[Editor’s note: This article is one of the most important VT ever published, and it remains every bit as relevant today, as the use of mini nukes with ‘clean’ designs continues unabated; and unless this becomes widely known and pressure applied by the masses of the people to stop it, they will continue to be used with impunity to kill innocent people en masse. Ian]
First published September 4, 2015
VT Nuclear Education: The Uranium Hydride Bomb
by Ian Greenhalgh with Jeff Smith
We are now at the fifth generation of nuclear weapons design which means that a number of different types of nuclear weapon exist and they differ substantially in effect from the weapons we are familiar with from all those 1950s newsreels of huge atomic blasts.
Modern 5th gen weapons are often of much lower yield than the Hiroshima and Nagasaki bombs with yields of less than 1 kiloton (1000 tonnes of TNT equivalent). They also have different manners of explosion – instead of one big bang they can have much longer burn times producing visual effects that look quite different.
This is why it is important to learn a little about these new weapons types and how their explosions would appear visually as it will better enable the identification of nuclear events in future; allowing people to discern between the explosion of a warehouse full of rocket fuel and a nuclear explosion.
Make no mistake, we have entered into a dangerous new age where the use of these advanced low yield nuclear weapons will become increasingly commonplace; therefore we all need to become better informed about these weapons so it becomes harder to use them covertly to commit acts of terrorism.
The uranium hydride bomb is a variant design of the atomic bomb first proposed as far back as 1939. It uses deuterium, an isotope of hydrogen, to act as a neutron moderator in a U235 based implosion weapon. The neutron chain reaction is a slow nuclear fission process vs a fast fission process. Due to the use of slower moving neutrons the bombs total explosive power is adversely affected by the thermal cooling of neutrons since it delays the neutron multiplication factor or Alpha production rate. Two uranium hydride bombs are known to have been tested back in the 1950’s,the Ruth and Ray test explosions in Operation Upshot-Knothole, 1956. The tests produced a yield comparable to about 200 ton of TNT or more each. However both tests were considered to be fizzle yields at the time. Since the trend was for weapons with a bigger bang, the technology was shelved for over 20 years.
In a delayed or slow fission nuclear weapon design the hydrogen deuterides in the form of uranium hydride (UH3) or plutonium hydride (PUH3) moderates (slows) the neutrons, thereby increasing the neutron cross section for neutron absorption. The result is a much lower required critical mass,and thus a smaller weapon thereby reducing the amount of U235 or plutonium PU239 needed for an explosion. The result in the original 1956 design was that the slower neutrons delayed the reaction time too much and reduced the efficiency of the weapon. Its effect was to increased the time between subsequent neutron generation events that are necessary for a rapid explosion and it creates a problem in the containment of the explosion; the inertia that is used to confine implosion type bombs will not be able to confine all of the reaction in time. The end result is a lower yield fizzle blast with a very long burn time instead of a big bang. The predicted energy yield would be in the order of 1kt or 1000 tons TNT up to 5 kt equivalent instead of a standard 20 kt blast for the same amount of fissile material used.
In a classical nuclear weapon design a critical mass is supposed to go more or less instantly into an explosion. However if half the critical mass, of U235 or U239 is undergoing delayed or moderated fission the process will continue so long as Alpha gain (neutron production) is positive. With huge numbers of atoms involved, even if there isn’t enough for a prompted explosion; There will always be enough energy given off as Bremsstrahlung radiation and the whole thing should get ‘burning hot’ before it detonates. Unburnt deuterium fuel will then be consumed by the fireball as neutron or Alpha production slowly decays. This is done by reducing the size of the critical mass by adding a mixture of Lithium6 and Deuterium hydrates to the core before it undergoes compression. The end result is what used to be called a fizzle yield. So instead of a 20KT blast you would only get about a 5KT blast but a much longer plasma fireball burn time.
