Radiation protection 101 for Fukushima 2

[trottiey]

There are two types of exposure to radiation: internal and external. External exposure refers to direct irradiation, like exposure to the sun. If you can shield it or walk away, the external exposure stops. Internal exposure refers to inhaling or ingesting radioactive contaminants, or otherwise absorbing them into your body. These will stay with you until your body can eliminate them, and they may come extremely close to your cellular DNA. Very small amounts of radioactive contamination that poses no external risk can become life-threatening if it becomes internal. Conversely, intense radioactive sources pose no internal risk as long as they are contained in a sealed container, but could be life-threatening to nearby personnel if the container does not provide adequate shielding.

In the course of normal operation of a nuclear plant or x-ray machine, everything is contained and external exposure is the greatest hazard. But in the Chernobyl and Fukushima crises, the greatest concern is with environmental releases and internal exposures. Highly radioactive fission products have been released into the environment through a combination of controlled venting, failure of containment, and fire. This is comparable to the release of toxic pollutants from major industrial disasters. Initial releases in the first three days of the crisis were low and unlikely to cause health effects in the population. The situation deteriorated on the fourth day.

Release of radioactive contaminants is best measured in units of becquerels or curies. Unfortunately, I have seen no such estimates to date; please post if you know a good source.

Microsieverts (µSv) and millirems (mrem) are units of radiation dose that measure biological effect on human tissue. The concept of "radiation dose" is primarily used to quantify external exposures, but it can be used to measure internal exposure if the nature of the contaminant and quantity ingested is known.

The worldwide average background dose for human beings subject to natural environments is 2400 µSv per year. An effective dose of 10,000 µSv carries with it a 0.056% chance of developing cancer or other health effect. For people who receive extremely high doses in short periods of time, (say, in excess of 500,000 µSv in a single day) there is an additional risk of acute radiation poisoning.

I welcome questions.

 

[oldfieldguy]

Help me get a handle on this dosage thing.

What small experience I have with radiation dosages was from training received in the 1970's in the army.

In that training, we measured exposures in "rads" ( radiation absorbed dose)

How do the two compare?

old field guy

 

[blacksmith37]

Not my area, but: there are different types of radiation that have signnifcantly affects- alpha, beta, gamma, neutrons, and ???. All with different energy levels . Can these all be reasonably measured on the same scale to determine risk to humans ?

 

[trottiey]

Rads and Greys are units of radiation dose that measure energy deposited by ionizing radiation. The energy deposited in your body is called your "absorbed dose". 100 rad = 1 Grey

Your absorbed dose, in units of rads,
converted to Greys by dividing by a factor of 100,
multiplied by the radiation weighting factor, in units of Sv/Gy
multiplied by the tissue weighting factor, unitless
is your "effective dose," in units of sieverts.

Typical radiation weighting factors range from 1 to 20 Sv/Gy, generally on the high side for internal contamination from nuclear fuel.

Typical tissue weighting factors range from 0.01 to 1.0, and are frequently uncertain for internal contamination. Conservative assumptions in the upper range are frequently made.

 

[trottiey]

Alpha, beta, gamma radiation can all be measured on the same scale. The differences between them is accounted for by the radiation weighting factor and the dose measurement techniques. Neutron radiation presents a bit more of a problem, but this is not relevant to the Fukushima accident.

Different enrgy levels are accounted for by the measurement of absorbed enrgy. A Geiger-Muller counter only measures counts per minute, which does not include the enrgy levels, which is why those measurements cannot be converted into doses. We have other tools for that.

Measuring internal and external doses on the same scale poses some complexities, but I don't really have time to explain further here. Maybe later.

 

[trottiey]

Lesson #2:
We're hearing many reports from Fukushima in terms of µSv per hour. Numbers given in these units only represent external dose rates. Some of these doses rates are due to contaminants released into the environment, but they only represent the external dose due to having the contaminant nearby; they do not include the internal dose rate you would receive if you breathed them in. A big reason for this is that internal contamination rates are not purely a function of your environment. They also depend on the rate of intake into your body, which in turn depends on how hard you are breathing, ambient humidity, how much you are drinking, how often you touch your hands to your mouth, etc.

