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Verification, validation and uncertainty quantification (VVUQ) have become acommon practice in thermal-hydraulics analysis. An important step in theuncertainty analysis is the sensitivity analysis of various uncertain inputparameters. The common approach for computing the sensitivities, e.g.variance-based and regression-based methods, requires solving the governingequation multiple times, which is expensive in terms of computational cost. Analternative approach to compute the sensitivities is the adjoint method. Thecost of solving an adjoint equation is comparable to the cost of solving thegoverning equation. Once the adjoint solution is available, the sensitivitiesto any number of parameters can be obtained with little cost. However,successful application of adjoint sensitivity analysis to two-phase flowsimulations is rare. In this work, an adjoint sensitivity analysis framework isdeveloped based on the discrete adjoint method and a new implicit forwardsolver. The framework is tested with the faucet flow problem and the BFBTbenchmark. Adjoint sensitivities are shown to match analytical sensitivitiesvery well in the faucet flow problem. The adjoint method is used to propagateuncertainty in input parameters to the void fraction in the BFBT benchmarktest. The uncertainty propagation with the adjoint method is verified with theMonte Carlo method and is shown to be efficient.

Application of discrete adjoint method to sensitivity and uncertainty analysis in steady-state two-phase flow simulations