Microfuel Elements Research Articles

New in the hydraulics of channels with permeable walls: indicator of the relative value of the dissipation

The factors that have led to the creation of the scientific direction “hydraulics of variable mass” to study the fluid movement laws in channels with permeable walls are indicated. The results of applying of the dynamics of a variable mass point for describing the flow in such pipelines are presented. It is noted unjustifiability of the second Newton’s law generalization to the case of motion of a variable mass point for hydrodynamics problems. The functionality of one-dimensional and multidimensional models of fluid motion in permeable channels based on the classical equations of fluid and gas mechanics is characterized. It is substantiated the dominance of one-dimensional models in engineering computational practice, and a number of contradictions in the description of fluid dynamics are shown (with the flow visualization). On the base of the new kinematic image (instead of the generally accepted “solid jet”, when fluid particles are separated or joined), it has been obtained a one-dimensional equation of fluid motion in a permeable channel in which the friction coefficient is an indicator of the relative magnitude of the energy dissipation of the flow. The dependence of the Coriolis coefficient on the flow regime is constructed. The structure of the friction drag coefficient of a permeable channel has been studied using the vector dimension of length. It is shown that the dissipation of the flow energy in a permeable channel is higher both during outflow and inflow of liquid than in channels with solid walls at the same flow rates. The results are in demand in the development of chemical technology devices, nuclear reactors with microfuel elements, filters and heat exchangers containing channels with permeable walls.

Investigation of heat generation in a pebble bed during induction heating

The use of fuel in the form of microfuel filling in nuclear reactors is a possible way to improve the safety and efficiency of nuclear power plants. In experimental studies, microfuel elements are simulated by stainless steel balls, and internal heat generation by high-frequency induction heating. The paper presents the results of a study of heat release in a pebble bed during induction heating, and features are revealed. The distribution function of the volumetric density of heat release along the height of the pebble bed is obtained.

Reduction of Hydraulic Losses in a Pebble-Bed Reactor for Nuclear Power Propulsion Systems for Advanced Spacecraft

An analysis of various schemes for coolant motion in a nuclear reactor with spherical microfuel elements (coated fuel particles, or pebbles) is presented. The conditions under which it is possible to reduce the pressure losses in the coolant keeping all the advantages of the spherical microfuel elements versus the fuel rods are considered. The rate of coolant flow and the pressure losses in the reactor during its longitudinal-axial and radial motion through the pebble bed are determined. An interchannel scheme of coolant motion is proposed, and the conditions are defined for which it has advantages over the radial scheme.

Prospective Micro-Encapsulated Fuel with Silicon Carbide Protective Coat

A promising direction in the development of the next-generation fuel is the development of fully ceramic micro-encapsulated fuel (FCM). Such fuel has proven itself well in high-temperature gas-cooled reactors (HTGR). The fuel consists of 200–700 μm in diameter microspheres made from the fuel material coated with a multi-layer ceramic coating. The standard four-layer coating used in HTGR reactors consists of three layers of pyrolytic carbon and a layer of silicon carbide. The technology developed at VNIINM for fabricating protective coatings consisting of only silicon carbide makes it possible to enhance the radiation and heat resistance of the coatings and, correspondingly, increase the fuel life. Calculations have confirmed the main advantages of the micro-encapsulated fuel with a silicon carbide coating over the conventional TRISO coatings. The use of such microfuel elements opens up the possibility of developing fuel elements capable of retaining fission products both under normal operating conditions and in design-basis and beyond-design-basis accidents. Variants of the design of rod-shaped accident-tolerant fuel (ATF) for water-cooled reactors based on micro-encapsulated fuel with silicon carbide coatings and cladding comprised of a composite of the type SiC–SiC are considered.

