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Saturday, October 12, 2013

Nuclear Weapons - Plutonium Production

After the CP-1 Reactor successfully proved the concept of a self-sustaining fission reaction, the US accelerated its development of nuclear weapons.  The Manhattan Project took a two-pronged approach to generating enough material to build the first bomb: Uranium Enrichment and Plutonium Production.  This post is about Plutonium Production.

The post about Uranium Enrichment is linked here.

Plutonium production is relatively straightforward.  You make a much larger, higher-power version of CP-1.  Engineers are able to take advantage of something that happens when fission occurs in natural uranium fuel, and this is the transmutation of Uranium-238 to Plutonium-239.

Naturally occuring Uranium is only 0.711% fissile U-235, while 99.284% is U-238.  In a large, high-power (meaning high thermal neutron population) graphite-moderated reactor, a significant number of neutrons will be captured by atoms of U-238, simply due to the large number of neutrons and U-238 atoms present.  What happens then is transmutation, which is changing of one element into another.

The U-238 nucleus captures a neutron and becomes U-239.  U-239 has a very short half-life of 23.5 minutes, and it beta decays to Neptunium-239.  Neptunium-239 has a half-life of 2.35 days and beta decays to Plutonium-239, the desired bomb-making product.  Plutonium has a much longer half-life of 24,400 years - reasonably stable, from a weapons design standpoint.

Plutonium-239 is more desirable than enriched Uranium-235 for nuclear weapons for a couple of reasons:  Firstly - when split, the Plutonium-239 nucleus produces more neutrons than U-235.  The significance of this is that more fission events will occur in a Plutonium weapon before the entire assembly is vaporized, generating a larger blast.  Secondly, Plutonium-239 is more likely to fission after absorbing a neutron than is U-235, again producing more fissions, and a larger blast.

For the above reasons, Plutonium-239 has the smallest critical mass of any substance.  A sphere only 4" across, weighing just 24 lbs, without a neutron reflector can be detonated.  By using a neutron reflector and some other techniques, a mere 12 lbs of plutonium can be made to become supercritical - and into a nuclear weapon.

Even with all of this knowledge, there are some serious technical challenges to overcome when designing a successful Plutonium-based nuclear device, starting with the manufacture of the Plutonium itself:
  • Because Plutonium-239 (Pu-239 from now on) is fissile itself, you cannot leave it in the reactor too long, or it will in turn be used up as fuel in the reactor.  Furthermore, Pu-239 can capture a second neutron and become Pu-240, which is a very bad contaminant for a weapon.  For this reason, there is an optimum time to keep the fuel in the reactor.  Too short and you don't create the most Pu-239 possible.  Too long, and the reactor may fission the valuable Pu-239, or convert it to Pu-240.
  • For the above reasons, a Plutonium production reactor must be continuously refueled while in operation.  Production reactors are unlike reactors used for electrical power in design and operation due to the requirement for online refueling.
  • High reactor power requires cooling, and in most cases water was used.  Introducing light water to the core results in modest neutron losses, so the reactor must be made larger, and contain more Uranium to compensate.  A few early Plutonium Production Reactors were cooled by using once-through air cooling.
  • When water is used to cool a graphite-moderated reactor, there is a bad side-effect:  Recall that water is a mild absorber of neutrons, and thus a mild poison.  If the water begins to boil, it will absorb fewer neutrons.  That means more neutrons very suddenly become available to create a fission in Uranium, which increases heat, which boils more water, and so you get a runaway heating/fission cycle.  This is called a "Positive Void Coefficient of Reactivity", and this is also what happened at the very end of the accident at Chernobyl.
Let's have a look at a couple of Plutonium Production Reactors.

Below is a cut-away of the X-10 Graphite Reactor at Oak Ridge, Tennessee.  This was the very first Plutonium Production Reactor built.  As can be seen on the drawing at the lower right, this reactor was air cooled.  X-10 went into operation in 1943, two years before the first nuclear device was detonated, and provided much of the material for it.  The X-10 reactor is decommissioned, and is on the historic registry.

Below, a photo of the same reactor, back when it was in operation.  This is a view of the front, also called the loading face.  As mentioned earlier, these reactors are refueled continuously while in operation.  These technicians are feeding a small slug of fresh fuel into the reactor.  The front side is a thick block of concrete to shield against gamma radiation.  It seems as if neutron and gamma radiation would shine out through the fuel loading holes...  It appears as though a few of the fuel channels have become monitor wells for thermocouples to measure in-core temperatures.

I believe this is a picture of the B Reactor at Hanford, Washington. This is pretty obviously a side-view, showing the water cooling arrangement. Each cooling channel has a valve to adjust the flow and control temperature. This reactor is also currently decomissioned and is on the historic registry.

The back side of a production reactor is also interesting. Every time a slug is pushed into the front of the reactor, an irradiated one pops out the back, and drops into a pool of water. 

And now comes the tricky part.  You have a very, very, radioactive chunk of Uranium metal and fission products, clad in aluminum, containing a few grams of plutonium that you badly want. 

The first step before processing was to give the the slugs a month in water to allow the shorter-lived radioactivity to die down.  Next, the slugs would be dissolved in acid and the Plutonium would be chemically separated, and then refined into metal. 

Unfortunately, the acids and other highly radioactive by-products were simply buried in underground storage tanks (to shield people from the radiation), and these tanks eventually leaked. Then, as now, people only worried about short-term objectives, not long-term damage.

But all of the above steps were merely hurdles toward the goal of getting enough Pu-239 to create a weapon.  And here is what all the engineering, effort, and pollution, was for:  A sphere of high-purity Pu-239 that could be used as the "pit" or core, of a Plutonium weapon.

The sphere below is the "Demon Core", a subcritical mass that became critical on two separate accidents (by using neutron reflectors), killing a scientist on each occasion.

Plutonium, while superior to Uranium for weapons (due to higher neutron yield and higher likelihood of fission), presents difficulties that Uranium does not.  Uranium does not generate a lot of free neutrons until it reaches critical mass.  Therefore to build a Uranium weapon, you may simply slap two sub-critical pieces together and inject a few neutrons from a separate source.  Explosion follows.

Plutonium from a nuclear reactor however, contains the contaminant Pu-240, which is prodigious neutron producer.  Pu-240 decays by spontaneously fissioning!  Since fission is already occuring in the bomb material, it is impossible to use the two-piece assembly method to get a detonation.

If the two-piece assembly method were used for a Pu-239 device, large levels of fission would occur  before the two pieces could physically made contact.  The device would melt and vaporize before a supercritical assembly could take form and you would get a "fizzle".   With Plutonium weapons, a different technique must be used to achieve the result.

The Pu-239 sphere is made hollow.  There is enough material in this sphere to create a nuclear explosion, but only if the sphere is rapidly compressed into a solid.  A large number of directional explosive charges surround the hollow sphere, and when detonated simultaneously, they rapidly crush it into a solid sphere, causing it to have the proper geometry to be instantaneously supercritical.

  It is essential that the Plutonium sphere is evenly crushed from all sides, and that the shock waves from each explosion converge on the center of the sphere at the exact same time.  The development of the trigger mechanism to converge shock waves from several explosive charges to within a millimeter or two was quite a feat at the time.

Below, the Trinity Device.  The first nuclear weapon, tested in New Mexico, July 16, 1945.  Note the dual fire control boxes, and the double wires going to each explosive charge on the sphere.

And here is the end result of all that scientific and engineering effort...  frightening and stunning footage.

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