Emergency preparedness

Radiation protection is mostly about routine work done carefully. Emergency preparedness is about the rare day when it is not routine. For this session the class joins an exercise at Meridian Nuclear Engineering's emergency operations centre, where Dr. Vasquez acts as the dose assessment lead and Hana runs the plume projection program. The scenario is a severe accident at a reactor unit with a release of noble gases and iodine to the atmosphere. The exercise tests whether the people inside the station, the off-site authorities and the public would be protected if it ever happened for real.

What can go wrong

Radiological emergencies range from small and local to very large. A spill of a radioactive solution in a laboratory, a radiography source stuck outside its container, a lost sealed source, a transport accident, a fire in a building storing radioactive material and, at the far end, a severe reactor accident with damage to the fuel and failure of containment. Each needs a plan scaled to the hazard. The principles are the same: protect people from deterministic effects at once, reduce stochastic risk as far as reasonably possible, and do not cause more harm with the response than the radiation itself would cause.

The lessons of past accidents shape modern plans. At Chernobyl in 1986 the lack of timely protective action and the failure to restrict contaminated milk led to many thyroid cancers in children from radioiodine. At Fukushima Daiichi in 2011 the evacuation itself, especially of elderly and hospital patients, caused deaths and great hardship, while the radiation doses to the public were mostly low. Both events show that protective actions must be prompt where they matter, but proportionate and well planned, with attention to vulnerable people.

Protective actions

Iodine thyroid blocking with potassium iodide (KI) is effective only for radioiodine and only if taken close to the time of exposure, ideally a few hours before or at its start; taken a day later it gives little protection. That is why plans near nuclear plants pre-distribute KI tablets to households, schools and businesses, so that they are on hand when an instruction is given. KI does nothing against other radionuclides or external dose.

Planning zones

In Ontario, the Provincial Nuclear Emergency Response Plan (PNERP), with implementing plans for each station including Darlington, divides the area around a nuclear power plant into zones. An automatic action zone close to the plant, of about 3 km, has protective actions taken by default from plant conditions. A detailed planning zone of about 10 km, which includes it, has detailed arrangements for sheltering, evacuation and iodine thyroid blocking. A contingency planning zone of about 20 km covers very unlikely severe accidents, with no default measures, and an ingestion planning zone of about 50 km protects food and drinking water. Under the CNSC's REGDOC-2.10.1, KI tablets are pre-distributed to residences and institutions in the detailed planning zone and made available to residents of the wider zone on request.

Intervention levels

Decisions to shelter, evacuate or distribute KI are guided by projected doses, the dose that would be received if no action were taken. Because there is uncertainty in any projection and the actions themselves carry risks and costs, the criteria are set in terms of dose averted or projected over a period, and are guides for decision makers rather than limits.

Projecting doses from a release

In an emergency, dose assessment staff estimate where the release will go and what doses it will give, using the source term from the plant, weather data and atmospheric dispersion models. The simplest model is the Gaussian plume. For a continuous ground-level release of \(Q\) becquerels per second in a wind of speed \(u\), the air concentration on the plume centreline at ground level a distance \(x\) downwind is

where \(\sigma_y\) and \(\sigma_z\) are the horizontal and vertical spreads of the plume, which grow with distance and depend on atmospheric stability. Multiplying the time-integrated concentration by a breathing rate and an inhalation dose coefficient gives the internal dose; multiplying it by a cloud-immersion dose coefficient gives the external dose from noble gases. Real systems use more advanced models with terrain, changing weather and deposition, but the Gaussian plume shows the key behaviour: concentrations fall roughly as the inverse of distance to a power between 1.5 and 2, and stable night-time weather with light winds gives the highest doses at a distance.

