By Ron B. Davis Jr., Georgetown University
The first and only nuclear devices ever deployed in wartime used extremely concentrated samples of fissile nuclei obtained through nuclear fission. Uranium-235 was used in the ‘Little Boy’ bomb dropped on Hiroshima. Plutonium-239 was used in the ‘Fat Man’ bomb dropped on Nagasaki—and a month earlier in a bomb test that took place in Alamogordo, New Mexico.
The Decay Chain
In both uranium and plutonium, a specific amount of weapons-grade material is necessary to achieve what is called ‘criticality’. Only when enough fissile material is packed into a small enough volume can we reach the condition where neutron flux is sufficiently high to cause a runaway chain reaction.
Most radioactive isotopes, which are lighter than uranium, emit alpha particles as part of their decay chain. The loss of two protons and two neutrons per alpha particle, successively, decreases the mass of the remaining nucleus on its way to a stable end point. But with uranium and elements to its right, there is a short-cut available, an alternative path to stability—the process of fission.
A Fissile Nucleus
Certain isotopes of these heavier actinoids are what scientists call ‘fissile’. A fissile nucleus can be split into two or more smaller nuclei of substantial size, usually because of a collision with a neutron. The less-common uranium-235, with a half-life of about 700 million years, is one such nucleus.
Thorium by and large undergoes radioactive decay to become the smaller, stable nucleus of lead. The same is true of the most abundant isotope of uranium, uranium-238.
However, when it comes to nuclear fission, uranium-235 can break into significantly large pieces, like the barium and krypton. Several neutrons accompany that process, since neither of the smaller resulting nuclei needed so many neutrons to find a home on the nuclear stability curve. The free neutrons can go on to promote additional fissions in a chain reaction.
Fission Chain Reaction
When fission chain reactions are desirable, even in controlled reactors, uranium is slightly enriched in the fissile isotope to ensure that there are enough fissile nuclei and free neutrons to maintain a chain reaction. So-called ‘fuel grade’ uranium, for example, is enriched in uranium-235 to a concentration of about 4%, or around five times greater than its natural abundance.
Thus, uranium-235 needs to be enriched as much as possible when we want the fast, explosive, all-out power of nuclear weapons.
The minimum quantity of a fissile material needed to achieve a nuclear explosion is commonly referred to as the ‘bare critical mass’ of that material. The bare critical mass of weapons grade plutonium is about ten kilograms, or 22 pounds. For U-235, a ‘bare critical mass’ is 52 kilograms, or almost 115 pounds.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Neptunium-based Atom Bomb
This brings up an interesting question. If uranium and plutonium can be used to produce nuclear fission reactions, what about neptunium? Neptunium is sandwiched right there in between uranium and plutonium—surely neptunium must have very similar properties? Yet to date, the manufacture of a neptunium-based nuclear bomb has never been publicly acknowledged by any government.
The fact is that production of a neptunium-based atom bomb is theoretically entirely possible. Just like many isotopes of plutonium, the fissile isotope neptunium-237 forms readily in nuclear reactors, with a half-life of about 2 million years making it long-lived enough to use in constructing weapons.
In principle, that could indeed be used to produce a device that creates a runaway chain reaction, just like the uranium in Little Boy or plutonium in Fat Man.
Neptunium’s Critical Mass
The reason that neptunium has never been used is a simple practical consideration—mass.
Neptunium isn’t easy to come by. Like plutonium it has to be made in so-called breeder nuclear reactors or harvested from spent nuclear fuel. And neptunium’s critical mass is six times that of weapons grade plutonium, meaning that even the smallest of neptunium devices would likely have to weigh considerably more and use much more costly nuclear fuel to achieve the same result.
Add to this the fact that development of the plutonium-based atom bomb was well underway before the fissile neptunium-237 nucleus was even discovered, and it is easy to see why neptunium was never used.
There were simply more effective devices already in development before its fissile isotope was ever available. Neptunium was essentially obsolete as a nuclear fuel source before it was even discovered.
The actinides are placed on the periodic table where the repulsive forces of packing more and more positive charge into the nucleus really begin to overwhelm the nuclear binding energy of atoms. That makes these elements progressively less stable as we move across the series.
The actinoids with odd atomic numbers, starting with actinium and protactinium, have roles in major nuclear decay chains make them of interest to nuclear scientists. But their vanishingly small abundance makes them relatively useless as raw materials.
Thorium and uranium, the smallest even-numbered elements of the series at 90 and 92, are different. Both have at least one isotope that is primordial and are made fairly abundant in Earth’s crust by the fact they are lithophilic. The remarkably long half lives of their most stable nuclei have also made them indispensable actors in reconstructing the geological history of our planet through radiological dating.
Synthetically Produced Plutonium
What is eye-opening is how plutonium, though virtually absent in nature, can be synthetically produced as long-lived isotopes by neutron bombardment.
During World War Two, refining enough uranium-235 from U-238 was difficult, so as a second source of fissile material, scientists took the depleted U-238 and bombarded it with neutrons to make plutonium that made possible additional bomb designs.
And of course, there’s poor neptunium, whose potentially useful 237 isotope was only uncovered after plutonium had rendered it effectively obsolete.
Common Questions about Uranium and Plutonium
Only when enough fissile material is packed into a small enough volume can we reach the condition where neutron flux is sufficiently high to cause a runaway chain reaction.
The production of a neptunium-based atom bomb is theoretically entirely possible. Just like many isotopes of plutonium, the fissile isotope neptunium-237 forms readily in nuclear reactors, with a half-life of about 2 million years making it long-lived enough to use in constructing weapons.
Plutonium, though virtually absent in nature, can be synthetically produced as long-lived isotopes by neutron bombardment.