Time, Distance, Shielding
Time, distance, and shielding are the three fundamental controls used to reduce radiation exposure: minimize the time near the source, maximize the distance from it (inverse-square law), and place shielding between the source and personnel.
Definition
The TDS trinity is the practical implementation of ALARA. Halving exposure time halves the dose; doubling distance reduces dose to one-quarter; appropriate shielding (lead, concrete, steel) can attenuate by orders of magnitude.
Related terms
- ALARA (As Low As Reasonably Achievable) — ALARA is the regulatory principle requiring that radiation exposures be kept "as low as reasonably achievable", applied through the trinity of time, distance, and shielding controls to minimize dose to workers and the public.
- Controlled Area — A controlled area is a defined zone around a radiation source where access is restricted and monitored to ensure dose rates remain below regulatory limits, established before any radiography operation begins.
- Half-Value Layer (HVL) — The half-value layer (HVL) is the thickness of a specified material required to reduce the intensity of a beam of radiation by half, used to characterize radiation penetration and design shielding.
More safety terms
Where this comes up in practice
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.