Enhanced Natural Circulation Modeling in the GNF BWR Core Simulator AETNA02 for ESBWR

Abstract

GE Hitachi Nuclear Energy’s (GEH) next evolution of Boiling Water Reactor (BWR) technology is the Economic Simplified Boiling Water Reactor (ESBWR). The ESBWR is a natural circulation reactor which employs numerous passive safety features while simultaneously offering a large power output for a relatively small plant footprint. The ESBWR is characterized with shortened fuel length and a tall, partitioned chimney region above the core to promote natural circulation core flow. The ESBWR is studied with Global Nuclear Fuel (GNF)’s advanced core-simulator AETNA02. AETNA02 is a static, three-dimensional coupled nuclear-thermal hydraulic computer program representing a BWR core. The new GNF lattice-physics core simulator package LANCR02/AETNA02 consists of the two-dimensional method of characteristics based lattice code, LANCR02, which generates cross sections over a range of plant conditions and passes them to the three neutron energy group coarse mesh nodal diffusion code AETNA02. For the thermal hydraulic solution, AETNA02 includes a model which explicitly solves for the flow in each channel, water rod, and bypass region which accompanies a fuel channel. For the solution of a natural circulation plant like the ESBWR, AETNA02 utilizes the Automatic Plant Thermal Hydraulics (APTH) model which includes models for the core, the chimney region, the separator, the dryer, and the downcomer. AETNA02 iterates on the fuel channel flow and the pressure drop by modeling each chimney partition explicitly with fuel channels and bypasses mapped to it in addition to a bypass mixing model for the chimneys. This is relevant because the peripheral channels (low power) will have a lower void fraction while the central channels (higher power) will have a higher void fraction. Thus, the chimneys connected to these channels will each have a different hydrostatic head. To verify and validate each of the APTH component models, code-to-code comparisons are performed with the GEH TRACG04 code as well as data comparisons with experiments. The TRACG04 computer program is a best-estimate two-fluid transient code. Code-to-code comparisons of the two different methods (two-fluid model in TRACG04 versus drift flux model in AETNA02) are made. In addition, this study attempts to quantify the impact on the core flow distribution that will affect the calculated thermal margins, including the Critical Power Ratio (CPR). The impact of the multiple chimney (MC) versus the single chimney (SC) model is studied. The multiple chimney partition modeling provides additional detail to the core flow distribution that is not considered in the single chimney model. The results confirm and add confidence that the multiple chimney partition modeling will provide improved accuracy in the ESBWR core design.