Dose Estimate for a Co-60 Valve Maintenance Job
A maintenance job requires two workers to refurbish a valve near a Co-60 source. The measured dose rate is 3.0 mSv/h at 1.0 m from the source. The work position is 0.75 m from the source, and the job is expected to take 3 hours with both workers present. The site administrative dose constraint for this job is 4 mSv per worker. Lead shielding is available; the lead half-value layer for Co-60 gamma radiation is 1.2 cm. The planning engineer proposes to do the job as-is because the workers can share the dose, and because the job is short. The radiation protection supervisor asks for an independent dose estimate and a decision.
1. Estimate the unshielded individual dose and determine whether the job meets the 4 mSv constraint.
Read the worked answer
The dose rate scales inversely with the square of distance. At 0.75 m: dose rate = 3.0 mSv/h × (1.0/0.75)^2 = 3.0 × 1.778 = 5.33 mSv/h. For a 3-hour job, the individual dose is 5.33 × 3 = 16.0 mSv per worker. This is four times the 4 mSv job constraint. Sharing the dose does not help unless the workers are present for different portions of the job; if both are present for the full 3 hours, each receives 16 mSv. The proposal is therefore not acceptable as-is, and the job must be redesigned before work starts.
2. Determine the lead thickness needed to meet the constraint, and state one additional ALARA action.
Read the worked answer
Required attenuation factor = 16.0/4.0 = 4.0. The number of half-value layers needed is n = log2(4.0) = 2.0, so the lead thickness is 2 × 1.2 = 2.4 cm. Using 2.4 cm gives approximately 4.0 mSv; a practical design would use 3.6 cm (three HVL), giving 16.0/8 = 2.0 mSv and margin for unexpected stay time. An additional ALARA action is to reduce occupancy time by pre-staging tools, rehearsing the job, and using remote or extended-handle tools so the workers can stand farther away. Distance and time are often more effective than adding shielding in a congested area; the final plan should combine shielding, time reduction, and distance, then re-estimate dose before issuing the work permit.