model of the Texas A&M Training, Research, Isotopes, General Atomics Reactor, in which an array of self-powered neutron detectors (SPNDs) was considered for input to the power synthesis methodology. The core power synthesis is conducted using a point-based iterative method with an iterative loop built in to ensure working equation consistency. The forward problem of SPND response to simulated perturbations in reactor power was solved for Gaussian peak-type perturbations in the reactor power distribution. These perturbations varied in variance, amplitude, and core location to assess their impact on synthesis error and to determine the number of iterations required for convergence. A relation between the unique resolvability limit and perturbation width was identified such that the maximum synthesis error increased rapidly when the peak width went beneath this limit (a width approximating half the reactor's fuel pin-to-pin pitch); this resolvability limit is specific to the SPND configuration and fuel segmentation considered herein. The synthesis error increased linearly with perturbation peak amplitude, whereas the convergence time increased nonlinearly. Perturbations located closer to the center of the core were synthesized more accurately, albeit with a higher number of required iterations. These findings provide a qualitative and quantitative understanding of the accuracy and speed at which different types of spatial power perturbations can be resolved in light-water reactors. • The Texas A&M TRIGA reactor is used to simulate power synthesis. • Simulated vanadium self-powered neutron detectors inform the synthesis algorithm. • Different perturbation variables are used to assess error and convergence. [ABSTRACT FROM AUTHOR]

This paper describes the parametric analysis of an optical fiber–based gamma thermometer (OFBGT) that is intended to be used to infer the power distribution in the Ohio State University Research Reactor (OSURR). The OFBGT measures the radial temperature difference Δ T between an optical fiber that is within the thermal mass and an optical fiber that is within the capillary tube that is attached to the exterior of the outer sheath of the OFBGT. This gas gap acts as a thermal resistance to volumetric gamma heating q ′ ′ ′ of the OFBGT thermal mass. Of the six that are analyzed, one is deemed most appropriate for operation in the OSURR Central Irradiation Facility. This design produces a maximum Δ T of ~50°C at full reactor power (450 kW). A comparison of the six OFBGT designs generally shows how modifications of the design that increase Δ T suffer from decreased spatial resolution. [ABSTRACT FROM AUTHOR]

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. • Reduction in solution error is achievable by increasing the number of SPNDs per string • However, this is at the cost of increased run time • Applying a stricter convergence criteria may have error reduction benefits • However, a stricter convergence criteria is often detrimental to the run time • Periodicity changes in the AP1000 power distribution cause resolvability issues [ABSTRACT FROM AUTHOR]

• Several molten fluoride pseudo-ternary salt system densities were estimated. • The estimation is based on Redlich-Kister expansion and Muggianu interpolation. • This method generally outperforms estimation by additive molar volumes. • Some unmeasured pseudo-binary system densities were also estimated. The development of next-generation molten salt reactors relies on accurate knowledge of the thermophysical properties of the candidate coolant and fueled molten salts. These thermophysical properties include density, viscosity, thermal conductivity, and heat capacity. Because of difficulties in measuring thermophysical properties of molten salts, there are many gaps in the current state of thermophysical property knowledge of these salts, particularly those that contain actinides or beryllium. Therefore, leveraging modeling techniques to estimate unknown molten salt thermophysical properties and guide future experimental measurements has high value for the nuclear industry. In this study, the densities of molten fluoride pseudo-ternary salt systems, which are of interest to the nuclear industry, were estimated using Redlich-Kister expansion and Muggianu interpolation techniques. The pseudo-ternary systems considered for estimation in this study were NaF-LiF-ZrF 4 , LiF-BeF 2 -ZrF 4 , LiF-BeF 2 -ThF 4 , NaF-LiF-BeF 2 , NaF-KF-BeF 2 , NaF-ZrF 4 -UF 4 , and NaF-BeF 2 -UF 4. This Redlich-Kister estimation approach accounts for nonideal mixing behavior based on pseudo-binary subsystem interaction parameters determined from experimentally measured pseudo-binary system density data sets. The Redlich-Kister estimation was compared with the method of additive molar volumes, which assumes ideal mixing. The Redlich-Kister approach was also used to determine previously unknown binary and ternary interaction parameters based on experimentally measured density data sets for select pseudo-ternary salt systems. The results of this study show improvement in density estimation using the Redlich-Kister approach for all systems considered compared with estimation by additive molar volumes. Furthermore, this analysis allowed for the estimation of nonideal density behavior in experimentally unstudied ZrF 4 -UF 4 and BeF 2 -UF 4 , as well as the quantification of ternary interaction in NaF-LiF-ZrF 4 , NaF-BeF 2 -UF 4 , and NaF-ZrF 4 -UF 4. [ABSTRACT FROM AUTHOR]

