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 center, 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 the United States, NRC rules in 10 CFR 50.47 require two emergency planning zones around each power reactor: a plume exposure pathway zone of about 10 miles (16 km) radius, where sheltering, evacuation and KI are planned in detail, and an ingestion pathway zone of about 50 miles (80 km), where food and water protection is planned. States and counties prepare the off-site plans, which FEMA reviews. A 2023 NRC rule, 10 CFR 50.160, allows small modular reactors and other new technologies to size their emergency planning zone from analysis of their accidents, which may give a zone as small as the site boundary, relevant to the BWRX-300 planned at Clinch River. KI is offered to residents of the plume zone by the states.

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 centerline 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 behavior: 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 meters."""
    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 centerline
    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.

The EPA PAG Manual gives emergency worker guidelines of 50 mSv (5 rem) for all activities, 100 mSv (10 rem) to protect valuable property necessary for public welfare, and 250 mSv (25 rem) for life-saving or protection of large populations, with higher doses only for fully informed volunteers. Plant emergency plans translate these into turn-back doses for each task.

Classification and response

NRC-licensed plants classify events in four levels: an unusual event, an alert, a site area emergency and a general emergency, the last indicating actual or imminent core damage with potential for release. The licensee must notify state and local authorities within 15 minutes of declaring an emergency and the NRC immediately afterwards, and in any case within an hour. A general emergency triggers protective action recommendations for the public, typically evacuation or sheltering of nearby sectors.

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 center 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.