Ericson, M. Nance | Search NASA

DOE OSTI2024

Impact of breech geometry and propellant flow on the release of large pellets for the ITER disruption mitigation system

Studies have been performed on the release mechanism for large pellets using high pressure gas in a shattered pellet injector. Typically, pellets are dislodged from the cryogenic surface and accelerated down a barrel using high pressure gas delivered by a fast-acting propellant valve. The pellets impact an angled surface which shatters the pellet into many small fragments before entering the plasma. This technique was initially demonstrated on DIII-D (Commaux et al 2016 Nucl. Fusion 56 046007) and is now deployed on JET, KSTAR, ASDEX-Upgrade, and other tokamaks around the world in support of ITER's disruption mitigation system design and physics basis. The large hydrogen, 28.5 mm diameter, 2 length-to-diameter ratio, pellets foreseen for ITER SPI operation have low material strength and low heat of sublimation, which cause the pellets to be fragile and highly reactive to the impact of warm propellant gas. Due to the size of the pellets, significantly more propellant gas is required to dislodge and accelerate them. This creates a potentially significant propellant gas removal issue as 2–6 bar-L of gas is expected to be required for release and speed control. The research presented in this paper is an in-depth exploration of the parameters that are keys to reliable pellet release and speed control. Computational fluid dynamics (CFD) modeling of propellant flows through various breech designs was conducted to determine the force generated on the back surface of a pellet. These simulations assumed the use of the ORNL designed flyer plate valve. CFD modeling combined with experimental measurements provide adequate insight to determine a path to an optimal valve and breech design for ITER SPI pellet release and speed control while minimizing propellant gas usage.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2023

Wireless System Development for GaN-Based Transmitters for Advanced Nuclear Reactor Environments

The market for consumer electronics is dominated by Si-based electronics. Many Si-based complementary metal–oxide–semiconductor (CMOS) devices are reaching their limitations in terms of switching frequencies, voltages, currents, and temperatures. Furthermore, these devices are widely known to be susceptible to ionizing radiation and neutrons. Advanced nuclear reactors, especially microreactors and small modular reactors, require wireless electronics that can survive larger neutron and gamma doses than Si CMOS. Because of their size, electronics will need to be placed closer to the reactor cores, and cabling capabilities may be limited. However, GaN high-electron-mobility transistors (HEMTs) have been shown to switch faster with higher gains, and they can achieve current and voltage limits beyond traditional Si CMOS. However, they have less market maturity and have undergone less testing. These GaN HEMTs have been shown to survive ionizing radiation with little effects, but the effects of neutron radiation remain widely unknown. In this report, a wireless electronic system capable of supporting multiple radiation-hardened (rad-hard) sensors is presented. The proposed transmission scheme was designed to be simple to increase the likelihood of success in the immature GaN HEMT process and in high-radiation environments. The transmission system was tested using software-defined radios to emulate the transmission scheme before the transmitter circuitry in the GaN process was developed.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