12 March 2011

Massive Explosion Rocks Japan Nuclear Plant- Radiation Facts

Posted by Dan Satterfield


NHK Japan

With the news this morning of an explosion at the Fukushima reactor in Japan, I thought I would repost something I wrote over a year ago on radiation. You are likely to hear about milli-rems and Sieverts on the news, and this should help you make sense of it.

A physicist on the BBC has been quoted as saying that the explosion was probably a result of a core breach coming into contact with water. This would imply at least a partial melt down. One expert also said the explosion MAY have released an enormous amount of radioactivity and that the situation was clearly “grave”. Not much seems to be known for sure as of 4:30 AM U.S. central time.

There was a report on NHK of radiation levels of 101 milli-rems outside the plant. That may have been BEFORE the explosion. A typical dental x-ray will give you 2-20 milli-rems and most people get 200-300 milli-rems a year from natural sources. (NHK says the radiation was measured just before the explosion)

1000 milli-rems= 1 rem

100 rems (1 Sievert) will make you quite ill and 300 rems is called LD50. 50% of people who get that much radiation will die within 60 days. If you start hearing of levels near 1 rem, that is bad. It will require mass evacuations.

Here below is my original post:


It’s an interesting question isn’t it? Something you should probably know.  A little bit if information that you HOPE you never NEED to know. As my sister once said, “Once you’ve seen one nuclear war, you’ve seen’em all!”

In general people are afraid of radiation, and that is a good thing. It can be dangerous. It can also kill off a cancer that is eating you up. It can, and in many places, does keep you warm on a cold winter day. Electricity is dangerous too, but we usually don’t hesitate to change the batteries in our flashlight!

 Radiation sources from the NRC
First of all a question. Will you will get a higher dose of radiation living next door to a big power plant that burns coal, or next to a nuclear power plant? The answer of course is you will get more radiation from the plant burning coal.


Radiation measurements can get complicated, but a simplified and generally correct summary is this. Radiation is measured in Seiverts. That’s the new term. The old unit is called REM. REM stands for Roentgen Equivalent for Man.

100 REM = One Sievert

Most of the time we are measuring radiation in thousandths of a REM. Called milli-rems. If you live on the East coast of the USA, you get about 50-60 milli-rems of radiation a year from natural sources. Some of it comes from radioactive material naturally in your body.

If you live in Denver, built on top of granite and a mile a high, you get about 150 milli-rems a year. You get more if you are an airline pilot and spend a lot of time at 35,000 feet. You also get about 40 milli-rems every time you get a whole mouth x-ray of your teeth. (I should be positively glowing in the dark by now!)

There are three types of radiation. Alpha Rays, Beta Rays and Gamma Rays. Alpha and Beta rays are easily stopped by paper, clothes and skin. Gamma Rays are the bad ones. It takes a few inches of lead or a few feet of concrete and dirt to slow or stop them. A detonating nuclear device will produce a LOT of gamma rays. Even if the blast doesn’t get you, the dose of radiation could be very high, and perhaps fatal.

Gamma Rays are the most dangerous type of radiation.A dirty bomb is not a nuclear explosion. It’s just taking some radioactive material and blowing it up with conventional explosives. It will not go far and it will spread highly concentrated material over a wide area, and thus greatly reduce the radiation. I worry very little about a dirty bomb and you shouldn’t either. If you don’t get hurt by the explosion, you are unlikely to be hurt by the radiation. (Unfortunately it appears the terrorists know this now, and it’s even more unlikely you will have to worry about it.) 

Cancer cells are very susceptible to radiation. More so than healthy cells, and that is why many cancer patients are given enough radiation to make them sick. Because of this, most people know the symptoms of radiation sickness. Nausea, fatigue, and loss of hair.

How much does it take to get sick? Usually more than 25 rems. That’s 2,5000 milli-rems. A lot more than the 40 millirems you get at the dentist.


If 100 people get a sudden dose of 350 rems of radiation, about 3.5 sieverts, then about half of them will die in 60 days. 350 rems is considered LD50/60. It means what you think. 50% death rate in 60 days. There is some disagreement about this, some say 3 Sieverts, and it depends on your whole body getting that much. If you get 1000 rems of radiation, you will be dead in about an hour.

If you are unlucky enough to get dosed with 500 rems, you may vomit right away, but the next day you will probably be ok. Experts call this the walking dead phase. You will most certainly not survive beyond two weeks, or so I am told.

Interestingly, if you go to the EPA web sites or local Emergency management sites, you will not easily find this information. Plenty of information on radiation, and the like, but nothing about how much it takes to kill you!

Gamma Ray exposure in USA from Nat. geog. Survey. Elevation above sea level is a key factor.
Gamma Ray exposure in USA from Nat. Geog. Survey. Elevation above sea level is a key factor.

One last thing. How much radiation to absolutely, positively give you cancer? Answer: Probably around 20 Sieverts. Notice anything? It only takes 5 Sieverts to kill you!

This is where the Linear Hypothesis comes in. If 20 Sieverts gives 100% of those exposed cancer, will 1/100th as much (20 rems) give 1% cancer. Seems that way.

The latest Science from the National Academies is that 100 milli-rems per year over a lifetime will cause one cancer in every 175 people. Since about 20% of the population dies of cancer anyhow, the risk goes to 20.05%.

Suppose someone sets off a dirty bomb and it will give me 100 milli rems a year if I stay in the house I love. My risk of cancer will go up by 1/2 of one percent. I’d take that risk. You think the government would let me?

Doubt it.

Congrats, you now know more about radiation than 95% of the population.


Dr. Robert Muller Lecture on Radiation U. Cal. Berkeley and PHYSICS FOR FUTURE PRESIDENTS also by him-on amazon btw


National Academies of Science, Prepublication Copy, Health Risks from Exposure to Low Levels of Ionizing Radiation BEIR VII Phase 2, June 29, 2005 page 500 Table 12-9.