Influence of ATF Cladding Tube Candidates on the Critical Heat Flux in a Vertical Rod Bundle During Flow Boiling of Water at Low Pressure

This paper presents an experimental investigation into the thermohydraulic performance of accident-tolerant fuel cladding materials in a rod bundle configuration under near-atmospheric pressure conditions, representative of simplified light water reactor scenarios. A chromium nitride (CrN)–coated Zircaloy-4 (Zr-4) rod bundle and a fully iron chromium aluminum alloy–clad rod bundle are compared against an uncoated Zr-4 reference, selected for their improved oxidation resistance and reduced hydrogen generation potential. Critical heat flux (CHF) is used as the primary benchmark for evaluating the onset of boiling crisis under steady-state conditions. The influence of alternative cladding materials on surface temperature and maximum heat transfer is assessed at both reduced and elevated operating conditions around a reference point [20 K subcooled mass flux of 200 kg∙m2∙s−1 at 1400 mbar(a)]. Heating is applied under consecutive steps until the CHF is reached. The test section consists of a directly electrically heated, magnetically compensated five-rod bundle enclosed in a square vertical 43.7 × 43.7-mm2 flow channel with controlled boundary conditions. ... mehrEach rod has a 9.5-mm outer diameter, 8.36-mm inner diameter, and 300-mm heated length, additionally with an 8-µm-thick CrN layer. CHF detection is enabled via sight glasses in the flow channel and high-speed imaging, synchronized with a ring buffer and transformer-triggered shutdown, enabling repeated nondestructive tests with various samples. The observed material effects on CHF behavior support the development of mechanistic models and enhance the predictive capabilities of multiphase CFD simulations.