Thorium Molten Salt Reactor Research Articles

Measurement on the flow structure of a gas-liquid separator applied in TMSR

The swirl flow in an axial gas-liquid separator is critical for the fission gas separation in the thorium molten salt reactor. Although the two-phase flow pattern was visualized in our previous studies, the liquid flow field is still unknown. To clarify the three dimensional fluid structure, we used the stereo PIV technique to explore the distribution of three velocity components. The velocity information was extracted from five typical planar positions covering the whole swirl chamber. The averaged tangential velocity profile indicates that a quasi-Rankine vortex dominates the swirl flow and the swirl intensity represented by the maximal tangential velocity firstly increases and then decays gradually. For the first time, we found that a secondary counter rotating vortex is contained in the core region of the swirl flow and the downstream orifice configured in the separator probably induces the variation of flow structure. The averaged axial velocity is distributed in a W type manner across the swirl chamber. The averaged radial velocity is more complicated due to the special configuration of the separator. The magnitude of radial velocity component is of the same order compared to other two velocity components and the distribution is asymmetrical in the first two observing fields and gradually becoming symmetrical in the downstream. All velocity distributions presented in the 3D waterfall constitute a complete picture of the swirl flow structure in the gas-liquid separator.

Numerical study of the dynamic characteristics of a single-layer graphite core in a thorium molten salt reactor

A reactor core in a thorium molten salt reactor uses graphite as a moderator and reflector. The graphite core is a multi-layered arrangement of graphite bricks that are loosely connected to each other using a system of keys and dowels. Consequently, the graphite core is a type of discrete stack structure with highly nonlinear dynamic behavior. Hence, it is important to investigate the dynamic characteristics of the graphite core. In this study, a three-dimensional single-layer graphite core model, which is a part of the thorium molten salt reactor side reflector structure, was analyzed using the explicit method in ABAQUS 2016 to study the core dynamic behavior when subjected to different excitations. The design parameters, such as the diameter of the dowel, the gap between key and keyway and the bypass flow gap between two adjacent bricks, were also considered in this model. To reduce excessive demands on available computational resources considering the effect of molten salt, the spring–dashpot model was applied to model the interaction forces between the molten salt and graphite bricks. Numerical simulation results show that the effect of molten salt is a reduction in the peak maximal principal stress, and a larger gap between two bricks is beneficial to maintain the integrity of the graphite core under earthquake loading. The results obtained by the simulation can be used as a reference for future designs of a molten salt graphite core.

Neutronics modeling and analysis of the TMSR-SF1 fuel lattice and full core with explicit fuel particle distribution and random pebble loadings

The first Solid Fueled Thorium Molten Salt Reactor (TMSR-SF1), a Fluoride-salt-cooled High Temperature Reactor (FHR) designed by the TMSR group of Chinese Academy of Science (CAS), is expected to be constructed in the near years. The combination of TRistructural-ISOtropic (TRISO) coated-fuel particles, pebble bed core, and Flibe coolant leads to various neutronics challenges, such as the double heterogeneity, distribution of TRISO particles, high density packed pebble bed, neutronics characteristics of Flibe salt, and etc. For the purpose of evaluating candidate neutronics codes, benchmark study will be conducted for the infinite fuel pebble lattice with explicitly described fuel particle distribution. For investigating key physics phenomena of the randomly packed pebble bed core, two coordinate generation methods will be explored in this study: Discrete Element Method (DEM) that accounts for inter-particle forces and a mathematical model (MM) that fills arbitrary domains through simple geometric rules on the addition of particles. While the former generates packing distributions closer to real-world scenarios, the latter is more computationally effective. The pebble loading effect on eigenvalues and fission power distribution will be studied. Emphasis is placed on the Flibe salt density reactivity effect -- the most unique feature of the FHR comparing to the conventional High Temperature Gas-cooled Reactors (HTGRs). Neutron balance based reactivity decomposition method will be employed for describing the underlying physics.

