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Formation mechanisms of insoluble Cs particles observed in Kanto district four days after Fukushima Daiichi NPP accident

Formation Mechanisms of Insoluble Cs Particles

Introduction

  • Observation: Insoluble Cs particles type A were first identified in Tsukuba city on 15 March 2011.

  • Formation Hypothesis: Initially thought to form by evaporation/condensation within the reactor pressure vessel (RPV) of Unit 2 based on the 134Cs/137Cs ratio.

  • New Explanation: It is proposed that these particles formed by melting glass fibers from the HEPA filter in the Standby Gas Treatment System (SGTS) due to the hydrogen detonation in Unit 3.

  • Impact: Large amounts of particles were moved seaward, but some were deposited on the lower elevations of Unit 3, leading to potential resuspension into the atmosphere due to steam generation.

Key Concepts and Findings

Types of Insoluble Cs Particles

  • Type A Particles:

    • Shape: Spherical

    • Size: 2-10 μm

    • Origin: Units 2 or 3

    • Location: Found in Kanto district, including Tsukuba and Tokyo.

  • Type B Particles:

    • Shape: Distorted

    • Size: 70-400 μm

    • Origin: Unit 1

    • Location: Primarily north side near Unit 1.

  • Composition: Type A particles are characterized by their high radioactivity and are covered by silicate glass, leading to their insolubility.

Formation Mechanism of Type A Particles

Evaporation/Condensation vs. Atomization

  • Evaporation/Condensation:

    • Not a feasible explanation for the composition of Type A particles due to the variety of components present.

  • Atomization Process:

    • Occurs when molten materials are fragmented into fine droplets by a high-speed air blast, like that generated in a hydrogen explosion.

    • Conditions for Type A formation included high temperatures (over 3000 K) and rapid cooling.

Events During the Fukushima Accident

Timeline of Significant Events

  • March 14, 2011 11:01 AM: Hydrogen explosion in Unit 3 leads to formation of Type A particles.

  • March 14, 2011 15:30 PM: Restart of cooling water leads to potential resuspension.

  • March 15, 2011 02:30 AM: Further resuspension of particles expected due to steam flow.

Environmental Monitoring

  • Cs Particle Distribution:

    • Measurements indicated that approximately 30% of measured Cs in Tsukuba was Type A, rising to 90% in Tokyo.

  • Wind Patterns: Initial west-southwest winds carried particles to the Pacific, changing to north-northeast, capturing particles back toward Kanto district by March 15 early morning.

  • Monitoring Data: High concentrations of Cs particles noted at Tokai, indicating successful transport of Type A particles from Fukushima.

Conclusion and Future Directions

  • Implications for Worker Safety: Understanding the formation and resuspension of Type A particles is crucial for minimizing radiation exposure during decommissioning.

  • Further Research Needed: Additional studies required to validate these mechanisms and quantify the fraction of Type A particles among total Cs releases.

  • Future Challenges: Real data confirmation and further environmental analysis are necessary to affirm the current hypotheses on Type A particle formation.