If the contaminant is a gas or fine dust, internal contamination rates are sometimes given in units of DAC. (derived airborne concentration) There is a strong parallel between this type of airborne exposure and external exposures: 1 DAC is equivalent to 10 µSv per hour. I have not seen any DAC numbers from Fukushima to date; please post if you see them. If the contaminant is a liquid or a dust that is heavy enough to settle, other measurements are used.

Regardless of whether the contaminant is a gas, dust, or liquid, and regardless of whether it is ingested or inhaled or absorbed, all internal doses (as opposed to dose rates) can be converted into µSv of effective dose, given enough data to do all the calculations. Once reduced to µSv, (as opposed to µSv/hour) internal doses are directly comparable to external doses in the same units.

Most people have little experience with artificial internal radiation, but we do routinely intake natural doses. For example, smoking a typical pack of 20 cigarettes will give you an internal dose of 108 µSv. Each ordinary natural banana that you eat gives you an internal dose of 0.1 µSv. Each bottle of gatorade you drink is 1.0 µSv.

In the first three days of the Fukushima crisis, very small amounts of radiological contaminants were released into the environment, probably too little to have any health impact on the public. After that, the situation deteriorated, and a more substantial amount of radioactive material may have been released. Low levels of contamination were detected in Tokyo, on civilians in the evacuation zone around the plant, and on US military personnel who had flown through plumes. It now seems plausible that the Fukushima incident may result in a mild increase in cancer rates in the general population. Enhanced attention to hygiene in the coming days may help mitigate the risks.

 

[MintJulep]

If you can shield it or walk away, the external exposure stops.

This neglects direct contact external contamination by radioactive materials and the associated external exposure.

If your skin, hair or clothing is contaminated then external exposure continues until the contamination is removed.

 

[unclesyd]

One way to resolve the worker's radiation exposure. One question is this an increase in radiation field or total radiation to the body.

"On Wednesday morning, the Japanese government raised the permitted radiation exposure for plant workers by 2.5 times to allow them to work longer, according to NHK TV".

 

[trottiey]

This increase in permitted dose sounds like a planned emergency measure. All the radiation protection regulations that I have seen allow radiation occupational limits to be raised by a pre-determined amount in the event of an emergency. Rules for this are then incorporated into radiation protection manuals and training.

For example, typical occupational limits are 20,000 µSv per year for radiation workers, and this would be automatically increased to 50,000 µSv per year if an appropriate level of emergency was declared. The general expectation is that many of these workers would see some period of unemployment or early retirement or switch jobs after this event, where they would take lower doses.

This limit and its increase applies to absorbed radiation by each individual worker, from the sum of all internal and external exposure. Each worker wears a small instrument that measures his own cummulative exposure to external radiation fields. So this will account for his movements behind varying degrees of shielding, and the variations in fields over time. Bioassays (urine samples and the like) will be used to measure the internal exposure of workers after the fact, but some rough estimates can usually be made before then.

Even higher occupational doses are permitted on a volunteer basis for urgent life-critical emergencies.

 

[trottiey]

I think I was wrong in my last post. It now sounds like their planned emergency measures automatically increased the allowable dose to 100,000 µSv per year, but the increase to 250,000 µSv is a brand new allowance. Such a dose, if any worker were to reach that limit, would carry with it a 1.4% chance of developing cancer or other health effects. It remains below any detectable acute effects.

 

[cloa]

Intense doses tend to be considerably worse than the same dose over a longer period due the body's ability to repair itself.

 

[unclesyd]

There was neat comparison of the dosage the workers are receiving with other things like it is equivalent to a CT Scan every hour or a chest A-Ray every minute.