Experimental study on natural circulation and its stability in a heavy liquid metal loop

Motivated by the increasing interest in heavy liquid metal (HLM) cooled fast reactors and accelerator driven system (ADS), the TALL test facility was designed and constructed at KTH to investigate the thermal-hydraulic characteristics of HLM. In this paper, the HLM natural circulation characteristics in a HLM loop were investigated with experiments in the TALL test facility. The study includes measurements on (1) start-up of natural circulation from different initial conditions; (2) stability of natural circulation; (3) effects of influencing parameters and (4) capability of natural circulation. The experimental data are compared to predictions with a relevant code (RELAP5). Significant natural convection flow was observed in the experiments. It was found that the natural circulation was easily established and stabilized. It took only a few minutes to have a stable natural circulation prevailing from cold conditions. The natural circulation flowrate depends on the loop resistance, and the temperature difference between the hot leg and the cold leg, as determined by the power level and the heat sink capacity. The experiments show that the maximum flowrate for the natural circulation is 0.5 kg/s (corresponding to 0.5 m/s in the heat exchanger), resulting in heat removal of 15 kW from the core tank, which is comparable to the capacity of 100 W/cm of the electric heater elements. The preliminary analysis performed with the RELAP5 code is in reasonable agreement with the experimental data.     

Neutronic performance of HYLIFE-II fusion reactor using various thorium molten salts

HYLIFE-II is one of the major inertial fusion energy reactor design concepts in which a thick molten salt layer (Flibe = Li₂BeF₄) is injected between the reaction chamber walls and the explosions. Molten salt coolant eliminates the frequent replacement of solid first wall structure during reactors lifetime by decreasing intense neutron flux. This study presents the neutronic analysis of HYLIFE-II fusion reactor using various liquid wall coolants, namely, 75% LiF–25% ThF₄, 75% LiF–24% ThF₄–1% ²³³UF₄ or 75% LiF–23% ThF₄–2% ²³³UF₄. Neutron transport calculations for the evaluation of neutron spectra were conducted with the help of Scale 4.3 by solving the Boltzmann transport equation in S₈–P₃ approximation. The effects of flowing liquid wall thickness and type of coolant on the neutronic performance of the reactor were investigated. Furthermore, radiation damage calculations at the first wall structure with respect to type and thickness of liquid wall were carried out. Numerical results showed that using the flowing liquid wall containing the molten salt, 75% LiF–23% ThF₄–2% UF₄ with a thickness of 70 cm maintained tritium self-sufficiency of the (DT) fusion driver and extended the first wall lifetime to the reactors lifetime (30 full power years). In addition significant amount of high quality fissile fuel was bred through (n, γ) reaction of ²³²Th. Moreover, energy multiplication factor (M) was increased to 12 by high rate fission reactions of ²³³U occurring in the flowing wall. On the other hand, it was concluded that using the other two coolants, 75% LiF–25% ThF₄ or 75% LiF–24% ThF₄–1% ²³³UF₄, as liquid wall did not satisfy the radiation damage and the tritium sufficiency criteria together at any thickness, so that these two coolants were not suitable to improve neutronic performance of HYLIFE-II reactor.     

Strain-induced corrosion cracking behaviour of low-alloy steels under boiling water reactor conditions

The strain-induced corrosion cracking (SICC) behaviour of different low-alloy reactor pressure vessel (RPV) and piping steels and of a RPV weld filler/weld heat-affected zone (HAZ) material was characterized under simulated boiling water reactor (BWR)/normal water chemistry (NWC) conditions by slow rising load (SRL) and very low-frequency fatigue tests with pre-cracked fracture mechanics specimens. Under highly oxidizing BWR/NWC conditions (ECP +50 mV_SHE, 0.4 ppm dissolved oxygen), the SICC crack growth rates were comparable for all materials (hardness <350 HV5) and increased (once initiated) with increasing loading rates and with increasing temperature with a possible maximum/plateau at 250 °C. A minimum K_I value of 25 MPa m^1/2 had to be exceeded to initiate SICC in SRL tests. Above this value, the SICC rates increased with increasing loading rate dK_I/dt, but were not dependent on the actual K_I values up to 60 MPa m^1/2. A maximum in SICC initiation susceptibility occurred at intermediate temperatures around 200–250 °C and at slow strain rates in all materials. In contrast to crack growth, the SICC initiation susceptibility was affected by environmental and material parameters within certain limits.     

