Analysis of particles containing alpha-emitters in stagnant water at torus room of Fukushima Dai-ichi Nuclear Power Station’s Unit 2 reactor

Abstract

Particles containing alpha (α) nuclides were identified from sediment in stagnant water in the torus room of the Fukushima Dai-ichi Nuclear Power Station(FDiNPS)’s Unit 2 reactor. We analyzed uranium (U), which is the main component of nuclear fuel, using scanning electron microscopy (SEM). Other α-nuclides (plutonium [Pu], americium [Am], and curium [Cm]) were detected by alpha track detection and the morphology of particles with α-nuclides were analyzed by SEM-energy dispersive X-Ray (EDX) analysis. Several uranium-bearing particles ranging from sub-µm to several µm in size were identified by SEM observation. These particles contained zirconium (Zr) and other elements which constituted fuel cladding and structural materials. The 235U/238U isotope ratio in the solid fractions that included U particles was consistent with what was found for the nuclear fuel in the Unit 2 reactor. This indicated that the U of similar fuel composition had made finer. The α-nuclide-containing particles identified by alpha track analysis were several tens to several hundred µm in size. The EDX spectra showed that these particles mainly comprised iron (Fe). Since the amount of α-nuclide material was very small, Pu, Am, and Cm were adsorbed on the Fe particles. This study clarifies that the major morphologies of U and other α-nuclides in the sediment of stagnant water in the torus room of FDiNPS’s Unit 2 reactor differed.

Analysis of particles containing alpha emitters in stagnant water in Fukushima Daiichi Nuclear Power Station’s Unit 3 reactor building

“…As indicated, 238 U was quantified in all fractions of all particle sizes, indicating its existence in various particle sizes. The concentration of U in both the torus and MSIV room samples in fractions greater than 10 μm is approximately one order of magnitude higher than that in the Unit 2 torus room samples (4.57 × 10 2 ppb) 19 . More than 99.8% of the U was in fractions larger than 10 μm.…”

“…As the main U isotopes ( 235 U and 238 U) in nuclear fuel have a long half-life and low specific activity, SEM–EDX was employed to detect U-rich particles. For detection of U-containing particle lager than 0.5 μm in diameter, particles containing more than 3% U by atomic ratio (hereafter “U particles”) were detected on the basis of elemental composition analysis, as previously reported 19 . Examples of the observation results for U particles in the torus room samples are shown in Fig.…”

“…Recently, our group analyzed particles containing α-emitters identified from sediment in stagnant water in a torus room of the FDiNPS Unit 2 reactor building 19 . In a previous study, several U-bearing particles ranging from sub-μm to several μm in size were identified by SEM.…”

The key role of sample analysis in Fukushima Dai-Ichi decommissioning, debris management, and accident progression investigation

“…Small, 0.5-3.0 μm particles containing Zr, U, and other actinides were found in sediments of stagnant water accumulated in the torus room (Yomogida et al, 2022). Analysis of this water revealed that U was mostly related to particles larger than 10 µm.…”

Uranium hydroxide/oxide deposits on uranyl reduction

“…[3][4][5][6][7] The retained water generated in a nuclear reactor from the accident at Fukushima Daiichi Nuclear Power Station (referred to as FDiNPS) was analyzed and particulate solids containing alpha nuclides, such as U were reported. 8 The formation mechanism is not clear. The system in a nuclear reactor is considered to be a non-uniform system and contains U, as shown below.…”