The Uranium Explosive Myth


Uranium is not an explosive

In January, 1957, Walt Disney Studios released the animated short film Our Friend the Atom. It was an instant sensation, shown in theaters and classrooms across America. In it, a nuclear chain reaction was demonstrated using ping-pong balls and mousetraps. A large box was lined across the bottom with mousetraps loaded with two ping-pong balls each. One additional ball was tossed into the box, tripping one mousetrap, sending two other balls flying to trip other traps, and soon all the mousetraps were tripped with ping-pong balls flying everywhere. This was a very good demonstration of how the nuclear chain reaction worked for bombs, but it didn’t work for reactors. The reason is two-fold; first, the Uranium used in bombs is highly concentrated in Uranium isotope 235 (U-235) while reactors use only a very dilute, non-explosive concentration of U-235. Bomb grade Uranium (U-235) is so concentrated (>90%) that the high-energy neutrons released from each initial fission immediately causes just enough additional fissions to make the "prompt, supercritcal" explosion happen.



However, the neutrons from fresh fissions are about a million times too energetic to produce a chain reaction in low-concentration reactor fuel. This was either unknown to Disney’s staff, or considered too technical for the audience. Whatever the reason, the mousetrap and ping-pong ball demonstration resulted in misleading the public's understanding of chain reactions in bombs and reactors. They seemed to be the same. But, they are not.

It’s the wrong kind of Uranium

Naturally-occurring Uranium cannot be used to make a bomb because it is not a natural explosive. Natural Uranium is a uniform mix of two isotopes, U-238 and U-235. Natural Uranium is 99.3% U-238 and 0.7% U-235. U-238 is such a poor neutron-induced fissioner, under any conditions, that we can correctly say it won't experience a chain reaction in any way,shape, or form. It’s the U-235 that makes the nuclear chain reaction possible because it is a very good fissioner, relative to U-238. However, U-235 doesn't fission very much when bombarded by high energy neutrons which is the kind of neutrons released out of the fission.

In order to make a bomb core that will actually explode, the U-235 concentration must be increased to in excess of 90% to produce enough immediate fissions to make an explosion possible. This highly concentrated form of U-235 is necessary for a detonation...anything less won't work. This is in no way a secret, at least not any more. The U-235 concentration needed to make a bomb that works can be easily found in library encyclopedias and numerous websites on the internet. Regardless, anything less than a 90% U-235 concentration, and you can’t make a weapon small enough for a deliverable bomb…even if launched by a powerful rocket.

In theory, a ridiculously enormous amount of Uranium with about a 20% U-235 concentration is possible, but in no way realistic; the bomb would be bigger in diameter than the Empire State Building is tall. Less than a 20% concentration of U-235 and a nuclear explosion is absolutely impossible, no matter how much of the material is amassed. However, this is the reason that 20% "enriched" Uranium and Plutonium are defined as "weapon's grade".  The 1-3% U-235, and/or Pu-239 in reactor fuels  cannot explode, regarless of how much is amassed.

Power plant reactors never use Uranium with a high U-235 concentration, in order to keep fuel costs manageable. Back in the 1960s, early power plant reactors used Uranium with concentrations of U-235 in the 3-5% range. These were relatively small power plants using cores so small that a concentration increase of U-235 from the natural level was needed to sustain a chain reaction sufficient to produce electricity. As plants got bigger and the reactor cores larger, the U-235 concentrations dropped to between 1 and 3%. The precious few nuclear power plants completed in America after the Three Mile Island accident were quite large, and did not need the natural abundance of U-235 changed much at all. They used what is essentially natural Uranium, but those levels have been increased in order to allow longer in-core lifetimes between refuelings. In all cases, the concentration of U-235 found in any reactor fuel is way-too dilute to produce anything like a nuclear explosion. No matter how severe a reactor accident that can possibly be imagined, the fuel cannot explode like an atomic bomb.

How can an explosive be made out of something that is not itself an explosive? Perhaps the best commonly-known example is Nitrogen. About 79% of each breath you take is Nitrogen. No one would mistake it for an explosive. It is the wrong form of Nitrogen to detonate. However, chemically transformed from a gas into another, non-gasseous molecular structure, the Nitrogen becomes the primary active ingredient in Nitroglycerine and Tri-Nitro Toluene (TNT), which are unquestionably explosive. Devastating explosives can be made out of Nitrogen, which is not-itself an explosive. With Uranium, the natural form of the element must be metallurgically transformed into a highly un-natural type of Uranium in order to become the primary ingredient in a nuclear bomb. A terrible explosive can be made out of Uranium, which is not-itself an explosive.

