Physics Question #2700

James Hope, a 18 year old male from Gloucester asks on April 8, 2005,

I have a physics paper here (don't worry i am not asking you for the answers!) that says each U-235 fission releases 206MeV like this: Fission Fragmens: 166 Gamma photons : 15 antineutrinos: 12 beta particles: 8 neutrons: 5 from this I see that only 15Mev is really released as usable energy, the fragments are obviously still atoms, the antineutrinos just fly off somewhere, the beta particles are just electrons that will get absorbed somewhere and the neutrons hang around to initiate more fissions. Also I see no heat being produced. So what is used in nuclear reactors to boil the water?

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The answer

John Elton McFee answered on August 4, 2005

[Editor: In fact a tremendous amount of heat is being produced as kinetic energy after nuclear fission occurs.] The fission fragments and betas, being charged particles, are slowed down in a short distance by electromagnetic interactions with atoms in the reactor core (the part of the reactor which contains the uranium, moderator and control rods). Their kinetic energy is absorbed by the medium that they are in and this heats the materials up. This heat can be transferred to a coolant by a variety of means. Neutrons, being uncharged, slow down by collisions with nuclei in the core, much like billiard balls scattering. They take a longer distance to slow down, but they too lose their kinetic energy until they are moving relatively slowly. That lost kinetic energy is again absorbed by the reactor core and surroundings and heats it up. If the core and surrounding water are large enough, the gamma rays get absorbed by a variety of mechanisms. The only energy that escapes is the 12 MeV belonging to the neutrinos.

Jeremy Whitlock answered on August 4, 2005

Actually, most of the energy in the list is recoverable. As your list shows, the largest fraction of fission energy (about 80%) is found in the kinetic energy of the two fission fragments. Since the fission events occur in a dense medium (the uranium fuel), the two fragments travel only a fraction of a millimetre before they are slowed by collisions and transfer their kinetic energy to the lattice, which shows up as heat.

Similarly, the gammas, betas, and free neutrons all contribute to the heating inside the reactor, generally within a metre or so of each originating fission event. In a Canadian heavy water CANDU power reactor, the heat which isn't lost to the shielding or the moderator is removed by the coolant and used to raise steam for electricity generation.

The only unrecoverable energy from fission is that few percent carried off by the neutrinos, which have an almost non-existent interaction probability with matter. Neutrinos stream out of a nuclear reactor by the billions per second, go right through the surrounding buildings and the earth itself, and head out into space to join the rest of the dark matter in the universe.

Where did all this energy come from? It was locked up in the original uranium nucleus: a ball of 92 positive charges (protons) trying to repel each other, but held tight by a nuclear "glue" that only works at close proximity (i.e. when protons and neutrons are "touching" each other). This battle between electrostatic repulsion and nuclear attraction is lost when a stray neutron enters the uranium nucleus and causes instability. Usually the uranium nucleus then begins to wobble and shake, until electrostatic repulsion takes over and flings two halves of the nucleus in opposite directions. The total mass of the two fragments plus other released particles is less than that of the original uranium nucleus (plus the stray neutrons), and the missing part is what shows up as (mostly kinetic) energy, through Einstein's famous mass-energy equivalence relationship.

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