For the first time, a model was developed to simulate the cooling of the Fukushima Daiichi Nuclear Power Plant reactor Unit 1-derived, ‘Type B’ radiocaesium bearing microparticles, distributed into the environment during the 2011 nuclear meltdown. By establishing an analogy between ‘Type B’ CsMP and volcanic pyroclasts, the presented model simulates the rapid cooling of an effervescent silicate melt fragment upon atmospheric release. The model successfully reproduced the bi-modal distribution of internal void diameters observed in ‘Type B’ CsMP, however, discrepancies resulted primarily due to the neglection of surface tension and internal void coalescence. The model was subsequently utilised to estimate the temperature within reactor Unit 1 in the instant preceding the hydrogen explosion—between 1900 and 1980 K. Such a model demonstrates the accuracy of the volcanic pyroclast—‘Type B’ CsMP analogue, and confirms radial variations in cooling rate as the cause of the vesicular texture of Unit 1 ejecta. The presented findings provide scope to further explore the comparison between volcanic pyroclasts and ‘Type B’ CsMP via experimentation, which will provide a deeper understanding of the specific conditions within reactor Unit 1 during the catastrophic meltdown at the Japanese coastal plant.

For the first time, a model was developed to simulate the cooling of the Fukushima Daiichi Nuclear Power Plant reactor Unit 1-derived, 'Type B' radiocaesium bearing microparticles, distributed into the environment during the 2011 nuclear meltdown. By establishing an analogy between 'Type B' CsMP and volcanic pyroclasts, the presented model simulates the rapid cooling of an effervescent silicate melt fragment upon atmospheric release. The model successfully reproduced the bi-modal distribution of internal void diameters observed in 'Type B' CsMP, however, discrepancies in the peak frequencies resulted primarily due to the neglection of internal void coalescence. The model was subsequently utilised to estimate the temperature within reactor Unit 1 in the instant preceding the hydrogen explosion, to be between 1,850 and 1,920 K. Such a model demonstrates the accuracy of the volcanic pyroclast - 'Type B' CsMP analogue, and confirms radial variations in cooling rate as the cause of the vesicular texture of Unit 1 ejecta. The presented findings provide scope to further explore the comparison between volcanic pyroclasts and 'Type B' CsMP via experimentation, which will provide a deeper understanding of the specific conditions within reactor Unit 1 during the catastrophic meltdown at the Japanese coastal plant.