"There is no safe level of radiation exposure. So the question is not: What is a safe level? The question is: How great is the risk?"
There have been three major theories as to how radiation damages living tissue, all set by physicians. All are approximations, and based on broad assumptions.
How does damage occur? In other words, how does radiation cause cancer?
A typical cell is around 0.02mm across, a cell nucleus is about 0.001mm.
When radiation, say a gamma, enters your body, there is a chance it will intersect with one of your cells. Inside any cell is a nucleus, which contains chromosomes. These are essentially DNA helixes. DNA looks like two entwined strings of nucleotides - the amino acids A, T, C, and G. Across strands they are paired A-T and C-G. A portion of DNA (a series of these acids) is called a gene. Genes exist along chromosomes, and they contain the data for proteins.
If the radiation happens to pass into the cell nucleus (which is a relatively large entity compared to the rest of the cell), one of 4 things can happen.
All exposure subjects cells to risk. In order of decreasing probability:
Possibility (3) is the one to watch out for. During division, the DNA strands stretch out, and it is during this time which your cells are most susceptible to damage.
It is also possible for the radiation to ionise the water in the cell cytoplasm, leading to the formation of free radicals, which can travel some distance. They can react chemically with the DNA in the nucleus, interfering with the chemical bonding along the helix.
Two types of damaging interaction can occur with the amino acids.
It is also possible to have compound breaks along the DNA, which is not easy for the cell to repair, unlike single strand or double strand breaks.
The cell and nuclear membranes are also susceptible to damage. This could be due to alterations in permeability/osmosis in the membrane due to the radiation-induced imbalance of ionised particles.
Once a certain threshold is exceeded, you will start saturating the cells. This lethal threshold serves to define two categories of radiation.
| EFFECT | NATURE | THRESHOLD? | DOSE DEPENDENCE |
|---|---|---|---|
| Stochastic (somatic or genetic) | Non-lethal mutations affecting single cells | No | Probability of effect increases with dose |
| Deterministic | Lethal mutations affecting large number of cells | Yes | Severity of effect increases with dose |
In cancer, a stem fails to stop its mitosis. It and its descendants divide uncontrolled, forming a tumour. A bit like a binary tree in cell multiplicity.
Oncogenes are genes which interfere with the cell division process. They are mutations of proto-oncogenes, whose role are to control cell growth and mitosis. It is thought radiation promotes creation of oncogenes.
There are also cancer-suppressing genes, which inhibit oncogene formation. The best known example is the Rb gene, which inhibits retinoblastoma.
After all of this, let me add a fourth idea on radiation damage:
(4) probability of hereditary genetic damage or cancer is a function of:
type of radiation (a,b,g,n) x energy of radiation x dose rate
Here you have 4 discrete degrees of freedom, and 2 continuous degrees: rate & energy. Assume that there is a cut-off energy for a unit of a particular type of radiation, E_max, such that if E > E_max a cell will die, and E < E_max the cell will survive (either in damaged or undamaged state). We are worried about the E < E_max cases. If E > E_max then you get radiation poisoning and you will definitely die if you get a large enough dose.
The probability of nucleus intersection is a function of radiation type. (The size of radiation varies considerably.)
The probability of a nucleus being hit twice or more is very low, unless the number of incident radiation approaches the sample size. In which case you get radiation poisoning and die anyway.
You get a 6D phase space of statistical mechanics. Supplement this with an action in path integral form. Plotted, you'd have a 6D graph, unlike your normal 3D graphs. It's worse than the 4D spacetime of general relativity. No wonder the physicicans only plot projections! You can trace out a person's history in this phase space, and then give them a final probability of cancer/hereditary damage.
Experts agree that the silvery, unstable metal plutonium-239, with a half-life of 24,000 years, is hazardous and sould be isolated from the biosphere. However, the risks posed to workers and communities by stored plutonium depend on the route of exposure as well as the particle size, isotope, and chemical form.
Weapons-grade plutonium outside the body presents little risk unless exposures are frequent and extensive. It emits primarily alpha particles, which cannot penetrate skin, clothing, or even paper. Nearly all the energy from plutonium is deposited on the outer, nonliving layer of the skin, where it causes no damage. The neutrons and the relatively weak gamma photons it emits can penetrate the body, but large amounts of weapons-grade plutonium would be needed to yield substantial doses.
Workers wearing only lead aprons can handle steel drums containing solid plutonium metal with no immediate untoward effects. However, as weapons-grade plutonium ages, it becomes more dangerous because some of the contaminating plutonium-241 is converted via beta decay to americium-241, which emits far stronger gamma radiation.