Simultaneous PIV/PLIF and Pulsed Shadowgraphy Measurement of Gas-Liquid Flows in a Swirling Separator

The gas-liquid separator is a key component in the gas removal system of the Thorium Molten Salt Reactor. Phase separation is driven by a swirling flow, and the fundamental principle is that dispersed bubbles are accumulated and coalesced into an air core to realize separation from the liquid phase. In this paper, simultaneous particle image velocimetry (PIV) and pulsed shadowgraphy techniques are applied to characterize the two-phase-flow patterns in the evolutionary process of the air core. The PIV technique utilizes fluorescent particles as tracers in the liquid flow field, and a charge coupled device (CCD) camera records the planar laser-induced fluorescence signal of the particles. Another camera simultaneously detects the shadow and motion of the air core via backlighting from an array of infrared light-emitting diodes. The signals originating from the different phases are separated by a beam splitter with a dichroic filter and optical filters, and only undisturbed signals from the shadow of the air core and fluorescence tracer particles of the fluid are effectively captured by the two CCD cameras, respectively. Experimental data are carried out for three Reynolds numbers Re for a range of outlet pressures Pout. The morphology of the air core tail periodically transforms from a linear type to a single-helix type to a double-helix type before reaching a stable state at the critical outlet pressures Pcout. The analysis of gas-liquid flow patterns indeed indicates that axial velocity has a strong influence on the air core evolution. The periodic fluctuation results from the magnitude and direction of axial velocity.

Numerical study of tritium transport characteristics in Thorium Molten Salt Reactor with Solid Fuel (TMSR-SF)

Abstract Tritium control is one of the most key issues for the development of Molten Salt Reactor (MSR). The tritium transport characteristics in 2MWth Thorium Molten Salt Reactor-Solid Fuel (TMSR-SF) proposed by Shanghai INstitute of Applied Physics (SINAP) is studied in this paper. Three works are conducted: Firstly, an integrated numerical model is established, including tritium production, adsorption, diffusion, corrosion and permeation. Secondly, the Tritium trAnsPort chAracteristics analysiS code (TAPAS) for TMSR-SF is developed and benchmarked. The results prove the fidelity and accuracy of TAPAS. Finally, the distributions of the key variables including TF, T2 and Cr2+ in TMSR-SF system at both steady and transient state are obtained. In addition, some methods for tritium mitigation are evaluated by TAPAS. Numerical results show that tritium can be efficiently removed from the molten salt via graphite absorption. The amount of T2 at steady state is two orders of magnitude more than TF. Tritium permeation can be prevented by increasing 7Li enrichment and reducing redox potential. Tungsten or similar low-permeability material is a candidate for tritium barrier when covered on the surface of the heat exchanger wall. This work provides some valuable considerations for the tritium control in the MSR.

Dynamic characteristics identification of two graphite bricks in molten salt reactor considering fluid-structure interaction

Abstract Graphite core plays a very important role in thorium molten salt reactor (TMSR) served as a reflector, a moderator as well as a structural material. The whole graphite core is submerged in molten salt during operation and consists of a large number of graphite bricks interconnected with keys and dowels. The molten salt as a fluid will affect the dynamic behavior of the graphite core. This phenomenon is called fluid-structure interaction (FSI). In order to maintain the integrity of the graphite core under a seismic event, it is essential to predict dynamic characteristics of the graphite bricks affected by FSI. In this paper a 1/4 scaled-down test model derived from similarity analysis will be presented. The nonlinear response of the graphite brick under the excitation of the sinusoidal harmonic is regarded as a single degree of freedom system. The dynamic characteristics of added mass and added damping can be obtained by fitting the time history of models’ displacement subjected to simple harmonic motion. The experimental results shown that the added mass and the added damping are strongly dependent on the gap size between two bricks. Furthermore, a three-dimensional finite element model has been derived for the dynamic analysis of two graphite bricks with molten salt and the modelling parameters are obtained from the experiment. We found that the results from the numerical method are in good agreement with the experiment.

