The NSTF at Argonne: Passive Safety and Decay Heat Removal for Advanced Nuclear Reactor Designs

Argonne National Laboratory’s Natural convection Shutdown heat removal Test Facility (NSTF) is a state-of-the-art, large-scale facility for evaluating performance capabilities of decay heat removal systems. NSTF’s purpose is to:

1. examine passive safety for future nuclear reactors;
2. provide a user facility to explore alternative reactor design concepts; and
3. generate benchmarking data to validate advanced computer simulations.

After nearly four years of construction, experimental testing of the 26-m tall NSTF began in early 2014. The test facility has since operated for over 2,200+ hours, producing sixteen accepted tests at NQA-1. Studies were performed at full-scale normal operation, design basis accident, as well as off-normal scenarios to help scientists understand the heat removal performance of the RCCS  concept under different conditions. As of July 2016, the NSTF has successfully concluded the air-based portion of the testing program, with conversion to a water-cooled design currently in progress. Startup testing is anticipated to begin in late 2017.

Natural circulation is a phenomenon observed in everyday life, where changes in density drive fluid motion and provide a means to transfer heat. This figure shows a simple box, where one side is cooled and the other heated. The fluid near the heated wall changes temperature and rises due to a decreased density, while fluid near the cold wall sees an increase in density and subsequently sinks. This dual acting rise – sink motion causes the fluid to circulate within the box and transfer heat between the two walls.

How can natural circulation be applied in nuclear reactor safety?

For critical applications, where truly passive and inherently safe means of heat removal are required, the natural circulation concept can provide a high level of performance with relative simplicity.

The next generation of nuclear reactors will incorporate passive safety systems, many in the form of natural circulation loops like the example box, to ensure safe and long-term operation. Omitting the need for fans, pumps or human intervention, these systems rely instead on natural forces to provide safe and dependable cooling during an emergency.

Originally built to aid in the development of General Electric’s Power Reactor Innovative Small Module (PRISM) Reactor Vessel Auxiliary Cooling System (RVACS), Argonne National Laboratory’s NSTF provided confirmatory data for the airside of the RVACS. Today, as a large-scale integral test facility (1/2 scale axial height of a typical full sized gas cooled reactor), the NSTF has been modified for the U.S. Department of Energy’s (DOE) Advanced Reactor Concept (ARC) program. When completed, the NSTF will confirm the performance of the Next Generation Nuclear Plant (NGNP) reactor cavity cooling systems (RCCS) and similar passive confinement or containment decay heat removal systems in Small Modular Reactors (SMR).

1. The overall facility height for flow testing is 26-m, where twelve riser ducts standing 6.7-m tall serve as the heated test section.
2. Optical fiber cables, spanning nearly 100-m of combined total length, allow for high-resolution temperatures measurements and will support future code validation efforts.
3. An array of radiant heaters supply up 220kW of power onto the test section and can be configured for axial or azimuthal shaping to accurate simulate the walls of a reactor pressure vessel. Instrumentation includes gas and wall thermocouples, inlet mass flow rates, pressure drop, and heat flux sensors.

We use specialized heat flux meters (shown below) to separate the radiative and convection contributions.

Click on the pins to learn how we measure heat transfer.

How do we measure heat transfer?

Convection Contribution Indicators

• Golden shiny surfaces
• Very low emissivity
• Reflect the majority of radiation and
• provide information about the convection heat transfer within the heated test section.

Radiative Contribution Indicators

• Matte black surfaces;
• Very high emissivity;
• Absorb all radiation in addition to the convection.

Measuring Heat Transfer

• We do a differential measurement of the golden and black surfaces to identify heat transfer split and their contribution amounts.

Sophisticated data acquisition and control tools ensure accuracy and safety during experiments performed with the NSTF. A LabVIEW-based data acquisition and Eurotherm power controller communications software and control system can measure and display a wide range of parameters, such as local gas and structure temperatures, bulk air flow rate, radiative and convective components of heat flux to cooling ducts, pressure drop, local weather data, and heater power.

We obtain through high resolution temperature mapping and sophisticated data acquisition unprecedented data accuracy to validate computer simulations. Many of the simulations employ advanced computational tools running on supercomputers at Argonne’s Leadership Computing Facility, home to a world-class computing capability dedicated to breakthrough science and engineering. Modeling capabilities in support of the NSTF mission include 1D system modeling in RELAP5-3D and CFD simulations with STAR-CCM+.