Heat transfer to supercritical pressure carbon dioxide flowing upward through tubes and a narrow annulus passage

A heat transfer test facility, SPHINX, which uses carbon dioxide as a medium at supercritical pressures, has been built at KAERI. A series of experiments are under way for various geometries including tubes of several diameters and narrow annulus passages of a concentric and eccentric layout. The experiments aim to collect heat transfer data and to provide an empirical heat transfer correlation required for a SCWR design. In this paper the test results for tubes of 4.4 mm and 9.0 mm IDs, and a concentric annular passage (8 mm × 10 mm × L1800 mm) are presented for certain combinations of the heat fluxes and mass fluxes. The heat transfer coefficients produced in the tests were compared with those from the existing heat transfer correlations with different media. A new correlation was introduced for the experiment data presented in this paper.     

Increased fuel burn up in a CANDU thorium reactor using weapon grade plutonium

Weapon grade plutonium is used as a booster fissile fuel material in the form of mixed ThO₂/PuO₂ fuel in a Canada Deuterium Uranium (CANDU) fuel bundle in order to assure the initial criticality at startup.Two different fuel compositions have been used: (1) 97% thoria (ThO₂) + 3%PuO₂ and (2) 92% ThO₂ + 5% UO₂ + 3% PuO₂. The latter is used to denaturize the new ²³³U fuel with ²³⁸U. The temporal variation of the criticality k_∞ and the burn-up values of the reactor have been calculated by full power operation for a period of 20 years. The criticality starts by k_∞ = 1.48 for both fuel compositions. A sharp decrease of the criticality has been observed in the first year as a consequence of rapid plutonium burnout. The criticality becomes quasi constant after the second year and remains above k_∞ > 1.06 for 20 years. After the second year, the CANDU reactor begins to operate practically as a thorium burner.Very high burn up could be achieved with the same fuel material (up to 500,000 MW·D/T), provided that the fuel rod claddings would be replaced periodically (after every 50,000 or 100,000 MW·D/T). The reactor criticality will be sufficient until a great fraction of the thorium fuel is burnt up. This would reduce fuel fabrication costs and nuclear waste mass for final disposal per unit energy drastically.     

Hydraulic flow tests of APWR reactor internals for safety analysis

A concept of radial neutron reflector of APWR brings about safety problems relevant to the flow induced vibration and thermal deformation. The CFD code has been expected to solve them by calculating pressure fluctuations of turbulent flow in the downcomer and the flow distribution into the neutron reflector. A series of hydraulic flow tests was conducted by NUPEC from 1998 to 2002 to demonstrate the new design of the neutron reflector and to obtain test data for validating the CFD code. The measured pressure fluctuations in the downcomer and their statistics were utilized for validating the specific turbulent model to be able to calculate a spectrum of pressure fluctuation such as the LES model. The measured flow rates at inlet holes of the lower core plate were utilized for validating for the general turbulent model, for example, the k– turbulent model. The calculated results with the LES model agreed well with the measured pressure fluctuations and their spectrum, but did not agree with the correlation between adjacent pressure fluctuations. On the other hand, the calculation results with the k– turbulent model agreed well with the measured flow rates at inlet holes of the lower core plate.     

Luminescence from low-energy impact of and ions (n = 1,2,3) on a Tore Supra carbon tile

The hydrogen Balmer series was observed when beams of H⁺, , and D⁺, , ions impinged at 100–1000 eV on a carbon fiber composite tile (Tore Supra). The excitation functions of the H_β (D_β) lines demonstrate the validity of the independent particle model. The logarithmic plot of the H_β (D_β) line intensity versus the reciprocal projectile ion velocity can be well reproduced by a model in which the hydrogen ion projectiles are neutralized in the entrance phase and subsequently electronically excited in collisions with carbon atoms. Finally, the excited hydrogen atoms leave the solid and emit the observed photons, unless they are quenched in non-radiative processes.     

Azimuthal dependence and accurate determination of the half-angle in RBS channeling

Rutherford Backscattering Channeling (RBSC) is often used in the studies of single crystalline materials with regard to issues hinging upon the location of specific atomic species within the crystal lattice. Results are deduced from the characteristics of an angular scan curve representing the yield of close collision events from the constant scattering of the probing projectiles from the crystalline sample in a sequence of directions lying in a plane perpendicular to one particular crystal lattice direction. Such angular scans exhibit dips near major channels, and their angular widths are of concern. On a fundamental level, accurate determination of these angular widths in the case of axial channeling and their interpretation come up against uncertainties arising out of the absence of a preferred choice for the plane of scan. Here, we shall illustrate these uncertainties in a representative case, channeling of 2 MeV He⁺ ions in the 1 0 0 direction of Si.     

