What do you think of when you hear someone talk of nuclear explosions? Most think of what they have seen on videos and what they have been told for years. A blinding flash. Extreme heat. A terrible blast pressure wave. A mushroom cloud. Radioactive fallout. Death and total destruction.
These are just a few of the phenomena that were observed when nuclear devices were exploded during a time of war. Many additional phenomena involve interactions with the device and the surrounding environment and can be detected and measured at great distances.
Nuclear explosive tests from the late 1940s onward were used to improve designs and device efficiency. The tests were carefully designed to gain many purely scientific advancements without the aforementioned disastrous consequences. In addition to benefiting military applications, the tests resulted in many scientific discoveries applicable to peacetime uses of nuclear energy. This article is written to lay the groundwork as to how the nuclear reactions occur that advanced science while giving you a glimpse of what goes into safely conducting a nuclear explosive test.
The very basic nuclear reaction that takes place involves the fission, or splitting of a susceptible marginally unstable atom, such as uranium 235 or plutonium 239. These particular atoms happen to be radioactive all by themselves, but only slightly so. The nucleus may simply decay, releasing an ‘alpha’ particle (a helium nucleus) or ‘beta’ particle (a high-speed energetic electron) as it rids itself of some of the excess energy causing its instability. This natural, or ‘spontaneous,’ decay is very slow, an interval of many thousands of years during which only half of the original amount of material will decay.
These susceptible atoms will also split if their internal amount of energy is made to exceed a certain threshold, tipping the nucleus irreversibly ‘over the edge’ of stability. Should the atom absorb a stray neutron, which happens to carry the excess energy needed to destabilize the nucleus, it can naturally split into two smaller pieces of somewhat similar size. It also emits ‘leftover’ neutrons that won’t fit into the newly created smaller atom’s nuclei, plus release a lot of energy in the form of gamma emissions that used to be a tiny part of the matter that made up the original atom. The amount of energy released is in accordance with Einstein’s E=mc2 formula.
If one were to personalize the reaction, it might go something like this: An atom, containing a nucleus with 92 or 94 protons, could be likened to a family of individuals that really don’t like each other at all, and try their best to escape from one another. But the neutrons contained in the same nucleus, all 143 (for uranium 235) or 145 (for plutonium 239) of them, intermingle with the unhappy protons and attempt to mediate the family feud, desperately holding everything together with just enough binding energy to keep the atom from destroying itself in a fratricidal frenzy. Then, suddenly a stray neutron carrying additional energy appears on the scene, and collides with the swarming melee that is the barely stable nucleus. Instead of being welcomed as additional reinforcements for the mediating forces of the neutron army, the extra energy the neutron carries instead destabilizes the whole conglomeration. This overwhelms the just-enough-to-keep-disaster-at-bay binding energy, thus opening the door for the unhappy family of protons to reorganize and escape as two new smaller and more manageable nuclei, satisfying the protons’ desires pent up for billions of unhappy years to fly apart. These smaller nuclei are again mediated by the peacemaker neutrons that now have a much better chance to keep the smaller troublemakers together, a little like sheepdogs keeping the flocks in a herd. In addition, two to three neutrons are kicked out of the leftover nuclei with nowhere to go, and the original nucleus, as it disintegrates, issues a final primordial scream as a gamma emission resulting from the conversion of a portion of the original mass of the nucleus to energy.
Why test? The reason for most nuclear explosive tests were to check on feasibility of various designs of a device to either verify an initial design or improve performance of a previously designed device. These could range from variations of component geometry inside, differing materials of structures, examination of fuel mixtures to proof of concept verification and safety tests. Rarely, if ever, were devices taken from active military delivery systems and detonated just to test stockpile reliability. Once a particular design has been successfully tested, further explosive tests are unnecessary. As internal components age, they can be replaced with new identical equipment, maintaining stockpile reliability and confidence. But for new designs and modifications, a test is absolutely critical as designs on paper sometimes didn’t work as expected, and it took the explosive test results to find out why.
What happens when the nuclear test countdown gets to zero? LOTS! Let’s look at the incredibly complex setup and functioning of a nuclear explosive test, simplified to the basics so we may understand the steps of the process as they occur.
Weeks before the scheduled test, the site is prepared and the device is physically emplaced in the test location. The location may be in a man-made mine shaft deep below a mountain, or in a specially drilled vertical shaft. If the test is to be an atmospheric or underwater test, a tower may be constructed or the device dropped from a plane, or a waterproof shell may be used. For high altitude testing, a rocket or missile is used to place the device at the desired altitude. All recent tests have been sited underground as a result of the Limited Test Ban Treaty (1963). The siting is determined by the reasons for the test as well as available real estate and geology to ensure no radioactive leakage occurs during or after the explosion.
