Energy From Thorium Discussion Forum

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PostPosted: May 23, 2014 10:07 am 
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Would it be useful to use a term like "Breeding ratio" to explain why some types of reactors are more fuel sustainable then others? From what I understand all reactors breed some of their fuel because they contain fertile atoms which will sometimes catch neutrons, turn into fissile ones and fission.

It would be interesting to know some numbers if possible for different types of reactors. I'm assuming water cooled ractors have a poor breeding ratio and because of that they aren't viewed as a sustainable large scale alternative in terms of available U-235. From what I understand a reactor like the IMSR are able to attain a higher breeding ratio which improves fuel sustainability. Breeders would obviously have a ratio of 100% or more and only require fissile startup and abundant fertile input as it operates.

As I was writing this and thinking about it I guess it's not a straightforward answer to this. One example would be that water cooled solid fueled reactors actually breed some fissile material that remains unused before fuel rods have to be replaced. (Mostly Pu-239 from my understanding)


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PostPosted: May 23, 2014 7:56 pm 
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From what I read light water reactors has the lowest breeding ratio of the comertial reactors, somehere between 8-20%.

In heavy water reactors the design was based on a high neutron economy in mind, so the breeding is somewhere 20-40%.

Fast breeders can be 100% which means it can replace all its fisile fuel with 239Pu.

To obtain a high breeding ratio is important to know the factors that make the neutrons not breeding new fuel and how to minimize the effect.

The problem with breeding is that the process is sometimes associated with nuclear weapons proliferation, however technically speaking breeding is a marvel of nature and breeding thorium into 233U can be the door to a new age in humanity.


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PostPosted: May 23, 2014 8:39 pm 
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I have seen projections that give HWRs breeding ratios approaching ~0.8.

For instance a CANDU fueled with only 1.2% enriched uranium will obtain a burnup of ~2.2 atom percent.
Which implies a high breeding ratio since the fuel must still contain something like ~0.4% fissile at the end.


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PostPosted: May 23, 2014 11:43 pm 
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There are a number of tables given in Wash-1097. Figures for U-235 and Pu-239 are also given for comparison with Th/U-233.
There are two types of figures
a. Gross Conversion ratios.
b. Net converted isotopes after burning up as part of fuel.
There are other points of equivalence:-
A. Fission cross sections at various energy levels. They are in order U-233, U-235 and Pu239 in thermal spectrum and U-233, Pu-239 and U-235 in fast spectrum.
B. Eta values. U-233 has highest in thermal and Pu-239 in fast reactors.
c. Unproductive absorption in fissile isotopes. It is least in U-233 and highest in Pu-239.
In Th-U-233 and U-235--Pu-239 fuels the consumed and created fissile isotopes are the same so these figures are not important for comparison.
Table 3.5 shows that minimum fissile is required in a U238/U233 fuel and maximum in a Th/U235 fuel.
Thorium in the blanket will generally result in a virtual breeder.


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PostPosted: May 24, 2014 4:55 pm 
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The breeding ratio is indeed an important figure.

But it does not only depend on the reactor type...the max is a MCFR with theoretical up to 1.6, followed by the Fast Sodium Reactor with a max. of 1.3 ...down to the commercial LWR with 0.55 - 0.6

-You can increase the breeding ratio in moderated reactors by using thorium - 233 U fuel
-You can increase the breeding ratio in fast reactors by using U - Pu fuel
-You can increase the breeding ration by increasing the reactor dimensions (less neutron losses)
-You can decrease the breeding ratio by increasing the burn-up at solid fuel reactors (higher burn-up means more fission products that absorb neutrons) Sometimes Gd is used as neutron absorber to adjust criticality.

The main challenges of nuclear power are green coated communists and the investment costs for new nuclear power plants. Hence the most important characteristic for a nuke are the overnight cost of installed kW.

Holger


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PostPosted: May 30, 2014 3:58 pm 
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Joined: Dec 13, 2011 9:57 pm
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No equipment can be designed to optimize all design parameters.
Some of the parameters to be considered are:
1-Breeding
2-Service Life
3-Capital cost
4-Fuel cost
5-Operation and Maintenance
6-Profiferation resistance
7-Safety
8-Spent fuel generation
9-Spent fuel reprocessing capability

For example, a high breeding yield design can have a positive reactivity making the reactor relatively less safer or can breed Pu making them relatively less proliferation resistant, however requires less enriched U reducing the cost of fuel.


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