Questions about fast reactors and nuclear waste

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In summary, the conversion ratio should be determined by the TU content at discharge of fuel, but not on the entire core, since some of the fuel is retained for one or more additional cycles. Also, if your objective is to burn transuranics, should you put just Plutonium and no Uranium in the fuel? Because if you put Uranium, then the U-238 transmutes to U-237 through n,2n, then decays beta into Np-237 which is very inconvenient because it has a very long half life.
  • #1
Cactor
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Hello, I have a few questions regarding the transmutation processes inside fast reactors. I would appreciate your help. I am doing some work at the university about SFR and ABR.

First of all, I know that fast reactors can operate in 3 modes: burner, breeder and converter (halfway between burner and breeder). My question is, can the same reactor operate in any of these ways, or are they only designed for one specific mode? If you put MOX fuel, does it mean that you want it to operate in breeder mode?

Another thing is, how do you calculate the transuranics conversion ratio? I am currently doing (mass of transuranics at EOC)/(mass of transuranics at BOC) but I don't know if this is correct.

Also, if your objective is to burn transuranics, should you put just Plutonium and no Uranium in the fuel? Because if you put Uranium, then the U-238 transmutes to U-237 through n,2n, then decays beta into Np-237 which is very inconvenient because it has a very long half life.

Also, could you please give me some good internet bibliography about SFRs? Google is not really helpful.

Thanks.
 
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  • #2
Cactor said:
Hello, I have a few questions regarding the transmutation processes inside fast reactors. I would appreciate your help. I am doing some work at the university about SFR and ABR.

First of all, I know that fast reactors can operate in 3 modes: burner, breeder and converter (halfway between burner and breeder). My question is, can the same reactor operate in any of these ways, or are they only designed for one specific mode? If you put MOX fuel, does it mean that you want it to operate in breeder mode?

Another thing is, how do you calculate the transuranics conversion ratio? I am currently doing (mass of transuranics at EOC)/(mass of transuranics at BOC) but I don't know if this is correct.

Also, if your objective is to burn transuranics, should you put just Plutonium and no Uranium in the fuel? Because if you put Uranium, then the U-238 transmutes to U-237 through n,2n, then decays beta into Np-237 which is very inconvenient because it has a very long half life.
The mode depends on the fuel design and core design (e.g., size of blanket), and proportions of Pu and TU isotopes, and enrichment. It's a matter of balancing Ʃf and Ʃa within the fuel assembly, among fuel assembly, and among regions of the core.

The breeder/conversion ratio should be determined by the TU content at discharge of fuel, but not on the entire core, since some of the fuel is retained for one or more additional cycles.

Fast reactors generally use MOX fuel with about 20% fissile content, although the fissile content may vary (+/-). Usually MOX is (U,Pu)O2, where the Pu(239,240,241,242) and perhaps some Am, Cm is diluted in a matrix of natural or depleted UO2, so one still has to deal with the transmutation of U235 and U238. Usually, for a breeder reactor, the intent is to convert U-238 to fissile Pu for future use.

Fast reactors can also use mixed carbide (MC) fuel, mixed nitride (MN) fuel, mixed carboxide (MCO) fuel, metal fuel (e.g., (U,Pu)Mo), and combinations, e.g., cermet, or cer-cer fuel. (U,Pu)ZrC is another possibility and represents fissile material dispersed in an inert matrix.

IAEA has some TECDOC's on fast reactor technology and experience, transmutation and fuel cycle matters. I'll try to post links later.

Update: Look for Recent Publications on this page - http://www.iaea.org/NuclearPower/FR/
Click on the links and one can find a link to a free pdf that one can download (use Save Target As)

See also - http://www.iaea.org/INPRO/publications/index.html - for some reports on Innovative fuel and fuel cycles
 
Last edited:
  • #3
Ok, I just realized that my formula to calculate the transuranics conversion ratio was wrong.

