12.2 Radiological Safety: Time, Distance, Shielding, and Dose Limits

The ASNT NDT Level II radiographic general exam lists Radiological Safety Principles Review as official RT topic 5. The paper is asking whether you understand how dose is created and how it is controlled. It is not a substitute for a radiographer card, an industrial-radiography license, or the employer's operating and emergency procedures. Field radiography of gamma sources and X-ray machines is regulated by the US Nuclear Regulatory Commission (NRC) or by an agreement-state program that administers compatible rules. Dose numbers in this section are NRC 10 CFR Part 20 facts. They are not ASNT-published exam secrets, and they are not a shop's permission slip to freelance a source.

ALARA is the working rule, not the leftover dose

ALARA means as low as reasonably achievable. NRC builds it into the radiation-protection program (10 CFR 20.1003 and 20.1101). The idea is simple and easy to fail on an exam: staying under an annual limit does not make extra dose acceptable. If a longer source-to-restricted-area distance, a collimator, a shorter exposure, or a concrete wall would cut dose without wrecking the technique, you take that cut.

ALARA is implemented with three physical levers and a stack of administrative ones:

• Time — less time in the field, less dose.
• Distance — inverse-square drop in intensity.
• Shielding — half-value and tenth-value layers, collimators, camera shielding, vault walls.
• Administration — written procedures, posting, surveys, two-person field practice where the license requires it, training, and stop-work authority.

A Level II who treats the annual limit as a budget to spend is not practicing ALARA.

Time

Absorbed dose from a constant dose rate is dose rate × time. If a position reads 20 mR/h and you stand there for 15 minutes (0.25 h), you take about 5 mR. Cut the time in half and you cut that increment in half. Practical time controls in radiography:

• Set up cold — markers, cassettes, collimator aim, and barriers in place — before the source is cranked out or the X-ray tube is energized.
• Use the shortest exposure the technique sheet allows. Raising energy to shorten time can hurt contrast; that is a technique decision, not a safety shortcut you invent on the pad.
• Do not linger at the crank or the tube head after the exposure. Retract or de-energize, then survey.
• Plan the shot list so you are not walking back into a live field to “just move a letter.”

Time is the lever you always have, even when you cannot add a wall.

Distance and the inverse-square law

For a compact source in air, intensity falls with the square of distance:

I₂ / I₁ = (D₁ / D₂)² or I₂ = I₁ × (D₁ / D₂)²

Double the distance and intensity falls to one-fourth. Triple it and intensity falls to one-ninth. Move from 10 ft to 20 ft and a 100 mR/h field becomes 25 mR/h. Move to 30 ft and it becomes about 11 mR/h. That is why a few extra metres of rope often buy more protection than an argument about pocket-dosimeter ticks.

Inverse square assumes a point-like source and no extra absorber. A collimator, a wall, or scatter from a large part changes the field. You still use inverse square to estimate a boundary, then you survey the real boundary with a meter. Exam stems that give two distances and one dose rate want the square, not a linear guess and not “gamma does not care about distance.”

Distance also protects the public side of the rope. If the procedure sets a restricted-area boundary so that dose rate at the rope supports public-dose control (industrial radiography commonly posts on the order of 2 mR/h, tied to the 10 CFR 20.1301 “0.002 rem in any one hour” public-hour cap and the license), you calculate, then you walk the rope with a meter. Calculated feet that were never surveyed are not a boundary.

Shielding: HVL, TVL, lead, steel, and concrete

A half-value layer (HVL) is the thickness of a stated material that cuts a stated radiation to one-half. A tenth-value layer (TVL) cuts it to one-tenth. After n HVLs, remaining intensity is I₀ / 2ⁿ. After n TVLs it is I₀ / 10ⁿ. Two TVLs leave 1%. Three HVLs leave 12.5%. One TVL is about 3.3 HVLs of the same material at the same energy.

HVL and TVL are energy- and material-specific. Iridium-192, cobalt-60, selenium-75, and a 200 kV X-ray beam do not share a single lead thickness. Cobalt-60 is harder to shield than Ir-192. Concrete and steel are the usual vault materials; lead is common in collimators, camera heads, and local shields. A steel vessel wall is shielding for the far side and scatter for the near side. Do not memorize a single millimetre value as “the” HVL for all RT.

Shielding rules that show up on principles items:

• Put shielding between the source and people, not only between the source and the film.
• A collimator is directional shielding. It shrinks the useful beam, cuts scatter, and shrinks the controlled area. It is an ALARA tool, not a cosmetic tube.
• Camera tungsten/depleted-uranium shielding protects you only when the source is fully retracted and latched. A source in the guide tube is not “in the camera.”
• Never assume a wall is a TVL if you have not been told the material, thickness, and energy.

NRC 10 CFR 20 dose limits — attribute NRC

ASNT does not publish these numbers as a certification table. NRC 10 CFR 20.1201, 20.1208, and 20.1301 do. Learn them as regulator facts.

