• BWR
• nuclear
• stability

The ongoing improvements in the analytical techniques based on more realistic assumptions, plant performance feedback, and the latest fuel designs have resulted in a significant increase in the calculated operational margins related to the safety analysis.
The power level is used, with other data, in many of the licensing analyses that demonstrate the safety of the plant. The increasing of the available safety analysis margin provides BWR plants with the potential for an increase in thermal power rating up to 20% (Power Uprate). In order to operate much more effectively and to more efficiently use Pu fissile materials, BWR plants expand the operating range in the region of operation with less flow than rated core flow. This extended operating domain for a plant that has uprated power greater than ~7% is identified as Maximum Extended Load Line Limit Analysis Plus (MELLLA+).
Since in the BWR design the stability must be evaluated over the entire operating domain where power oscillations could occur, the MELLLA+ domain must be investigated. New licensing bases, hardware and software specifications are required to prevent plants from exceeding their Safety Limit Minimum Critical Power Ratio (SLMCPR) due to oscillations in fuel channel power. The “Detect and Suppress Solution - Confirmation Density” (DSS-CD) [Ref.1] is a GE Hitachi Nuclear Energy (GEH) proprietary and US NRC approved methodology. DSS-CD is a stability Long Term Solution (LTS) which consists of hardware and software that provide for reliable, automatic detection and suppression of stability related power oscillations. It is designed to identify unstable power oscillations upon inception and initiate control rod insertion to terminate the oscillations prior to any significant power amplitude growth that would threaten the
SLMCPR.
The computer code TRACG [Ref.4], the GEH proprietary version of the Transient Analysis Reactor Code (TRAC), is used to evaluate the Minimum Critical Power Ratio (MCPR) performance of anticipated instability events that are reasonably limiting in ii order to perform best-estimate event simulations for a specified range of operating conditions and anticipated oscillation modes. TRACG [Ref.4] is a multi dimensional twofluid Thermal-Hydraulic (T-H) model with a three dimensional (3-D) kinetics model capable of simulating the reactor core and the systems relevant for establishing boundary conditions for the reactor vessel. TRACG has been extensively qualified against test data and BWR actual plant data from instability events.
The stability performance of the core is highly dependent on the core configuration and operating state. Each DSS-CD stability evaluation consists of a steady-state analysis performed, using the GEH proprietary 3-D core simulator PANACEA, to establish the core condition immediately prior to a transient event that is modeled using TRACG.
The scenarios considered in the analysis are loss of core flow events due to recirculation pump trip events, starting from the expected most limiting steady-state operating conditions, at the highest Power/Flow (P/F) ratios in the MELLLA+ domain.
The main objective of this dissertation is to demonstrate that the DSS-CD solution provides positive margin to the safety limit MCPR at the time of the suppression of the power oscillations. The power suppression by DSS-CD for high growth instability events occurs within a few oscillation periods from the time the instability is sensed by a period based oscillation counting detection algorithm, confirmed by multiple Oscillation Power Range Monitors (OPRMs), combined with the detected oscillations reaching a low amplitude discriminator setpoint.
In addition some sensitivity analyses are performed in order to evaluate the impact of a few parameters on the stability behavior of the core. These parameters were identified to be relevant based on preliminary analysis and the T-H stability literature.
• Sensitivity analysis on the axial nodalization of the bypass region to study the impact of the bypass voiding on the T-H feedback in the TRACG 3-D neutronkinetics solution for two different top guide geometries.
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• Sensitivity of the stability behavior of the core to the channel grouping. The impact of the channel grouping on stability applications during two recirculation pump trip (2RPT) events is performed. The stability performances of the core in 2RPT events requires evaluation of two aspects; 1) onset of oscillations; and 2) mode of oscillations.
• Noise to the system applied to evaluate the impact on the onset of oscillation.
• A more realistic UO2 conductivity model that takes into consideration exposure
and gadolinium concentration was used in order to understand the impact on stability applications of the fuel rod thermal time constant.
Finally, the use of automation tools for nuclear analysis and in particular to perform stability evaluation is discussed. The benefits of the automation tools are considered in the context of productivity and prevention of human errors to better assure the delivery of quality analysis products.