Thermohydraulic analysis of U-tube steam generators

Abstract

Recent trends in plant safety analysis reveal a need for benchmark analytical representations of the steam generators to aid in the improvement of system codes and of fast codes for operator assistance. A model for such applications should exhibit four characteristics. First, it should be capable of representing the entire unit. Second, it should be based on detailed physical models, supplemented by well-tested empirical correlations and utilize a reliable numerical method, while still allowing for the assessment of potentially simplifying assumptions. Third, it should be validated. Fourth, it should provide a basic framework for expansion to severe transient (accident) analysis.

A model satisfying these characteristics has been developed. The downcomer, evaporator, and riser are treated by the twofluid, three-dimensional code THERMIT. A zero-dimensional calculation closes the natural circulation loop by linking the riser to the downcowaer. Effects included are: condensation, flashing, structure and liquid heat sinks and compressibility in the steam dome. The primary-side representation allows for any number of tubes per secondary-side computational cell. For each tube, four temperatures are calculated: primary fluid, primary wall, intermediate wall, and secondary wall.

The capability for calculating paraoeter distributions in steady-state at full and half power has been verified. Results are in excellent agreement with measurements conducted at the Westinghouse Model Boiler No. 2, as well as with calculations by the ATHOS code. Global paraiaeter computations for both mild and severe operational transients have also been verified. Calculations compare well with plant start-up data gathered at the Arkansas Nuclear One-Unit 2 facility.

The present research has produced the first integrated U-tube steam generator model which both utilizes the porous body two-fluid formulation and has validated capability of application to operating transients.