Fission-Fusion hybrid to destroy nuclear waste From a challenge to an opportunity

1. Neutron shield Poloidal Coils 100 MW CFNS core UT-IFS Super-X Divertor Fission-Fusion hybrid to destroy nuclear waste From a challenge to an opportunity M. Kotschenreuther1,S. Mahajan1, P. Valanju1, and E. Schneider2 1Institute for Fusion Studies, 2Department of Mechanical Engineering The University of Texas at Austin PPPL April 30, 2009

2. Scientist and Businessman - A rare meeting of minds Jim Hansen - Tell Obama the Truth-The Whole Truth: • However, the greatest threat to the planet may be the potential gap between that presumption (100% “soft”energy) and reality, with the gap filled by continued use of coal-fired power. Therefore it is important to undertake urgent focused R&D programs in both next generation nuclear power and --- • However, it would be exceedingly dangerous to make the presumption today that we will soon have all-renewable electric power. Also it would be inappropriate to impose a similar presumption on China and India. Exelon CEO John Rowe Interview - Bulletin of American Scientists: • We cannot imagine the US dealing with the climate issue, let alone the climate and international security issues without a substantial increment to the nation’s nuclear fleet • I think you have to have some federal solution to the waste problem ---- If it (the Federal Government) ultimately cannot, I do not see this technology fulfilling a major role Renaissance of Fission Energy is emerging as a global imperative - everyone is talking! A believable technical solution to the nuclear waste problem- a scientific imperative

3. Nuclear Energy Resurgence - what could fusion add? • Simplest would be to provide unlimited, low waste andcarbon freeenergy • Promise so attractive that its pursuit had and has a mandate in spite of difficulties and enormous times expected to be spent in this quest • Two major developments in the last decade have redefined the overall relevant scales in the “energy debate”: • Broader recognition of the specter of anthropogenic global warming, caused by carbon-based fuels, haunting our civilization • Drastic boosts in energy consumption due to rapidly increasing affluence in sections of developing societies => We must produce lot more energy while our conventional sources of energy production (coal, natural gas…) have proved unfriendly to the planet => => Nuclear Energy must be an integral component of any desirable carbon-free future energy mix (with renewables - some inherently intermittent) Can fusion neutrons aid nuclear resurgence?- clean the reactor mess? Is it time to revive the age-old but hybernating Fission-Fusion Hybrid?

4. Generic hybrid (H) and the fission only path (FR) • Discussion limited only to transuranic (TRU) waste -destruction applications - it is assumed that no alternative environmentally/ socially sound and affordable “waste disposal” paths exist • Hybrid here connotes a fission system (generally an FR) coupled to a source of external neutrons • FR , in this context, is a “relatively mature” technology (NAS studies -1990s)- fusion is a technology in the making-The fission-fusion hybrid is yet to make its debut • How does one, then, make a case for H (vis a vis FR?)

5. Case for a Hybrid - confronting the shortcomings • A generic hybrid is very expensive • Fusion driver similar to ITER- reduced size implies cost ~1/3 of $12b • Typically FR cost is about $3b=> cost of H is double (or more) of FR • The differentials FR-LWR~$1b, H-LWR~$4-5b ( for the same Power) • Huge issues of Complexity reliability and availability (Mike) • Higher Risk, Time and Cost for develpoment • FR can be fielded for waste destruction in ~20 yrs. • Though physics of a generic hybrid may be easier, the technology difficulties are very similar to that of a fusion reactor • Unique safety issues (sub-critical operation notwithstanding) emerging from the marriage of two complicated technologies- One with a large magnetic field and the other with a very high power density core with liquid metal coolant. It will surely take several major ideas and innovations to beat these daunting handicaps

6. Figures of Merit • The Support ratio S • advanced number of light water reactors (LWR) whose TRU waste can be destroyed by a single advanced reactor (H or FR) • both of these are more expensive than LWRs • The Unit cost per advanced reactor C • necessarily more for H than FR ----- Ch > Cfr • The reprocessing factor R • measures the amount of reprocessing needed in all steps required to burn the waste to ~99% of the original • Complexity, safety, and dependability of the technology An attractive Hybridsolution must, then, be not too complex, and • Minmize Ch - Cfr , and Rh • Maximize Sh/Sfr to such an extent that the waste destruction system based on H is (much) cheaper than the system based on FR

