High Support Ratio Fusion-Fission Hybrid System ~ Fuel Cycles Swadesh Mahajan for The Texas Group DOE Fusion-Fission Hybrid Workshop Sept.30-Oct2, 2009

Outline Objective: provide an overview of FFH-enabled high support ratio fuel cycles that mitigate the burden placed by the most difficult-to-transmute actinides on other thermal or fast spectrum transmutation systems. Fuel Cycle –Zero net TRU production: Fusion Fission Transmutation System (FFTS) concept incorporating LWR transmutation – IMF and Corail based transmutation paths Fission Systems- a glimpse with Tritium Breeding figures.

The Generic High support Ratio Fuel Cycle TRU Burning in SFR Current Practice LWR Preburn TRU U, FP TRU Burning in FFTS LWR: Uranium Oxide Fuel UOX SF Reprocess IMF SF FR-FFH TRU Reprocess FP SF TRU Maximal Utilization of the LWR consistent with obvious constraints

Hybrid is a part of the FR family Obvious Hybrid is a Sub critical Fast Reactor Not so Obvious Sub criticality, being a mode of FR operation, is an additional channel for optimization

UT Reference = IMF- FF Hybrid LWR: Inert Matrix Fuel TRU U, FP TRU Burning in FFTS LWR: Uranium Oxide Fuel UOX SF Reprocess IMF SF Fission Fusion Hybrid TRU/TRU* Reprocess FP SF TRU With 75% burn in the LWR-IMF step the support ratio is high ~ 5% thermal burn in the Hybrid. BUs of more than 50-60% require heterogeneity; enriched U needed to support the reactivity of IMF fuel. Support ratio is further boosted by sending Pu242 back to LWR TRU*=TRU-Pu242

High Support Ratio SFR Based Fuel Cycles Extensive thermal-spectrum burn is also compatible with some SFR fuel cycles. One example* uses the CORAIL sustained Pu recycle scheme to stabilize Pu inventories and passes the MA recovered from CORAIL fuel to a sodium-cooled fast reactor. * M. Stillman and R. Hill, “Effect of Thermal-Spectrum Transmuter Deep Burnup of Transuranics on Fast- Spectrum Transmuter Performance,” Proc. GLOBAL 2003, New Orleans, LA, Reprocess LWR: Uranium Oxide Fuel Reprocess CORAIL fuel in LWRs Sodium Cooled FR, CR ~ 0.5 Spent Fuel U Pu U Np Am Cm Fission Products +loss Geological Repository Geological Repository Fission products +loss U Np Am Cm U Pu Np Am Cm Under this scheme about 75% of the TRU discharged from the UOX fuel is eventually fissioned in CORAIL fuel within the LWRs. The remaining 25%, MA from UOX as well as CORAIL fuel, is sent to a low-fertile FR. This scheme leads to a FR thermal power share of around 12%. Corails+Hybrid: wait Cm out - No U => run an Am only Hybrid-send Pu238 back to LWR support ratio gain ~ 3-4

Reprocess LWR: Uranium Oxide Fuel Reprocess FR: Sodium Coolant, CR ~0.5 Fusion Fission Hybrid Spent Fuel U Pu Np Am Cm Fission Products +0.1% loss 20% Burn Geological Repository Geological Repository Up to 50% burn Fission products +0.1% loss Np Am Cm U Pu Np Am Cm Reprocess LWR: Uranium Oxide Fuel Reprocess LWR: Inert Matrix Fuel (IMF) Fusion Fission Hybrid Spent Fuel Pu Np Pu Np Am Fission Products +0.1% loss Geological Repository Geological Repository Up to 50% burn Fission products +0.1% loss Pu Np Am Cm Decay Storage Cm Pu- 238, “Double Stratum” FR and FFH Transmutation 2. Am Bypass; Cm Partitioning and Decay Storage Additional Fuel Cycles Am Up to 75% burn 1-2 passes

