You Might Want to Know: What’s the other fissile fuel? (Answer: Plutonium) How is it made?
Part IV of IV this week
Part IV You Might Want to Know: What’s the other fissile fuel? How is it made?
“Fissile fuel” is fuel than can be used in an atomic bomb because it can generate a chain reaction of nuclear fission. The chain reaction is caused by neutrons that are thrown off when nuclei in the fissile fuel are “fished” by other neutrons. This fission releases the energy that, if it happens in a short enough time—no more than a millionth of a second—produces the explosion.
The fissile fuel used in Little Boy, the bomb the U.S. dropped on Hiroshima, was highly enriched uranium. The other fissile fuel, the fuel used in the Trinity test on July 16, 1945 to produce the first atomic explosion ever on earth, and the one used in the bomb dropped three weeks later on Nagasaki, was plutonium.
Uranium exists naturally. Plutonium doesn’t, except in trace amounts. An American chemist named Glenn Seaborg discovered plutonium in 1940 in the products of uranium that had been bombarded in Ernest Lawrence’s cyclotron at the University of California-Berkeley with the nuclei of heavy hydrogen. Soon afterwards, the scientists discovered that plutonium was fissile. When it “fished,” it threw off neutrons than might cause a chain reaction. A war was on in Europe. They decided not to publish their discovery. We weren’t in it yet but might be soon.
Since plutonium does not exist, for all practical purposes, in nature, we have to make it. It is made by bombarding uranium with neutrons. Uranium is being bombarded by neutrons in every operating nuclear reactor. That means that every operating reactor is making plutonium as one of the products of the fission that is going on in the reactor’s fuel elements.
In the Manhattan Project, which was established in 1942 after we had joined the war, the third site to be established--Site W at Hanford in eastern Washington State--was entirely devoted to the production of plutonium. Site W was established in 1943, after Site X in Oak Ridge, Tennessee for enriching uranium and Site Y in Los Alamos, New Mexico for bomb design.
Plutonium has several isotopes. For a nuclear bomb the isotope that works best is Pu239, which has 94 protons and 145 neutrons. After the fuel elements have been irradiated in a reactor, the Pu239 must be separated from the other plutonium isotopes and radioactive fission products. This can be done in a chemical process.
At Hanford, the irradiated fuel elements were conveyed to big buildings called “canyons” that were automated and heavily shielded. The process produced, besides the Pu239, large amounts of highly radioactive liquid waste which had to be rendered safe somehow or stored. We didn’t (and don’t) have any way of rendering this waste safe, so it had to be stored. Wherever such waste is stored, its high levels of radioactivity will make it dangerous for thousands of years. At Site W, on the Columbia River eastern Washington, it was put into big tanks, eventually over 170 of them. We hoped it would stay there. It didn’t.
The first plutonium delivered to Site Y at Los Alamos arrived in February 1944. It came not from Site W in Hanford but from Site X at Oak Ridge, which had its own reactor and chemical separation facility.
It was a tiny amount of Pu, but enough to allow the scientists at Site Y to realize that plutonium could not be used as the fissile fuel in a gun-type bomb like Little Boy. It was too fissile. It couldn’t be slapped together fast enough to keep the nuclear chain reaction from starting before the plutonium had been compressed into a supercritical state. The result would be not a mighty explosion but a radioactive “fizzle.”
The scientists at Site Y now realized they would have to focus on a bomb design different from the straightforward gun-design for the Hiroshima bomb. In this new design, conventional explosives would be designed to explode inward and placed around a ball of plutonium. This might, the scientists thought, if the explosion was uniform, allow the plutonium to be compressed quickly enough to put it into the supercritical state that would generate the nuclear explosion.
If the design of a bomb using HEU as the fissile fuel was no big challenge, getting the implosion design to work was. The Manhattan Project physicist Luis Alvarez wrote, before they figured it out how to do it, that “making [plutonium] explode is the most difficult technical job I know.”
Everybody knows now that while it may not be easy, it’s do-able.
By early 1945, when the scientists at Los Alamos began to get the first shipments of plutonium from Hanford, they thought they were on the way to solving this “most difficult” technical problem.
On July 16, 1945, at the White Sands Test Range in southern New Mexico, in the test called Trinity, the implosion design that used plutonium as the fissile fuel was tested. It worked, and the first-ever atomic explosion on earth resulted. The bomb dropped on Nagasaki on April 9, 1945, was this kind of bomb. It was the second atomic bomb to be used in combat. And, so far, the last one.
The good news, it turned out, was that the implosion design generated more efficient nuclear explosions than did the gun design. That meant we would need less of the very expensive fissile fuel for our nuclear bombs, whether plutonium or highly enriched uranium.
There was some bad news. The scientists at Los Alamos had realized, even before the Trinity test, that plutonium, the other fissile fuel, which we were now manufacturing in the nuclear reactors at Hanford as fast as we could, was, if a particle was inhaled, one of the most toxic substances on earth.
Between the end of World War II and 2003, according the 2005 report “Global Supplies of Nuclear Explosive Material,” more than one-thousand eight-hundred tons of plutonium had been produced in the thirty-five countries that had operating nuclear reactors. Over two-hundred fifty tons were weapons-grade plutonium, defined as containing less than 7% Pu 240. Enough for two-hundred fifty-thousand bombs.
Every year, these totals grow.
Next: How many nuclear tests have we conducted?
Hello all. Apologies for the typo in the fifth to last paragraph, to which I was alerted by alert reader David D Robert’s. The correct date for the Nagasaki bombing is of course August (not April) 9th. Thanks, DD.
There's a big typo in the fifth-to-last paragraph: Nagasaki was bombed on August 9, 1945, not April 9.