Optimization of Th-U fuel breeding based on a single-fluid double-zone thorium molten salt reactor

Molten salt reactor (MSR) shows great promise of high thermal-electric conversion efficiency, inherent safety, and on-line reprocessing. Furthermore, fuel breeding can be realized in both thermal and fast spectrum reactors with Th-U fuel cycle, which can make use of the abundant resource of thorium.In order to combine the advantages of high breeding ratio (BR) in fast spectrum and low fissile inventory for criticality in thermal spectrum, a single-fluid double-zone thorium-based molten salt reactor (SD-TMSR) core configuration is proposed. First, assemblies in both the inner and the outer zones are optimized by adjusting the ratios of molten salt and graphite with consideration of BR, 233U inventory, double time (DT), and temperature coefficient of reactivity (TCR) at the startup time, aiming to have a minimal DT for fuel breeding and a negative TCR for security. The results show that DT has the optimum value when the ratios of molten salt and graphite in the inner zone and the outer zone are 0.357 and 1.162, respectively. And the TCR can be improved to −2 pcm/K when the side length of the graphite hexagonal prism is 7.5 cm. Then, based on the optimized geometry, the burn-up calculations with a series of on-line reprocessing rates are carried out with a self-developed MSR reprocessing sequence (MSR-RS). The results show that on-line reprocessing is beneficial to the Th-U fuel breeding. When the reprocessing rate is 200 l/d, iso-breeding can be achieved. When the reprocessing rate is 5 m3/d, the performance of Th-U breeding is improved significantly, and only 16 years is needed for the 233U doubling. The results also show that TCR remains negative and inherent safety is satisfied during all the operation time.

Research on chemical durability of iron phosphate glass wasteforms vitrifying SrF2 and CeF3

A part of the radioactive waste produced in the Thorium Molten Salt Reactor (TMSR) will be present as fluorides. Because of some characters of fluoride, it is undesirable for immobilizing the waste as other usual oxide waste in borosilicate glass that often produces immiscibility. However, iron phosphate glasses have been made with up to 30 wt% SrF2 and 20 wt% CeF3, and about 20 wt% SrF2 and CeF3 combined. The iron phosphate glass matrix mixed with fluoride waste could be melted at temperatures between 1050 °C and 1200 °C in only 30–60 min, its melting temperature is lower than that of borosilicate glass. Powder X-ray diffraction (XRD) showed that the glassy wasteforms that contains more than 20% wt% SrF2 and CeF3 mixture, or more than 20 wt% CeF3 would start crystallizing. The product consistency test (PCT) was used to identify the chemical durability of the glass. Glass powder (−100 to +200 mesh) after washing was soaked in 10 ml of deionized water at 90 °C for 7 days. The normalized elemental mass release of elements Sr, Ce and F were calculated at the level of 10−3 g/m2, through the concentration of element measured after PCT, that was equivalent to or better than that of borosilicate glass wasteforms. Iron phosphate glasses are concluded to be a practical alternative for vitrifying the radioactive waste of TMSR.

Thermodynamic Evaluation of NaF-MF n (M=Be, U, Th) Systems for Molten Salt Reactor

The phase diagrams of NaF-BeF2, NaF-ThF4 and NaF-UF4 systems were assessed based on thermodynamic principles, and diverse thermodynamic models were adapted to different systems. Associate solution model (ASM) was used to describe the Gibbs energies of liquid phase of the NaF-BeF2 system, whereas other systems(NaF-ThF4 and NaF-UF4) were treated with the substitutional solution model(SSM) and intermediate compounds were described as stoichiometric compounds according to the Neumann-Kopp rule. All the thermodynamic model parameters were optimized by the least squares procedure until good coincidence was achieved between the calculated results and the experimental data. The derived thermodynamic parameters will be merged into the NaF-BeF2-ThF4-UF4 database to develop the thorium molten salt reactor(TMSR) project.

