Flow Instability Tolerance Simulation Heater for SMR and Other Rod Bundle CHF Tests Under Low Pressure, Low Flow, and High Quality Conditions

Abstract

In order to properly define the critical heat flux (CHF) limits for any fuel design, electrically heated simulation heater rods (SHR) are generally designed to allow CHF testing to be done with various representative heat flux profiles and bundle assembly configurations. To achieve high heat flux density and accurate CHF measurement representative of the reactor, with constant input of heat flux during the CHF transient event, and without the non-conservative uncertainty resulting from internal heat conduction, direct simulation heater rods (DSHR) with electrical current applied directly to the outer metal cladding for Joule heating are preferred over indirect simulation heater rods (ISHR). DSHR are normally designed to allow for gradual thermal expansion as CHFs under high pressure conditions are often very localized DNB events with relatively small accumulated expansion throughout the CHF transient process. However, under low pressure, low flow (LPLF) conditions, the system is subjected to flow instability induced CHF events that occur rapidly and could happen throughout the entire channel lengths rapidly within very short period of time (often in seconds or less) and often result in heater rod failure. In addition, such rapid flow and pressure oscillations often result in loss of flow measurement and control. Due to such experimental challenges, there are very few rod bundle CHF data obtained under LPLF conditions. With the development of SMRs, a method of obtaining safe and accurate measurements of rod bundle CHF under LPLF conditions has become imperative.

An innovative simulation heater, a Flow Instability Tolerant Simulation Heater (FITSH) is presented with supporting test data in this paper utilizing special designed dynamic balancing mechanism to handle the fast transient event and allow safe and accurate measurements of flow instability induced CHF under low pressure (as low as 1.2 bar), low flowrate, and high-quality conditions. Additionally, initial testing and benchmarking performance results of the FITSH are presented, including response time evaluation and LPLF CHF data.