Cabling from the control point remote from the site is connected and thoroughly tested. The cabling, routed to the nuclear explosive device as well as the diagnostic equipment, carries data in two directions. The encrypted signal to ensure device and other test equipment is working properly and the signals needed to safely arm and fire the device are sent from the control center. The return path carries safety and function responses and diagnostic results that actually measure the device performance back to recorders to be compared to the expected result.
The site, when all checks have been successfully completed and all data verified, is sealed, or ‘stemmed’, with material such as gravel and concrete to form an impenetrable ‘plug’ to contain the high pressure dangerous radioactive products formed by the test.
When all is confirmed to be in the proper configuration, the ‘go’ signal is forwarded from the test director, and the final countdown begins. A few seconds before zero, the final device preparation and arming sequence completes. This can involve remotely removing inhibitors and safety system locks to allow the explosion to take place. Inhibitors can be electronic in nature, or physical barriers inside the device. Materials with a very high neutron absorption character, or large cross section which would prevent the fission chain reaction from starting, are withdrawn from the core. Boost gasses such as tritium are injected into the core. Final electronic checks of the safety and firing systems are completed and electrically reported back to the control center. Safety and continuity checks continue right up to the last few milliseconds prior to firing.
At time zero, the firing signal is sent to the device via a secure encrypted message, which is authenticated via very special software inside the device’s permissive action link (PAL). The computer authenticates the final command and, if all is correct, the internal firing sequence and the diagnostic equipment recording begin. If the code is not correct, the nuclear weapon firing system is mechanically disabled and must be extensively refurbished before returning to service. The firing sequence is exquisitely choreographed to the fraction of a microsecond to ensure all the high explosive charges initiate simultaneously to create an incredibly strong and symmetrical crushing shockwave rushing inward toward the fissile core. The shockwave first encounters a ‘tamper’ device, which is a very dense and perfectly machined shell, often of a very heavy and dense metal that surrounds the fissile core. The tamper absorbs the shockwave, and smooths out the shock into a perfect sphere as it begins to compress toward the center of the device. The tamper also serves as a neutron reflector to help amplify the soon-to-start runaway fissioning of the fuel, and as an inertia to help hold the core contents in a super-critical state for as long as possible before the extreme energy produced blows the device apart.
At the center of the core is a neutron source awaiting the high voltage jolt from the firing sequencer, delayed fractions of a microsecond from the high explosive firing. The neutron source timing is especially critical, as the shower of neutrons it produces needs to meet the onrushing rapidly compressing fissile core at just the right time to efficiently begin the runaway chain reaction. Too soon, and the compressed density is not yet enough to ensure a high probability of continuing the chain reaction, as critical neutrons can ‘leak’ out of the fuel without interaction. Too late, and the source is crushed out of existence before it can contribute its neutrons. Either case can cause a ‘fizzle’, or incomplete detonation of the device at far below its designed goal. Timing the neutron initiator led to the US’ so called “dial-a-yield” nuclear weapons in which the timing determined the total weapon yield and allowed military commanders the ability change the weapon yield in the field.
When the neutron source receives its jolt, it generates the starter neutrons which begin fission in the now incredibly compressed and dense fuel. This sequence runs away rapidly as the now super compressed tiny volume of tamper and core leaves little room for the neutrons to escape unhindered and unreacted. The fissioning fuel generates further showers of neutrons, absorbed by surrounding fuel, cascading the process over a few nanoseconds. Each fuel atom fission releases energy absorbed by the fission fragments and additional neutrons to be absorbed by not yet reacted fuel, and hard radiation such as gamma, beta and alpha. The temperature jumps to millions of degrees within fractions of a microsecond as trillions of fissile atoms each contribute their fission result and mass converts to energy. Inertia keeps the device in place for a fraction of a microsecond more, but the gamma and x-ray flux produced has already left the center of the device at the speed of light, traveling about 12 inches per nanosecond.
The device casing by now glows a brilliant white as the gamma and x-ray flux interact with it, causing it to fluoresce and glow thousands of times brighter than the brightest flashbulb ever observed, and it awaits the onrushing shock of the now expanding millions of degrees’ hot plasma in its center. The vast majority of the nuclear fuel has been consumed in mere nanoseconds, before the casing even ruptures. The nuclear explosion has occurred and begins to produce phenomena detectable at a great distance as it interacts with its surrounding environment.
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