Power plant reactors use a very dilute concentration of U-235 in their fuel, a level which can never cause a nuclear detonation. It’s not even weapon's grade. Power reactors cannot explode like a nuclear bomb.


Slowing down neutrons

As mentioned above, before freshly-released neutrons in a power plant reactor can cause fissions themselves, they must be de-energized. U-235 might not fission very often with high energy neutrons, but it fissions very well when engulfed in a field of low energy neutrons. To those in the nuclear sciences, it is described as "making thermal neutrons out of fast neutrons". The two technical terms for the process are thermalization and moderation. Freshly-released neutrons are roughly a million times too energetic to efficiently re-enter an un-fissioned U-235 atom, make its nucleus split (fission) and release 2 or 3 new neutrons (average of 2.5 per fission). To allow the chain reaction to proceed, the "fast" neutrons must be "slowed down" (moderated).

There are several ways to slow down neutrons. In nearly all types of power plant reactors the material that slows down neutrons is the water flowing through the fuel core. Water is made up of two atoms of hydrogen and one of oxygen. The mass of a hydrogen atom is almost exactly the mass of a neutron. The fast neutrons rapidly migrate out of the fuel pellets  and into the water because s a whole lot of space on the atomic level and neutrons have no electromagnetic charge. Once in the water, the fast neutrons collide with the nuclei of the hydrogen atoms, which results in a very efficient energy transfer. After a fast neutron undergoes roughly a dozen of these collisions, the energy level has been reduced by a factor of about a million.
 
235 in reator fuels cannot explode. Although exceedingly brief, the fraction of a second needed to slow down a neutron after it is released by a fission is way too long to produce the prompt supercriticality needed for an explosion. 

Four important points should be made. First, it is generally believed that the worst possible water-moderated reactor accident would be a sudden loss of all the water flowing through the reactor core. One thing is undeniable; without the water, there can be no chain reaction. A hypothetical complete-loss-of-water accident will completely stop the chain reaction itself. There are other severe issues to be confronted when this happens, but the reactor is totally shut down. (More on this in the page on TMI)
 
Second, the two pieces of the Uranium nucleus left behind after fissioning are actually the nuclei of smaller atoms, such as Krypton, Xenon, Iodine, and more than 40 other elements including all rare earths and numerous semi-precious metals. These new elements are known as "fission fragments" or "fission products" in nuclear jargon. To the rest of the world, they are known as high-level nuclear waste (see the page "Nuclear Waste...Is It?").
 
Third, before nearly all of these highly radioactive fission fragments can migrate out of the fuel and be possibly released into the atmosphere outside of the power plant, the fuel cell must be severely damaged. The only two elements that can escape the fuel without fuel-cell damage are the gasses Krypton and Xenon because they are chemically inert  and will not combine or react with any other substance. Nearly 100% of all routine radioactive releases from nuclear power plants are these two elements . The rest of the "waste" elements are not chemically inert, and are trapped in the toughest, densest material on Earth; Uranium. (For more on this, see the page "Confusion about Fallout" on this website)
 
Finally, there has been some speculation that the explosion that decimated Fukushima daiichi unit #3 on March 14, 2011, appeared to be a small nuclear detonation. This misconception was because the debris cloud thrown in the air by the blast roughly took the shape of an elongated mushroom.
 
Mushroom clouds are not only produced by nuclear blasts. In fact, the vast majority are not. Large, powerful explosions which propagate vertically, without anything to get in the way, will routinely produce a mushroom shape. Anyone who has seen large explosions in action movies has seen this happen. F. Daiichi unit #3's explosion propogated vertically, and it was an unquestionably large detonation, sufficient to shatter and scatter the upper story of a large unreinforced concrete building. That it produced what looked like a mushroom cloud should come as no surprise. But, it was not a nuclear detonation.
 
Summation:
1. Uranium is not a natural explosive.
2. Reactor fuels are 30-90 times too dilute in fissionable isotopes to make a bomb.
3. It is impossible for any power plant reactor to experience a nuclear explosion.
4. Walt Disney's "ping pong balls" demonstration of a chain reaction only works for bombs.

References:

  1. Langer, Mark; Disney’s Atomic Fleet; Animation World Magazine, Issue 3.1;
    http://www.awn.com/mag/issue3.1/3.1pages/3.1langerdisney.html;  April 1998
  2. Light Water Reactors; Hyperphysics; Department of Physics and Astronomy, Georgia State University; http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/ligwat.html#c1; 2005
  3. Montgomery, Jerry, PhD and Rondo, Jeffery, PhD; Asymmetrical Fission Products; Unclear to Nuclear; http://www.unclear2nuclear.com/asymFission.php; 2008