Thermal, Fluid, and Structural Analysis of a Cermet Fuel Element

Abstract

This paper presents results and interpretation of a high-fidelity, multidisciplinary simulation of a cermet fuel element. The fluid, thermal, and structural equations are solved for a symmetric sub-region of a fast spectrum reactor core; detailed neutronic simulations provide heat deposition rates. Creep effects on stress are also simulated, as stresses are beyond elastic limits. The intent of this work is to predict maximum fuel temperature, variations in coolant exit temperature between channels, coating stress, and stress sources to evaluate and optimize cermet fuel element performance for Nuclear Thermal Propulsion reactors. The sources of mechanical stress in a fuel element are identified and their individual contributions are quantified. Temperature gradients and cladding-fuel thermal expansion mismatch are major sources of stress. Thick fuel element edge walls contribute to high fuel temperatures, high stresses, and low flow through edge coolant channels. NomenclatureA = constant in creep equation, MPa -n s -1 C p = specific heat, J/kg-K CTE = coefficient of linear thermal expansion, m/m-K E = modulus of elasticity (Young's modulus), GPa g = acceleration due to gravity, m/s 2 I sp = specific impulse, s k = coefficient of thermal conductivity, W/m-K k eff = thermal conductivity, effective value of cermet mixture, W/m-K k m = thermal conductivity of matrix (higher thermal conductivity metal matrix) , W/m-K k p = thermal conductivity of fuel particles, W/m-K %ΔL/L o = percentage change in length due to linear thermal expansion, % m = mass, kg = mass flow, kg/s MW = molecular weight, kg/mol MW t = megawatts of thermal energy, MW n = power law exponent in creep equation Q = activation energy, J/mol Re d = Reynolds number based on channel diameter R u = universal gas constant, J/mol-K T, T M , T ref = temperature, melting temperature, reference temperature, K %TD = percentage of theoretical density, % U = structural displacement, m V = fluid velocity, m/s V f,p , V f,1 = percent volume fraction of fuel particles, percent volume fraction of first mixture component, % ΔV= velocity increment for trajectory burn, m/s y + = normal boundary layer grid spacing at wall, normalized 2 z, r,θ = coordinates: axial, radial, and circumferential, m, deg α, α eff = coefficient of linear thermal expansion (CTE), effective CTE, m/m-K ε = emissivity, relative ability of a planar surface to emit radiation over all wavelengths = creep rate, 1/s γ = ratio of specific heats µ Poisson = Poisson's ratio µ = viscosity, Pa-s ρ, ρ o = density, theoretical density, kg/m 3 σ = stress, MPa Block = three symmetric sub-elements that combine to form the fuel element cross-section FE = fuel element A -25% B = metal matrix/ceramic particle mixture of metal A and ceramic B, % volume A /25% B = alloy of metals A and B, % volume % = compositions are percentage by volume, unless otherwise marked as % weight X 0 = subscript for theoretical density

Cycle Analysis of a 200MW Class CERMET Based NTR Engine

“…The thermal energy deposition rates for the various engine components are first calculated via a neutronics analysis which models the detailed geometry of the entire reactor core and simulates the interaction of neutrons with component materials in a Monte Carlo simulation using the MCNP program 11 . Thermal energy deposition rate data for the reactor core fuel elements, axial and radial reflectors, control drums, and filler elements are input into the NESS program, where it is used to determine both engine component requirements, thermodynamic state points along the hydrogen flow path, and engine system level performance 13,14 .…”

A Comparison of Materials Issues for Cermet and Graphite-Based NTP Fuels

“…It is similar to conventional chemical propulsion except that it uses a nuclear fission reaction-not a chemical reaction-to 1 Senior Research Engineer, 21000 Brookpark Road, MS VPL-3, AIAA Member. 2 Post Office Box 2274, Idaho Falls, ID, 83403, AIAA Senior Member.…”

“…N heat the hydrogen propellant. Equation (1) shows that, for an ideal gas, the low molecular weight, MW, of the hydrogen propellant-2/18 of LOX/LH2-gives significantly better efficiency, or specific impulse I sp , than chemical propulsion.…”

“…With NTP, fission heat is generated in the solid fuel and must be transferred to the gaseous propellant through coolant channels in the fuel; this solid fuel must remain intact and not melt, so the fuel is designed to thermal and structural limits. Reference [1] explains the how high specific impulse, I sp , is achieved and gives a history.…”

A Methodology for Producing Uniform Distribution of UO2 in a Tungsten Matrix

“…However, many questions on the nuclear fuel performance and affordability still remain. A key enabling technology for future NTP systems is the fabrication of a stable high temperature nuclear fuel form that can operate at temperatures up to 2,500 º C. The fuel materials need to be chemically compatible with the coolant/propellant, typically hydrogen, and able to withstand stresses introduced from thermal gradients [2]. The focus of this paper is on cermet fuels composed of ceramic fuel particles embedded in a refractory metal matrix.…”