MOCUP is a system of external processors that allow for a limited treatment of the temporal composition of the user-selected MCNP cells in a time-dependent flux environment. The ORIGEN2 code computes the time-dependent compositions of these individually selected MCNP cells. All data communication between the two codes is accomplished through the MCNP and ORIGEN2 input/output files, the MOCUP Processor Output files, and two user supplied tables. MOCUP is either command line or interactively driven. The interactive interface is based on the portable X11 window environment and the Motif tool kit. MOCUP was constructed so that no modifications to either MCNP or ORIGENZ were necessary. Section 4 of the writeup contains the input instructions needed to set up the MOCUP run. MOCUP is extremely useful for analysts who perform isotope production, material transformation, and depletion and isotope analyses on complex, non-lattice geometries, and uniform and non-uniform lattices.

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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

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