The occupational whole-body number candidates must not mix up is 5 rem TEDE per year, not 0.1 rem (that is the public annual cap) and not 50 rem (that is skin, extremity, or organ). Lens is 15 rem. Embryo/fetus is 0.5 rem for the pregnancy after written declaration — and ALARA still applies; the licensee also avoids a large pulse early in the pregnancy. A declared pregnancy does not erase the worker's own occupational limits; it adds the fetus limit.

Units on the exam: 1 rem = 10 mSv. 5 rem = 50 mSv. Stems may print either. Do not treat millirem and millisievert as interchangeable without the factor of ten.

Restricted, radiation, high-radiation, and very-high-radiation areas

These are NRC 10 CFR 20.1003 definitions, taught on every industrial-radiography principles review. They are about access and posting, not about whether a weld is acceptable.

A restricted area is a control class (the licensee decides who enters). A radiation area or high radiation area is a dose-rate class. Field radiography routinely creates a high radiation area near the collimator while the source is out. That is why barriers, flashing lights or alarms where required, and a survey after retraction exist. Do not stand in a high radiation area to hold a cassette.

Instruments: survey meter, pocket dosimeter, badge / TLD / OSL

Three tools, three jobs. Mixing them is a classic topic-5 miss.

Survey meter (ion chamber or Geiger-Müeller, as the procedure specifies). This is a dose-rate instrument. You use it to find the boundary, to confirm the source is in the camera after crank-in, to check a storage container, and to walk an emergency. A meter that was not on, was left in the truck, or was never function-checked is not a survey. After every gamma exposure, survey the camera, the guide tube, and the collimator. A retracted crank handle is not proof the source is home.

Pocket dosimeter (direct-reading ion chamber) or an electronic personal dosimeter. This is an immediate personal dose reading for the shift. Industrial-radiography licenses typically require it to be charged/zeroed, worn, and read. It tells you that something unexpected happened today. It is not the legal lifetime dose of record by itself.

Film badge, TLD (thermoluminescent dosimeter), or OSL (optically stimulated luminescence) dosimeter. This is the usual dose of record, processed by an accredited processor, worn on the trunk (and elsewhere if the procedure says so). It does not alarm. It does not replace the pocket dosimeter or the survey meter. You cannot read it in the field to decide whether the source is out.

Field radiography licenses also commonly require an alarm ratemeter that screams when dose rate jumps — a last-ditch warning that you walked toward a live source. That alarm is not a survey and not a badge.

Wear what the license and the procedure name, in the location they name. A badge in a lunchbox is not monitoring.

Field controls: collimation, barriers, crank-out, and the source you never pick up

Collimate the beam to the area of interest. A wide-open guide-tube shot paints a useless cone of radiation across a plant aisle and raises scatter. Collimation is safety and image quality at the same time.

Barriers are calculated, then surveyed, then posted with the signs the procedure and 10 CFR 20 require. A helper is not a barrier. A parked truck is a barrier only if you surveyed the far side and control the space. Do not start an exposure because “the night shift is thin.”

Crank-out source control (gamma camera):

1. Survey and set the area before the source moves.
2. Expose only when people are outside the boundary and communication is live.
3. Retract the source fully and lock the camera.
4. Survey the camera, tube, and collimator. If the meter is still hot, the source is not in the camera — treat it as an emergency, not as a film problem.
5. Never leave a camera unattended with the key in and the source usable.

Never pick up an unshielded source. A disconnected or stuck pigtail is a very high local dose-rate object. Hands, a rag, and bravery are not tongs, not a shielded pig, and not the emergency procedure. The correct action is withdraw, maintain control of the area, and follow the licensee's emergency procedure (which exists because NRC/agreement-state licenses require it). Topic 5 will offer “pick it up quickly because time is ALARA” as a trap. Time is ALARA for a surveyed field, not for a 100-curie pellet in your palm.

X-ray cabinets and vaults have interlocks and warning lights. Defeating an interlock to “save a setup” is not a Level II initiative. It is a license violation.

What the Level II exam is — and is not

A passing RT general paper shows you reviewed principles: ALARA, inverse square, HVL/TVL, NRC limit classes, instruments, and the rule that you do not handle an exposed source. Field radiography still requires:

• An NRC or agreement-state license (the company) and the local operating procedures written under that license (often 10 CFR Part 34 or the state equivalent for industrial radiography).
• Radiographer and assistant radiographer credentials the license recognizes — commonly a state card or ASNT IRRSP (Industrial Radiography Radiation Safety Personnel). That card is a radiation-safety credential. It is not the same document as ASNT NDT Level II.
• The employer's emergency procedure, leak-test and maintenance rules, and two-person practice where required.

If a stem asks whether NDT Level II certification authorizes you to operate a camera on a pipeline right-of-way, the answer is no. If it asks whether the general exam replaces 10 CFR 20 compliance, the answer is no. Principles review makes you a safer interpreter and a safer assistant. It does not print a radiographer card.

Work topic-5 items by naming the lever (time, distance, shielding), naming the instrument (rate versus personal versus record), and naming the regulator when a rem number appears. Guessing “ASNT says 5 rem” is the wrong attribution even when the number is right.