The Uranium Deuteride design is a hydride type device, which relies on moderated fission (UH3 or UD3) rather than fast fission. The hydride designs work well but their slow fission process due to the use of moderated fission reactions is extremely limiting to their yield. This meas a much lower blast yield but a very long plasma burn time. If the purpose of the weapon is a very small yield with a very long thermal burn time. I.E. a nuclear thermobaric weapon with a limited yield of less than 5 to 6 kt and a maximum blast radius damage of less than 1 mile with little fallout; Then this is the weapon of choice.
In a slow burn nuclear weapon over 75% or more of the energy is now released as thermal and neutron radiation instead of a shock wave blast. It starts out as a very small fireball that quickly grows in size until full detonation level is achieved at about the 200 to 300 ton level. Then you get the classical explosion (the flash) and blast wave from the rapidly expanding fire ball. After the massive explosion, the fireball continues to burn and consume its unburnt nuclear fuel that is now in a fully plasmatized state. The nuclear fuel in a plasma form will continue to burn at a slower and slower rate until the alpha or neutron production goes to zero.
In the newer moderated weapon designs. The heat of the fission reaction produces fusion of the hydrogen isotopes surrounding the pit. Thus releasing a second burst of neutrons. This causes even more uranium atoms to fission, creating a much larger nuclear chain reaction that doubles the yield of the weapon with only a small percent of the total energy released being contributed by the original fission reaction. Bremsstrahlung radiation emitted from the plasma fireball is also called free / free radiation. This refers to the fact that the radiation is created by the charged particles that are set free both before and after the explosion that produced there emissions.
Pre-initiation is only a problem in the older WW2 era solid core devices (fatman) with a high pure fission yield because it is only in these types of bombs that you have an intrinsic disassembly limited time yield, and thus the requirement that a very high “alpha” (neutron multiplication rate) exist at disassembly time. Alpha ramps up as implosion proceeds, and if you integrate the neutron multiplication rate over the implosion time you find that in a high yield pure fission device the total number of doubling intervals that occur between criticality and full assembly is a large number, over 80 to 100 or more, while the number required to generate enough energy to disassemble the bomb at full yield is only about 80. This is why disassembly in the older solid core weapon designs is a problem; Any neutrons introduced in the first 20 doubling intervals will cause disassembly before maximum criticality of 80 doublings or shakes can occur. A “shake” is basically one doubling of a neutron generation.
However, in a modern boosted weapon the pure fission yield is only about 300 tons with boosting kicking in around 200 tons, and the total number of integrated doubling intervals between criticality and full criticality is less than the number of intervals required to take one neutron generation (a shake) up to the population level required to produce a full 200 ton boosted yield. This means that not only is pre-detonation not a problem in a modern boosted weapon design, but you can design the bomb in such a way that you actually have to inject a large number of neutrons into it to get it to explode at full yield. This is called a subcritical device and it is the basis of all mini or micro nukes operation.
Boosting requires millions of neutrons to flood the fissile core very rapidly with lots of heat and compression needed. But if you’re requirements are much more modest, (only a few kilotons or less) then just a small amount of neutrons are only needed to spark a single fission chain reaction. Then the quantity of fusion generated with a spherical shock implosion from an HE shell can produce enough fission to start the fusion chain reaction. The purpose of the deuterium hydrates is to act as a moderator for the neutrons, which will allow a much smaller quantity of fissile material to be used for a given critical mass. Unfortunately, moderating implies slowing of the blast detonation sequence, just what you don’t want in a big device. That’s why they fizzle. But for a small device it will work just fine. IE a micro-nuke.
The main benefit of using a moderator in a neutron bomb is that the amount of fissile material needed to reach criticality is greatly reduced. The slowing of fast neutrons will increase the total cross section for neutron absorption, thus greatly reducing the critical mass and allowing for a much smaller amount of fissile material to be used in a weapon. A side effect of this is that as the chain reaction progresses, the moderator will be heated, losing its ability to properly control the neutrons velocity. Another effect of moderation is that as the time between subsequent neutron generations is increased, Alpha is slowed down thus slowing down the total reaction time. Producing a lower yield. This makes the containment of the explosion a problem; the inertia that is used to originally confine the…