BWR Anticipated Transients Without Scram in the MELLLA+ Expanded Operating Domain, Part 2: Sensitivity Studies for Events Leading to Instability (NUREG/CR-7180)

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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

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Abstract

This is the second in a series of reports on the response of a BWR/5 boiling water reactor, operating in the expanded operating domain “MELLLA+,” to anticipated transients without reactor scram (ATWS). Herein, ATWS events initiated by a turbine trip are considered at two points in the fuel cycle: the beginning of cycle (BOC), and at peak hot excess reactivity (PHE, close to the middle of the cycle). We evaluate the effects of modeling the gap conductance (between the fuel pellet and clad), the turbine bypass fraction, and the initial core flow rate. We compare two limiting fixed values of gas-gap conductance, a low value of 5,000 W/m²-K at BOC and a high value of 161,000 W/m²-K at PHE with corresponding base cases that utilize a dynamic gas-gap model. We analyze two turbine bypass fractions, viz., 10% and 25% at PHE, and compare them with the 100% bypass base case. A reduced core flow of 75% of the rated core flow is analyzed at PHE and compared with the base case of 85%.

The simulations were carried out using the TRACE/PARCS code system and models developed for a previous study with all relevant BWR/5 systems. Modeling in the core is particularly detailed (four types of fuel rods are included in each fuel assembly, and 382 thermal-hydraulic channels to represent all assemblies, taking into account half-core symmetry) so as to capture the complex neutronic thermal-hydraulic coupling during periods of instability.

The study offers insights into the behavior of the reactor during these events, including the impact of the operator’s assumed actions on its oscillatory behavior due to instabilities in the reactor, and on its eventual shutdown. It shows the effect of gap conductance, turbine bypass fraction, and initial flow rate.