Gamma-rays shielding properties of xPbO:(100 − x)B2O3 glasses system at 662 keV

The mass attenuation coefficients, total interaction cross-sections and effective atomic numbers of xPbO:(100 − x)B₂O₃ (where 30  x  70% weight) glass system have been investigated on the basis of the mixture rule. The results are good agreement with the theoretical values, calculated by WinXCom. Mass attenuation coefficients were increased with increase PbO content, due to increase effective atomic number of glass samples, which increase probability of photoelectric absorption in glass. However, Compton scattering gives dominant contribution to the total mass attenuation coefficients for studied glass samples. Their shielding properties of glass samples are also better than ordinary shielding concretes and commercial window glass which can be used with advantage as transparent in visible region. These results are indicating the potential of glasses in radiation shielding materials.     

Electron-impact excitation and ionization cross sections for ground state and excited helium atoms

Comprehensive and critically assessed cross sections for the electron-impact excitation and ionization of ground state and excited helium atoms are presented. All states (atomic terms) with n4 are treated individually, while the states with n5 are considered degenerate. For the processes involving transitions to and from n5 levels, suitable cross section scaling relations are presented. For a large number of transitions, from both ground and excited states, convergent close coupling calculations were performed to achieve a high accuracy of the data. The evaluated/recommended cross section data are presented by analytic fit functions, which preserve the correct asymptotic behavior of the cross sections. The cross sections are also displayed in graphical form.     

Tribological performance of MoS2 coatings in liquid helium and at high loads

The main part of a narrow support element (NSE) of the W7-X superconducting coil system is an aluminium bronze pad, PVD coated on its spherical surface with MoS₂, which slides against the flat surface of the stainless steel coil housing, coated with MoS₂ spray. The operational requirements of the NSEs are: vacuum of p < 10⁻⁶ mbar, temperature T  4 K, maximum load P 1500 kN, typical displacement ≤5 mm, smooth sliding and no stick-slip events. The paper describes test results obtained with a downscaled NSE at T = 4.2 and 77 K. During the test the NSEs were submerged in liquid helium and nitrogen, respectively. Whereas the LN₂ test ran smoothly for up to 15,000 cycles, the test in LHe showed stick-slip from the very first cycle. The stick-slip disappeared after 50 cycles. Post mortem analysis of the tested parts revealed that in case of LHe the sprayed MoS₂ film was removed during the first 30–100 cycles by blistering and flaking. The reason for the loss of adhesion at LHe temperature is not known, several possible causes are under discussion. Further experiments under vacuum and at T 4 K are being prepared which are expected to help in clarifying the issue.     

Effects of interfacial shear condition and trailing-corner radius on the wake vortex of a bubble

Comparative effects between the interfacial shear condition and the trailing-corner radius () on the wake vortex of a bubble are studied. In the investigation, the standard k– model is employed, and the two types of bubble: solid and gaseous, have different interfacial boundary condition. Namely, for solid bubbles the no-slip condition is imposed, resulting in a non-zero interfacial shear condition, while for gaseous bubbles the free-slip condition is imposed, yielding a zero interfacial shear condition. The flow condition is set for a slug flow with the bubble drifting at a terminal velocity corresponding to the Reynolds number of 35,000. The results show that, the flow can be roughly divided into two flow regimes: the small- and large- regimes. In the small- regime, the trailing-corner radius plays a dominant role and the difference in the interfacial shear condition has little effects on the wake vortex, causing the wake vortices of the two bubble types to be similar in shape, size, and circulation. In contrast, in the large- regime, the interfacial shear condition can manifest and affect flow separation and the wake vortex, causing significant differences between the wake vortices from the two bubble types. Namely, as is increased towards the large- regime, the wake vortex of the solid bubble changes relatively little while that of the gaseous bubble significantly decreases in size. At small- the circulations around the wake vortex of both types of bubble are almost identical initially. However, as is increased towards the large- regime, the circulation of the gaseous bubble decreases with increasing at a more pronounced rate than that of the solid bubble. These results show that it is the absence of interfacial shear in the large- regime that causes the wake vortex to be more sensitive to the trailing-corner radius.     

