On Half Life and Radiation Part III
I do hope this will be the last of this topic for a while, honestly, it's getting pretty dull for me even! I should start where I left off last time, here is a decay chain that happened all the time inside the Hanford nuclear reactors in Hanford Washington during World War II (it happens as well at other reactors but the historical significance of what happened at Hanford is too much to ignore).
So what is going on here? Well we start with Uranium (red circle, I've colour coded by type of atom), now natural Uranium is more than 99% U-238, we bombard that Uranium with a neutron (the +n, no charge) that increases the weight by 1, to U-239 and still 92 protons.
Then one neutron breaks apart into a proton and an electron (called a Beta particle) the beta is so energetic it zooms off into space, thus beta radiation. The proton sticks around and our atom has one extra proton, so 92+1 = 93, which is Neptunium. A second Beta decay occurs and our Neptunium becomes Plutonium.
Notice the weight only changed when we added a neutron, under beta decay the atomic weight (rounded off) is unchanged, the weight actually does change, but beta particles are so light compared with protons and neutrons the apparent atomic weight is effectively the same.
Plutonium 239 has some interesting properties, it's got a long half life, over 24,000 years. (It's the longest lived synthetic material, that I'm aware of, that comes out in the waste stream of a nuclear reactor). It is toxic if ingested, and if you hit it with a neutron odds are the atom will split and in so doing, release A LOT of energy, and a few new neutrons. In fact, the more energetic the neutron that hits the plutonium 239, the more likely it will not only split, but release more neutrons.
Nuclear weapons use very energetic (or fast spectrum) neutrons, most nuclear reactors us low energy (or thermal spectrum) neutrons. Plutonium 239 is great stuff for building bombs with.
In the early 1940s it became very apparent bombs could be built using Uranium-235 or Plutonium-239. U-235 is rare, it occurs about 0.7% of the time in natural Uranium and you cannot chemically separate it from the far more abundant U-238 since it's all Uranium, but Plutonium is a different story.
As America started ramping up her war effort after the attack on Perl Harbor it was decided to build a number of large water cooled nuclear reactors on the Columbia river in Washington state. The reactors would split Uranium-235 using thermal neutrons, each time a U-235 was split two or three new neutrons would be released. Some neutrons would go on to split more U-235 some would be absorbed by U-238.
These reactors would use graphite to bring the the neutrons into thermal equilibrium (hence thermal spectrum neutrons). Such thermal neutrons tend to split U-235 and often bounce off (or deflect from) U-238 but from time to time the U-238 would absorb the neutron and the result was the decay chain I outlined above.
The fuel would stay inside Hanford for about 8 weeks, after that time, it would be ejected from the reactor and then undergo a process of chemical separation, Plutonium was kept, everything else was considered waste and put into containment.
Ultimately the plutonium was used, first in the Trinity test bomb detonated in the New Mexico desert early in the morning of July 16, 1945, and then a second plutonium bomb was dropped on Nagasaki Japan on August 9, 1945. (The Hiroshima weapon used nearly pure U-235, after two years of enrichment the United States had enough U-235 for one bomb, the Hiroshima bomb design was never tested.)
By the time the bomb was dropped it was estimated that the reactors (there were three operational several more would come online after the war) at Hanford could make enough Plutonium for two or three bombs every month. By the time President Kennedy was elected Hanford was producing several tons of Plutonium every year (a typical bomb needs less than ten pounds of plutonium).
Besides the massive the cleanup that is now in progress (and will be for decades at Hanford) another legacy of our arms race was all that weapons grade plutonium (and enriched uranium). If ever there was an argument for nuclear energy, surely the fact that we can convert this weapons material into electricity has to be a leading reason.
In my next entry I hope to discuss neutron cross sections, which I alluded to here. In the meantime here is a fascinating documentary about the construction of the Hanford Site during the war (or click on the image below).
So what is going on here? Well we start with Uranium (red circle, I've colour coded by type of atom), now natural Uranium is more than 99% U-238, we bombard that Uranium with a neutron (the +n, no charge) that increases the weight by 1, to U-239 and still 92 protons.
Then one neutron breaks apart into a proton and an electron (called a Beta particle) the beta is so energetic it zooms off into space, thus beta radiation. The proton sticks around and our atom has one extra proton, so 92+1 = 93, which is Neptunium. A second Beta decay occurs and our Neptunium becomes Plutonium.
Notice the weight only changed when we added a neutron, under beta decay the atomic weight (rounded off) is unchanged, the weight actually does change, but beta particles are so light compared with protons and neutrons the apparent atomic weight is effectively the same.
Plutonium 239 has some interesting properties, it's got a long half life, over 24,000 years. (It's the longest lived synthetic material, that I'm aware of, that comes out in the waste stream of a nuclear reactor). It is toxic if ingested, and if you hit it with a neutron odds are the atom will split and in so doing, release A LOT of energy, and a few new neutrons. In fact, the more energetic the neutron that hits the plutonium 239, the more likely it will not only split, but release more neutrons.
Nuclear weapons use very energetic (or fast spectrum) neutrons, most nuclear reactors us low energy (or thermal spectrum) neutrons. Plutonium 239 is great stuff for building bombs with.
In the early 1940s it became very apparent bombs could be built using Uranium-235 or Plutonium-239. U-235 is rare, it occurs about 0.7% of the time in natural Uranium and you cannot chemically separate it from the far more abundant U-238 since it's all Uranium, but Plutonium is a different story.
As America started ramping up her war effort after the attack on Perl Harbor it was decided to build a number of large water cooled nuclear reactors on the Columbia river in Washington state. The reactors would split Uranium-235 using thermal neutrons, each time a U-235 was split two or three new neutrons would be released. Some neutrons would go on to split more U-235 some would be absorbed by U-238.
These reactors would use graphite to bring the the neutrons into thermal equilibrium (hence thermal spectrum neutrons). Such thermal neutrons tend to split U-235 and often bounce off (or deflect from) U-238 but from time to time the U-238 would absorb the neutron and the result was the decay chain I outlined above.
The fuel would stay inside Hanford for about 8 weeks, after that time, it would be ejected from the reactor and then undergo a process of chemical separation, Plutonium was kept, everything else was considered waste and put into containment.
Ultimately the plutonium was used, first in the Trinity test bomb detonated in the New Mexico desert early in the morning of July 16, 1945, and then a second plutonium bomb was dropped on Nagasaki Japan on August 9, 1945. (The Hiroshima weapon used nearly pure U-235, after two years of enrichment the United States had enough U-235 for one bomb, the Hiroshima bomb design was never tested.)
By the time the bomb was dropped it was estimated that the reactors (there were three operational several more would come online after the war) at Hanford could make enough Plutonium for two or three bombs every month. By the time President Kennedy was elected Hanford was producing several tons of Plutonium every year (a typical bomb needs less than ten pounds of plutonium).
Besides the massive the cleanup that is now in progress (and will be for decades at Hanford) another legacy of our arms race was all that weapons grade plutonium (and enriched uranium). If ever there was an argument for nuclear energy, surely the fact that we can convert this weapons material into electricity has to be a leading reason.
In my next entry I hope to discuss neutron cross sections, which I alluded to here. In the meantime here is a fascinating documentary about the construction of the Hanford Site during the war (or click on the image below).
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