New research facility to support nuclear reactor power uprates

A new thermal hydraulics research facility at the University of Wisconsin–Madison is generating crucial data to support safely expanding the operating capacity of current U.S. nuclear power plants.

The PHILUS (high-Pressure High-temperature annuLUS) facility recently began operating in February 2026 under the direction of Juliana Pacheco Duarte, Associate Professor in the Department of Nuclear Engineering and Engineering Physics. The lab is supported by funding from a U.S. Nuclear Regulatory Commission Faculty Development Grant and a U.S. DOE Distinguished Early Career Program grant awarded to Duarte, who was one of only five faculty in the United States to receive the award in 2023.  

PHILUS was designed and built to investigate post-critical heat flux, a heat transfer accident regime of water-cooled nuclear reactors. Critical heat flux represents the maximum amount of heat that the water can safely remove from nuclear fuel rods. At critical heat flux, a film of vapor forms around the fuel rods, acting as an insulating blanket that prevents heat from escaping. This leads to high temperatures that can cause damage to the rods. 

The Nuclear Regulatory Commission has traditionally recognized critical heat flux as the thermal hydraulic limit for U.S. nuclear power plants. Many argue that this limit imposes overly conservative safety margins for most operating conditions, especially with the development of accident tolerant fuels that enhance safety in recent designs. 

Now, time-at-temperature criteria have emerged as a realistic alternative. These criteria would relax the traditional safety margins and support growing interest and financial incentives for implementing power uprates across the current fleet of U.S. light-water reactors, allowing plants to increase their operating power. 

In order to define and implement the proposed time-at-temperature criteria, certain modeling and experimental gaps must be investigated. A recent report from the Electric Power Research Institute identified that the transient temperature behavior of the reactor cladding shortly after reaching critical heat flux is one such area of uncertainty and limited data. 

PHILUS is a unique facility designed to provide that data. By investigating the temperature behavior after the point of critical heat flux, the facility will generate prototypical experimental data that can be used to develop new models and significantly improve system codes used to predict post-critical heat flux transient cladding temperatures. 

Similar experiments have been conducted at lower pressures, but PHILUS is one of the few facilities at a U.S. university to perform these experiments at the high pressures and temperatures characteristic of true reactor conditions and can achieve uniquely accurate temperature measurements.

Duarte investigated the post-critical heat flux conditions of small modular reactors as a PhD student in the University of Wisconsin–Madison Department of Nuclear Engineering and Engineering Physics, working with Wisconsin Distinguished Professor Emeritus Mike Corradini. Twelve years later, Duarte is a leader in the field and mentors many early career researchers like PhD student Cole Dunbar and postdoctoral researcher Dr. Donkoan Hwang who will help run the experiment.

“Post-critical heat flux heat transfer is a topic of large uncertainties as it involves complex vapor and liquid dynamics,” says Duarte. “PHILUS will bring unique high-resolution data that we can use to develop new models and significantly improve our capability to capture time-at-temperature clad behavior.”