Behavior of tungsten under irradiation and plasma interaction

The Soreq Applied Research Accelerator Facility (SARAF) is under construction in the Soreq Nuclear Research Center at Yavne, Israel. When completed at the beginning of the next decade, SARAF will be a user facility for basic and applied nuclear physics, based on a 40 MeV, 5 mA CW proton/deuteron superconducting linear accelerator. Phase I of SARAF (SARAF-I, 4 MeV, 2 mA CW protons, 5 MeV 1 mA CW deuterons) is already in operation, generating scientific results in several fields of interest. The main ongoing program at SARAF-I is the production of 30 keV neutrons and measurement of Maxwellian Averaged Cross Sections (MACS), important for the astrophysical s-process. The world leading Maxwellian epithermal neutron yield at SARAF-I (
 $5 \times 10^{10}$
 epithermal neutrons/s), generated by a novel Liquid-Lithium Target (LiLiT), enables improved precision of known MACSs, and new measurements of low-abundance and radioactive isotopes. Research plans for SARAF-II span several disciplines: precision studies of beyond-Standard-Model effects by trapping light exotic radioisotopes, such as 6He, 8Li and 18, 19, 23Ne, in unprecedented amounts (including meaningful studies already at SARAF-I); extended nuclear astrophysics research with higher energy neutrons, including generation and studies of exotic neutron-rich isotopes relevant to the rapid (r-) process; nuclear structure of exotic isotopes; high energy neutron cross sections for basic nuclear physics and material science research, including neutron induced radiation damage; neutron based imaging and therapy; and novel radiopharmaceuticals development and production. In this paper we present a technical overview of SARAF-I and II, including a description of the accelerator and its irradiation targets; a survey of existing research programs at SARAF-I; and the research potential at the completed facility (SARAF-II).

The Soreq Applied Research Accelerator Facility (SARAF): Overview, research programs and future plans

The development of suitable shielding material is a key challenge in the advancement of spherical tokamak fusion reactors. Tungsten carbide (WC) is a promising candidate material owing to its low neutron and gamma attenuation lengths resulting from the combination of high-Z and low-Z elements, but its behaviour under prolonged exposure to fusion neutrons is poorly understood. Here, we shed light on the microstructural evolution of WC under neutron irradiation by investigating the formation and clustering of defects using density functional theory atomic simulations. It is found that deviation from stoichiometry is accommodated entirely by C defects (vacancies and interstitials) while the disorder induced by radiation damage may be accommodated by three competing processes (C-Frenkel, anti-site and Schottky) with similar energetics. Vacancy clusters involving a combination of both tungsten and carbon vacancies show increasingly favourable binding energies with increasing cluster size and may lead to the fo...

Radiation-induced evolution of tungsten carbide in fusion reactors: accommodation of defect clusters and transmutation elements

Hydrogen blister formation in single crystal and polycrystalline tungsten irradiated by MeV protons

A new version of a beam dump was tested at SARAF Phase I. The beam dump consisted of a grid of tungsten pins welded to water-cool copper backing. The beam-induced heat was dissipated mainly through thermal radiation with a smaller amount via thermal conductance. Beam rastering was performed for better control of the irradiated area and beam density. A test irradiation using a 4 kW protons beam power demonstrated that the current design could readily accept average 300 W/cm2 beam power density while achieving low prompt and residual radiation levels with no blistering damage. Local melting of pins was observed at beam power density of 800 W/cm2. The results of the irradiation test are presented and analyzed and the plans for future improvements are discussed.

I. Gavish Segev

Papers

Blister formation at subcritical doses in tungsten irradiated by MeV protons

Hydrogen blister formation in single crystal and polycrystalline tungsten irradiated by MeV protons

Fracture morphology and mechanical properties of blisters in poly- and single crystal tungsten irradiated by MeV protons