Neil E. Todreas

Hydrodynamic models and correlations for bare and wire-wrapped hexagonal rod bundles — Bundle friction factors, subchannel friction factors and mixing parameters

Abstract Consistent hydrodynamic models for subchannel friction factors and mixing parameters in wire-wrapped rod bundles have been developed for use in subchannel analysis codes. Both flow regime and geometry effects are taken into account in these models. The laminar, transition and turbulent flow regimes are covered for the range of liquid metal fast breeder reactor (LMFBR) rod bundles of design interest. Correlations based on the models for the subchannel friction factor and mixing parameters are calibrated by the available world data. This data includes recent 37-pin hydrodynamic experiments with a geometry between that of typical fuel and blanket assembly design performed by the authors. Specific correlations are presented for subchannel and bundle friction factors, flow split, enhanced eddy diffusivity and the peripheral wire induced swirl velocity.

An experimental investigation of a passive cooling unit for nuclear plant containment

Abstract A set of condensation experiments in the presence of noncondensables (e.g. air, helium) was conducted to evaluate the heat removal capacity of a passive cooling unit in a post-accident containment. Condensation heat transfer coefficients on a vertically mounted smooth tube have been obtained for total pressure ranging from 2.48×10 5 Pa(abs) to 4.55×10 5 Pa(abs) and air mass fraction ranging from 0.30 to 0.65. An empirical correlation for heat transfer coefficient ( h ), has been developed in terms of a parameter group made up of steam mole fraction ( Xs ), total pressure ( P t ), temperature difference between bulk gas and wall surface (d T ). This correlation covers all data points within 20%. All data points are also in good agreement with the prediction of the diffusion layer model (DLM) with suction and are approximately 2.2 times the Uchida heat transfer correlation. Experiments with an axial shroud around the test tube to model the restriction on radial flow experienced within a tube bundle demonstrated a reduction of the heat transfer coefficient by a factor of about 0.6. The effect of helium (simulating hydrogen) on the heat transfer coefficient was investigated for helium mole fraction in noncondensable gases ( X He /X nc ) at 15, 30 and 60%. It was found that the condensation heat transfer coefficients are generally lower when introducing helium into noncondensable gas. The difference is within 20% of air-only cases when X He /X nc is less than 30% and total pressure is less than 4.55×10 5 Pa(abs). A gas stratification phenomenon was clearly observed for helium mole fraction in excess of 60%.

Distributed resistance modeling of wire-wrapped rod bundles

Abstract Three-dimensional flow hydrodynamic distributed resistance models for rod bundles were developed. The models specifically account for the presence of the wire-wrap spacer and may be used for any lumped parameter thermohydraulic analysis numerical program. Validation studies of the hydrodynamic resistance models were also performed using a subchannel code ASFRE. The models were tested against subchannel velocity and temperature data taken from bundles of triangular rod array configurations with wire-spacers. Overall the models performed satisfactorily predicting the most important qualitative trends for flows in wire-wrapped rod bundles.

A porous body model for predicting temperature distribution in wire-wrapped fuel rod assemblies

Abstract A porous body model, new in its application for predicting temperature distributions in wire-wrapped fuel rod assemblies, has been developed. The model developed for thermal transport in wire-wrapped rod bundles is similar in principle to the one which has long been successfully used for heat transfer in fixed beds of packed solids. Although the model is applicable to bundles in forced and mixed (combined forced and free) convection, attention in this paper is confined to bundles operating in forced (negligible natural) convection only. The results obtained from this analysis were found to predict available data with as good a precision as does the more complex analysis.

An assessment of two-phase pressure drop correlations for steam-water systems

Abstract Eighteen two-phase friction pressure drop models and correlations were tested against about 2220 experimental steam-water pressure drop measurements under adiabatic conditions and about 1230 in diabatic flow conditions. The data represented several geometries and had the following property ranges: Pressure 1.7–10.3 MN/m2 (250–1500 psia); Mass velocity 270–4340 kg/m2sec (0.2–3.2 Mlb/ft2hr); Quality Subcooled to 100%; Equivalent diameters 2.3–33.0 mm (0.09–1.3 in.). The four models and correlations which were found to have the best performance were the Baroczy correlation, the Thom correlation and the homogeneous model two-phase friction multipliers, φ 2 fo = 1+X V fg V f and φ 2 fo = 1+X V fg V f 1+X μ g μ f −1 0.25 The correlations were also evaluated with the data being sub-divided into sets which were based on properties and flow conditions.