Fuel Cycles - Additional Comments The previous slide showed that Cm decay storage could ease the fuel fabrication burden on the hybrid and enhance the utilization of IMF. The curium isotopes present in spent LWR fuel at discharge are predominantly: Cm-242 (T 1/2 = 163 d) decay to Pu-238 which has a favorable effect on the thermal-spectrum neutron balance, Decay of the strong heat emitter Cm-244 (T 1/2 = 18.1 yr) to Pu-240 This plutonium would be harvested and returned to the IMF. In addition, if Am utilization in LWRs proves to be untenable, the Am (predominantly Am-241 at that stage) would bypass the IMF recycle step and pass directly to the hybrid. While fabrication issues tied to volatility remain, the Am discharged from IMF is overwhelmingly Am-243, a relatively benign(less radioactive) isotope from the standpoint of fuel handling.

Maximal Thermal Spectrum Transmutation: Synergies with the FFTS Because of their non-fertile matrices, inert matrix fuels (IMFs) enable light water reactors (LWRs) to meaningfully reduce transuranic (TRU) inventories. Very deep (> 50%) burn up in the IMF- LWR step (+ additional flourishes like waiting Cm out (sending Pu240 to LWR), running only an Am reactor in the second step)may preclude fuel cycle closure using unaided FRs because of the high minor actinide (MA) content of the residual TRU: safety and stability issues. Million dollar questions- how bad does the fuel have to be that the best optimization may be attained through a sub-critical assembly? Does making such bad fuels (that go into FR) bring enough advantages?

Inert Matrix Fuel Forms This is Jim Tulenko’s subject. Yttrium stabilized ZrO 2 (YSZ) and SiCO 2 are typical, –Fabrication techniques are similar to those used for MOX and UOX: dry milling of constituent oxides or co-precipitation to form a solid solution, followed by compaction and sintering of resulting grains. –Mechanical and thermal properties of YSZ based IMF have been reported in several studies as has durability under irradiation. –Initial results from in-pile testing at Halden (57% burnup; to update models for fission gas release, thermal conductivity and fuel swelling) have been published. Fission gas release issues have been noted Magnesia-zirconia may offer superior performance

What Can be Accomplished in a Thermal Spectrum? Most IMF single-pass deep burn studies (e.g. Herring, Nucl. Tech., 2004) attain IMF burnups of up to 55-60% –Achievable in contemporary PWRs with minimal modifications (e.g. heterogeneous assemblies with 1/9 IMF pins, remainder 4.95% enriched UOX) CONFU and CONFU-B assembly designs investigated at MIT have the potential to ultimately attain near-complete TRU burn-down via multi recycle in PWRs –Challenges related to high IMF loading (power peaking, DNBR) Certain species ( 242 Pu, 243 Am, 244 Cm) transmute exceptionally slowly in a thermal spectrum, so that a practical limit upon what can be accomplished in a thermal spectrum may exist

A Practical Burnup Limit Figure. IMF Burnup versus Fluence Burnup in a thermal spectrum is limited because an inflection point occurs near 75% burnup. Additional incineration requires large residence times because the remaining actinide isotopes are relatively transparent to thermal neutrons.

Residual TRU Following IMF Burn It may be plausible to achieve 75% TRU burnup in a single IMF pass given perturbations from existing single pass schemes (e.g. increased 235 U enrichment, 4/3 IMF-bearing / all-UOX assembly cycle reload pattern) The more transmutation that is accomplished in LWRs, the fewer fast spectrum systems that will be required. The isotopic content (a/o) of the residual TRU after 75% burn is shown in the table at right. This is the feed to the FFTS.

IMF/FFTSIMF/FFTS with Cm decay storage 3 Pass MOX Pu- Np-Am/ FFTS Double Stratum FR + FFTS LWR - UOX 77.7% % % % 1.0 Thermal Recycle 17.0% % % Fast Reactor 21.1% FFTS 5.3% % % % Thermal Power Split (%) TRU production [kg TRU produced / kg TRU produced in UOX] CORAIL+Hybrid is very similar to IMF/FFTS: Mixing CORAIL philosophy (Send all Pu back to LWR) works wonders for the support ratio.

