Investigation of downward facing critical heat flux with water-based nanofluids for In-Vessel Retention applications

Abstract

In-Vessel Retention ("IVR") is a severe accident management strategy that is power limiting to the Westinghouse AP1000 due to critical heat flux ("CHF") at the outer surface of the reactor vessel. Increasing the CHF level by altering the cooling fluid would increase the safety margin of current design power or allow for higher power. The modification to current licensed design to implement a new cooling fluid would not require significant changes to the containment and associated systems. Previous research at MIT and other institutions has demonstrated that CHF of water on a heated metal surface can be increased from 30% to 200% with the introduction of nanoparticles. Alumina has shown the best CHF enhancement of the nanoparticles tested to date at MIT. Alumina nanoparticles and water based nanofluids have also shown long term stability in solution, which is important for the long time frame (hours to days) of IVR. To measure the CHF of geometry and conditions relevant to IVR for the AP1000, a two-phase flow loop has been designed and built. The test section designed to have hydrodynamic similarity to the AP 1000 and allows for all angles that represent the bottom surface of the reactor vessel. Research completed herein measured CHF for varied conditions of orientation angle, pressure, mass flux, fluid type, and surface material. Results for stainless steel with water based alumina 0.001% by volume nanofluid indicate an average 70% CHF enhancement with a range of 17% to 108% for geometry and conditions expected for IVR. Experiments also indicate that only about thirty minutes of boiling time is needed to obtain CHF enhancement. Implementation could involve storage tanks of high concentration nanofluids installed in containment. Once the IVR strategy is initiated with flooding of the vessel cavity with water from the In-containment Refueling Water Storage Tank ("IRWST"), the nanofluids would be released to mix as the natural circulation flow sets up along the gap between the vessel and the insulation mounted to the concrete wall in the vessel cavity. Boiling then plates nanoparticles onto the surface enhancing CHF.