Technique for Determining Local Nucleate and Film Boiling Correlations for Large Diameter Tubes

The boiling behaviour on the surface of large diameter tubes is known to be strongly dependent on the local orientation of the surface around the circumference. Understanding such local variations in boiling behaviour is of particular interest in the CANDU® nuclear industry, where part of the heat removal path for cooling the fuel under postulated accident conditions is via pool-boiling on the surface of the 132 mm diameter calandria tube that is immersed in a pool of the heavy water moderator. While the average pool boiling behaviour of the calandria tubes has been well studied with integrated experiments, local boiling correlations have not been developed for prototypical diameter calandria tubes. Local boiling correlations will allow for more detailed modelling of postulated accident scenarios and better quantification of safety margins. In this study, the nucleate and film pool-boiling characteristics of a large diameter Zircaloy-2 CANDU® calandria tube were tested in a pool of subcooled water. A novel technique is developed to derive the local boiling curve along the circumference of the tube, involving the local heating, measurement of the wall temperature, and heat flux. The adequacy of the technique is determined by comparing the local boiling curves with previous experiments in which averaged pool boiling characteristics were obtained.

Detection of background thermal neutrons in a modified low-background germanium gamma-ray spectrometer

The paper presents the detection of background neutrons using the 558.4 keV gamma line emitted from excited 114Cd nuclei after neutron induced processes. Stable cadmium and 60 L water moderator were placed inside low background shield of high purity germanium (HPGe) based spectrometer. The method was applied to study diurnal mean variation of background neutron flux. For this purpose 1 h gamma background spectra of modified HPGe spectrometer were collected subsequently for 50.5 days and then summed in 24-h-cycle. Results for the 558.4 keV line are presented along with those of main background lines. The total gamma-ray spectrum was also analysed and results are presented and discussed. No cyclic structure was noticed for all examited lines but 2223 keV, for which some day-night effect is suggested to exist. Test of another moderator made of 108 kg of graphite was done, but the results obtained with water are more promising.

Optimization of the Canadian SCWR Core Using Coupled Three-Dimensional Reactor Physics and Thermal-Hydraulics Calculations

The Canadian supercritical water-cooled reactor (SCWR) design is part of Canada's Generation IV reactor development program. The reactor uses batch fueling, light water above the thermodynamic critical point as a coolant and a heavy water moderator. The design has evolved considerably and is currently at the conceptual design level. As a result of batch fueling, a certain amount of excess reactivity is loaded at the beginning of each fueling cycle. This excess reactivity must be controlled using a combination of burnable neutron poisons in the fuel, moderator poisons, and control blades interspersed in the heavy water moderator. Recent studies have shown that the combination of power density, high coolant temperatures, and reactivity management can lead to high maximum cladding surface temperatures (MCST) and maximum fuel centerline temperatures (MFCLT) in this design. This study focuses on improving both the MCST and the MFCLT through modifications of the conceptual design including changes from a 3 to 4 batch fueling cycle, a slightly shortened fuel cycle (although exit burnup remains the same), axial graded fuel enrichment, fuel-integrated burnable neutron absorbers, lower reactivity control blades, and lower reactor thermal powers as compared to the original conceptual design. The optimal blade positions throughout the fuel cycle were determined so as to minimize the MCST and MFCLT using a genetic algorithm and the reactor physics code PARCS. The final design was analyzed using a fully coupled PARCS-RELAP5/SCDAPSIM/MOD4.0 model to accurately predict the MCST as a function of time during a fueling cycle.

OPTIMIZATION OF COLLIMATOR NEUTRON DESIGN FOR BORON NEUTRON-CAPTURE CANCER THERAPY (BNCT) BASED CYCLOTRON 30 MeV

<span>This research in BNCT has a goal to design a collimator that can be used for cancer therapy. Simulations were carried out by MCNPX software. A collimator is designed by cyclotron 30 MeV as a neutron generator. Independent variables varied were material and thickness of each collimator’s component to get five of IAEA’s standard of the neutron beam. The result is two collimator designs that can pass all IAEA’s standard. Those designs are cyclotron collimator I and cyclotron collimator II. Collimator designs obtained are tube collimator consisting of a cylindrical target </span>⁷<span>Be length of 1.4 cm and radius 1 cm, a lead wall with thickness 23 cm, cylindrical heavy water moderator (D</span>₂<span>O) with radius 3 cm. Filter Cd-nat for cyclotron collimator I with a thickness of 1 mm and a radius 3 cm. Cyclotron collimator II uses </span>⁶⁰<span>Ni with a thickness of 5 cm as a filter. The radius aperture is 3 cm. These two collimator designs can be used for cancer treatment with BNCT. Dosimetry calculation and manufacture of prototypes are needed to test the application of this design.</span>

A PRELIMINARY CATHENA THERMALHYDRAULIC MODEL OF THE CANADIAN SCWR FOR SAFETY ANALYSIS

The supercritical water-cooled reactor (SCWR) is one of six reactor concepts under development in the Generation-IV International Forum (GIF). As a member of GIF, Canada is developing a pressure-tube type SCWR, which has the potential to fulfill all major GIF goals on enhanced safety, sustainability, economics, and proliferation resistance. The system thermalhydraulics code CATHENA will be used in the safety analyses for the Canadian SCWR. Based on the current conceptual design of the Canadian SCWR, a CATHENA idealization has been developed. This model includes all 336 fuel channels with a detailed model of heat transfer in the reactor core. Also modeled are the main pumps, inlet plenum, outlet plenum, turbines, and heavy water moderator. In this paper, the CATHENA idealization of the Canadian SCWR conceptual design is described. Simulation results for steady-state normal operations are also presented for the current Canadian SCWR conceptual design.

Coupled Neutronics/Thermal-Hydraulics Analysis of PT–SCWR With 54-Element Bundle Design

The radial power distribution is uneven in the Pressure Tube type SCWR bundle because of the higher fuel enrichment and the separation of light water coolant and heavy water moderator. The uneven profile will usually result in high cladding temperature. In order to optimize bundle design, the analysis of coupled neutronics/thermal-hydraulics was carried out. The neutronics code WIMS-AECL and the subchannel code ATHAS are selected to conduct the coupled neutronics/thermal-hydraulics analysis for the PT-SCWR 54-elements bundle design. The analysis of the reference case showed that the maximum cladding surface temperature of the bundles at both Beginning of the Cycle (BOC) and End of the Cycle (EOC) exceed the design limits. The optimization was carried out through (1) adopting staggered radial uranium enrichment profile and (2) optimizing the pitch-circle diameters of element rings. The results showed that the optimized bundle can achieve lower maximum cladding surface temperature, which is well below the limiting criteria, and meet the criteria of neutronics demands.

Design of Moderators and Reflectors for Chinese Spallation Neutron Source

Chinese Spallation Neutron Source (CSNS) is a significant research facility for studies of structure and dynamics of materials, which can play an important role in research and education. Moderators and reflectors system is one of the core components in CSNS. There are three moderators totally, namely, the decoupled and poisoned hydrogen moderator, the coupled hydrogen moderator and the decoupled water moderator. Moderators are inserted into an aluminum alloy vessel which contains beryllium and stainless steel reflectors. The moderators and reflectors are designed into a whole plug, MR Plug, so as to move the plug into or out of the core vessel of CSNS easily. The structure design of moderators and reflectors system is just finished now, including moderators, reflectors and their container. Neutronics requirements and strength criterions are satisfied, manufacture of system is feasible. Structure strength analyses of some parts are also presented.