Geant4-Based Radiological and Shielding Assessment of Plasma-Immobilized Intermediate-Level Radioactive Waste

Abstract

Plasma-based treatment provides a compact high-temperature route for conditioning intermediate-level radioactive waste (ILW), particularly nitrate-bearing liquid streams generated during nuclear fuel cycle operations. In high-frequency torch (HFT) plasmatron systems, liquid waste is introduced into a high-temperature plasma region, where rapid evaporation, nitrate decomposition, melt formation, and radionuclide incorporation occur within a compact reactor geometry. Because the radioactive inventory is distributed inside a finite molten phase rather than represented by a point source, quantitative radiation transport modeling is required to evaluate internal energy deposition, operator exposure, and shielding performance.This work presents a Geant4-based Monte Carlo framework for the radiological assessment of a laboratory HFT plasmatron used for plasma immobilization of ILW. The model represents the quartz discharge chamber, water-cooled steel body, quartz crucible, molten NaCl–NaNO3–CsNO3 phase, surrounding air region, and a tissue-equivalent operator scoring volume positioned 1 m from the crucible center.The principal source term is 137Cs modeled as a uniformly distributed volumetric 661.7-keV gamma source within the molten phase for total activities of 1.2 MBq, 6 MBq, and 24 MBq. A monoenergetic parametric neutron sensitivity source, colocated with the molten ILW volume, is also considered to explore possible actinide-related contributions without assuming a fully specified actinide inventory. The simulations show pronounced intrinsic self-shielding by the melt-crucible-reactor assembly. Approximately 43.35% to 43.36% of the emitted photon energy is absorbed within the internal reactor components.For the reference 6-MBq case, energy deposition is dominated by the reactor shell and crucible, while the operator scoring volume receives only about 37.9 eV per decay. The calculated operator gamma dose rates scale linearly with activity, increasing from 5.03 × 10−3 to 1.01 × 10−1 μSν·h−1 over the investigated activity range. Additional lead shielding further reduces the normalized transmitted detector response.These results demonstrate that the finite molten source geometry, intrinsic structural attenuation, and engineered shielding can be evaluated reproducibly using the developed Geant4 workflow, providing a basis for shielding optimization and operator dose assessment in plasma-based ILW treatment systems.