The microstructure evolution of type 17-4PH stainless steel during long-term aging at 350 °C

The microstructure evolution of 17-4 precipitation hardening (17-4PH) stainless steel during long-term aging at 350 °C was studied mainly by X-ray diffraction and transmission electron microscopy. The results showed that the matrix is lath martensite and the precipitation of nano-metric particles of -Cu phase after the alloy has been subjected to solution and temper treatment. When the alloy is aged at 350 °C to 9 months, some reversed austenite is formed and the -Cu precipitates are coarsening according to ripening process. When the alloy aged from 9 to 12 months, some bulk secondary carbides, M₂₃C₆, are precipitated. With the age time extended to 15 months, an amount of reversed austenite is transformed and the G-phase, a kind of intermetallic compound, precipitation occurs nearby the -Cu precipitates in the matrix at this intermediate temperature.     

Influence of the void fraction profile on the distribution parameter C0 for a bubbly gas–liquid flow in a horizontal round pipe

This paper presents the results of a numerical experiment on the influence of the void fraction profile on the distribution parameter C₀ in a horizontal bubbly two-phase flow. It was shown that for non-symmetric void fraction profiles, which occur normally in horizontal flows, the distribution parameter may be less than 1. In this case, the ratio of the volumetric flow quality β to the average void fraction α can also be less than 1.     

Experiments on in-vessel melt coolability in the EC-FOREVER Program

This paper reports the results from the experiments conducted on the coolability of corium melt during a severe accident scenario when the bottom head is full of the core melt, undergoing natural circulation. These experiments are part of the EC-FOREVER Program in which vessel failure experiments have also been performed. The experiments are performed in a 1/10th scale vessel (400 mm diameter and 15 mm wall thickness) and the oxidic melt employed is the mixture CaO + B₂O₃ at 1400 K, representing the corium melt mixture of UO₂ + ZrO₂.The experiments employed an initial phase, during which uniform volumetric heating of the melt was provided and the vessel was pressurised to 25 bar, for several hours, to generate maximum creep deformation of 5%, in order to provide the conditions for the formation of a gap between the melt-pool crust and the bottom head wall. After this phase, the vessel was flooded with water.Data were obtained on only the vessel and the melt pool temperatures in one of the EC-FOREVER experiments reported here. In the second experiment, however, besides the temperature data, additional data were obtained on the steam flow rate and the heat transfer to the water, at the upper face of the melt pool, as a function of time.It was found that the gap cooling mechanism was not effective in reducing the vessel wall temperatures after water flooding. Post-test examinations revealed that the water ingression extended to the depth of only 60 mm in the melt pool. The character of the heat transfer to the water from the melt pool upper surface was found to be similar to that observed in the MACE tests for the coolability of an ex-vessel melt pool flooded by water at the top.     

Droplet entrainment correlation in vertical upward co-current annular two-phase flow

Upward annular two-phase flow in a vertical tube is characterized by the presence of liquid film on the tube wall and entrained droplet laden gas phase flowing through the tube core. Entrainment fraction in annular flow is defined as a fraction of the total liquid flow flowing in the form of droplets through the central gas core. Its prediction is important for the estimation of pressure drop and dryout in annular flow. In the following study, measurements of entrainment fraction have been obtained in vertical upward co-current air–water annular flow covering wide ranges of pressure and flow conditions. Comparison of the experimental data with the existing entrainment fraction prediction correlations revealed their inadequacies in simulating the trends observed under high flow and high pressure conditions. Furthermore, several correlations available in the literature are implicit and require iterative calculations.Analysis of the experimental data showed that the non-dimensional numbers, Weber number (We = ρ_gj_g²D/σ(Δρ/ρ_g)^1/4) and liquid phase Reynolds number (Re_f = ρ_fj_fD/μ_f), successfully collapse the data. In view of this, simple, explicit correlation was developed based on these non-dimensional numbers for the prediction of entrainment fraction. The new correlation successfully predicted the trends under the high flow and high pressure conditions observed in the current experimental data and the data available in open literature. However, in order to use the proposed correlation it is necessary to predict the maximum possible entrainment fraction (or limiting entrainment fraction). In the current analysis, an experimental data based correlation was used for this purpose. However, a better model or correlation is necessary for the maximum possible entrainment fraction. A theoretical discussion on the mechanism and modeling of the maximum possible entrainment fraction condition is presented.     

