Nuclear Weapon’s Technology
Fusion weapons
Nuclear weapons exploit two principle physical, or more specifically nuclear, properties of certain substances: fission and fusion.
Fission is possible in a number of heavy elements, but in weapons it is principally confined to what is termed slow neutron fission in just two particular isotopes: 235U and 239Pu. These are termed fissile, and are the source of energy in atomic weapons. An explosive chain reaction can be started with relatively slight energy input (so-called slow neutrons) in such material.
Isotopes are ‘varieties’ of an element which differ only in their number of neutrons. For example, hydrogen exists as 1H 2H and 3H — different isotopes of the same chemical element, with no, one, and two neutrons respectively. All the chemical properties, and most of the physical properties, are the same between isotopes. Nuclear properties may differ significantly, however.
The fission, or ’splitting’ of an atom, releases a very large amount of energy per unit volume — but a single atom is very small indeed. The key to an uncontrolled or explosive release of this energy in a mass of fissile material large enough to constitute a weapon is the establishment of a chain reaction with a short time period and high growth rate. This is surprisingly easy to do.
Fission of 235U (uranium) or 239Pu (plutonium) starts in most weapons with an incident source of neutrons. These strike atoms of the fissile material, which (in most cases) fissions, and each atom in so doing releases, on average, somewhat more than 2 neutrons. These then strike other atoms in the mass of material, and so on.
If the mass is too small, or has too large a surface area, too many neutrons escape and a chain reaction is not possible; such a mass is termed subcritical. If the neutrons generated exactly equal the number consumed in subsequent fissions, the mass is said to be critical. If the mass is in excess of this, it is termed supercritical.
Fission (atomic) weapons are simply based on assembling a supercritical mass of fissile material quickly enough to counter disassembly forces.
The majority of the energy release is nearly instantaneous, the mean time from neutron release to fission can be of the order of 10 nanoseconds, and the chain reaction builds exponentially. The result is that greater than 99% of the very considerable energy released in an atomic explosion is generated in the last few (typically 4-5) generations of fission – less than a tenth of a microsecond.
This tremendous energy release in a small space over fantastically short periods of time creates some unusual phenomena — physical conditions that have no equal on earth, no matter how much TNT is stacked up.
Plutonium (239Pu) is the principal fissile material used in today’s nuclear weapons. The actual amount of this fissile material required for a nuclear weapon is shockingly small.
Below is a scale model of the amount of 239Pu required in a weapon with the force that destroyed the city of Nagasaki in 1945:
In the Fat Man (Nagasaki) weapon design an excess of Pu was provided. Most of the remaining bulk of the weapon was comprised of two concentric shells of high explosives. Each of these was carefully fashioned from two types of explosives with differing burn rates. These, when detonated symmetrically on the outermost layer, caused an implosion or inward-moving explosion.
The two explosive types were shaped to create a roughly spherical convergent shockwave which, when it reached the Pu ‘pit’ in the center of the device, caused it to collapse.
The Pu pit became denser, underwent a phase change, and became supercritical.
A small neutron source, the initiator, placed in the very center of this Pu pit, provided an initial burst of neutrons – final generations of which, less than a microsecond later, saw the destruction of an entire city and more than 30,000 people..
Nearly all the design information for weapons such as these is now in the public domain; in fact, considering the fact that fission weapons exploit such a simple and fundamental physical (nuclear) property, it is no surprise that this is so. It is more surprising that so much stayed secret for so long, at least from the general public.
A neutron reflector, often made of beryllium, is placed outside the central pit to reflect neutrons back into the pit. A tamper, often made of depleted uranium or 238U helps control premature disassembly. Modern fission devices use a technique called ‘boosting’ (referred to in the next section), to control and enhance the yield of the device.
Today’s nuclear threat lies mostly in preventing this fissile special nuclear material (often referred to as SNM) from falling into the wrong hands: once there, it is a very short step to construct a working weapon.
What we do now to keep these devices out of the hands of groups like Al-Qaeda is vital to civilized peoples.
Fusion weapons
Fission weapons discussed above are ultimately limited in their destructive capability by the sheer size a subcritical mass can assume — and be imploded quickly enough by high explosives to form a supercritical assembly. The largest known pure fission weapon tested had a 500 kiloton yield. This is some thirty-eight times the release which destroyed Hiroshima in 1945. Not satisfied that this was powerful enough, designers developed thermonuclear (fusion) weapons.
Fusion exploits the energy released in the fusing of two atoms to form a new element; e.g. deuterium atoms fusing to form helium, 2H + 2H = 4He2 , as occurs on the sun. For atoms to fuse, very high temperatures and pressures are required. Only fusion of the lightest element, hydrogen, has proven practical. And only the heavy isotopes of hydrogen, 2H (deuterium) and 3H (tritium), have a low enough threshold for fusion to have been used in weapons successfully thus far.
The first method tried (boosting) involved simply placing 3H in a void within the center of a fission weapon, where tremendous temperatures and high pressures were attendant to the fission explosion. This worked; contributing energy to the overall explosion, and boosting the efficiency of the Pu fissioning as well (fusion reactions also release neutrons, but with much higher energy).