INVESTIGATION OF THERMOHYDRAULIC AND NEUTRON -PHYSICAL CHARACTERISTICS OF PERSPECTIVE FUEL ASSEMBLY FOR LOW-POWER REACTOR FACILITY

The paper presents the results of a numerical study of hydrodynamics and a temperature field in the model of the fuel assembly for the KLT-40S reactor installation in the transition to a fuel consisting of spherical microfuels. The optimal design of a model of a f uel assembly with microfuels for a reactor installation KLT-40S is proposed, from the position of hydrodynamics and heat transfer. The fuel assembly by external dimensions completely corresponds to the traditional assembly with rod fuel elements. The optim al parameters of the perforation of the covers that retain the pebble bed of microfuels are determined. The results of the evaluation of neutron-physical characteristics are presented. The working capacity of the KLT-40S reactor installation is shown when using fuel from microfuel elements.

FORMATION OF CROSS-CUTTING STRUCTURES WITH DIFFERENT POROSITY ON THICK SILICON WAFERS

Through three−layered structure has been formed on silicon wafers 500 microns thick by electrochemical etching in a solution of hydrofluoric acid without using additional deletions monocrystalline layers. The resulting structures are divided into two types. The first type pass−through structure comprises two outermost macroporous silicon layers 220—247,5 microns thick with a pore diameter 7—10 microns and an average mesoporous silica layer 5—60 microns thick with a pore diameter of 100—150 nm. The second type pass−through structure includes macroporous silicon layer 250 microns in thickness, interlocking in the depth of the silicon wafer to form a cavity the size of 4—8 microns. The developed technology will allow forming monolithic structures of membrane−electrode assembly microfuel elements in an easier and more reliable manner.

Development of a thermal–hydraulic analysis code for the Pebble Bed Water-cooled Reactor

The Pebble Bed Water-cooled Reactor (PBWR) is a water-moderated water-cooled pebble bed reactor in which millions of tristructural-isotropic (TRISO) coated micro-fuel elements (MFE) pile in each assembly. Light water is used as coolant that flows from bottom to top in the assembly while the moderator water flows in the reverse direction out of the assembly. Steady-state thermal–hydraullic analysis code for the PBWR will provide a set of thermal hydraulic parameters of the primary loop so that heat transported out of the core can match with the heat generated by the core for a safe operation of the reactor. The key parameters of the core including the void fraction, pressure drop, heat transfer coefficients, the temperature distribution and the Departure from Nucleate Boiling Ratio (DNBR) is calculated for the core in normal operation. The code can calculate for liquid region, water-steam two phase region and superheated steam region. The results show that the maximum fuel temperature is much lower than the design limitation and the flow distribution can meet the cooling requirement in the reactor core. As a new type of nuclear reactor, the main design features with a sufficient safety margin were also put forward in this paper.

Method of nondestructive monitoring of fuel-element seal tightness

A method of monitoring the seal tightness of fuel elements is described. The method is based on determining leaks of gaseous fission products during annealing of preirradiated samples. The basic steps using this method are described and mathematical expressions for determining the constants of the activation model of the leakage of gaseous fission products, which is used for processing and analyzing experimental results, are presented. The measurements results obtained using the method develop are compared with the results obtained in reactor tests of spherical fuel elements. An example using the technique is described: simulators, for which the temperature dependence of 135 Xe leakage (to 1400 K) is obtained and a quantitative relation between the number of microfuel elements with damaged coatings and leakage of gaseous fuel products, are tested. It is found that technological contamination of graphite of the simulators by uranium is present.

Formation of multifunctional barriers to increase the radiochemical resistance of the protective coatings of HTGR fuel elements

The radiation-size changes of pyrocarbon protective coatings on HTGR microfuel elements are analyzed. It is shown that there is a relationship between the microstructural inner pyrolytic layers and the formation of cracks in these layers as the irradiation dose accumulates. The effect of cracks in the inner pyrocarbon layers on the damage to the silicon carbide layer is examined. It is determined that incorporating into the inner pyrocarbon layer or forming on the inner pyrocarbon-silicon carbide interface compositions, for example, silicon carbide-carbon, Ti 3 SiC 2 , ZrC, TiC, and nitrides of Zr, Ti, and Al creates an obstacle to interior cracks, increasing the radiation-chemical resistance of the carbide layer and the microfuel as a whole.

