11.3 Radiation Safety, Applications, Capabilities & Limitations

Working Safely with Ionizing Radiation

RT is the only common NDT method whose hazard can be lethal, so a Level III must know dose limits, the ALARA principle, and area-control rules as thoroughly as the inspection technique itself. In the United States these are enforced by the NRC under 10 CFR Part 20 (or by Agreement-State equivalents).

Dose Units and Limits

Radiation quantities use both conventional and SI units.

The sievert/rem apply a quality factor to reflect biological harm, whereas the gray/rad measure only the energy deposited. 10 CFR 20.1201 sets these annual occupational limits:

• Total effective dose equivalent (TEDE): 5 rem (50 mSv) per year
• Lens of the eye: 15 rem (150 mSv) per year
• Skin and extremities: 50 rem (500 mSv) per year
• Members of the public: 0.1 rem (1 mSv) per year

ALARA - Time, Distance, Shielding

ALARA ('As Low As Reasonably Achievable') is implemented through three levers:

1. Time - dose is proportional to exposure time, so minimize the time spent near the source.
2. Distance - the inverse-square law makes distance the most powerful and cheapest control; doubling the distance quarters the dose rate.
3. Shielding - interpose dense, high-Z material (lead, concrete, or depleted uranium). Barrier thickness is sized in half-value or tenth-value layers.

Survey Meters and Personnel Monitoring

Every radiographer carries three instruments: a survey meter (ion chamber or Geiger-Muller) to read dose rate and confirm the source is shielded after each exposure; a pocket dosimeter or electronic alarming dosimeter for an immediate cumulative reading; and a film badge or TLD/OSL badge for the legal dose-of-record. A rate alarm (chirper) warns the worker when entering a high field. The survey meter is used to verify the source has fully returned to its shielded position - a stuck source is the classic overexposure accident.

Controlled and High-Radiation Areas

Regulations grade areas by dose rate:

• Radiation area: could exceed 5 mrem/hr at 30 cm.
• High radiation area: could exceed 100 mrem/hr at 30 cm.
• Very high radiation area: could exceed 500 rad/hr at 1 m.

Boundaries must be posted and roped off; the radiographer sets the restricted boundary where the dose rate drops below the regulatory limit (commonly 2 mrem/hr for an unrestricted area).

Worked inverse-square safety example: a survey meter reads 80 mR/hr at 3 ft from an exposed source. What is the rate at 6 ft? The distance doubles, so intensity falls to 1/(2^2) = 1/4: 80 / 4 = 20 mR/hr. To find where the rate reaches the 2 mR/hr boundary, solve 80 x (3/d)^2 = 2, giving (3/d)^2 = 0.025 and d = 3 / sqrt(0.025) = about 19 ft. This is exactly how radiographers rope off a restricted boundary before a shot.

Applications

RT is a volumetric method - it examines the full through-thickness of a part, making it ideal for internal, three-dimensional discontinuities: porosity, slag and tungsten inclusions, voids, shrinkage, incomplete penetration, and internal weld defects in castings, weldments, and pipe. It is a workhorse for boiler and pressure-vessel welds (ASME), pipelines (API 1104, evaluated over any 12-in / 300-mm weld length), and structural welds (AWS D1.1, which permits no cracks at all).

Capabilities

• Produces a permanent, archivable record (film or digital file) that others can review later.
• Detects subsurface / buried discontinuities that surface methods (PT, MT) cannot reach.
• Works on most materials - metals, composites, and plastics - and on complex assemblies.
• Yields a real image showing the type, size, and location of internal flaws.

Limitations

• Orientation-sensitive: RT easily misses tight, planar discontinuities (cracks, laminations, lack of fusion) lying roughly perpendicular to the beam, because they change the through-thickness very little. Such cracks are found reliably only when the beam is nearly parallel to their plane (contrast with UT, which excels at planar cracks).
• Requires access to both sides - source on one side, detector on the other.
• The radiation hazard demands licensing, trained personnel, exclusion zones, and often a production shutdown.
• Slow and costly compared with PT, MT, or VT; film also needs chemical processing.
• Thickness-limited by source energy - very thick sections need Co-60 or a high-energy linac.
• Poor at measuring the depth of a discontinuity, since a radiograph is a two-dimensional shadow of a three-dimensional object.

Biological Effects and Program Controls

A Level III should recognize two categories of radiation harm. Deterministic (non-stochastic) effects - skin burns, cataracts, radiation sickness - have a threshold dose and grow more severe as dose rises; they drive the extremity and eye-lens limits. Stochastic effects - cancer and genetic damage - are assumed to have no threshold, and their probability (not severity) increases with dose, which is why every dose is kept ALARA even below the legal limits. A radiation-safety program therefore layers engineering controls (collimators, shielded projectors, interlocks), administrative controls (procedures, surveys, restricted-area postings, and training), and personnel dosimetry, all overseen by a Radiation Safety Officer (RSO) under the facility's NRC or Agreement-State license. Radiographers must also plan source-retrieval steps in advance, because a source that fails to fully retract creates an immediate high-radiation area that the survey meter must catch before anyone approaches.