GREENCDT Waste Hydrogen Explosion Management in Decommissioning Nuclear Facilities

The long term storage of spent fuel, raw waste, final products and decommissioning provide scenarios across the NDA estate where hydrogen is a significant concern. There are many instances where hydrogen generated from nuclear waste (via radiolysis or chemical reaction) are collected in silos, vessels, containers or pipes at NDA estate sites. Mixed with air, these have the potential to ignite and result in damaging overpressures via deflagration or detonation. Hydrogen explosions generally start as a deflagration but can transition to detonation (DDT) with destructive effects. In decommissioning and storage scenarios it is hard to justify a complete absence of ignition sources. It is therefore necessary to evaluate ignitability of non-standard hydrogen-air mixtures. Further, if the ignition consequences can be justified to be low or can be mitigated it can lead to effective decommissioning strategies. As NDA estate goes into decommissioning and long term storage new challenges in managing hydrogen are being met where being able to underpin the reduction in conservatisms can lead to better informed safety cases which will reduce complexity, cost and timescales at Sellafield, Dounreay and Magnox sites. Therefore, hydrogen management becomes of paramount importance to spent fuel storage, raw waste storage and decommissioning scenarios.

The proposed work aims to evaluate ignitability of a range of hydrogen-air mixtures, and obtain a greater understanding of Deflagration-to-Detonation mechanism of these mixtures. This is urgently needed for UK nuclear waste disposal sites, e.g. Sellafield Ltd., where hydrogen explosion is a significant safety concern in the decommissioning and storage scenarios. The project objectives include:

• Examine electrostatic discharge ignition sources occurring in the nuclear decommissioning and storage scenarios;
• Evaluate the ignitability of hydrogen-air mixtures (e.g. nitrogen rich) and determine their flammability boundary under various ignition energy levels, temperatures and pressures;
• Map the regime of hydrogen flame instability and quenching boundary (or envelop) under various turbulence levels and diluents (e.g. Helium, Nitrogen and Argon and CO2);
• Construct a detonation peninsula using measured auto-ignition delay times, combined with computed heat release times and excitation times;
• Determine the proneness and onset of DDT of exotic hydrogen-air mixtures using updated detonation peninsula diagram.
• Investigate the waste hydrogen DDT in the large-scale duct or silo using CFD tool with the detonation peninsula diagram.

A logical progression of the present work will extend the fundamental science required to underpin prediction of damage to nuclear process plant, storage containers and structures resulting from deflagration and detonation of hydrogen. The research outcomes will lead to new methods of predicting, measuring and ultimately mitigating the hydrogen atmospheres encountered to provide safer, more cost effective and expedient remediation of the NDA estate.