An analysis of the hydrogen explosion in the Fukushima-Daiichi accident

Analysis of Hydrogen Explosion in the Fukushima-Daiichi Accident

Overview

  • Authors: J. Yanez, M. Kuznetsov, A. Souto-Iglesias

  • Institutions: Karlsruhe Institute of Technology, Technical University of Madrid

  • Publication Date: Received October 13, 2014, Accepted March 29, 2015

  • Keywords: Nuclear safety, Combustion, Explosions, Numerical simulation, Detonation

Abstract

  • The study estimates 130 kg of hydrogen (H2) participated in the explosion during the Fukushima-Daiichi nuclear power plant accident.

  • The explosion intensity was significant, indicating a devastating incident would have occurred even with less hydrogen.

Introduction

  • Historical Context: Three major nuclear accidents occurred:

    • Three Mile Island (1979, USA)

    • Chernobyl (1986, Ukraine)

    • Fukushima Daiichi (2011, Japan)

  • Hydrogen Sources: Generated from reactions after severe accidents:

    • Oxidation of Zircaloy by steam

    • Radiolysis of water

    • Reaction between water and boron carbide

    • Interaction of molten core with containment concrete

  • Hydrogen's Behavior: Accumulates in stratified layers, potentially igniting and causing structural damage if ignited.

Event Sequence of the Fukushima Accident

  • Earthquake: On March 11, 2011, a 9.0 magnitude earthquake occurred, prompting a shutdown of the Fukushima plant.

  • Tsunami Impact: A subsequent tsunami struck, disabling emergency power and leading to critical damage.

  • Pressure Increase: Water pumps failed; the core pressure rose beyond design limits necessitating venting.

  • Hydrogen Accumulation: Resulted from the oxidation of fuel rods, leading to eventual detonation.

  • Explosion Timing:

    • Unit 1: 24.8 hours after the event

    • Unit 3: 68.2 hours after

    • Unit 4: 87.9 hours after

Hydrogen Explosion Analysis

Generation Estimates

  • Hydrogen Produced: Estimates range from 800-1000 kg during the cooling system failure (Sehgal et al., 2012).

  • TEPCO's Estimates: Hydrogen quantities vary based on zirconium oxidation and concrete interaction.

Evaluation Methodologies

  1. Shock wave analysis to determine the lower bounds of hydrogen amounts.

  2. Examination of combustion product size providing upper bound estimates.

Combustion Wave Propagation

  • Shock Wave Speed: Estimated speed around 360 m/s with a Mach number of approximately 1.05.

  • Pressure over time: Over-pressure from the shock wave estimated at 28 kPa.

Damage Assessment & Simulation**

Numerical Simulations

  • Calculation Parameters: Used COM3D code to simulate hydrogen deflagration under realistic conditions.

  • Results:

    • Estimated combustion yielded significant overpressures capable of damaging structures.

    • Simulations considered variable amounts of hydrogen: 10, 80, and 200 kg.

Damage Diagrams

  • Provided a relationship between maximum over-pressure and impulse duration to evaluate structural damage.

  • Results indicated serious risk to reactor building structures with specific hydrogen amounts causing potential devastation.

Conclusions

  • Hydrogen Quantities: Estimated between 50 kg and 270 kg involved in the explosion, refining previous estimates.

  • Design Recommendations: Emphasizes need for nuclear power plants to withstand potential hydrogen detonations in future designs to enhance safety.