• Thermal conductivity of molten eutectic LiF-NaF-KF is measured from 834 to 1195 K with the variable gap technique. • Thermal conductivity of LiF-NaF-KF is predicted with a kinetic theory derived model and equilibrium molecular dynamics. • Temperature dependence of the thermal conductivity of LiF-NaF-KF is discussed from an experimental and theoretical viewpoint. • Inconsistencies between experimental data on thermal conductivity of molten LiF-NaF-KF reported in literature are discussed. • Mapping of the thermal conductivity of LiF-NaF-KF is proposed in the entire range of temperature from 750 K to 1300 K. Molten salts are being proposed for numerous advanced energy applications, including advanced nuclear reactors, concentrating solar power plants, thermal energy storage, and fusion reactors. Accurate knowledge of the thermophysical properties of molten salts directly impact the performance of these energy systems and are essential for design and safety analyses. Thermal conductivity data for fluoride molten salts and mixtures are especially lacking. In this work, experimental measurements of thermal conductivity using the steady-state variable gap technique were performed on eutectic LiF-NaF-KF from 834 to 1195 K. The experiment accounts for radiative, convective, and conductive heat losses. In addition, theoretical and molecular dynamics models are used, from 750 K up to 1300 K, to estimate the thermal conductivity for comparison with the experimental results. The results of experiments show a weak negative deviation of thermal conductivity with temperature, unlike previous experimental results in the literature. The measured thermal conductivity magnitudes agree with the theoretical and molecular dynamics predictions, aside from the data above 1100 K, where heat losses and radiative errors are the most significant, having a 16% maximum deviation from theory. These experimental results provide new thermal conductivity data for the LiF-NaF-KF system and further validation of the predictive models. The theoretical model was used to map the composition and temperature dependent thermal conductivity of LiF-NaF-KF and the mapping's deviation from a linear additivity estimation of thermal conductivity. This mapping showed the highest deviations from linearity for KF-LiF rich mixtures and increasing deviation with temperature. Notably, the deviation from linearity near the LiF-NaF-KF eutectic composition was around 25%. [ABSTRACT FROM AUTHOR]

• Techniques used for the measurement of thermal conductivity are reviewed. • A steady state instrument designed to measure the thermal conductivity is presented along with the measurement theory. • Tested on helium and KNO 3 NaNO 3 molten salt from 300 °C to 500 °C. • Effects of convection and radiation were mitigated using this technique. • Conductive heat losses will need to be minimized to extend operability above 500 °C. High-temperature fluids such as molten salts, liquid metals, and gasses are being proposed for many advanced energy systems including thermal energy storage devices, concentrating solar plants, and advanced nuclear reactor designs. However, the chemical behavior and thermophysical properties of many of these fluids have not been well characterized, which hinders the design, modeling, safety analysis, and deployment of these systems. Thermal conductivity is a property that is especially limited by existing measurement capabilities, which are subject to errors caused by convection, material interaction, radiative heat transfer, and instrument degradation. Therefore, there is a lack of standard, systematic measurement techniques for high-temperature, reactive, and corrosive fluids. In this work, the development of a variable gap thermal conductivity measurement system is detailed. The system is designed to measure the thermal conductivity of highly corrosive and reactive fluids, and survive operation between 100 °C and 800 °C. The effects of convection are minimized by limiting the thickness of the specimen to thin sizes (<0.3 mm). Corrections for radiative heat transfer were included in the working equations to consider specimens with varying optical properties. The design, construction, instrumentation, operating principles, and data analysis techniques are discussed in detail. The system was tested up to 500 °C using helium gas and molten KNO 3 NaNO 3 to verify the measurement technique and determine the sources of error. At 300 and 400 °C KNO 3 NaNO 3 , results showed maximum relative error of 6% when compared to results in the literature. The helium results were within 13% of those in the literature at 300 and 400 °C. Higher errors were observed at 500 °C for both fluids, and the sources of these errors are discussed. [ABSTRACT FROM AUTHOR]

The use of single-crystal sapphire optical fibers has been considered to extend fiber-optic sensing to the extreme temperature (>1000 °C) environments encountered in nuclear applications. However, before these sapphire fiber–based sensors can be deployed, their optical transmission and dimensional stability (which impacts drift of some sensors) must be characterized under representative testing conditions. Data regarding the optical transmission of sapphire following high-dose neutron irradiation at temperatures >100 °C is extremely limited. This work provides measurements of optical density (i.e., attenuation) and directional dimensional changes in bulk single-crystal sapphire materials irradiated to a fast neutron fluence of 2.4 × 1021 n/cm2 (3.5 displacements per atom) at temperatures ranging from 95 to 688 °C. Optical density measured after irradiation at 95 and 298 °C showed ultraviolet and visible absorption bands corresponding to known defect centers and temperature trends that were generally consistent with previous ex situ and in situ measurements made at much lower neutron fluence. However, optical density measured after irradiation at 688 °C was as much as two orders of magnitude higher, indicating that the fundamental mechanism for radiation-induced attenuation changes at this irradiation temperature. Additional analysis and comparison with previous works suggest that the attenuation may result from void formation, leading to increased Rayleigh scattering losses in the material and increased swelling that would also result in drift of Bragg grating-based sensors in sapphire fibers. These results pose serious questions regarding the feasibility of sapphire fiber–based sensors for high-temperature nuclear applications. [ABSTRACT FROM AUTHOR]