Consideration of critical heat flux margin prediction by subcooled or low quality critical heat flux correlations

The accurate prediction of the critical heat flux (CHF) margin which is a key design parameter in a variety of cooling and heating systems is of high importance. These margins are, for the low quality region, typically expressed in terms of critical heat flux ratios using the direct substitution method. Using a simple example of a heated tube, it is shown that CHF correlations of a certain type often used to predict CHF margins, expressed in this manner, may yield different results, strongly dependent on the correlation in use. It is argued that the application of the heat balance method to such correlations, which leads to expressing the CHF margins in terms of the critical power ratio, may be more appropriate.

Design strategy and constraints for medium-power lead-alloy-cooled actinide burners

Abstract We outline the strategy and constraints adopted for the design of medium-power lead-alloy–cooled actinide-burning reactors that strive for a lower cost than accelerator-driven systems and for robust safety. Reduced cost is pursued through the use of (1) a modular design and maximum power rating to capitalize on an economy of scale within the constraints imposed by modularity, (2) a very compact and simple supercritical-CO2 power cycle, and (3) simplifications of the primary system allowed by the use of lead coolant. Excellent safety is pursued by adopting the integral fast reactor approach of achieving a self-controllable reactor that responds to all key abnormal occurrences, including anticipated transients without scrams, by a safe shutdown without exceeding core integrity limits. The three concepts developed are the fertile-free actinide burner for incineration of all transuranics from light water reactor (LWR) spent fuel, the fertile-free minor actinide (MA) burner for preferential burning of MAs working in tandem with LWRs or gas-cooled thermal reactors, and the actinide burner with thorium fuel aimed also at reducing the electricity generation costs through longer-cycle operation.

Conceptual design and analysis of a semi-passive containment cooling system for a large concrete containment

Abstract An internal evaporator-only (IEO) concept has been developed as a semi-passive containment cooling system for a large dry concrete containment. The function of this system is to keep the containment integrity by maintaining the internal pressure not to exceed ultimate design pressure, i.e. 0.83 MPa (120 psia) in the absence of any other containment cooling following a severe accident, which postulates core damage and hydrogen combustion. The ability of the concept to protect the containment was evaluated for the design basis accident (DBA) large break loss of coolant accident (LB LOCA) and severe accident scenarios (LB LOCA without Emergency Core Cooling System (ECCS) and containment spray flow, 100% zirconium oxidation and complete hydrogen combustion). All were modeled using the GOTHIC computer code. It was concluded that a practical system requiring four IEO loops could be utilized to meet design criteria for severe accident scenarios.

Analytic formulae for the effective conductivity of a square or hexagonal array of parallel tubes

Abstract A set of analytic formulae are presented for the effective thermal conductivity of either a square or a hexagonal array of parallel tubes which have distinct core, tube and fill conductivities. The formulae are based on a generalization of Rayleighs [Phil. Mag. 34(5), 481–502 (1892)] method to include hexagonal (as well as square) arrays, tubes (as well as solid rods), and higher-order terms in the analytic series. The accuracy of the analytic formulae is determined by comparison with essentially exact numerical calculations. The formulae are applied to the problem of an array of dry, spent, nuclear fuel rods.

Thermal stress initiated fracture as a fragmentation mechanism in the UO2-sodium fuel-coolant interaction

Abstract An analytic study was carried out to determine the applicability of the concept of thermal stress fragmentation to the UO 2 -Na fuel-coolant interaction. Major emphasis was put on the fracture mechanics approach to assess whether or not the solidifying UO 2 would fracture under the thermally-induced stresses. It was found that the stress levels were sufficient to generate K I values substantially in excess of the UO 2 fracture toughness K IC . Thus, rapid instantaneous propagation of inherent flaws is anticipated.