Internal dose

External dose stops the moment a worker walks away from the source. Internal dose does not. Once a radionuclide is inside the body, it irradiates tissue until it decays or is excreted, and the worker cannot step back from it. The lesson begins with a case Marcel handled years ago: a mechanic who removed a respirator early to wipe sweat from his face during work on a contaminated pump, and whose routine bioassay the following week showed an intake. The dose was small, but the investigation took a month and changed the station's respirator procedure. Internal dosimetry is how such intakes are measured, assessed and, above all, prevented.

How radionuclides get in

There are four routes of intake. Inhalation of airborne particles, gases or vapors is the most important at nuclear facilities. Ingestion, usually from contaminated hands or food, is uncommon at work but important for the public after accidental releases. Absorption through intact skin is significant for a few substances, above all tritiated water vapor, which is taken up through the skin at a rate comparable with breathing it. Entry through wounds can give a direct and sometimes large intake and requires immediate medical and radiological follow-up.

After intake, the material's fate depends on its chemical form and particle size. Inhaled particles deposit in different parts of the respiratory tract according to their size; small particles reach the deep lung. Soluble material passes quickly into the blood; insoluble material may stay in the lungs for months or years. From the blood, each element goes where its chemistry takes it: iodine to the thyroid, strontium and radium to bone, cesium throughout the soft tissues, and tritiated water into all body water. Excretion through urine, feces, sweat and breath removes it at rates described by biological half-lives.

Effective half-life

A radionuclide in an organ disappears by two processes at once: radioactive decay, with physical half-life \(T_p\), and biological elimination, with biological half-life \(T_b\). If both are exponential, the effective half-life is

For tritium the biological half-life controls everything, because the physical half-life is so long. Drinking more fluids shortens the biological half-life of tritiated water by speeding the turnover of body water, which is why workers with a tritium uptake are sometimes advised to increase their fluid intake.

Committed dose and dose coefficients

The committed effective dose \(E(50)\) is the total effective dose delivered over 50 years by an intake, assigned to the year of intake as lesson 2 explained. In practice it is calculated by multiplying the intake \(I\), in becquerels, by a dose coefficient \(e(50)\), in sieverts per becquerel, which the ICRP publishes for each radionuclide, chemical form, route and particle size:

The dose coefficient for tritiated water is a thousand times smaller than for iodine-131, because tritium's beta particles are so weak and it spreads evenly through the body rather than concentrating in one organ. A tritium intake must be very large to give a significant dose, but at a CANDU station the inventory of tritium in the heavy water systems is large, so tritium is still the main internal hazard.

Annual limits on intake and derived air concentrations

Two derived quantities make internal limits practical. The annual limit on intake (ALI) is the intake that would give a committed dose equal to the annual limit, \(\text{ALI} = 0.02\ \text{Sv} / e(50)\) if a 20 mSv reference is used, or 0.05 Sv for the full annual limit. The derived air concentration (DAC) is the concentration in air that, breathed by a reference worker for a working year of 2000 hours at 1.2 m³/h, would give one ALI:

In the United States, Appendix B of 10 CFR Part 20 tabulates ALIs and DACs for each radionuclide, based on ICRP Publication 30. Exposure to one DAC for one hour, one DAC-hour, corresponds to 1/2000 of the ALI, about 0.025 mSv (2.5 mrem) of committed effective dose equivalent when the ALI is based on the 5 rem stochastic limit. An area where concentrations could exceed 1 DAC, or 12 DAC-hours in a week, is an airborne radioactivity area under 20.1003 and must be posted.

# Internal dose: effective half-life, committed dose, a tritium urine estimate and DAC-hours.
import math

def t_eff(tp, tb):
    return 1 / (1 / tp + 1 / tb)

for name, tp, tb in (('tritium (HTO)', 12.3 * 365.25, 10.0), ('I-131 thyroid', 8.02, 80.0), ('Cs-137', 30.1 * 365.25, 110.0)):
    print(f"{name:14s}: effective half-life {t_eff(tp, tb):7.1f} days")

# Committed dose = intake x dose coefficient (approximate ICRP values, Sv/Bq)
e50 = {'HTO': 1.8e-11, 'I-131 ingestion': 2.2e-8, 'Cs-137 ingestion': 1.3e-8}
for k, e in e50.items():
    print(f"1 MBq of {k:17s} -> {1e6*e*1000:7.3f} mSv committed")

# Tritium urine result: body water about 42 L, urine concentration equals body water concentration
C_urine = 120e3            # Bq/L
body = C_urine * 42.0      # Bq
E = body * e50['HTO']
print(f"urine {C_urine/1e3:.0f} kBq/L -> body content {body/1e6:.1f} MBq -> about {E*1000:.3f} mSv")
# Dose still to come falls with the 10-day effective half-life; faster turnover shortens it
for tb in (10.0, 5.0):
    print(f"  biological half-life {tb:4.1f} d -> committed dose {body*e50['HTO']*tb/10*1000:.3f} mSv (scaled)")