7. A Scenario uniquely suited to a Hybrid • Judicious Choice of the fuel cycle (IMF-LWR, H) • Deep Pre-Burn in LWR getting rid of the bulk of TRU~75% • Invoke Inert Matrix Fuel=> IMF-LWR phase in the Texas reference case • The H-phase burning the remaining highly recalcitrant 25% residue • Though smaller in mass, still contains most of the original radioactivity and long-term biohazard • Cannot be stably and safely destroyed in FR • Unique territory for the hybrid • New fuel cycle => Sh ~ 16-25while Sfr ~2-4 => Sh / Sfr~7-8 • Rh/Rfr also substantially decreases • A huge reduction in the number of advanced reactors and the reprocessing costs make the H system considerably cheaper than the FR system. If we could just make a near term, reliable, and not too expensive fusion module powering a safe Hybrid

8. A Replaceable (and repairable) Fusion Module • Fusion driver as a replaceable module that fits within a fission blanket, but is not physically connected to it • Very different form a generic hybrid in which the fission blanket is an integral part of the driver, and is located inside the TF coils • The fusion module may be replaced (as a unit) at the time of fuel shuffling- both maintenance operations could take 4-6 weeks • The material constraints become much less stringent- the driver components have to withstand fusion neutrons for ~1-2 years << for generic hybrids, fusion reactors- much shorter testing times. • Hugely reduced MHD issues since the metal coolant samples only the poloidal field aligned essentially along the coolant flow. • H Design greatly simplified-Fusion fission systems physically decoupled • Any reasonable compact high power density fusion source ok. Portability adds somewhat to the cost Nevertheless, it is the Deep Pre-burn concept plus the Replaceable fusion module that could turn the Hybrid into a potential winner

9. The Fusion DriverA Compact High Power Density Fusion Neutron Source(CFNS)

10. What may constitute a reference fusion driver? • Fusion power levels similar to a CTF • ~100 MW • Choose an ST for engineering advantages when marrying to fission • Coil losses not very important for a hybrid with our fuel cycle • Other advantages outweigh this • High power density with low coil mass and low capitol cost • easy maintenance - a much more important consideration • Slight neutron shielding on center TF to extend life to ~ 1-2 years • To make room, aspect ratio A~ 1.8 is on high side for ST

11. Core physics operation assumed to be conservative at this point • Below No-wall limit • estimates by Jon Menard quoted in Jeff Freidberg’s book: • use TROYON definition <>N with correction for q* • H-mode confinement (H ~ 1) • Te = Ti (no enhancements to reactivity for hotter ions) • Densities far below Greenwald limit (< 0.3) • Minimum q above 2 (avoids worst NTMs) • CD efficiency: I neR/Ph= 0.2 x 1020 (<Te>/10kev) A/Wm2 • Most uncertain core physics parameter?- to be investigated by NSTX

12. B CFNS gross parameters

13. Modest Core Physics Demands • CFNS can use operating modes and “normalized” performance quality which is reliably experimentally demonstrated on many present tokamaks - only because SXD allows high power density without degrading the core

14. Hybrid closer to Today’s experimental achievements than ITER or a pure fusion reactor The Hybrid fusion source has a higher power density compared to current experiments and ITER - needSXD

15. The CFNS divertor is implausible without the Super-X divertor • From Stangeby: sheath limited if S’ = Q||u /n1.75 L0.75 > 1 x10-27 • Benchmark with SOLPS- define S = Q||u (Bdiv/Bu) /n1.75 L0.75 / 3 x10-27 If S > 1, reliably sheath limited (typically Te plate > 100 eV) • This would give: • Negligible radiation/high heat flux • Unacceptable erosion sputtering • Low neutral pressure and very likely unacceptable He exhaust Operating window becomes substantial only with SXD

16. CFNS: conservative design CD power= 50MW Pfus =100 MW • At moderate density, no wall stable regime • At very low density: • too much current => • poor MHD stability • Add core radiation to make H = 1 and “save” divertor when possible • Only SXD has S<1

17. CFNS: more advanced scenarios CD power= 50MW • Only SXD has S<1 • 400 MW fusion at <N> ~ 4.5 • Assume H = 1.3 attainable • More core radiation to “save” divertor if possible • Required H is high for 400 MW fusion power

