Fuel Cycle Options

• Weapons material (²³⁹Pu) as sole source
• Fission waste as sole source
• Weapons material as makeup feed
• Minor actinides only (no Pu or U in feedstock)

Disposition scenario

• Power producing mode (high conversion ratio)
• Moderate destruction mode (conversion ratio of 0.5-1)
• Maximum destruction mode (non-uranium fuel, high burnup reactivity loss)
• Pu denaturing (i.e., producing radioactive byproducts that contaminate the Pu)

Processing mode

• batch vs. continuous processing
• once-through vs. multiple recycle

Engineering Concepts Previously Considered

Key Neutronic Performance Parameters

• Conversion ratio (ratio of production to destruction of actinides)
• Peak and average ²³⁹Pu discharge burnup (MWd/g)
• Consumption rate (kg/yr)
• Loading rate (kg/yr)
• Discharge fraction of ²³⁹Pu
• Fraction of original Pu destroyed
• Fission-to-capture ratio
• External neutron source strength (MW)

Engineering Performance Parameters

• Power density (linear power, surface heat flux, volumetric heating)
• Fluence limits and lifetime
• In-system inventory of 239Pu and total actinides
• Time dependence and spatial nonuniformity of blanket behavior

Example: Na-cooled integral fast reactor

• Metallic fuel, relatively hard spectrum
• Multiple recycling of the fuel is used to allow complete actinide destruction
• Burnup of ~10% per pass is achieved
• The minimum ²³⁹Pu fraction achieved is ~50%

Thermal power: 840 MW per module
Peak allowable fast fluence: 3.8x10²³ n/cm² (cladding limited)

* could be tailored for TRU consumption =0
** TRU=transuranic
*** 231 kg/yr ~ 1 g/MWd = maximum achievable

Example: HTGR

• GA PC-MHR
• Pu oxide fuel in multiple layers of pyrolytic carbon and SiC
• Use of a burnable poison to enable high once-through burnup (Er₂O₃)
• Relatively soft spectrum (peak at ~0.1 eV)
• Several scenarios considered: "Pu spiking", "spent fuel" level of irradiation, and maximum "Pu destruction"

Processing mode	once-through, no recycle
Thermal power	450 MW per module
TRU consumption rate (kg/yr)	250
Peak discharge burnup (MWd/g)	0.785
Average discharge burnup (MWd/g)	0.590
Fuel cycle length (months)	36
.
239Pu burnup	90-95%
Total Pu burnup	65-72%
Discharge 239Pu fraction	<30%
Peak fast fluence (goal)	4.2x1025 n/cm2

Preliminary Conclusions

1. There are many different transmutation fuel cycles and many different blankets proposed, each having its own unique characteristics.

2. There is no established set of criteria for transmutation reactors. Point-design comparisons are tied to authors' assumptions and are inconclusive. Most concepts could be re-optimized under a different set of assumptions.

3. Most performance parameters are more dependent on blanket and fuel cycle choices than the source of neutrons.

4. The most fundamental distinction between these systems is whether or not an external source of neutrons is provided - i.e., critical vs. subcritical blanket operation. With an external neutron source and no requirement for criticality, deeper burnup is possible without fuel recycling.

5. Both fusion and accelerators provide external sources. If depth of burnup is the chief reason for using an external source, then accelerators are likely to be superior to DT fusion because the need to breed tritium restricts the depth of burnup for fusion systems. We should consider DD.

6. Subcritical assemblies offer operating advantages over critical assemblies. Spectrum-related performance differences are probably secondary in importance; fast-flux fission cores can perform pretty well.

7. Operating a subcritical assembly starting from an initial loading to complete burnup implies a very wide range of operating conditions. A combination approach, in which only the latter stages of burnup are achieved using an external neutron source, offers a sensible alternative.