I suppose there's another nightmare in my
closet, but my bed's not big enough for three.
from: There's A Nightmare In My Closet
by: Mercer Mayer
Given the hype and misunderstanding about radiation, and Renee’s buying into the Armageddon story in Japan, I thought I’d share some basics on radiation exposure:
A ‘rem’ is: “a basic unit used to measure the amount of biological damage caused by various types of ionizing radiation, equal to the dose that produces the same amount of damage in human tissue as one roentgen of X-rays or gamma rays (0.01 sievert)”
In english: a measure of damage to people by radiation.
For the last 50 years the US Navy and the US government has generally limited exposure to 5 rem/year. There is no noticeable health effects at that level. During my 9.5 years in the navy, with about ½ that time on submarines I received a total of about 1.5 rem.
How much radiation is safe (there lots of opinions – and you know, opinions are like belly buttons – everyone has one and most have lint) (http://www.radiationanswers.
· 1 rem received in a short period or over a long period is safe—we don’t expect observable health effects.
- 10 rem received in a short period or over a long period is safe—we don’t expect immediate observable health effects, although your chances of getting cancer might be very slightly increased.
- 100 rem received in a short time can cause observable health effects from which your body will likely recover, and 100 rem received in a short time or over many years will increase your chances of getting cancer.
- 1,000 rem in a short or long period of time will cause immediately observable health effects and is likely to cause death.
Just like the rest of the world is dorked up and uses the metric system instead of inches, feet and miles – they don’t measure radiation exposure with rem, they use Sieverts.
1 rem = 0.01 sieverts (we are going to come back to this)
Everyone is exposed to ‘background’ radiation all the time – from the sun, rocks, and man-made stuff around us (like computer screens or medical x-rays).
On average in the USA we are exposed to 620 mrem/year (that is .62 rem/year). There have been studies done (thank you Dr. Sharp) that indicate if you take away that background radiation – there is actually a health risk.
OK, so I mentioned what I was exposed to on US nuclear submarines, now let’s look at other exposures we are subjected to:
· Chest x-ray
· 12hrs in a jetliner
· Living within 50 miles of a coal fired electric plant: .03 mrem/year
· Living within 50 miles of nuclear plant:
o yep – under normal conditions, a coal fired plant is 3x more ‘radioactive’ than a nuclear plant; coal slag or coal ash found at a nuclear power plant would be considered nuclear waste
The other day there was a photo of a radiation detector in Tokyo showing a reading of .6 micro Sievert/hr. Doing the math, that is the same as .06 mrem/hr = 525 mrem/year. Remember that normal background radiation is 620 mrem / yr! I think that some of what they were detecting was background radiation.
There is no question that closer to the damaged reactors there is significantly more radiation. I have not heard numbers, just things like “radiation levels surge,” or they are “high.” It is possible that some workers are being exposed to 50-100 rem.
But even given those high numbers – the effect on human health – even if exposed to large populations will be trivial compared to 10,000+ killed in the tsunami.
I’ve also been asked about the mechanics of the BWR (boiling water reactor) that they are having problems with.
The nuclear fuel is in rods encased in zirconium. Other rods with boron or other neutron ‘absorbers’ are used to control the reaction. Sometimes the fuel moves, and sometimes the control rods move – I don’t know which was used in Japan. But it does not matter – in all cases a loss of power results in the reactor ‘scramming’ or shutting down – it is very rapid and even in an earth quake it would shut down. However, shutting down only means no more fission – there is a butt-load of radio-isotopes created and slowly decaying from the previously operating reactor. This decay of fission product daughters (that’s what they are called) also gives off heat – quite a bit of heat at first and then less and less over the next several decades.
So because of decay heat it is necessary to maintain cooling water flow in a shut down reactor. Typically there are two main ways that cooling water is pumped thru the reactor – using electricity from the power grid or using electricity from emergency diesel generators. After the earthquake the power grid went down, then after the tsunami the emergency generators were swept away. So decay heat was not being removed – eventually boiling off any water remaining in the reactor vessel. Usually there is a third way to get cooling to the reactor for cases where there is no electricity – natural circulation of sea water or river water. I assume that even this method was damaged or degraded in the quake/tsunami.
The overheating fuel will melt the zirconium and cause highly radioactive crapola into the primary coolant – also the highly radioactive environment will case the disassociation of water – creating hydrogen and oxygen. This is where the hydrogen came from that caused the explosions (not from very hot water – as the press keeps reporting).
The spent fuel (fuel that was removed from the reactor years ago) also has low levels of decay heat being generated – so low that if they spent fuel just hangs out in a pool of water it is fine. But apparently what happened was that this water was also lost and not able to be replaced. This spent fuel, when uncovered, also can melt. Apparently that also happened.
What a mess.
1 comment:
I wonder if your brother knows my dad - he was in the navy and is also a nuclear engineer. Ask him if he knows Craig Enos. :)
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