Because 3H is a gas at room temperature, it can be easily ‘bled’ into the central cavity from a storage bottle prior to an explosion, and impact the final yield of the device. This is still used today, and allows for what is termed ‘dial-a-yield’ capability on many stockpiled weapons.
Multistage thermonuclear weapons — the main component of today’s strategic nuclear forces — are more complex. These employ a ‘primary’ fission weapon to serve merely as a trigger. As mentioned above, the fission weapon is characterized by a tremendous energy release in a small space over a short period of time. As a result, a very large fraction of the initial energy release is in the form of thermal X-rays.
These X-rays are channeled to a ’secondary’ fusion package. The X-rays travel into a cavity within a cylindrical radiation container.
The radiation pressure from these X-rays either directly, or through an intermediate material often cited as a polystyrene foam, ablates a cylindrical enclosure containing thermonuclear fuel (shown in blue at left); this can be Li2H (lithium deuteride).
Running along the central axis of this fuel is a rod of fissile material, termed a ’sparkplug’.
The contracting fuel package becomes denser, the sparkplug begins to fission, neutrons from this transmute the Li2H into 3H that can readily fuse with 2H (the fusion reaction 3H + 2H has a very high cross-section, or probability, in typical secondary designs), heat increases greatly, and fusion continues through the fuel mass.
A final ‘tertiary’ stage can be added to this in the form of an exterior blanket of 238U, wrapping the outer surface of the radiation case or the fuel package. 238U is not fissionable by the slower neutrons which dominate the fission weapon environment, but fusion releases copious high energy neutrons and this can fast fission the ordinary uranium.
This is a cheap (and radiologically very dirty) way to greatly increase yield. The largest weapon ever detonated — the Soviet Union’s ’super bomb’, was some 60 MT in yield, and would have been nearer 100MT had this technique been used in its tertiary. Again, to control the yield precisely, 3H may be bled from a separate tank into the core of the primary, as shown in the hypothetical diagram on the left of a modern thermonuclear weapon.
This primary/secondary/tertiary or multistage arrangement can be increased — unlike the fission weapon — to provide insane governments with any arbitrarily large yield.
Fusion, or thermonuclear weapons, are not simple to design nor are they likely targets of construction for would-be terrorists today.
Many aspects of the relevant radiation transport, X-ray opacities, and ultra-high T and D equations-of-state (EOS) for relevant materials are still classified to this day (though increasing dissemination of weapons-adaptable information from the inertially-confined fusion (ICF) area may change this in time). Keeping such information classified makes good sense.
With Rigards from Simplethinking…
Retinal Scan
The human retina is a thin tissue composed of neural cells that is located in the posterior portion of the eye. Because of the complex structure of the capillaries that supply the retina with blood, each person’s retina is unique. The network of blood vessels in the retina is so complex that even identical twins do not share a similar pattern.
Although retinal patterns may be altered in cases of diabetes, glaucoma or retinal degenerative disorders, the retina typically remains unchanged from birth until death. Due to its unique and unchanging nature, the retina appears to be the most precise and reliable biometric. Advocates of retinal scanning have concluded that it is so accurate that its error rate is estimated to be only one in a million.
A biometric identifier known as a retinal scan is used to map the unique patterns of a person’s retina. The blood vessels within the retina absorb light more readily than the surrounding tissue and are easily identified with appropriate lighting. A retinal scan is performed by casting an unperceived beam of low-energy infrared light into a person’s eye as they look through the scanner’s eyepiece. This beam of light traces a standarized path on the retina. Because retinal blood vessels are more absorbent of this light than the rest of the eye, the amount of reflection varies during the scan. The pattern of variations is converted to computer code and stored in a database.
The idea for retinal identification was first conceived by Dr. Carleton Simon and Dr. Isodore Goldstein and was published in the New York State Journal of Medicine in 1935. The idea was a little before its time, but once technology caught up, the concept for a retinal scanning device emerged in 1975. In 1976, Robert “Buzz” Hill formed a corporation named EyeDentify, Inc., and made a full-time effort to research and develop such a device. In 1978, specific means for a retinal scanner was patented, followed by a commercial model in 1981.
Retinal scanners are typically used for authentication and identification purposes. Retinal scanning has been utilized by several government agencies including the FBI, CIA, and NASA. However, in recent years, retinal scanning has become more commercially popular. Retinal scanning has been used in prisons, for ATM identity verification and the prevention of welfare fraud.
Retinal scanning also has medical applications. Communicable illnesses such as AIDS, syphilis, malaria, chicken pox and Lyme disease as well as hereditary diseases like leukemia, lymphoma, and sickle cell anemia impact the eyes. Pregnancy also affects the eyes. Likewise, indications of chronic health conditions such as congestive heart failure, atherosclerosis, and cholesterol issues first appear in the eyes.
