Radiation and health

The question that drew the most hands at the information session was the simplest one: is it safe to live near a nuclear plant? Hana passes it to Lina Morales, Meridian's radiation protection specialist, who has spent twenty years measuring doses at reactors, hospitals and industrial sites. Lina's view is that the honest answer needs numbers, and that people handle numbers well when they are given a sense of scale. This lesson builds that sense of scale.

Ionizing radiation

The radiation from radioactive decay and fission is ionizing: each particle or photon carries enough energy to knock electrons out of atoms, creating ions. Visible light, radio waves and the microwaves in a kitchen oven do not carry enough energy per photon to do this. In living tissue, ionization can break chemical bonds, including those in DNA. Cells repair most such damage quickly, but a small fraction of damage is repaired incorrectly, and over years this can occasionally lead to cancer. Very large doses kill cells outright and cause the rapid effects known as radiation sickness.

The four main kinds of ionizing radiation behave differently. Alpha particles deposit their energy densely over a very short path and cannot pass through skin, so they matter mainly if alpha emitters are taken into the body. Beta particles travel a little further and can affect skin and the lens of the eye. Gamma rays and x-rays penetrate the whole body. Neutrons, present around operating reactors, also penetrate deeply and are especially damaging per unit of energy. Shielding for each type must be chosen to match.

Measuring dose

The absorbed dose is the energy deposited per kilogram of tissue, measured in grays (Gy), one joule per kilogram. Because different radiations cause different amounts of biological harm for the same energy, the equivalent dose multiplies the absorbed dose by a radiation weighting factor: 1 for gamma, x-rays and beta, 20 for alpha, and between about 2.5 and 20 for neutrons depending on their energy. The effective dose then weights each organ by its sensitivity and adds them up, so that a partial-body exposure can be compared with a uniform one. Both are measured in sieverts (Sv). Everyday doses are small, so the millisievert (mSv), one thousandth of a sievert, is the usual unit.

Here \(D\) is the absorbed dose in grays, \(w_R\) is the radiation weighting factor, \(H\) is the equivalent dose in sieverts, \(w_T\) is the tissue weighting factor and \(E\) is the effective dose. Radiation protection uses effective dose to set limits and compare exposures.

Doses in everyday life

Everyone is exposed to natural radiation all the time: cosmic rays from space, radon gas from the ground, potassium-40 and carbon-14 in the body, and uranium and thorium in rock and building materials. Natural background in Canada averages about 1.8 mSv a year, varying with geology and altitude; the world average is about 2.4 mSv. Medical imaging adds a significant amount for many people: a chest CT scan gives about 7 mSv.

Against that background, the dose to someone living next to an operating nuclear plant from the plant's routine releases is typically well below 0.01 mSv a year, less than one percent of the natural background, and less than the extra cosmic radiation from a single long-haul flight. Plants are licensed by the Canadian Nuclear Safety Commission (CNSC) with release limits set so that the dose to the most exposed member of the public stays far below the public dose limit of 1 mSv a year from licensed activities, and actual doses are a small fraction of that limit.

Health effects and risk

Radiation effects fall into two groups. Deterministic effects, also called tissue reactions, occur only above a threshold dose, and their severity rises with dose. Skin reddening needs a few grays to the skin; acute radiation syndrome begins above about 1 Gy to the whole body, received in a short time; a dose of about 4 to 5 Gy received rapidly is fatal to about half of those exposed without medical treatment. Such doses occur only in serious accidents or mishandled high-activity sources, and almost never to the public.

Stochastic effects, mainly cancer, have no clear threshold; the probability rises with dose, but the severity does not. Studies of the survivors of the 1945 atomic bombings, of patients given radiation treatment and of workers show a clear increase in cancer risk above about 100 mSv. For radiation protection, the International Commission on Radiological Protection (ICRP) assumes a linear no-threshold relationship below that: a small risk proportional to dose, about 5 percent per sievert for fatal cancer in a general population. The assumption is deliberately cautious and widely used for planning, though the true effect of very low doses is too small to measure directly.

Dose limits and ALARA

Radiation protection rests on three principles. Justification: any practice involving radiation must do more good than harm. Optimization: doses must be kept as low as reasonably achievable, a principle known as ALARA, taking costs and benefits into account. Limitation: individual doses must not exceed set limits. Under the Nuclear Safety and Control Act and the regulations made under it, the CNSC sets the limits that apply to nuclear sites in Canada.

