Press Room - Modeling a boron dilution transient in a pressurized water reactor

	Modeling a boron dilution transient in a pressurized water reactor Tom Keheley, Advisory Engineer, Framatome ANP
	The nuclear industry routinely uses standard problems to demonstrate the adequacy of computer codes for predicting plant transients. The Committee on the Safety of Nuclear Installations within the Organization for Economic Cooperation and Development (OECD) sponsors these International Standard Problems, based on tests performed in research facilities around the world. One of these, International Standard Problem ISP-43, was used to demonstrate the ability of computational fluid dynamics (CFD) techniques to predict boron mixing in the downcomer of a reactor vessel.

	When a pressurized water reactor (PWR) is shut down, boron is dissolved in the coolant. Because the boron solution absorbs neutrons, it assures that the reactor stays subcritical. The event modeled here is the inadvertent startup of a charging pump, which adds un-borated water to the core and may result in an unplanned criticality. The test was performed at the University of Maryland 2x4 Thermal-Hydraulic Loop. This loop is a scale model of the Three Mile Island Unit 2 pressurized water nuclear reactor. The test was performed by holding the reactor vessel at 347 K and injecting water into one cold leg at a lower temperature and then using thermocouples to determine the mixing. Note that no boron was injected into the vessel; mixing was determined strictly by the change in temperature. The measure of success was defined as predicting the transient average temperature of the thermocouples at the exit of the downcomer, labeled as Level 4 in the test. Framatome ANP used STAR-CD to simulate ISP-43. Since Framatome ANP’s fuel groups world wide have been using STAR-CD for several years as their CFD code. The code is currently being used in the United States, Germany, and France to model and analyze fuel assembly components. pro-am was used to model the vessel. The template option was chosen because cells around certain parts of the vessel need to be specially sized to capture the flow phenomena around area changes. The model itself had about 640,000 trimmed cells. A clipped view is shown in Fig. 1. STAR-CD’s AMG solver and the RNG k-epsilon turbulence model were chosen. Two different approaches were taken for the transient analysis of the problem. The first computation used constant fluid properties and no buoyancy. This was followed by a second providing temperature-dependent density and viscosity through user subroutines, and with buoyancy on. The two analyses were compared with test results at level 4 as shown in Fig. 2. While the constant-property analysis is good, using buoyancy STAR-CD predicts the Level 4 average temperature more accurately and within the uncertainty of the test results. The analysis was performed using six LINUX boxes with 2.0 GHz Xeon processors. The constant-property analysis took 56 minutes for 100 time steps of 0.05 seconds, while the buoyancy analysis took 56 minutes for 80 time steps of 0.05 seconds. This analysis is used to demonstrate that STAR-CD can adequately analyze a boron dilution transient. It also serves to demonstrate that Framatome ANP users are sufficiently experienced with the code to be able to apply it successfully.	Fig. 1: pro-am model of the test vessel Fig. 2: Constant property and buoyancy analysis comparisons to test data