A bulging drum at 3.6% hydrogen: storage, repackaging and a five-year deferral
The low-level radioactive waste store at a Hong Kong site holds about 1,200 200-litre drums of cemented ion-exchange resin and evaporator concentrate, some dating from the early 1990s. During a routine inspection one drum is found with a bulging lid, a surface dose rate of 2.4 mSv/h, and a headspace hydrogen concentration of 3.6% v/v measured with a sniffer through a temporary piercing. The lower flammability limit of hydrogen in air is 4% v/v. Eleven other drums show corrosion staining and dose rates above the 2 mSv/h store acceptance criterion. The store has passive ventilation and no hydrogen monitoring. The operator proposes to move all 1,200 drums into a new modular store with forced ventilation and to defer inspection of the remainder for five years. The regulator has asked for a justification, and the site manager wants the move completed before the typhoon season.
1. Set out the immediate actions for the bulging drum and explain the mechanisms generating hydrogen inside it.
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Immediate actions, in order:
- Treat the drum as a hydrogen hazard: exclude ignition sources (no hot work, no vehicles, no non-Ex-rated instruments) within a defined zone, post warning signs and restrict access.
- Do not move the drum. Movement can disturb the crust and release a hydrogen-rich pocket and can breach a corroded lid.
- Ventilate the area and monitor continuously with a calibrated hydrogen meter; confirm the 3.6% reading and check for stratification under the lid.
- Vent the drum in a controlled way through a filtered, flame-arresting vent, under a written procedure, with hydrogen monitoring and a radiological survey. Do not puncture a pressurised drum without a plan.
- Notify the RSO and the regulator, record the finding, and sample the headspace gas (H2, CH4, O2) and, if safe, the drum contents.
Mechanisms:
- Radiolysis. Gamma and beta radiation decompose the water in the cement and resin, producing H2 with a primary yield of roughly G(H2) ≈ 0.45 molecules per 100 eV, plus H2 from resin degradation. The rate is low but the drum is sealed for decades, so the inventory accumulates.
- Anaerobic corrosion. Fe + 2H2O → Fe(OH)2 + H2. Once oxygen is consumed and the drum is damp, corrosion of the steel and of any galvanised components can dominate.
3.6% against a 4% LFL is not a margin: it sits within measurement uncertainty and the gas can be locally richer. The drum is outside any safe storage envelope and must be brought under control before any campaign begins.
2. Assess the proposal to move all 1,200 drums and defer inspection for five years. What waste acceptance criteria and safety case content would you require?
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The blanket move is not acceptable; a staged, criteria-based approach is.
- The bulging drum must be vented and repackaged or placed in a vented overpack. It cannot be moved as it stands.
- The other 11 suspect drums need inspection, gas sampling and non-destructive examination of lid and weld before any handling.
- The remaining ~1,188 drums can be moved only after a sampling/NDE programme gives confidence they meet the new store's acceptance criteria. A defensible package is a 100% visual and dose-rate survey plus a random sample (say 5–10%) with a stated statistical acceptance rule.
- Deferring inspection of the rest for five years is not justified: the failure mode (corrosion plus gas generation) is time-dependent and the oldest drums carry the highest risk.
Waste acceptance criteria:
- Radiological: contact and 1 m dose rate, removable contamination, worker and public dose from the move.
- Gas and chemistry: hydrogen generation rate per drum (measured or bounding), free liquid, reactive metals, oxidisers, chelating agents.
- Mechanical: drum integrity, weight, lifting points, stackability, corrosion allowance.
Safety case content: source term and inventory; normal and accident doses; fire and explosion assessment with ventilation rates, hydrogen monitoring and alarms, ignition control, detection and suppression compatible with radioactive waste; criticality (not credible for resin, but state and justify); ageing management with inspection intervals shorter than the observed degradation time; transport arrangements; emergency plan; QA and records. A quarantine area with its own safety case is needed for non-conforming drums.
3. What does 'characterisation' mean in this context, and what minimum data set would you require before the drums are moved?
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Characterisation means establishing, for each drum or batch, the properties that govern how it can be stored, moved and eventually disposed of - not just a dose-rate reading.
Minimum data set:
- Radionuclide inventory: gamma spectrometry plus scaling factors for alpha and beta emitters, and tritium and C-14 where relevant.
- Total activity and the resulting dose rates at contact and at 1 m.
- Physical and chemical form: cemented? free liquid? organic content? pH?
- Gas generation: headspace composition, hydrogen generation rate, whether the drum needs a filtered vent.
- Drum condition: wall thickness, corrosion, lid deformation, seal condition.
- Mass and dimensions for handling and stacking.
I would require a documented sampling plan stating the confidence achieved (for example, 95% confidence that fewer than 1% of drums exceed the hydrogen-generation criterion), chain of custody for samples, and a QA record linking each result to a drum serial number. Historical records from the early 1990s should be reviewed and their reliability stated; where records are missing, the drum is characterised by measurement, not assumption. Any drum that cannot be characterised is held in quarantine and is not moved into the new store.