# Gaussian plume projection for a ground-level release (illustrative exercise values).
import math

def sigmas(x):
    """Briggs open-country spreads for neutral (class D) conditions, x in metres."""
    sy = 0.08 * x / math.sqrt(1 + 0.0001 * x)
    sz = 0.06 * x / math.sqrt(1 + 0.0015 * x)
    return sy, sz

Q = 1e11          # Bq/s of I-131
u = 3.0           # m/s
T = 7200.0        # s, release duration
BR = 3.3e-4       # m3/s adult breathing rate (light activity)
DCF_THY = 1.5e-7  # Sv/Bq thyroid equivalent dose per Bq inhaled (illustrative)
KI_CRIT = 0.050   # Sv thyroid, approximate criterion for iodine thyroid blocking

for x in (1000, 3000, 5000, 10000, 20000):
    sy, sz = sigmas(x)
    chi = Q / (math.pi * sy * sz * u)          # Bq/m3 on the centreline
    H = chi * T * BR * DCF_THY                 # Sv to the adult thyroid
    rec = 'KI and shelter or evacuate' if H > KI_CRIT else 'below KI criterion'
    print(f"x = {x/1000:5.1f} km  sy = {sy:6.0f} m  sz = {sz:5.0f} m  chi = {chi:9.2e} Bq/m3  "
          f"thyroid {1000*H:7.1f} mSv  {rec}")

# Doubling the wind speed halves the concentration
sy, sz = sigmas(3000)
print("ratio chi(u=6)/chi(u=3) =", round((Q/(math.pi*sy*sz*6)) / (Q/(math.pi*sy*sz*3)), 2))

x =   1.0 km  sy =     76 m  sz =    38 m  chi =  3.67e+06 Bq/m3  thyroid  1306.4 mSv  KI and shelter or evacuate
x =   3.0 km  sy =    210 m  sz =    77 m  chi =  6.57e+05 Bq/m3  thyroid   234.1 mSv  KI and shelter or evacuate
x =   5.0 km  sy =    327 m  sz =   103 m  chi =  3.16e+05 Bq/m3  thyroid   112.5 mSv  KI and shelter or evacuate
x =  10.0 km  sy =    566 m  sz =   150 m  chi =  1.25e+05 Bq/m3  thyroid    44.6 mSv  below KI criterion
x =  20.0 km  sy =    924 m  sz =   216 m  chi =  5.33e+04 Bq/m3  thyroid    19.0 mSv  below KI criterion
ratio chi(u=6)/chi(u=3) = 0.5

Emergency workers

People who respond to an emergency, plant staff, firefighters, police and ambulance crews, may need to accept higher doses to save lives, prevent a severe accident from getting worse or protect large populations. Plans set guidance for those doses, require that volunteers be informed of the risks, and provide dosimetry, protective equipment, KI and briefings.

Under section 15 of the Radiation Protection Regulations, the effective dose to persons taking part in the control of an emergency is limited to 500 mSv, and the equivalent dose to the skin to 5000 mSv, except for actions to save lives, where informed volunteers may exceed it. Station plans set lower turn-back doses for most tasks.

Classification and response

Station emergency plans classify events into graded categories so that the right level of response is called at once, and notify the province, which leads the off-site response under the PNERP, and the CNSC. The provincial emergency operations centre directs public protective actions, informed by the station's dose projections and by field monitoring teams who measure dose rates and collect air, water and soil samples.

Exercises are where plans are proved. The Meridian Nuclear Engineering exercise ends with a debrief in which Hana's group reports that the plume program gave its first projection eighteen minutes after the release started, but that the weather feed lagged by half an hour and the initial projection had to be corrected when field teams reported dose rates twice the prediction in one sector. The corrective action is to move field team deployment earlier in the procedure. That kind of finding, small and concrete, is exactly why exercises are held.

Public information

In every emergency, people need clear, consistent and timely information: what has happened, what they should do and why. Plans therefore include pre-written public messages, arrangements for a joint information centre where the licensee and the authorities speak with one voice, and outreach before any emergency so that residents near a plant know how they will be alerted and what sheltering means. After Fukushima, surveys showed that fear and distrust caused much of the harm; honest communication about doses, including saying plainly when doses are low, is part of protection.

Recovery after an emergency is planned too. Once a release has ended, monitoring maps the deposited activity, food and water restrictions are applied or lifted on the basis of measurements, and decisions on relocation and return balance the dose averted against the harm of keeping people away from their homes. International guidance now treats recovery as an existing exposure situation, with reference levels, typically in the range of 1 to 20 mSv per year, used to guide cleanup rather than as limits.

For the Meridian Nuclear Engineering class, the exercise's final lesson is personal: each of them now has a defined emergency role, a dosimeter and a briefing card, and knows where to report if the real alarm ever sounds.