ALARA and job planning

Time, distance, shielding — in order of cost

The three controls are usually recited as equals. They are not. Distance is free and acts as the square: stepping from one metre to two cuts the dose rate to a quarter, and the same job can then be done in four times the time for the same dose. Time costs schedule but no money. Shielding costs money, weight, and the handling dose incurred putting it in place — which is why a lead blanket installed to save 50 µSv can cost more dose than it saves.

This ordering is why a planning calculation should start by asking how far back the work can be done, and only then how long it will take and what needs shielding.

Where inverse square stops working

The 1/d² law is a statement about a point source radiating into open space. It is an excellent approximation once you are several source dimensions away, and a poor one closer in. Near a line source — a contaminated pipe run, a loaded transfer line — the falloff is closer to 1/d. Near a large contaminated plane or a tank wall it is nearly flat, because moving back brings more of the surface into view at the same time as it increases the distance to each part of it. Backing away from a spill on the floor buys far less than the same step away from a sealed source.

The practical test is whether the distance you are moving to is large compared with the size of the source. If it is not, measure rather than calculate.

Stay time and its honest use

Stay time is the dose budget divided by the dose rate, and it is the simplest calculation in radiation protection. Its failure mode is not arithmetic but the assumption that the dose rate is constant. Rates change when a shield is opened, when a source is moved, when a worker leans in, or when the job goes differently than planned. A stay time computed from a single pre-job reading is a planning figure, not a control — the control is a dosimeter with an alarm.

Building in margin is normal practice: plan to a fraction of the budget so that an unexpected rate does not immediately consume it.

Collective dose, and what it hides

Collective dose sums the dose over everyone involved, in person-millisieverts. It is the right quantity for comparing two ways of doing the same job, and it is the quantity an ALARA programme is usually judged on. It also contains an incentive worth being aware of: because it adds people together, it can be reduced by using fewer workers for longer, which pushes individual doses up while the total falls.

Optimisation that only looks at the collective figure will drift in that direction. The correct practice is to check both — the collective total for comparing options, and the highest individual dose against the individual constraint. The per-worker figure alongside the total in this calculator is there for that reason.

Half- and tenth-value layers

A half-value layer halves the dose rate and therefore doubles the stay time; a tenth-value layer divides by ten. Both are quoted at a specific photon energy and depend strongly on it, and handbook values are broad-beam figures that already include scattered radiation. Narrow-beam values computed from attenuation coefficients alone are thinner and will underestimate the shielding a real installation needs.

Worked examples

How long a repair can take near a source

A valve has to be repaired 1 m from a pipe where the survey meter reads 1.8 mSv/h, and the job has a dose budget of 0.5 mSv per person. There is no single ALARA formula; the calculator chains two simple ones. The stay time is the budget divided by the dose rate, t = D / Ḋ, and the dose rate at another distance follows the inverse square law, Ḋ₂ = Ḋ₁ × (d₁/d₂)². Enter the rate in the “Stay time” mode, then use the “Distance” mode to ask where the rate falls to 0.5 mSv/h.

Stepping back from 1 m to 2 m quarters the dose rate and so quadruples the time available. Long-handled tools that keep the hands and body 1.9 m away would allow a whole hour on the same budget. The inverse square law holds only once the distance is several times the size of the source; for a long pipe, measure the rate again at the new position rather than trusting the calculation.

Collective dose for a three-stage job

The “Job dose” mode opens with a typical job: setup by 2 people for 0.5 h at 80 µSv/h, the main work by 3 people for 2 h at 250 µSv/h, and cleanup by 2 people for 1 h at 40 µSv/h. Collective dose is the sum over tasks of workers × hours × dose rate, in person·mSv.

The main work carries almost all of the dose, so that is where the planning effort belongs. Doing it from 2 m instead of 1 m (enter 62.5 µSv/h for that task) cuts the job total by 68%, more than halving every rate on the job would, and lowers each main worker's dose from 0.5 to 0.125 mSv. Shielding the setup and cleanup would hardly change the total. Check the individual doses as well as the sum: a low collective dose can still come from one person receiving most of it.

FAQ

How do I calculate the stay time in a radiation area?

Divide the dose you are prepared to accept by the dose rate where the work is done: t = D / Ḋ. With a 0.5 mSv budget at 1.8 mSv/h that is 16.67 minutes. Use a measured dose rate at the actual working position, and leave a margin for the time spent getting in and out.

Does doubling the distance really cut the dose rate to a quarter?

For a source that is small compared with the distance, yes: the inverse square law gives Ḋ₂ = Ḋ₁ × (d₁/d₂)². Close to an extended source such as a pipe, a tank or a contaminated floor the rate falls more slowly, so measure at the new position instead of relying on the law.

What is collective dose and what is it used for?

Collective dose is the sum of individual doses for a group, in person·mSv, here worked out as workers × hours × dose rate for each task. It is a planning tool for comparing ways of doing the same job. ICRP Publication 103 advises against using it to estimate health effects from very small doses spread over many people.

Which of time, distance and shielding should I try first?

Usually distance and time, because they cost the least: long-handled tools, rehearsing the job away from the source and moving people out of the area between steps. Shielding takes time and dose to put in place, so it pays off mainly for long jobs or high dose rates. The job-dose mode shows which task carries most of the collective dose, which is where any of the three matters most.

Is a dose under the legal limit automatically ALARA?

No. Dose limits are a ceiling, not a target. ALARA asks whether the dose could reasonably be lower, taking cost and practicality into account, and many organisations set lower dose constraints or investigation levels for that reason.