MELT-3, Thermohydraulics and Neutronics, Fast Reactor Transients with Feedback

1 - Description of problem or function: MELT3 is a multichannel, neutronics, thermal-hydraulics digital computer program developed to investigate the transient behavior of a fast reactor during postulated transient overpower conditions. Reactivity feedback resulting from Doppler broadening, coolant density change and expulsion, bulk core expansion, and fuel movement are explicitly taken into account. The bulk of the modeling detail has been addressed to the in-vessel portion of the reactor plant, although the friction and inertial aspects of up to three separate closed primary loops can be simulated. A wide variety of accident conditions may be investigated. Particular modeling emphasis has, however, been placed on the simulation capabilities required for an unprotected transient overpower accident sequence. 2 - Method of solution: The energy equations are solved by a Crank- Nicholson type implicit scheme. The momentum equations are solved by an iterative implicit scheme. 3 - Restrictions on the complexity of the problem: Calculations for up to 20 channels (e.g., 20 pins representative of subassembly clusters) and 20 axial segments within the fuel region may be performed simultaneously. A total of 10 different axial coolant flow zones having different flow areas (comprising a total of 75 nodes) can be explicitly modeled in the axial direction. Radial heat transfer calculations within each pin axial segment are performed using up to 12 radial fuel nodes (including one surface node), 3 cladding nodes (including inner and outer surface nodes), a coolant node, and a structural node