1. A Fission-Fusion Hybrid Reactor in Steady-State L-Mode Tokamak Configuration with Natural Uranium Mark Reed FUNFI Varenna, Italy September 13 th, 2011

2. PART I: The Issue PART II: Fission PART III: Fusion PART IV: Conclusions

3. PART I: The Issue Why this might be a good idea

4. Contention Fission-fusion hybrids could actually be more viable than stand-alone fusion reactors and obviate some challenges of fission.

5. Constraints D-T tokamaks Fully non-inductive (steady-state) Low confinement mode (L-mode) operation Pebble bed blanket with helium coolant Natural or depleted uranium Lithium-lead eutectic layer for tritium breeding (one triton per fusion neutron)

6. PART II: Fission The maximum natural uranium blanket power gain

7. Basic Layout Li-Pb natural uranium with He coolant shield

8. Neutronics Methodology Developed a subcritical Monte Carlo code (benchmarked with MCNP) Treated uranium and lithium layers as elongated toroidal shells (quartic solutions for neutron path lengths) ENDF cross-sections and other nuclear data

9. Blanket Variables Uranium toroidal layer thickness Lithium toroidal layer thickness Relative positioning of toroidal layers Homogenized uranium density (different pebble designs) Lithium enrichment Major and minor tokamak radii

10. Layer Thickness Optimization

11. Subcritical Neutron Multiplication k = 0.27 k 0 = 1.19

12. Total Power Composition

13. Fusion-Born Neutron Fate

14. Fission Results Blanket power gain of 7 Tritium breeding ratio of 1.05 Uranium layer thickness of 18 cm Lithium enrichment of 90% 6 Li Helium coolant velocity ≈ 10 m/s

15. PART III: Fusion The minimum tokamak size for steady-state L-mode operation

16. 0-D Tokamak Model Volume-averaged parameters Simply relate R, a, B, q*, P fus, and Q fus Current limit and safety factor (q* > 2) Greenwald density limit Troyon no-wall pressure limit (β N < 3) L-mode operation (H-89 scaling) Fully non-inductive (f NI ≈ 1) Solenoid flux approximately twice plasma flux

17. Fusion power surface density P F /A S and fixed B max uniquely define each operating point 2 < R/a < 4 1 2 3 4 5 6 0-D Tokamak Relations

18. Stand-Alone Fusion Reactor Q = 40, R/a = 2.6, B max = 15 T, P F /A S = 5 MW/m 2.

19. Fission-Fusion Hybrid Reactor Q = 6.3, R/a = 3.1, B max = 15 T, P F /A S = 3 MW/m 2.

20. Fusion Results Major radius of 5.2 m Aspect ratio of 2.8 Maximum on-coil magnetic field of 15 T Fusion gain of 6.7 Total fusion power of 1.7 GW Safety factor of 3.0 H 89 = 1.48 (L-mode)

21. PART IV: Conclusions What this all means

22. Fission-Fusion Advantages Fully non-inductive L-mode operation at small scale (low capital cost relative to pure fusion devices) Subcritical operation (flexibility and safety) Control of fission blanket indirectly through control of the tokamak plasma – fission blanket gain increases with time due to plutonium breeding No uranium enrichment (non-proliferation) Enhanced transmutation of long-lived fission products through (n,2n) reactions

23. Conclusion Instead of complicating the already difficult challenges of fission and fusion, fission-fusion hybrids could actually simplify many difficult aspects of fission and fusion. A profusion of pro-fusion sentiment?

24. Acknowledgements Prof. Ron Parker (fusion) Prof. Ben Forget (fission) M. Reed, R. Parker, B. Forget. “A Fission-Fusion Hybrid Reactor in L- Mode Tokamak Configuration with Natural Uranium”. PSFC/RR-11-1 (2011). MIT Plasma Science and Fusion Center (PSFC) report:

25. Extra Slides

26. L-mode and H-mode H-mode has rough profiles that create edge- localized modes (ELMs), the bane of current fusion research. L-mode does not give rise to ELMs but has lower power density. Some current hybrid designs are based on ITER (H-mode).

27. Hybrid Power The fission blanket augments the fusion power.

28. At large size, increases in temperature lead to operation at the maximum D-T rate coefficient. T near maximum provides inherent stability (negative reactivity coefficient) Absolute maximum limits feasible parameter space 66 keV T= 10 keV 100keV Log (D-T rate coefficient)