Advantages
- Low occurrence of false positives
- Extremely low (almost 0%) false negative rates
- Highly reliable because no two people have the same retinal pattern
- Speedy results: Identity of the subject is verified very quickly
Disadvantages
- Measurement accuracy can be affected by a disease such as cataracts
- Measurement accuracy can also be affected by severe astigmatism
- Scanning procedure is perceived by some as invasive
- Not very user friendly
- Subject being scanned must be close to the camera optics
- High equipment costs
DNA Fingerprinting
People everywhere expected the new millennium to bring surprises. But the particular shock and horror that rippled through the international viticulture community in 2000 was most unexpected. It had been found that sixteen of the most highly prized varieties of wine-making grapes were the product of mating between the classic Pinot and the classically undervalued Gouais grape.
This blew the proverbial cork off the industry because the Gouais was considered such an inferior specimen that there were even attempts to ban its cultivation in France during the Middle Ages. This proves that humble origins can still produce superior quality. More practically, though, knowledge of their heritage allows improved breeding of highly desirable subspecies of grape. And viticulturists everywhere had DNA fingerprinting technology to thank.
DNA fingerprinting is a term that has been bandied about in the popular media for about fifteen years, largely due to its power to condemn and save, but what does it involve? In short, it is a technique for determining the likelihood that genetic material came from a particular individual or group. 99% of human DNA is identical between individuals, but the 1% that differs enables scientists to distinguish identity. In the case of the grapes, scientists compared the similarities between different species and were able to piece together parent subspecies that could have contributed to the present prize-winning varieties.
The DNA alphabet is made up of four building blocks – A, C, T and G, called base pairs, which are linked together in long chains to spell out the genetic words, or genes, which tell our cells what to do. The order in which these 4 DNA letters are used determines the meaning (function) of the words, or genes, that they spell.
But not all of our DNA contains useful information; in fact a large amount is said to be “non-coding” or “junk” DNA which is not translated into useful proteins. Changes often crop up within these regions of junk DNA because they make no contribution to the health or survival of the organism. But compare the situation if a change occurs within an essential gene, preventing it from working properly; the organism will be strongly disadvantaged and probably not survive, effectively removing that altered gene from the population.
In the non-coding regions of the genome, sequences of DNA are frequently repeated giving rise to so-called VNTRs – variable number tandem repeats. The number of repeats varies between different people and can be used to produce their genetic fingerprint. In the simple example shown above, person A has only 4 repeats whilst person B has 7. When their DNA is cut with the restriction enzyme Eco RI, which cuts the DNA at either end of the repeated sequence (in this example), the DNA fragment produced by B is nearly twice as big as the piece from A, as shown when the DNA is run on a gel (right). The lane marked M contains marker pieces of DNA that help us to determine the sizes. If lots of pieces of DNA are analysed in this way, a ‘fingerprint’ comprising DNA fragments of different sizes, unique to every individual, emerges.
But why bother? After all, I know where my wine comes from – Tesco’s, right? Well, there are many relevant applications of DNA fingerprinting technology in the modern world, and these fall into three main categories: To find out where we came from, discover what we are doing at the present, and to predict where we are going.
In terms of where we came from, DNA fingerprinting is commonly used to probe our heredity. Since people inherit the arrangement of their base pairs from their parents, comparing the banding patterns of a child and the alleged parent generates a probability of relatedness; if the two patterns are similar enough (taking into account that only half the DNA is inherited from each parent), then they are probably family. However, DNA fingerprinting cannot discriminate between identical twins since their banding patterns are the same. In paternity suits involving identical twins – and yes, there have been such cases – if neither brother has an alibi to prove that he could not have impregnated the mother, the courts have been known to force them to split child care costs. Thankfully there are other, less “Jerry Springer-esque”, applications that teach us about our origins. When used alongside more traditional sociological methodologies, DNA fingerprinting can be used to analyse patterns of migration and claims of ethnicity.
DNA Fingerprinting can also tell us about present-day situations. Perhaps best known is the use of DNA fingerprinting in forensic medicine. DNA samples gathered at a crime scene can be compared with the DNA of a suspect to show whether or not he or she was present. Databases of DNA fingerprints are only available from known offenders, so it isn’t yet possible to fingerprint the DNA from a crime scene and then pull out names of probable matches from the general public. But, in the future, this may happen if DNA fingerprints replace more traditional and forgeable forms of identification. In a real case, trading standards agents found that 25% of caviar is bulked up with roe from different categories, the high class equivalent of cheating the consumer by not filling the metaphorical pint glass all the way up to the top. DNA fingerprinting confirmed that the ‘suspect’ (inferior) caviar was present at the crime scene.
Finally, genetic fingerprinting can help us to predict our future health. DNA fingerprinting is often used to track down the genetic basis of inherited diseases. If a particular pattern turns up time and time again in different patients, scientists can narrow down which gene(s), or at least which stretch(es) of DNA, might be involved. Since knowing the genes involved in disease susceptibility gives clues about the underlying physiology of the disorder, genetic fingerprinting aids in developing therapies. Pre-natally, it can also be used to screen parents and foetuses for the presence of inherited abnormalities, such as Huntington’s disease or muscular dystrophy, so appropriate advice can be given and precautions taken as needed.
Cordially thanks to Dalya Rosner from Cambridge University.
