MUCHA1, Fuel Rod Pair Thermohydraulics During LOCA and ECCSA for LWR. MUCHA2, Primary Coolant Thermohydraulics During LOCA and ECCS for LWR

1 - Description of problem or function: MUCHA1 predicts the thermal and hydraulic behavior of any two reactor fuel rods and their associated parallel flow channels during a loss-of-coolant accident using as boundary conditions plenum fluid conditions determined by MUCHA2 or a system-oriented code. MUCHA2 predicts the thermal and hydraulic behavior of the primary coolant within the reactor pressure vessel during a loss-of-coolant accident. The heated core is represented by two equivalent fuel rods and their associated parallel flow channels. A plenum model has been developed to predict the upper and lower plenum fluid conditions throughout the accident sequence for use as boundary conditions for the parallel core channels. 2 - Method of solution: In MUCHA1 an explicit, finite-difference technique is used to solve the conservation equations describing the fluid behavior in the parallel core channels. Fluid thermodynamic properties are determined from property tables using an explicit, finite-difference technique. In MUCHA2 an explicit finite-difference technique is used to solve the conservation equations describing the fluid behavior in parallel core channels and plena. Fluid thermodynamic properties are determined from property tables incorporated in the code. Heat conduction in the fuel rod is determined using an explicit, finite-difference technique. 3 - Restrictions on the complexity of the problem: In MUCHA1 the two fuel rods can be represented by up to a total of 15 radial nodes. Each fuel rod and its associated flow channel may be represented by up to 12 axial segments. MUCHA2 applies the same restrictions to each pair of equivalent fuel rods