Thorium and Molten Salt Reactors: Essential Questions for Classroom Discussions

A little-known type of nuclear reactor called the “molten salt reactor” (MSR), in which nuclear fuel is dissolved in a liquid carrier salt, was proposed in the 1940s and developed at the Oak Ridge National Laboratory in the 1960s. Recently, the MSR has generated renewed interest as a remedy for the drawbacks associated with conventional uranium-fueled light-water reactors (LWRs) in use today. Particular attention has been given to the “thorium molten salt reactor” (TMSR), an MSR engineered specifically to use thorium as its fuel. The purpose of this article is to encourage the TPT community to incorporate discussions of MSRs and the thorium fuel cycle into courses such as “Physics and Society” or “Frontiers of Physics.” With this in mind, we piloted a pedagogical approach with 27 teachers in which we described the underlying physics of the TMSR and posed five essential questions for classroom discussions. We assumed teachers had some preexisting knowledge of nuclear reactions, but such prior knowledge was not necessary for inclusion in the classroom discussions. Overall, our material was perceived as a real-world example of physics, fit into a standards-based curriculum, and filled a need in the teaching community for providing unbiased references of alternative energy technologies.

Large eddy simulation of unsteady flow in gas–liquid separator applied in thorium molten salt reactor

Axial gas–liquid separators have been adopted in fission gas removal systems for the development of thorium molten salt reactors. In our previous study, we observed an unsteady flow phenomenon in which the flow pattern is directly dependent on the backpressure in a gas–liquid separator; however, the underlying flow mechanism is still unknown. In order to move a step further in clarifying how the flow pattern evolves with a variation in backpressure, a large eddy simulation (LES) was adopted to study the flow field evolution. In the simulation, an artificial boundary was applied at the separator outlet under the assumption that the backpressure increases linearly. The numerical results indicate that the unsteady flow feature is captured by the LES approach, and the flow transition is mainly due to the axial velocity profile redistribution induced by the backpressure variation. With the increase in backpressure, the axial velocity near the downstream orifice transits from negative to positive. This change in the axial velocity sign forces the unstable spiral vortex to become a stable rectilinear vortex.

Numerical investigation on the bubble separation in a gas-liquid separator applied in TMSR

The fission gas removal system plays a critical role in the development of the Thorium Molten Salt reactors. An axial gas-liquid separator is adopted in the gas removal system. To predict the bubble trajectories in swirling flow is essential for designing such gas-liquid separator, since the separation efficiency is closely related to the bubble trajectory. In this paper, we proposed a numerical method to predict the bubble motion. This method is a modified Lagrangian approach in that the velocity of the continuous phase is obtained by approximating the velocity profiles from CFD. Combing the known velocity distribution with explicit mathematical expression and the force model for a single bubble, a mathematical model to calculate the bubble motion is well posed. Calculations with various bubble sizes and Reynolds numbers were carried out. By comparing the simulation results with the experimental data, we concluded that the numerical results agree well with the experimental data. The maximum error of the separation length is less than 10%, which is accurate enough for the determination of the dimension of the separator.

Investigation on the effect of geometrical parameters on the performance of a venturi type bubble generator

The venturi type bubble generator adopted in the fission gas removal system for the liquid-fuel thorium molten salt reactor plays a crucial role in strengthening the mass transfer rate. Aiming at clarifying how these three main geometrical parameters, including the injection hole diameter, injection hole number, and divergent angle, influence the bubble size distributions, an experimental study is carried out. The bubble size distribution is extracted from the images captured by the high speed visualization. Post processing algorithm is developed to detect the bubble edges and compute their Sauter mean diameters for each flow condition. By changing the value of each geometrical parameter, the variation of the bubble size distribution is obtained and discussed. It is shown that the injection hole diameter and number basically have little effect on the bubble size distribution while the divergent angle is proved to be sensitive. Finally, the reasons that account for the bubble size variation with three geometrical parameters are explained by the bubble turbulence breakup theory.