On the other hand, plutonium inside the body is highly toxi. Solid plutonium metal is neither easily dispersed nor easily inhaled or absorbed into the body. But if plutonium metal is exposed to air to any degree, it slowly oxidizes to plutonium oxide (PuO2), which is a powdery, much more dispersable substance. Depending on the particle size, plutonium-239 oxide may lodge deep in the alveoli of the lung where it has a biological half-life of 500 days, and alpha particles from the opxide can cause cancer. Also, fractions of the inhaled plutonium oxide can slowly dissolve, enter the bloodstream, and end up primarily in bone or liver.
Plutonium oxide is weakly soluble in water. If it is ingested in food or water, only a small fraction (4 parts per 10,000) is absorbed into the gastrointestinal tract. However, it may take just a few millionths of a gram to cause cancer over time. In animals, small doses induce cancer, especially in lung and bone.
In published studies of plutonium's effects on humans, most subjects were exposed to multiple sources of radiation. Some researchers say the available health data on plutonium workers have not yet been used to do careful epidemiological studies, because researchers have been denied access to much of the data on workers and military personnel exposed to plutonium. In the studies done so far, plutonium workers do not show major excesses of any type of cancer.
Becuase of the relative lack of human data, the risks of chronic exposure to plutonium are uncertain. Exposure standards in the U.S. are based partly on studies of survivors of Hiroshima and Nagasaki and partly on animal experiments. A 1991 White House Office of Science & Technology Policy studye says that "sufficient human data are not available to provide accurate risk assessment of exposure."
Then next phenomenon is the supersonic blast front. You see it before you hear it. The pressure front has the effect of blowing away anything in its path. Heavy steel girders were found bent at 90 degree angles after the Japanese bombings.
After the front comes the overpressure phase. It would feel like being under water a few hundred metres. At a few thousand metres under the sea, pressurised hulls implode. The pressure gradually dies off, and there is a negative overpressure phase, with a reversed blast wind. This reversal is due to air rushing back to fill the void left by the explosion.
The air gradually returns to room pressure. At this stage, fires caused by electrical destruction and ignited debris, turn the place into a firestorm. Just like Dresden in WWII. It is estimated over fifty thousand died in the first few days of the Hiroshima bombing.
Then come the middle term effects such as keloid formation and retinal blastoma.
Genetic or hereditary damage can show up up to forty years after initial irradiation.
The following diagram is of blast zone radii, courtesy of Outlaw Labs. Note that damage from blast pressure falls off as a function of 1/r^3.
.
. .
. . .
. .
[5] [4] [5]
.
. . . .
. . . .
. [3] _ [3] .
. . [2] . .
. _._ .
. .~ ~. .
. . [4] . .[2]. [1] .[2]. . [4] . .
. . . .
. ~-.-~ .
. . [2] . .
. [3] - [3] .
. . . .
. ~ ~ .
~
[5] . [4] . [5]
.
. .
. .
.
[3 different bomb types]
____________________________________________________________________________
______________________ ______________________ ______________________
| | | | | |
| -[10 KILOTONS]- | | -[1 MEGATON]- | | -[20 MEGATONS]- |
|----------------------| |----------------------| |----------------------|
| Airburst - 1,980 ft | | Airburst - 8,000 ft | | Airburst - 17,500 ft |
|______________________| |______________________| |______________________|
| | | | | |
| [1] 0.5 miles | | [1] 2.5 miles | | [1] 8.75 miles |
| [2] 1 mile | | [2] 3.75 miles | | [2] 14 miles |
| [3] 1.75 miles | | [3] 6.5 miles | | [3] 27 miles |
| [4] 2.5 miles | | [4] 7.75 miles | | [4] 31 miles |
| [5] 3 miles | | [5] 10 miles | | [5] 35 miles |
| | | | | |
|______________________| |______________________| |______________________|
The surrounding cooler air exerts some drag on this rising air, which slows down the outer edges of the cloud. The unimpeded inner portion rises a bit more quicker than the outer edges. A vacuum effect occurs when the outer portion occupies the vacuum left by the higher inner portion. The result is a smoke ring.
The inner material gradually expands out into a mushroom cloud, due to convection. If the explosion is on the ground, dirt and radioactive debris get sucked up the stem, which sits below the fireball.
Collisions and ionisation of the cloud particles result in lightning bolts flickering to the ground.
Initially, the cloud is orange-red due to nitrous oxide formation (cf car smog). This reaction happens whenever air is heated.
When the cloud cools to air temperature, the water vapour starts to condense. The cloud turns from red to white.
In the final stages, the cloud can get about 100km across and 40km high, for a megaton class explosion.
To learn more about air explosions, see the Reference by Kinney and Graham, "Explosive Shocks in Air".
The Red Phoenix, 1994.