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I have a couple of questions about nuclear reactors used for electricity generation.

a) If the spent fuel is still radioactive and quite hot, why is it disposed off ? Why can't its energy be used / harvested to do any more work ?

b) Do all reactors use the radioactive fuel to heat a working fluid to propel a turbine ? Specifically, are there technologies / methods of directly harvesting the momentum of the fission particles and the radiation to do work ?

Thanks.

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a) To convert heat energy to electricity the heat energy has to be at sufficiently high temperature, as the latter limits the percentage of heat energy that can be converted to other energy forms. Very roughly the efficiency $\epsilon$ of an idealised heat machine is given by:

$$\epsilon\:\text{(%)} \approx 100 \times \big(1-\frac{T_H}{T_L}\big)$$

Where $T_H$ is the hottest point of the cycle and $T_L$ the coolest. Spent fuel just isn't hot enough and of course also dangerous to handle, further decreasing profitability.

b) Almost all civilian nuclear energy production relies on turning the fission energy into high temperature steam, used to drive turbines to generate electricity. Some minor specialist applications turn heat directly into small amounts of electrical energy.

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In answer to your first question, the fuel in standard nuclear reactors is Uranium-235. The chain reaction that produces the heat is started by bombarding slow moving neutrons at U-235 nuclei - http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fission.html

The speed of these neutrons, and the choice of U-235 is very specific to the reaction. The spent fuel roads in a reactor have used most of the uranium content, leaving more stable fission products that can't be induced into a chain reaction - http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/u235chn.html

The fuel rods coming out of the reactor may be hot because they've come out of a reactor with a temperature of a few hundred degrees, but theyre no longer contributing to maintaining that heat since they're not a part of the chain reaction anymore. They'll quickly lose this heat once they leave the reactor, so theyre not much use in terms of energy production.

Fuel rods coming out of a reactor can be very radioactive, as the fission products are usually unstable. These fuel rods are often called 'hot' by nuclear physicists, but that's in reference to the energy of the radiation released from them, not their temperature, and this radiation is hard to harness efficiently into energy.

In answer to your second question, the heat produced in the reactor is a measure of the increase in kinetic energy of the particles within the reactor, and the transfer of momentum from these particles to the coolant passing through it. So essentially the process is already directly harnessing the momentum of the particles.

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  • $\begingroup$ By directly harvesting their momentum, I mean using the particle directly to produce electricy, either by the Seebeck effect (see answer by Gert below) or by some variation of the photo-electric effect. Also, what is the temperature of the spent rods ? I get the impression that they are hot enough to boil the water they are immersed in ? $\endgroup$ – firdaus Jan 19 '16 at 1:13
  • $\begingroup$ The temperature inside the average nuclear reactor is around 300-400 degrees, so yes spent fuel rods are hot enough to boil water, but as they are spent fuel rods, obviously they only have that temperature because they've been sitting in a hot reactor, once removed they will lose that heat pretty quickly, not very useful in maintaining the steady flow of steam necessarry to produce reliable energy. $\endgroup$ – Robin Jan 19 '16 at 4:42
  • $\begingroup$ Posted my comment early, I'll continue in a new one for clarity. With regards to your momentum question: there isn't much of an analogy to be made of slow moving neutrons to the photoelectric effect, as in that process a charged particle is released by the incidence of light. where as in U-235 reactors, slow moving neutrons are absorbed by Uranium nuclei, inducing them to break into fission products and release other neutrons. $\endgroup$ – Robin Jan 19 '16 at 4:51
  • $\begingroup$ It's worth, noting that for a chain reaction to occur in a U-235 reactor, the neutrons scattered among the reactor must be travelling a relatively low speeds (see my earlier reference), this requires the fuel rods to be contained within a dense material known as a moderator, often this is carbon. So this would make any complicated set up involving isolating the particle's momentum directly quite difficult. $\endgroup$ – Robin Jan 19 '16 at 4:53
  • $\begingroup$ With regards to the seebeck effect, even with a constant massive water flow through these reactors, they still need constant checking with boron rods that absorb excess neutrons and slow down the reactor. The temperature range of 300-400 degrees within a reactor would make it quite a hostile environment to try to introduce large conducting materials. $\endgroup$ – Robin Jan 19 '16 at 5:00

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