[Donaldson Tan]

Thermonuclear plants provides 17% of the electrical energy used in the United States. Clearly, nuclear power is the future and the alternative to fuel. There are many R&D carried out in extracting energy efficiently from nuclear reactors and also scaling down the size of nuclear reactors and development of nuclear microbatteries to power small devices.

I am trying to understand the gist of how the LWR nuclear reactor operates. It involves UO2 rods arrange in bundles to heat up water.

Fission of U-235 can be induced by bombarding it with slow neutrons. U-235 undergoes fission to produce 2 daughter nuclei and 3 nuetrons. For civilian use, it is important to ensure that the uraninium rods are maintained at its critical mass, whereby only 1 neutron from each fission reaction attack another U-235 nucleus.

The thingy here is that only U-235 is fissible, not U-238. Uranium exist as 0.7% U-235 and 99.3% U-238. The UO2 is enriched industrially by centrifugation to remove U-238 to increase the proportion of U-235 to 3.5%. The fission reaction described by the above paragraph only works for U-235, so what happen to the rest of the uranium, ie. U-238?

Does nuetron from the fission reaction of U-235 attack U-238 to produce P-239? Plutonium 239 is known to be fissible. If we can convert U-238 to P-239, does nuclear fuel enrichment plants equip with capabilities to convert U-238 to P-239? Otherwise, alot of U-238 will be throwned away. The energy from U-238 can be harnessed by converting it to P-239

Centrigation is the current industrial method to seperate U-235 and U-239. However, a more modern techniques employs gaseous diffusion of UF6 to seperate the 2 isotopes. Why is the former technique more preferred? Centrifugation is definitely WWII technology, borned out of the renowned Manhatten Project. Why isnt the gaseous diffusion technique catching on? Gasesous diffusion is so much more faster..

[jdurg]

Actually, gaseous diffusion of UF6 is what was used during WWII.  The problem with it is that you need a HUGE amount of space for the gaseous diffusion plants, and you also need to work with a lot of fluorine gas.  Two things that most people aren't too keen about.  So centrifigation doesn't require as much space or time to perform as does UF6 gaseous diffusion, therefore it's much cheaper.

In regards to the situation with the 3.5% U-235, the remaning U-238 in there is highly likely to absorb a neutron and form Np-239 which then becomes Pu-239.  Some of this Pu-239 will fission as well and provide more energy to the overall reaction, or the U-238 will bounce the neutrons back into the U-235 and ensure that they fission.  As long as you have a critical mass of fissionable U-235 in the reactor core, spontaneous fission will result.  It doesn't matter how close together the atoms are.  If they're too close together, then you get an uncontrolled chain reaction and a nuclear explosion.  You don't want that in a power plant.  

The problem with the breeder reactor plan is that you're producing a lot of impure Pu-239.  So after the fuel rods have met their lifespan, you have to refine the nuclear waste product to remove the Pu-239.  Pu-239 is also VERY easily made into a nuclear bomb so you then need to have MASSIVE security measures and deal with the international stigma of producing fissile plutonium.  All of that costs a lot of money and the cost/benefit ratio isn't that great.  Plus, you will need to use a lot of fossil fuels to transport the waste material and then refine it.  So the overall fuel savings aren't as great as they initially appear.  (This is a main problem that most people overlook.  They think that the fuel can be refined and transported without the use of any other fossil fuels.  This is simply not the case.  The refinement of the raw ore to get fissile fuel for a nuclear power plant is a long process that requires a lot of energy to successfully peform.)

[Donaldson Tan]

ok. so what happens in a typical nuclear plant is that they have 2 nuclear reactors. The first nuclear reactor harness energy from the fission of U-235. The spent fuel from the the first reactor is processed to convert it to P-239. The 2nd nuclear reactor then  harnesses energy from the processed spent fuel (mainly P-239). the waste from the 2nd nuclear reactor is then treated to remove any heavy metal or toxic radioactive isotopes before disposing into the environment.

am I right?

[jdurg]

The problem is, when a spent fuel rod is removed from the reactor, it is so insanely radioactive that if any unprotected person got within a good ten yards of it they would drop dead immediately.  Spent fuel rods are mechanically removed and forced to 'sit' for quite a long time in order to let the intensely radioactive isotopes in there decay before any type of processing is done on them.  The processing of spent nuclear fuel rods is a very dangerous and time consuming process.  Also, I believe that standard uranium fueled reactors are able to use plutonium rods as well, so having two reactors would just be more costly as you'd need to have two well trained crews to run both reactors and the dangers associated with running a nuclear power plant would double.  Again, not all too cost effective.

[constant thinker]

I happen to remember another process for harnessing nuclear energy. It directly absorbed particles (I don't remember which type) and used it as electricity. I believe this process is used in deep space satellites because it's more compact and the satellites don't require tremendous amounts of power.

[Corvettaholic]

Are you thinking of RTGs? I believe those work by having a radioactive isotope sitting in the box, and the said box has a bunch of thermocouples so when those get nice and toasty you get electricity.

[Donaldson Tan]

we were discussing abt RTG-based nuclear batteries in an earlier post. Here's the link. You may look at it for more information: http://www.chemicalforums.com/index.php?board=9;action=display;threadid=2742