Numerical simulation of tritiated bubble trajectory in a gas-liquid separator

The gas-liquid separator is a critical component for the online tritium gas removal system in thorium molten salt reactor. In this paper, an isolated bubble trajectory in a single-phase swirling flow field of the separator is investigated theoretically. The mathematical modeling of the forces acting on the bubble is established, and the motion equations of bubble are solved numerically. The axial and circumferential velocities of the swirling flow are approximated by the combination of 2 co-flow Batchelor vortices. Notably, it is a promising way to obtain the analytical expression of velocity field in this gas-liquid separator. The results indicate that the separation length will increase as decreasing diameter of bubble and swirl number. Hence, the numerical simulation provides an effective tool to predict bubble trajectory and optimize the design of the gas-liquid separator.

Measurements of the total cross section of nat Be with thermal neutrons from a photo-neutron source

Abstract The total neutron cross sections of natural beryllium in the neutron energy region of 0.007 to 0.1 eV were measured by using a time-of-flight (TOF) technique at the Shanghai Institute of Applied Physics (SINAP). The low energy neutrons were obtained by moderating the high energy neutrons from a pulsed photo-neutron source generated from a 16 MeV electron linac. The time dependent neutron background component was determined by employing the 12.8 cm boron-loaded polyethylene (PEB) (5% w.t.) to block neutron TOF path and using the Monte Carlo simulation methods. The present data was compared with the fold Harvey data with the response function of the photo-neutron source (PNS, phase-1). The present measurement of total cross section of natBe for thermal neutrons based on PNS has been developed for the acquisition of nuclear data needed for the Thorium Molten Salt Reactor (TMSR).

Evaluation of the fraction of delayed photoneutrons for TMSR-SF1

The 10 MWth solid-fueled thorium molten salt reactor (TMSR-SF1) is a FLiBe salt-cooled pebble bed reactor to be deployed in 5–10 years, designed by the TMSR group. Due to a large amount of beryllium in the core, the photoneutrons are produced via (γ, n) reactions. Some of them are generated a long time after the fission event and therefore are considered as delayed neutrons. In this paper, we redefine the effective delayed neutrons into two fractions: the delayed fission neutron fraction and the delayed photoneutron fraction. With some reasonable assumptions, the inner product method and the k-ratio method are adopted for studying the effective delayed photoneutron fraction. In the k-ratio method, the Monte Carlo code MCNP6 is used to evaluate the effective photoneutron fraction as the ratio between the multiplication factors with and without contribution of the delayed neutrons and photoneutrons. In the inner product method, with the Monte Carlo and deterministic codes together, we use the adjoint neutron flux as a weighting function for the neutrons and photoneutrons generated in the core. Results of the two methods agree well with each other, but the k-ratio method requires much more computing time for the same precision.

Transient analysis for liquid-fuel molten salt reactor based on MOREL2.0 code

A molten salt reactor (MSR) is characterized by simultaneously using liquid fuel salt as both the nuclear fuel and coolant. The redistribution of delayed neutron precursors (DNPs) makes the transient behavior of MSRs different from traditional solid-fuel reactors. In this study, a 3D coupled neutronics/thermal hydraulics code, MOREL2.0, was employed to analyze a liquid-fuel Thorium Molten Salt Reactor (TMSR-LF) under perturbations of fuel pump start-up and coast-down and by overheating and overcooling the inlet fuel temperature. Some transient processes were simulated to provide guidance for the future design and optimization of TMSR-LFs. In response to the perturbations, reactivity was lost and gained in the pump start-up and coast-down, respectively. Overheating the inlet fuel temperature introduced negative reactivity, and TMSR-LF maintained a safety state, while overcooling the inlet fuel temperature resulted in positive reactivity. Overcooling by 70 K produced a supercritical transient condition and a rapid increase in power within a short period, which was followed by a decrease in power due to negative temperature feedback. The transient results demonstrate that the negative temperature feedback coefficients guarantee TMSR-LF inherent safety and the variation range of temperature stay within the safety margin.

