Airborne contamination and compromised respiratory protection

Priority order is people, then spread, then the area.

1. Stop and hold the workers at the step-off pad. Neither leaves the controlled area. The 1,800 and 400 Bq/cm² readings are 36 and 8 times the release limit; walking out would spread contamination through the building.
2. Have them doff outer clothing and respirators slowly, under survey, with the surveyor controlling each item. Bag and label all clothing as contaminated waste. Do not let them remove respirators and then touch their faces.
3. Frisk hands, face, hair and shoes; take nasal swabs before any washing of the face, then decontaminate by washing and re-survey. Nasal swabs are time-sensitive - once the worker blows their nose or showers, the evidence of intake is degraded.
4. Control the room: post barriers, restrict entry, keep ventilation running and inform the ventilation operator if the damaged filter affects exhaust filtration.
5. Notify the shift supervisor, the radiation protection manager and the station radiation safety officer, and start the event log with times.
6. Only then investigate the job and the prefilter change.

Taking the dosimeter reading (0.12 mSv) as reassurance is the common error - it measures external dose only and says nothing about intake.

The permit dose estimate assumed a protection factor of 10, so 25 DAC-hours would have been recorded as 2.5 DAC-hours. That assumption rests on a properly fitted and worn respirator. The statement that the straps were loose under the hood, combined with the contamination pattern and the alarm, invalidates it for that worker at least.

Conservative assessment: credit no protection factor and record the full 25 DAC-hours. At a typical committed effective dose conversion of roughly 20 µSv per DAC-hour for a mixed fission/activation product aerosol, that is on the order of 0.5 mSv committed effective dose - small compared with annual limits, but reportable and several times larger than the permit estimate.

Following actions:

• Nasal swabs, urine sample and, where available, a whole-body count for both workers; document sample times relative to exposure.
• Record the dose as a planned special or revised estimate with the actual protection assumed, and correct the permit dose record.
• Notify the radiation safety officer and assess whether the exposure meets the station's investigation and regulatory notification thresholds.
• Add the event to the dosimetry record so the worker's lifetime dose reflects the estimate, not the permit assumption.

Causes separate into job design, respiratory protection, and work practice.

• Job design: opening a ventilation housing containing a torn, loaded prefilter releases activity. The job should have been planned with a glovebag or local tent and HEPA vacuum, and the housing should have been surveyed before opening.
• Respiratory protection: a protection factor of 10 was assumed without confirming fit. Loose straps under a hood are a fitting failure, not bad luck; fit testing, a user seal check at the point of use, and taping the hood to the respirator facepiece should be mandatory for this job class.
• Supervision and pre-job brief: contractors working under a routine permit with no radiation protection presence during the filter change is a planning gap.
• Air monitoring: the CAM alarm functioned, but the job proceeded without a portable air sampler or continuous survey at the work location.

Corrective actions: revise the job hazard analysis and permit to require tenting, a portable sampler and a radiation protection technician present; require documented fit checks; retrain both workers and the supervising contractor; brief the shift on the event. Before work resumes, verify by a mock-up or a trial with a non-radioactive filter that the containment method works.