Concrete Preparation in a Contaminated Radwaste Room
An underground radioactive waste transfer room has 120 m² of concrete wall coating with 30% blistering and peeling. The specification requires 'abrasive blast to SSPC-SP 10' and a chloride limit of 5 µg/cm². Field tests show surface chloride 22 µg/cm², pH 4.5, and moisture vapor emission 5 lb/1000 ft²/24 h. The room is contaminated at 1,500 dpm/100 cm² smearable; open abrasive blasting is prohibited by the radiological work permit. The coating vendor requires a minimum concrete profile of CSP 4 and a sound substrate. The crew proposes to spot blast the blistered areas only, then apply a high-build epoxy primer. The outage window is 10 days, and ventilation is limited to HEPA-filtered negative air.
1. Evaluate the crew's proposal to spot-blast only the blistered areas and apply primer. What surface preparation and verification steps are required before coating?
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The proposal is inadequate. Spot blasting leaves 70% of a failed coating and unknown bond; new primer over old coating creates a weak interface. The existing coating must be removed to sound concrete over the full 120 m², then prepared to CSP 4 by vacuum-assisted shot blasting or scarification. Because open blasting is prohibited, use HEPA-shrouded equipment and containment.
Soluble chloride is 22 µg/cm² against a 5 µg/cm² limit; wash and rinse, then re-test until below the limit. pH 4.5 is acidic and must be neutralized and verified. Moisture vapor emission of 5 lb/1000 ft²/24 h must be compared with the primer's limit; a moisture-tolerant system or extended drying may be needed. After preparation, verify profile, cleanliness, salt, pH, moisture, and substrate soundness. Repair defects before priming. The 10-day window may not support full removal, but schedule does not justify coating over an unprepared substrate.
2. What radiological and quality controls should govern the surface preparation, and what acceptance evidence belongs in the package?
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Controls include the work permit, contamination surveys, fixative if needed, HEPA negative air, vacuum-shrouded blasting, personal dosimetry, and waste segregation. Prevent spread and bag spent abrasive. QC hold points should include the pre-cleaning survey, chloride/pH/moisture tests after washing, CSP profile comparison, visual inspection, and final cleanliness. Record test locations, methods, calibration, and results. If any reading exceeds limits, do not coat; repeat cleaning.
The package should show the original failure cause, extent of removal, environmental conditions during preparation, and that the new coating was applied over verified sound concrete. Because contamination limits access, independent verification and photographic or video evidence are useful. Acceptance depends on all criteria, not just profile.