Nuclear Fission: A ‘Critical’ Inquiry

Fission of U-235

Fission of U-235 -

In the previous post, I described the basic principles behind radioactivity.  In today’s post, I will describe nuclear fission reactions – the technique through which we can deliberately induce heavy atoms to break apart into smaller fragments, releasing energy in through radiation.  In the previous post, we talked about half-lives and what happens to radioactive atoms if one were to leave them alone and let them naturally decay.  As it turns out, there are other ways to make atoms break apart; one can slam atoms with proton and neutrons to make them more unstable, causing them to fragment.  The energy released from this fragmentation can be harnessed in a controlled manner in nuclear reactors, or can be deployed destructively in the form of a nuclear fission bomb.

Shattering an Atom

Recall the major fundamental force that act on particles inside an atom: strong nuclear, weak nuclear, and electromagnetic forces.  The strong nuclear force acts to keep an atom’s protons and neutrons bound together inside the nucleus, and the electromagnetic force causes like charges to repel and unlike charges to attract.  The weak nuclear force causes nucleons to change into other particles.  Leaving aside the weak nuclear force for now, let us look at the other forces that act on nucleons.  Protons in a nucleus are constantly trying to repel each other.  The strong nuclear force is trying to pull together and hold all the nucleons in the nucleus.  But what happens as the nucleus gets bigger?  The strong nuclear force tends to act very locally; hence, as we add more protons to a nucleus, making it bigger, the nuclear force is less able to hold them together against the repulsion which they mutually feel from each other.  When the nucleus is big enough, the forces are balanced on a knife’s edge: even a slight disturbance to the nucleus would cause it to unravel.  This is what causes natural radioactive decay.  We can hasten this process by artificially destabilizing the nucleus.  If we were to inject a proton or neutron into the nucleus, the forces could be deliberately thrown off-kilter, causing the nucleus to tear itself apart.  The resulting particles would be more stable than the initial nucleus.  A more stable arrangement has lower potential energy than a less stable arrangement.  To think of this, imagine a waterfall.  At the top of a waterfall, the water is in a gravitationally disagreeable position.  It has excess gravitational potential energy.  In falling down, that energy is converted to the roaring kinetic energy of the waterfall.  The resulting water at the bottom is warmer due to friction and traveling faster but has lower potential energy.  Similarly, an unstable atomic configuration has more energy than a set of smaller stable ones.  The excess energy is converted to gamma rays (photons) or is expended through kinetic energy by launching the resulting fragments at high speeds.

A Closer Look at Uranium

As an example, let us look at a fissile isotope of uranium called Uranium-235 (U-235).  When U-235 is gently hit by a neutron, the uranium nucleus captures it and rapidly becomes unstable.  It disintegrates to more stable products producing a barium atom, a krypton atom, two or three fast-moving neutrons and lots of gamma rays.  This process is called U-235 fission.  The total amount of energy released in the form of kinetic energy of the product fragments and in the gamma rays is minuscule per atom, but there are a lot of atoms in even a gram of uranium.  If all the atoms in a gram of uranium were be fissioned at once, about 70 gigajoules of energy would be released.  How much is this?  This is the equivalent to the energy released if 5000 tonnes of TNT were detonated.  This is the awesome power of atomic energy.

Chaining Them Together

U-235 Chain Reaction

U-235 Chain Reaction -

In the previous example, I had just assumed that one would, in some magical way, be able to fire neutrons into all the atoms in 1 gram of uranium.  This is not a trivial task, as it is quite hard and quite expensive to inject that many neutrons into a sample of uranium and correctly make them hit all the nuclei.  Instead, we just hijack the neutrons released by the uranium atom itself during fission.  Remember how U-235, when it fissioned, released 3 neutrons?  We can use these neutrons to hit other U-235 atoms to cause them to fission too.  One atom fissioning can be utilized to make 2 or 3 more atoms fission!  These three atoms can then fission and cause 9 other atoms to fission.  Those 9 can take out 27 and pretty soon, you have an avalanche of fissions.  This is known as a chain reaction.  There is a fly in the ointment though.  Uranium fission needs slow-moving neutrons to cause fissions.  The neutrons produced during U-235 fission are fast-moving neutrons.  They can still cause some fissions but they have a harder time doing so and most of them simply escape from the material.  In nuclear bombs, this problem is solved by reflecting neutrons back into the fissile sample using a neutron reflector to give them a better chance at causing fissions.  In a nuclear power plant, it is done by introducing materials that act to slow down neutrons called neutron moderators.  Using reflectors and moderators cause more neutrons to interact with uranium atoms causing the reaction to proceed faster and more efficiently.  During a nuclear chain reaction, if the number of fissions (without any further assistance) remains constant over time, the reaction is said to be critical.  If the number of fissions decrease, it is known as being subcritical and if it increases with time, it is known as being supercritical.

Diablo Canyon Power Plant in San Luis Obispo, CA

Diablo Canyon Power Plant in San Luis Obispo, CA -

A Fine Balancing Act

Without these (and even with these) measures, it is extremely hard to make U-235 actually cause a nuclear explosion.  When uranium starts fissioning in an uncontrolled, uncooled fashion, it heats up, melts and causes the surrounding material to expand and move further apart and change shape.  This causes the reaction to go slower as neutrons produced are less likely to interact with other uranium atoms and cause further fission.  When this happens in a nuclear bomb, which by the way uses conventional explosives to get the party started, the end result is called a nuclear fizzle.  The conventional explosives simply spread the hot uranium in a dust form over a wide area and contaminate it, in essence, turning the fission bomb into a dirty bomb. When the same result happens in the confines of a nuclear power plant, the result is informally called a core meltdown.  The end result is a giant hot pool of radioactive uranium emitting lots of radiation for many, many years.  It is generally next to impossible to get a nuclear explosion in a nuclear power plant and it is extremely difficult to engineer one to deliberately occur as part of a nuclear bomb detonation.

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