# DAC-hours: 1 DAC-h = 1/2000 ALI; with ALI tied to 50 mSv this is 0.025 mSv
dac_hours = 3 * 4.0 + 0.5 * 6.0       # 3 DAC for 4 h, then 0.5 DAC for 6 h
print(f"exposure {dac_hours:.0f} DAC-h -> {dac_hours * 50.0 / 2000:.2f} mSv committed")

tritium (HTO) : effective half-life    10.0 days
I-131 thyroid : effective half-life     7.3 days
Cs-137        : effective half-life   108.9 days
1 MBq of HTO               ->   0.018 mSv committed
1 MBq of I-131 ingestion   ->  22.000 mSv committed
1 MBq of Cs-137 ingestion  ->  13.000 mSv committed
urine 120 kBq/L -> body content 5.0 MBq -> about 0.091 mSv
  biological half-life 10.0 d -> committed dose 0.091 mSv (scaled)
  biological half-life  5.0 d -> committed dose 0.045 mSv (scaled)
exposure 15 DAC-h -> 0.38 mSv committed

Bioassay: measuring what is inside

Internal dose cannot be measured directly; it is inferred from measurements of what is in the body or what leaves it. In vivo measurements count gamma rays from outside the body with whole-body counters, lung counters or thyroid probes, using the detectors described in NUC-310. They work well for gamma emitters such as cobalt-60, cesium-137 and iodine-131. In vitro measurements analyze urine, feces or breath; they are needed for pure beta and alpha emitters, including tritium, carbon-14, strontium-90 and the actinides. Tritium in urine is measured by liquid scintillation counting, and at CANDU stations routine urine sampling, often weekly or more frequently for workers in tritium areas, is the backbone of internal dosimetry.

Interpreting a bioassay result means working backwards from a measurement on one date to the intake that caused it, using a biokinetic model that predicts the fraction of an intake present in the body, or excreted per day, at each time after intake. The answer depends on when the intake happened, whether it was a single event or chronic, and the chemical form. Uncertainty is far larger than for external dosimetry, often a factor of two or more for insoluble materials. Investigation levels in the bioassay program trigger follow-up sampling and a formal dose assessment.

Preventing intakes

The hierarchy of controls applies just as it does for any hazard. First, contain the material: closed systems, glove boxes, tents with negative pressure around open work, and drying of heavy water systems before maintenance. Second, ventilate: local exhaust at the work point, airflow from clean areas toward contaminated ones, and filtration with HEPA filters. Third, control contamination on surfaces, as lesson 8 describes. Last, protect the individual: respirators for particulates, and for tritium, plastic suits supplied with breathing air, because tritiated water vapor passes through ordinary filters and skin alike.

At TVA plants, tritium is a smaller internal hazard than at CANDU stations, but air sampling during work on open reactor coolant systems, steam generator work and spent fuel pool activities checks particulate and iodine concentrations, and respirator use follows 10 CFR 20 Subpart H.

Respirators are not free. They reduce comfort and vision, slow work and increase heat stress, so they can raise external dose by lengthening a job. The ALARA balance between external and internal dose is one of the most common practical decisions in radiation protection, and regulations in both countries allow licensees to consider total effective dose when deciding whether a respirator is justified.

Internal dose to the public

The same methods apply to members of the public, with dose coefficients for different age groups. After a release, people may inhale radionuclides from the passing cloud and later ingest them in food and water. Dose coefficients for infants and children are higher per becquerel than for adults for many nuclides, especially iodine-131, because their thyroids are small and active. Environmental monitoring programs, covered in NUC-310, measure concentrations in air, water, milk and produce, and dose assessment combines them with consumption rates and dose coefficients to estimate public dose.

The EPA drinking water standard for tritium is 20 000 pCi/L, about 740 Bq/L, derived from an older dose model; concentrations in drinking water near United States plants are typically far lower.

When an intake is suspected

If an intake is suspected, for example after a respirator failure, a contamination on the face or a wound in a contaminated area, the response is prompt. Nasal swabs and facial smears give a quick indication of inhalation; a special bioassay sample, or an in vivo count for gamma emitters, is arranged within hours; and for wounds, the wound is flushed, the activity measured, and a physician consulted about decontamination and, for some radionuclides, medical treatment to speed excretion. The dose assessment that follows records the intake, the method, the assumptions and the uncertainty, and the worker is told the result and what it means.

Marcel's mechanic from the opening of this lesson received a committed dose of about 0.3 mSv from inhaled cobalt-60 oxide. The station changed its procedure so that respirators are removed only at the step-off pad after outer clothing, and installed a cool-air supply for jobs in hot areas so that workers would not be tempted to break the seal of a mask to wipe sweat away.