Corrosion resistance of microfuel elements in air during contact with austenitic steel fuel-assembly components

A problem arising in the development of light-water reactors with a microfuel-based core is guaranteeing safety in accident regimes where the temperature of the outer surfaces of the microfuel elements reaches 1200-1400°C [1-3]. It has been determined experimentally [4] that unirradiated microfuel elements with a silicon carbide outer protective layer possess high corrosion resistance and integrity in a vapor-gas medium at temperatures up to 1600°C and have successfully passed heat-resistance tests by preheating in air up to 800-1600°C followed by rapid cooling in water. However, these tests were performed with no contact with the structural materials of fuel assemblies, while during a serious accident the chemical interaction between the metal and microfuel elements can destroy the protective outer layer of silicon carbide. Investigations of the interaction of sintered silicon carbide with zircalloy, austenitic steel, and oxides of various elements which are constituents of steel (FeO, Fe 3 O 4 , Cr 2 O 3 , MnO, and others) in air at high temperature 1450-1800°C established that silicon carbide is unstable under conditions where it comes into contact with type 316 austenitic steel at its melting temperature of 1450°C [5]. However, calculations have shown [1] that the most likely maximum temperature in a reactor core during a loss-of-coolant accident is ∼1200°C (for nominal VVER-1000 parameters). At the present time, there were no data on the behavior of the outer silicon carbide coating of microfuel elements in oxidative media in the case of contact with austenitic corrosion-resistant steel at the indicated temperature. It should also be noted that the structure and physical properties of sintered silicon carbide differ substantially from those of the carbide obtained by chemical gas phase deposition in a fluidized bed [5]. In this connection, investigations were performed on the corrosion resistance and integrity of silicon carbide coatings on microfuel elements coming into contact with austenitic stainless steels 08Kh18N10T and EI-847 (05Kh15N16M3B) in air at 1100-1200°C. In addition, the corrosion resistance of this steel under the same conditions was evaluated. Microfuel elements from two batches with different uranium oxide kernel diameters (300 and 500 μm) and multilayer cladding consisting of pyrolytic carbon and silicon carbide were chosen for the investigations. The outer protective silicon carbide coating on the microfuel elements was -50 pm thick in the first batch and -140 μm thick in the second batch. The nominal characteristics of the microfuel elements are presented in [4].

Supercritical steam cycle for nuclear power plant

Revolutionary improvement of the nuclear plant safety and economy with light water reactors can be reached with the application of micro-fuel elements (MFE) directly cooled by a supercritical pressure light-water coolant-moderator. There are considerable advantages of the MFE as compared with the traditional fuel rods, such as: Using supercritical and superheated steam considerably increases the thermal efficiency of the Rankine cycle up to 44-45%. Strong negative coolant and void reactivity coefficients with a very short thermal delay time allow the reactor to shutdown quickly in the event of a reactivity or power excursion. Core melting and the creation of corium during severe accidents are impossible. The heat transfer surface area is larger by several orders of magnitude due to the small spherical dimensions of the MFE. The larger heat exchange surface significantly simplifies residual heat removal by natural convection and radiation from the core to a subsequent passive system of heat removal.

Pyrocarbon coatings of microfuel elements of high-temperature gas reactors

Pyrocarbon coatings of microfuel elements of high-temperature gas reactors

Fission product escape from HTGR-fuel and microfuel elements

Fission product escape from HTGR-fuel and microfuel elements

Reactions of oxide fuel with pyrocarbon coatings on microfuel elements

Reactions of oxide fuel with pyrocarbon coatings on microfuel elements

A mathematical model for calculating stresses in the microfuel elements

A mathematical model for calculating stresses in the microfuel elements

Methods of calculation of the stress-strain state of microfuel elements of high-temperature helium-cooled reactors and the choice of their design

Methods of calculation of the stress-strain state of microfuel elements of high-temperature helium-cooled reactors and the choice of their design