The Nuclear Reactor Program (NRP) at NC State University is developing a suite of specialized facilities to investigate the performance of materials and instruments within the extreme radiation, chemical, and thermal environments of Molten Salt Reactors (MSRs). These facilities bridge the gap between theoretical models and real-world reactor conditions by providing both static and dynamic testing environments.

Static Molten Salt Irradiation Facility

Developed as the initial phase of the NRP’s molten salt capabilities, this facility allows for baseline experiments in stationary melts. It is designed to evaluate material behavior and facilitate post-irradiation examination (PIE) through the use of resealable, reusable components.

The system features an electric heater assembly (see figure below) specifically engineered for irradiation environments, housed within a standpipe outer shell. Each sample capsule can contain up to 300 g of salt and includes three internal eyelets for suspending material samples directly in the melt. A thermocouple is integrated via a port in the capsule lid to provide direct, continuous measurement of the melt temperature during irradiation. The resealable design allows salt samples to be stored and subsequently remelted within the same container, reducing corrosion risks and preserving the ability to extract samples for later study. The facility’s operability was successfully demonstrated in the integrated testing in January 2024.

Molten Salt Flow Facility (MSFF)

The MSFF is designed to simulate the representative flow and radiation environments of proposed advanced reactor concepts. It moves beyond static testing to study the transport of radionuclides and the impact of salt flow on wetted materials.

The MSFF is a vertical flow loop contained in a standpipe shell that extends to the center plane of the reactor core. A lobe-type positive displacement pump, positioned roughly 180 cm above the reactor core plane, regulates flow through the irradiation ‘experiment region’. During operation, the salt flow is conducted axially between an upper and lower recirculation duct, with a maximum irradiation length of 305 mm centered on the core mid-plane (see the figure below).

There are two experimental access points for samples, probes, and other experimental instruments. The first one is the primary shaft, which is a vertical shaft with an inner diameter (ID) of approximately 22.2 mm allows for the insertion of various payloads—such as graphite shapes, metal plates, or electrochemical probes—directly into the irradiated region. This is illustrated in the figure below. Two additional 9.2 mm ID ports provide access for non-irradiation experiments or supplemental salt monitoring instruments (e.g., pressure or electrochemical sensors).

Molten Salt Off-Gas Measurement Facility

The Molten Salt Off-Gas Measurement Facility is designed to enable the study of off-gases from molten salt and dissolved nuclear fuel at a controlled temperature during irradiation in order to further understand the diffusion behavior of the fission products within and out of the molten salt.

The system has a sample container that allows a stream of sweep gas (e.g. helium) flow across the surface of the molten salt and bring the fission products as well as any other volatiles to a gamma spectrometer and tritium monitors at the pool top. The sample container is made of low cobalt stainless steel and can hold up to ~60 g of FLiBe or FLiNaK salt. The chamber is wrapped with resistive heating filaments and surrounded by custom-made porous alumina-silica to achieve a steady temperature of 700-800 °C during irradiation. The released products may include Xe, Kr, tritium, volatile fluorides, and aerosols. To prevent the condensation of volatiles to the inner surface, the off-gas transport tube is also wrapped with heater tapes to keep its temperature at approximately 250 °C, which is above the vaporization temperatures of most of the fluorides. The whole sample heating assembly and the gas transport tubes are held by a 4” intermediate pipe, which is placed inside a 9” standpipe to prevent any direct heating to the pool water. The flow rate of the sweep gas is controlled with a digital mass flow controller that varies the flow rate from 0.01 to 1 slpm, which allows fission products to reach the pool top detectors from within 30 seconds to tens of minutes in order to differentiate short-lived and long-lived isotopes.

The measured gamma specs, hence, the amount of released gases/volatiles, will be calibrated using a separate LIBS system (collaboration with TAMU) and with Multiphysics simulations. The transport efficiency of the off gases as well as the release to birth ratio (R/B) can be well estimated and compared with experimental data to verify some of the fundamental parameters that have never been accurately measured before.