The explosions at Fukushima Daiichi Unit 3 and Unit 4 and implications on the evaluation of 1F3 accident

Introduction

• Investigates the explosions at Fukushima Daiichi Unit 3 and Unit 4

• Focus on understanding the explosion events during the accidents

Keywords

• Hydrogen Fireball

• Explosion

• Fukushima Daiichi

• BSAF

• MELCOR

• Severe accident

Overview of Explosions

• Three explosions occurred during the nuclear accidents:

  • Unit 1 (1F1) at 3:36 PM on March 12, 2011

  • Unit 3 (1F3) at 11:01 AM on March 14, 2011

  • Unit 4 (1F4) at 6:14 AM on March 15, 2011

• Videos of 1F1 and 1F3 show different explosion dynamics; 1F4 remains unknown due to lack of video.

Hydrogen Generation Estimates

• Total hydrogen equivalent needed for 1F3 explosion estimated at 1450 kg (after rounding).

• For both 1F3 and 1F4 explosions combined, approximately 2100-2400 kg of hydrogen equivalent must be generated.

• Estimate suggests that 71.3~81.5% of potential hydrogen sources could have contributed.

Methodology for Estimating Combustible Gases

• Estimation based on:

  • Hydrocarbon-based explosion database

  • Empirical correlations predicting fireball size of hydrocarbon fuel explosions

• Fireball explosion data should supplement severe accident analyses.

Explosion Dynamics

• Notable dynamics for each explosion:

  • 1F1: Fast deflagration; approx. 130 kg of hydrogen burned at about 8-9% concentration.

  • 1F3: More powerful mushroom-shaped fireball lasting over 9 seconds; characterized by multi-mode combustion including notable convection.

• 1F4 likely a similar deflagration to 1F1 but specifics remain unclear.

Conditions Leading to Explosions

• Three time windows identified for gas migration from drywell to the reactor building:

  1. 5-hour period during the second water injection (drywell pressure > 4 bars)

  2. 3-hour period during the third unsuccessful venting

  3. 4-hour period during the fourth unsuccessful venting

• Investigates factors for the 1F3 explosion including:

  1. Pre-explosion ignition conditions

  2. Contribution of H2 from vented gas during 1F3 venting

  3. Role of PCV head failure during the explosion

In-Vessel and Ex-Vessel Hydrogen Generation

• In-vessel hydrogen from metal oxidation and ex-vessel hydrogen generation during core-concrete interactions critical for explosions.

• Must analyze gas generation rates related to timing of vessel failures to understand explosion phenomena better.

Comparison with BSAF Phase 2 Study

• BSAF Phase 2 comparison shows variability in hydrogen generation across organizations:

  • Ranges: 600 kg to 1220 kg (in-vessel) and 0 kg to 1200 kg (ex-vessel)

  • Total generation estimates range from 600 kg to 2420 kg.

• One analysis closely aligns with the estimated value.

Conclusion

• Explosions at Fukushima Daiichi were significantly influenced by combustible gases derived from hydrogen generation.

• Understanding these explosions provides insights into severe reactor core meltdown accidents within BWR design.

• Emphasizes improvements required in accident analysis models, particularly relating to hydrogen production mechanisms.