Activity level of gross α and gross β in airborne aerosol samples around the Qinshan NPP

The monitoring results of gross α and gross β activity from 2001 to 2005 for environmental airborne aerosol samples around the Qinshan NPP base are presented in this paper. A total of 170 aerosol samples were collected from monitoring sites of Caichenmen village, Qinlian village, Xiajiawan village and Yangliucun village around the Qinshan NPP base. The measured specific activity of gross α and gross β are in the range of 0.02 - 0.38 mBq/m^3 and 0.10 - 1.81 mBq/m^3, respectively, with an average of 0.11 mBq/m^3 and 0.45mBq/m^3, respectively. They are lower than the average of 0.15 mBq/m^3 and 0.52mBq/m^3, of reference site at Hangzhou City. It is indicated that the specific activity of gross α and gross β for environmental aerosol samples around the Qinshan NPP base had not been increased in normal operating conditions of the NPP.     

CFD simulations of light gas release and mixing in the Battelle Model-Containment with CFX

In this validation work two turbulence models (k– and SST model) and two grids (a finer hybrid grid and a tetrahedral coarser grid) are considered in order to model helium release and dispersion. Simulation results are compared against an experiment of jet release phenomena in the Battelle Model Containment facility (BMC), a multi-compartment facility with a total volume of about 560 m³. In the selected test, HYJET Jx7, helium was released into the containment at a speed of 42 m/s over a time of 200 s. Although the k– model is the most commonly used turbulence model in most Computational Fluid Dynamics (CFD) applications, it does not provide the most accurate predictions for this application. Alternatively the SST turbulence model has been employed giving more accurate results. This investigation provides a further confirmation that the validation of commercial CFD codes is always required in order to select the more suitable physical models and computational grids for each specific application.     

Effects of working fluid and shaft rotation speed on the performance of HTR plants and the size of CBC turbo-machine

Investigated are the effects of the molecular weight of the working fluid, reactor exit temperature, and shaft rotation speed on the size and number of stages of the turbo-machine as well as the performance of high temperature reactor (HTR) plants with actively cooled reactor pressure vessel and direct or indirect Closed Brayton Cycles (CBCs). The present analyses for working fluids of helium (4 g/mol) and the 15 g/mol He–Xe and He–N₂ binary mixtures are performed for a reactor thermal power of 600 MW, shaft rotation speed of 3000–9000 rpm, and reactor exit temperature from 973 K to 1223 K. For the plants with indirect CBCs, the analyses assume a temperature pinch of 50 K in the IHX. Results show that the CBC compression ratio is relatively low (2.6 for He and He–Xe and 3.2 for He–N₂), increases very little with increasing the reactor exit temperature and near the maximum thermal efficiency of the plant. For the plants with a direct helium CBC, the thermal efficiency increases from 42% to 51% as the reactor exit temperature increases from 973 K to 1223 K, respectively, versus 37% to 47% for the plants with indirect He-CBC. The HTR plants with indirect He–Xe and He–N₂ CBCs and operating at a turbine inlet temperature of 1123 K have slightly higher thermal efficiencies (45.9% and 45.8%) than the He plant with indirect CBC (45.6%), generating 1.6 MWe more electrical power. The molecular weight of the working fluid has a very small effect on the plant thermal efficiency, but significantly reduces the size and number of stages of the CBC turbo-machine. Increasing the shaft rotation speed also decreases the size and number of stages of the CBC turbo-machine.     

Novel porous media formulation for multiphase flow conservation equations

Multiphase flows consist of interacting phases that are dispersed randomly in space and in time. An additional complication arises from the fact that the flow region of interest often contains irregularly shaped structures. While, in principle, the intraphase conservation equations for mass, momentum, and energy, and their initial and boundary conditions can be written, the cost of detailed fluid flow and heat transfer analysis with explicit treatment of these internal structures with complex geometry and irregular shape often is prohibitive, if not impossible. In most engineering applications, all that is required is to capture the essential features of the system and to express the flow and temperature field in terms of local volume-averaged quantities while sacrificing some of the details. The present study is an attempt to achieve this goal by applying time averaging after local volume averaging.Local volume averaging of conservation equations of mass, momentum, and energy for a multiphase system yields equations in terms of local volume-averaged products of density, velocity, energy, stresses, and field forces, together with interface transfer integrals. These averaging relations are subject to the following length scale restrictions:

dℓL,