Underwater arc discharge is being studied as one of the remote dismantling processes for fuel debris retrieval
at the Fukushima Daiichi Nuclear Power Plant. In this study, the effect of admixture of N2 molecules to argon
arc is investigated to make the plasma with higher enthalpy. A steady-state DC arc thermal plasma is generated
using a DC voltage between the anoxic copper anode and the 2%-thoriated tungsten cathode. The optical
emission spectrum of the plasma plume was measured through a quartz window. The spectral analysis of Ar I
lines was performed. The arc current and voltage were recorded. The total flow rate of the discharge gas was 25
L/min, where the nitrogen partial pressure ratio was adjusted in the range of 0 – 32%. It was found that the
resistance of the arc plasma column increased as the nitrogen mixture ratio in the discharge gas increased. The
electron temperature Te became increased up to approximately ~ 2 eV with increasing the volumetric ratio of
N2 gas. On the other hand, it was found that the electron density Ne generally increases several times when
nitrogen is mixed. The pH value of the irradiated water was found remarkably to decrease in the course of
plasma irradiation in the open atmosphere.