BWR Anticipated Transients Without Scram in the MELLLA+ Expanded Operating Domain, Part 1: Model Development and Events Leading to Instability (NUREG/CR-7179)

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Publication Information

Manuscript Completed: April 2014
Date Published: June 2015

Prepared by:
Lap-Yan Cheng, Joo Seok Baek, Arantxa Cuadra, Arnold Aronson,
David Diamond, and Peter Yarsky*

Nuclear Science and Technology Department
Brookhaven National Laboratory

*U.S. Nuclear Regulatory Commission

Tarek Zaki, NRC Project Manager

NRC Job CodesV6150 and F6018

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

Availability Notice


We developed models of a BWR/5 boiling water reactor for use with TRACE/PARCS and thereafter to analyze anticipated transients without scram (ATWS). We established the models for three different times during a fuel cycle, and included therein all systems needed for events initiated by turbine trip or the closure of a main steamline isolation valve. They include the standby liquid-control system, recirculation pumps, feedwater and water level control, reactor core isolation cooling system, safety and relief valves, suppression-pool cooling, and other systems. The modeling in the core that we undertook is detailed relative to state-of-the-art models, with four different fuel-rod types included in each fuel assembly, and 382 channels to represent all assemblies, taking into account half-core symmetry. The models we developed can be used for multiple ATWS applications, and for many transients with the reactor trip operational.

The models were applied to ATWS events initiated by a turbine trip while operating in the expanded operating domain “MELLLA+.” Sensitivity calculations were undertaken at the beginning-of-cycle to determine the effect of bypass fraction (10%, 25%, 50%, and 100%) and also at peak-hot-excess-reactivity to determine the effect of including a spectral-history correction on the void density. A calculation at end-of-full-power-life allowed us to compare three different times in the fuel cycle.

The regulatory purpose of the current work is to demonstrate the use of TRACE/PARCS in evaluating the potential consequences of ATWS events in BWRs operating under MELLLA+ conditions. Our study offers insights into the reactor’s behavior during these events, and in particular the impact of assumed operator actions on the observed oscillatory behavior caused by the reactor’s instability and on the eventual shutdown of the reactor. In addition, our study examined the ability of the TRACE/PARCS code system to calculate the complex phenomena during these events. We also gained insights into effective modeling of those phenomena. Hence, this study may be used as a basis for developing rigorous modeling guidance for similar application.

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