Multi-Pass IMF: CONFU To fission fusion hybrid or continued IMF burn-down Masses in kg TRU First IMF Pass: 60% TRU Burnup Looking again at multi-pass IMF in a CONFU-type strategy, Blending with fresh TRU is needed to support reactivity A mass flow diagram for one two-pass blending strategy utilizing homogeneous (TRU-Zr)O 2 assemblies is shown below Full-core loading of homogeneous IMF assemblies may not be feasible (core physics / safety constraints) Figure source: Taiwo, T. and R. Hill, “Comprehensive Summary of AAA and AFCI Transmutation Analysis Studies,” ANL-AFCI-198, 2007.

Conclusions A small number of fusion-fission transmuters can reduce the burden placed upon other thermal or fast spectrum systems by the most difficult-to-transmute species –This talk is by, no means, comprehensive We think the main challenges facing the proposed fuel cycle are –Maturity of LWR transmutation technologies –High-efficiency separation of advanced fuels –Fabrication of high TRU content fuels for high-fluence irradiation –Materials durability – magnets, cladding, structural materials Semi infinite number of research and development issues need to be investigated- some of these being studied by UT+collaborators

Fission Systems

Pb 80Al- 20D 2 O 6 Li 2 TiO 3 Graphite FissionBlanket 90Cu- 10D 2 O FFTS Geometry 3 m

Pb 80Al- 20D 2 O 6 Li 2 TiO 3 Graphite FissionBlanket 90Cu- 10D 2 O Alternate Configuration: Tank-type Design 0.85 m Na

3 H Breeding: Central Coil and Divertor Region Tritium self-sufficiency can be reached by placing Li 2 TiO 3 blankets in two locations where shielding of sensitive components is required: the vicinity of the upper and lower divertor plates; surrounding the central coil. Additional breeding (~20%) is obtained when the lithium titanate is backed by a graphite moderator. 3 H production rate, [atoms/sec] (% of consumption rate) Divertor Blankets Central Blanket Total 2.46E19 (69%) 1.86E19 (52%) 4.32E19 (121%) Note: divertor plate DPA rate is 2.3 dpa/fpy in this configuration. Replacement of a portion of the Li 2 TiO 3 with B 4 C can reduce the damage rate to as low as 0.38 dpa/fpy.

Limitations Imposed by TRU Isotopics In the full TRU recycle strategy, the metal fuel TRU fraction is limited by the decay power of 244 Cm to 25% –This limitation is imposed to avoid the need for active cooling of fuel assemblies during fabrication –In most strategies considered by AFCI, 241 Am rather than 244 Cm constrains TRU loading The low TRU fraction may support extended burnup but results in a large total TRU inventory and lower-than-optimal power density –lattice pitch can be adjusted, but this pushes fuel geometry into regimes where little previous work has been done; an investigation is ongoing An option that avoids this issue and utilizes the thermal spectrum more effectively is post-reprocessing Cm storage –Under this strategy, stored Cm is milked for Pu (overwhelmingly 240 Pu) which is returned to the IMF feed –Limited by ability to partition Am and Cm: a focus for AFCI in the coming year

Neutron Balance An aggressive fission / fusion thermal power ratio of 30 may be attainable –To maintain this ratio, k eff must remain between 0.91 and ~0.93 (upper limit tied to void reactivity coefficient) –The burnup reactivity behavior of the 242 Pu, 243 Am and 244 Cm rich fuel makes this possible, so that with 3 batch shuffling there is no neutronic obstacle to achieving ~50% burnup at this power ratio Material constraints will be limiting unless the state of the art improves: –fuel performance at high burnup fraction (note that AFCI test metal fuel has exceeded 30% burnup in ATR) –Structural damage imposing a nominal fast fluence limit of 4x10 23 n/cm 2 (limiting burnup to ~20%; this constraint is also limiting in present-day FR designs)

Global Neutron Balance GainsLossesNet Source (n,Xn) (mostly in Pb) fission N/A* Capture in nonactinide fission blanket components Capture in actinides Capture in Li 2 TiO Other captures escape Total Normalized to one fusion neutron. * fission neutron production turned off: only fusion and (n,Xn) neutrons included.