Control rod drop dynamic analysis in the TMSR – SF1 based on numerical simulation and experiment

The scram function renders the core subcritical in response to plant upsets, which is realized by fast drop of the control rods. In the solid fuel (SF) thorium molten salt reactor system (TMSR), denoted as the TMSR – SF1 project, the drop time of the rod is obviously affected by the driving mechanical drag and the hydraulic resistance of the liquid environment. An inner-house code is developed to analyze the rod drop in the TMSR – SF1. In the code, the total transmission efficiency of the driving mechanism is deduced as a relationship with dropping velocity of the rod. With the experimental and theoretic analysis, the motion process of the rod drop is simulated in this study. And it turns out that the drop time in the molten salt is satisfied with the limiting drop time in the TMSR – SF1. In addition, a hydraulic buffer is designed to alleviate the dropping shock from the scram and the calculation results are presented in the paper as well.

Experimental study on the bubble trajectory in an axial gas-liquid separator applied for tritium removal for molten salt reactors

In the tritium removal system applied in the thorium molten salt reactor, an axial gas-liquid separator was developed to separate the dilute dispersed bubbles from the reactor coolant. In order to quantify the separation efficiency for the separator and provide useful guidelines to design the separator, the separation trajectories of bubbles with different sizes in the swirl flow inside the separator are needed. To do this, an experimental study using the high speed visualizing and image processing was implemented to investigate the separation trajectories of bubbles under various bubble sizes and Reynolds numbers. A special bubble generator which is capable of generating single bubbles with controllable diameters was devised and incorporated into the separator test. By adjusting the parameters of the single bubble generator, i.e. the diameter of the needle and the rotation speed of the peristaltic pump, the trajectories of bubbles with diameters ranging from 0.2mm to 1.1mm, Reynolds numbers from 40,000 to 100,000 are recorded by the high speed camera and processed by a specialized image processing and analyzed. The results indicate that the bubble presents a convergent spiral path in the separating duration and eventually is entrapped by the air core. The separation length defined as the axial distance the bubble takes from the periphery of the separator to the air core will increase when decreasing the bubble diameter or increasing the Reynolds number. The experimental data obtained in this paper provides a detailed benchmark for the modeling of the bubble dynamics in the swirling flow.

Impact of photoneutrons on reactivity measurements for TMSR-SF1

The solid-fueled thorium molten salt reactor (TMSR-SF1) is a 10 MWth test reactor design to be deployed in 5–10 years by the TMSR group. Its design combines coated particle fuel and molten FLiBe coolant for great intrinsic safety features and economic advantages. Due to a large amount of beryllium in the coolant salt, photoneutrons are produced by (γ, n) reaction, hence the increasing fraction of effective delayed neutrons in the core by the photoneutrons originating from the long-lived fission products. Some of the delayed photoneutron groups are of long lifetime, so a direct effect is resulted in the transient process and reactivity measurement. To study the impact of photoneutrons for TMSR-SF1, the effective photoneutron fraction is estimated using k-ratio method and performed by the Monte Carlo code (MCNP5) with ENDF/B-VII cross sections. Based on the coupled neutron–photon point kinetics equations, influence of the photoneutrons is analyzed. The results show that the impact of photoneutrons is not negligible in reactivity measurement. Without considering photoneutrons in on-line reactivity measurement based on inverse point kinetics can result in overestimation of the positive reactivity and underestimation of the negative reactivity. The photoneutrons also lead to more waiting time for the doubling time measurement. Since the photoneutron precursors take extremely long time to achieve equilibrium, a “steady” power operation may not directly imply a “real” criticality.