An implicit method for a reconstructed discontinuous Galerkin (RDG) method is presented to solve compressible flow problems on tetrahedron grids. The idea is to combine the accuracy of the RDG method and the efficiency of implicit methods to obtain a better numerical algorithm in computational fluid dynamics. A least-squares reconstruction method is presented to obtain a quadratic polynomial representation of the underlying linear discontinuous Galerkin solution on each cell via an in-cell reconstruction process. The devised in-cell reconstruction is able to augment the accuracy of the DG method by increasing the order of the underlying polynomial solution. A matrix-free GMRES (generalized minimum residual) algorithm with an LU-SGS (lower-upper symmetric GaussSeidel) preconditioner is presented to solve an approximate system of linear equations arising from the Newton linearization. The implicit method is used to compute a variety of three-dimensional problems on tetrahedron grids to assess its accuracy and robustness. The numerical experiments demonstrate that the implicit reconstructed discontinuous Galerkin method can obtain an overall speedup of more than two orders of magnitude for all test cases compared with multi-stage Runge-Kutta reconstructed DG methods. The numerical results also indicate that this implicit RDG(P 1 P 2 ) method can deliver the desired third-order accuracy, while maintaining advantage in cost over the implicit DG(P 2 ) method. Nomenclatureof the left-hand side matrix A e = total energy per mass F = fluxes of mass, momentum and energy GMRES = generalized minimum residual L = lower part of the left-hand side matrix A LU-SGS = lower-upper symmetric Gauss-Seidel n j = unit outward normal vector to the cell boundary P = preconditioning matrix p = pressure R = residual vector S = entropy t = time U = conservative state vector Ũ = cell-averaged value of conservative state vector U = upper part of the left-hand side matrix A V = mass matrix W = test function vector u i = Cartesian velocity in i direction x, y, z = Cartesian coordinates x c , y c , z c = center coordinates of the cell Ω e = volume of the cell Γ e = boundary of the cell ε = value used in perturbing the solution γ = ratio of the specific heats η = span-wise location ρ = density ∆t = time step

Existing regulations supporting control room modernization activities do not fully address novel system development activities in nuclear process control. The need for the additional guidance is evident in novel system development for new functionality intended for existing nuclear power plants as well as entirely new control systems intended for advanced reactor designs. In particular, the evaluation and use of simulation to support the design process differences between the human factors activities performed for traditional control room modernization and a representative novel system that changes the fundamental concept of operations. We review such an example, namely a thermal power dispatch system intended for an existing commercial reactors that supports steam uses beyond electricity generation. We present the more expansive scope of human factors activities and the reliance on multiple types of simulation-based evaluations needed to support the design process.

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