Root cause study on hydrogen generation and explosion through radiation-induced electrolysis in the Fukushima Daiichi accident

Hydrogen Generation and Explosion Mechanisms in the Fukushima Daiichi Accident

Overview

• Title: Root cause study on hydrogen generation and explosion through radiation-induced electrolysis in the Fukushima Daiichi accident.

• Author: Genn Saji

• Published in: Nuclear Engineering and Design, 2016.

Key Highlights of Research

• Analysis of Light Water Reactors (LWRs) concerning hydrogen generation through water radiolysis during normal and severe conditions.

• It was concluded that water radiolysis is not likely the root cause of the hydrogen explosions at Fukushima.

• Predictive models indicate that tens of thousands of cubic meters of hydrogen gas accumulated before explosions in Units 1-4.

• Emphasis on the necessity of early disposal of hydrogen from reactor pressure vessels (RPVs) during station blackout (SBO).

Background of the Study

• The investigation is based on the author’s theory of radiation-induced electrolysis (RIE), which incorporates Faraday’s law of electrolysis into material balance equations.

• The theory originally aimed to study accelerated corrosion phenomena but extends to hydrogen generation during accidents.

Noteworthy Findings from the Accident

• A total of approximately 29,400 m³ of hydrogen gas was estimated to have accumulated in Unit 1 before the explosion.

• Immediate factors during SBO were examined, particularly the rapid initiation of electrolysis above 40 °C in the spent fuel pools (SFP).

• It implies that under certain conditions, especially involving radioactivity, a potential difference could create favorable conditions for electrolysis.

Water Radiolysis Mechanisms in LWRs

• Definition: Water radiolysis refers to the breakdown of water (H₂O) into hydrogen (H₂) and oxygen (O₂) caused by ionizing radiation.

• Key products of water radiolysis include radicals and stable molecular products formed during complex chain reactions but do not lead to large amounts of hydrogen generation rapidly.

• Studies indicated that the generated hydrogen was effectively not retained in the reactor water during the incident.

• Critical Hydrogen Concentration (CHC): Concentration of dissolved hydrogen to control oxidation products in pressurized water reactors (PWRs).

Mechanisms of Hydrogen Generation

In BWRs (Boiling Water Reactors)

• Hydrogen generation through water radiolysis occurs during routine operations; however, incidents of hydrogen explosions have been documented due to improper gas extraction in some plants.

• Notable Incidents: References to prior hydrogen explosions due to system malfunctioning in Germany (Brunsbüttel BWR explosion) and Japan's Hamaoka-1 BWR.

Investigative Methodology

• Modeling Studies: Simulation between different reactor units to predict the hydrogen generation and assess various reactor responses.

• Multiple reactors showed significant discrepancies between theoretical estimations and actual operational data.

Conclusions Drawn

• RIE identified as a more probable root cause of hydrogen generation than conventional water radiolysis theories.

• The necessity for operational protocols focusing on the early disposal of hydrogen during SBO is emphasized.

• Further exploration of spent fuel management during the operation and maintenance of LWRs in highly radioactive conditions is recommended.

• Proposals were made for integrating improved safety measures and operational adjustments based on the retrospective analysis of the accident.

Future Directions

• Explore the incorporation of advanced modeling techniques and rigorous experimental validation to understand the dynamics of hydrogen generation through RIE and its implications in severe nuclear accidents.