Development and testing of the three dimensional, two-fluid code THERMIT for LWR core and subchannel applications. Progress report, October 1978-September 1979

The report describes the effort involved in the development and assessment of the two fluid computer code THERMIT for light water reactor core and subchannel analysis. The developmental effort required a reformulation of the coolant to fuel rod coupling, found in the original THERMIT code, as well as an improvement in the fuel rod modeling capability. With these modifications, THERMIT now contains consistent thermal-hydraulic models capable of traditional coolant-centered subchannel analysis. As such, this code represents a very useful design tool for LWRs. The advantages of THERMIT are that it contains the sophisticated two-fluid, two-phase flow model as well as an advanced numerical solution technique. Consequently, mechanical and thermal non-equilibrium between the liquid and the vapor can be explicitly accounted for and, furthermore, no restrictions are placed on the type of flow conditions. However, the formulation of the two-fluid model introduces interfacial exchange terms which have controlling influence on the two-fluid equations. Therefore, the models which represent these exchange terms must be carefully defined and assessed. In view of the importance of these interfacial exchange terms, a systematic evaluation of these models has been undertaken. This effort has been aimed at validating THERMIT for both subchannel and core-wide applications