It should be:

TRU.CR.= (HMD-TRUD)/TRUD

Where HMD is total heavy metal mass destroyed and TRUD is total transuranic mass destroyed. Or, to make it simple, it is like:

TRU.CR.= U-238 destroyed / Pu-239 destroyed
 
  • #4
Cactor said:
Ok, I just realized that my formula to calculate the transuranics conversion ratio was wrong.

It should be:

TRU.CR.= (HMD-TRUD)/TRUD

Where HMD is total heavy metal mass destroyed and TRUD is total transuranic mass destroyed. Or, to make it simple, it is like:

TRU.CR.= U-238 destroyed / Pu-239 destroyed
That doesn't seem quite correct.

The conversion ratio should be related to quantity of TRU destroyed divided by the initial quantity or the total TRU, which would include the initial and that which is produced during the process.

U-238 does undergo fast fission as well as neutron capture. The destruction of U-238 should not be considered in TRU conversion directly, but only the portion which becomes TRU during the process, which is the TRU production part.
 
  • #5
Astronuc said:
That doesn't seem quite correct.

The conversion ratio should be related to quantity of TRU destroyed divided by the initial quantity or the total TRU, which would include the initial and that which is produced during the process.

U-238 does undergo fast fission as well as neutron capture. The destruction of U-238 should not be considered in TRU conversion directly, but only the portion which becomes TRU during the process, which is the TRU production part.

I made a very coarse approximation in the second formula. It's an hipotethical case in which you only have U-238 and Pu-239. All Uranium transofmrs into Plutonium, and all Plutonium undergoes fission. This approximation is not so far from reality.

The TRU.CR is, in theory = TRU atoms produced / TRU atoms destroyed through the cycle. You don't care about the initial or final amount, you just want what's been produced divided by what's been destroyed.

My source of information is:

Description of Transmutation Library for Fuel Cycle System Analyses
Steven J. Piet
Samuel E. Bays
Edward A. Hoffman
August 2010

Page 16

(sorry I can't put links)

I know that U-238 can undergo fission, but I lack that information for my calculations, so I approximate and say that they only absorb neutrons.
 

1. What is a fast reactor?

A fast reactor is a type of nuclear reactor that uses fast neutrons to sustain the chain reaction that produces energy. Unlike traditional nuclear reactors, which use slow or thermal neutrons, fast reactors use a high energy neutron spectrum. This allows for more efficient use of nuclear fuel and has the potential to produce less nuclear waste.

2. How do fast reactors differ from traditional nuclear reactors?

Traditional nuclear reactors use slow or thermal neutrons to sustain the chain reaction, while fast reactors use fast neutrons. This difference allows for a more efficient use of nuclear fuel and the potential to produce less nuclear waste. Additionally, fast reactors have the ability to use a variety of fuels, including recycled nuclear fuel, and can even produce more fuel than they consume.

3. What is the purpose of fast reactors?

The primary purpose of fast reactors is to provide a sustainable and efficient source of nuclear energy. They have the potential to produce less nuclear waste and can use a variety of fuels, including recycled nuclear fuel, making them a more environmentally friendly option for nuclear energy production. Fast reactors also have the potential to reduce the amount of long-lived radioactive isotopes in nuclear waste, making it easier to handle and store.

4. How do fast reactors handle nuclear waste?

Fast reactors have the ability to use recycled nuclear fuel, which reduces the amount of nuclear waste produced. Additionally, they can use a variety of fuels, including plutonium and other transuranic elements, which are long-lived radioactive isotopes that make up a significant portion of nuclear waste. By utilizing these elements, fast reactors can reduce the amount of long-lived radioactive isotopes in nuclear waste, making it easier to handle and store.

5. Are fast reactors safe?

Like any nuclear technology, fast reactors carry some inherent risks. However, fast reactors have been designed with safety measures in place, such as passive cooling systems and multiple barriers to prevent the release of radioactive material. Additionally, the use of fast reactors has the potential to reduce the amount of long-lived radioactive isotopes in nuclear waste, making it safer to handle and store. Overall, with proper regulation and safety protocols in place, fast reactors can be a safe and efficient source of nuclear energy.

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