In practice, ALARA matters more than the limits. Most nuclear plant workers receive less than 1 mSv a year from their work, and plants set internal targets well below the regulatory limits. Every job in a radiation area is planned in advance: Lina's team surveys the area, estimates the dose, chooses shielding and tools, rehearses the task if needed and issues each worker a dosimeter that records the actual dose.

Time, distance and shielding

Three simple tools control external dose. Spend less time near the source, and the dose falls in proportion. Increase the distance, and for a small source the dose rate falls with the square of the distance, the inverse square law. Put shielding between the source and the person, and each half-value layer of shielding halves the dose rate.

The inverse square law follows from geometry: radiation spreading out from a point covers a sphere whose area grows as \(r^2\). For gamma rays from cobalt-60, the half-value layer is about 1.2 cm of lead, about 2 cm of steel or about 6 cm of concrete. The program applies these rules to plan a maintenance task.

# Time, distance and shielding: dose from a small gamma source.
import math
# A point source gives 2.0 mSv/h at 1 m (illustrative industrial source)
D1 = 2.0
print("Dose rate against distance (inverse square law)")
for r in (0.5, 1, 2, 4, 10):
    print(f"  {r:4.1f} m: {D1 / r**2:7.3f} mSv/h")

# Shielding with half-value layers (HVL): each HVL halves the dose rate
hvl_lead_cm = 1.2       # approximate HVL of lead for cobalt-60 gamma rays
for t_cm in (0, 1.2, 2.4, 6.0):
    print(f"Behind {t_cm:3.1f} cm of lead at 2 m: {D1/4 * 0.5 ** (t_cm/hvl_lead_cm):.4f} mSv/h")

# Planning a task: stay below 0.1 mSv for one worker
limit = 0.1
for r in (1, 2, 3):
    rate = D1 / r**2
    print(f"At {r} m the worker may stay {limit/rate*60:.0f} minutes")

# Annual dose compared with public and worker limits
background = 1.8         # mSv per year, rough average natural background
print(f"\nNatural background: {background} mSv/yr; public limit from licensed sources: 1 mSv/yr")
print("Occupational limit: 50 mSv in any one year")

Dose rate against distance (inverse square law)
   0.5 m:   8.000 mSv/h
   1.0 m:   2.000 mSv/h
   2.0 m:   0.500 mSv/h
   4.0 m:   0.125 mSv/h
  10.0 m:   0.020 mSv/h
Behind 0.0 cm of lead at 2 m: 0.5000 mSv/h
Behind 1.2 cm of lead at 2 m: 0.2500 mSv/h
Behind 2.4 cm of lead at 2 m: 0.1250 mSv/h
Behind 6.0 cm of lead at 2 m: 0.0156 mSv/h
At 1 m the worker may stay 3 minutes
At 2 m the worker may stay 12 minutes
At 3 m the worker may stay 27 minutes

Natural background: 1.8 mSv/yr; public limit from licensed sources: 1 mSv/yr
Occupational limit: 50 mSv in any one year

Internal exposure and contamination

Radioactive material taken into the body, by breathing, swallowing or through a wound, irradiates tissue from inside until it decays or is excreted. The dose depends on the nuclide, its chemical form and how long the body retains it. Iodine concentrates in the thyroid, which is why people near the Chernobyl and Fukushima accidents were advised to take stable potassium iodide tablets, which fill the thyroid with ordinary iodine and block the uptake of radioactive iodine. Caesium spreads through soft tissue and leaves the body over a few months. Protection against internal exposure relies on containment, ventilation, protective clothing and respirators, and on monitoring of air, surfaces and people.

Monitoring and emergency planning

Around every nuclear plant, environmental monitoring programmes sample air, water, soil, milk and fish, and fixed gamma monitors record the dose rate continuously. The results are published, and the CNSC runs its own independent sampling. For Lina, this is the most persuasive answer to the question about living nearby: people can look up what has been measured, not just what has been predicted. Measurements around Darlington show nothing that can be distinguished from natural background beyond the plant boundary.

Emergency planning is the last layer of protection. Plans set out how the public would be warned and, if needed, told to shelter indoors, take potassium iodide tablets or evacuate. In Ontario, residents within 10 km of a nuclear plant receive potassium iodide tablets in advance, and those within 50 km can order them free of charge. The aim is that, even in a severe accident, doses to the public stay low enough that deterministic effects do not occur and the added cancer risk is minimized. Lesson 10 shows that, at Fukushima, the harm from a hurried evacuation of elderly and vulnerable people was greater than the harm from the radiation itself, and that planning now pays close attention to that balance.