Abstract: Allyl-palladium dithiocarbamate complexes, [Pd(allyl)(S2CNR2)], have been prepared from the addition of dithiocarbamate salts to [Pd(allyl)(μ-Cl)]2 and TGA and DSC studies have been carried out in order to assess their potential as MOCVD precursors to palladium sulfides. For comparison [(η3-C4H7)Pd(S2PPh2)] and [Pd(S2CNMeR)2] (R=Bu, Hex) have also been prepared and tested as precursors. The unsymmetrical dithiocarbamate complex, [(η3-C3H5)Pd(S2CNMeHex)], which has a melting point of 65°C was selected as the best single source precursor and thin films of predominantly Pd2.8S were deposited on glass slides. The crystal structures of [(η3-C4H7)Pd(S2CNMe2)], [(η3-C4H7)Pd(S2CNPr2)], [(η3-C4H7)Pd(S2PPh2)] and [Pd(S2CNMeBu)2] are reported. All except [(η3-C4H7)Pd(S2CNPr2)] show weak intermolecular S⋯H or Pd⋯H interactions. [Copyright &y& Elsevier]

A 4-step data analytic methodology has been devised for the purpose of inferring the distribution of power in a reactor core, based on the response of an array of optical fiber based gamma thermometers (OFBGTs). This data analytic methodology is crucial for the development of a system of OFBGTs for the purpose of calibrating local power range monitors in boiling water reactors. Such a system would be an improvement to the present calibration system in boiling water reactors, in terms of safety, efficiency, and permanence. The first step of this methodology is to establish an energy balance method. In this method, one uses MCNP to determine response functions, which allow one to convert from gamma thermometer response to power. The gamma thermometers and the reactor core are segmented, such that each gamma thermometer segment provides an estimate of power for each reactor core segment. The estimates of power for the reactor core segments (hereafter referred to as fuel assembly segments) are calculated based on the measured response of the gamma thermometers. The second step of the methodology is to employ a weighting scheme to combine the estimates of the power of the various fuel assembly segments, based on the response of the various OFGBT segments and the incremental dose rates of the various fuel assembly segments to the OFBGT segments. The third step is to iteratively calculate the estimates of the power of the fuel assembly segments, until a convergence criterion is met, which indicates that the calculation has converged. The fourth and final step of the methodology is to estimate the uncertainty of the power for each fuel assembly segment. The mathematical basis for this step is not the focus of this paper. We have used a 3-D homogeneous reactor model to demonstrate the data analytic methodology; and have found that the data analytic methodology operates as intended. • A data analytic methodology was developed to infer the power distribution in a reactor, based on optical fiber-based gamma thermometer responses. • The data analytic methodology is based on energy balance, and the weighting of the data is based on incremental dose rates. • A 3-D model was used to provide an example of how the methodology works, when the modeled data and "measured" data disagree. [ABSTRACT FROM AUTHOR]

The primary concern for implementing amorphous fused silica (a-SiO 2) fiber optic sensors in a nuclear environment is the radiation-induced attenuation (RIA) of the light signal due to the formation of radiation-induced color centers. In addition, Bragg grating sensors drift under irradiation due to radiation-induced compaction of the a-SiO 2 structure. This work provides new data regarding RIA and radiation-induced compaction of a-SiO 2 samples irradiated to a fast neutron fluence of 2.4 × 1021 n/cm2 at temperatures of 95, 298, and 688 °C. Results show that RIA may be approaching saturation for the range of photon energies evaluated in this paper and that the hydroxyl content has a significant impact on RIA when the irradiation temperature is increased to 688 °C. A model was developed for predicting radiation-induced compaction, and the resulting signal drift for Bragg grating sensors, as a function of neutron fluence and temperature. • a-SiO 2 samples irradiated to high neutron dose at 95, 298, and 688 °C. • Radiation-induced compaction and optical attenuation measured. • Model developed to predict temperature- and neutron-dependent compaction. • Compaction estimated to cause significant drift of fiber Bragg grating sensors. • Attenuation is significantly reduced at higher temperatures. [ABSTRACT FROM AUTHOR]