18. SXD-from theory to experiment • Worldwide plans are in motion to test SXD • MAST upgrade now includes SXD • NSTX: XD and future SXD? • DIII-D SXD test experiments, possibly next year • Long-pulse superconducting tokamak SST in India designing SXD • SXD: enables power exhaust into much lower neutron damage region • Much of ITER divertor technology be used (H2O cooled Cu substrate- steady Q < 10MW/m2, 20 MW/m2 transient) SXD for MAST Upgrade

19. CFNS Unknowns - Plasma wall interaction • SXD is promising, but needs testing • Success of SXD still leaves further PMI issues • Tritium retention • Effect of loss of wall conditioning on plasma performance? • Will material surfaces evolve acceptably at long times (e.g., will erosion / re-deposition lead to wall flaking & plasma disruptions?) • Will surfaces survive a rare disruption without unacceptable damage? • Liquid metal on porous substrate looks like a promising potential solution to all of these • NSTX might be able to test it sometime in the future?

20. The challenge of devising an attractive fission-fusion hybrid to transmute waste

21. LWR: Uranium Oxide Fuel UOX Spent Fuel (SF) Temporary Storage Direct Disposal Geological Repository Using Fast Reactors Generic fission-fusion hybrid: same as FR Unburned TRU Unburned TRU Spent Fuel Spent Fuel LWR: Uranium Oxide Fuel LWR: Uranium Oxide Fuel Fast Reactors (FR) Fission-Fusion Hybrid TRU in Fertile Matrix TRU in Fertile Matrix Spent Fuel Spent Fuel Reprocess Reprocess Reprocess Reprocess Geological Repository Geological Repository U, Fission products U, Fission products Fission products Fission products Generic Nuclear Waste Management Schemes Hybrid variant: use mostly FR with a few hybrids - for a minority of problematic TRU (minor actinides)

22. Recent history of transmutation schemes • National Academy of Sciences (NAS) investigated both fission only (critical FR) and external neutron driven (subcritical) schemes in 1990s • Considered critical fast reactors & accelerator neutron sources- but not fusion! Recommended against transmutation schemes • they were all too costly (hundreds of billions of dollars) and • took too long (two centuries to reduce transuranics (TRU) by ~99%) • 2001-2008: fast reactor transmutation schemes were refined • Recent congressional testimony (2005-2006) on FR approaches: same objections as NAS study, and objections to the proliferation dangers of reprocessing

23. Devising a winning hybrid based scheme • “Generic” hybrid scheme has no clear advantage over proposed FR schemes in cost, proliferation or time • However, a generic hybrid has obvious disadvantages; the fusion driver adds • Substantial extra cost • Major complexity • Major new technology development • Increased complexity leads to new failure modes and safety issues A winning Hybrid strategy must • find a “way” that hybrids make possible and advantageous • find a way to minimize fusion caused (substantial) disadvantages

24. Usual advantage claimed for a hybrid: safety • Hybrid is “safer”- the fission blanket operates sub-critically BUT: FR community claims good passive safety while burning TRU from LWRs - using advanced geometry, materials • FR safety (criticality accidents)- not raised as a major issue in the NAS study or recent congressional testimony • Major issues with FR approach - COST, reprocessing, time • Hybrid makes cost worse, slightly improves time • Safety advantage of hybrid is hard to argue persuasively- whereas disadvantages are clear-cut The Hybrid concept must find a decisive advantage way beyond anything the “generic” concept offers

25. Analysis of TRU from LWR • FR safety is made problematic by particular TRU isotopes which only fission from very fast neutrons (~ 1 Mev) • Problematic fuel- low fission cross section at lower energy ~ 100 keV • Such fuel leads to unacceptable controllability of the chain reaction- high void reactivity, low Doppler stability, low delayed neutrons, etc. • A rough measure of fuel quality: fission cross section f at ~ 100 keV This mixture of TRU can, indeed, be burnt in an FR But the cost is too high

26. FR can’t burn very low quality fuel - only Hybrids can • So we constructed an optimal fuel cycle to exploit hybrid’s advantage • Minimizes total system cost: • Burn as much TRU as possible in least expensive reactors - LWRs • These would entail little extra cost- no new reactors must be built! • Use hybrid reactors only for the unburned residual • This minimizes the number of expensive hybrids • Only low quality TRU are left-residual cannot be burned in an FR • A symbiotic relationship - each reactor type does what it does best • LWR- burns high & medium quality TRU cheaply and quickly • Hybrid- burns very problematic material safely, but is only used for the worst TRU that really need it

27. How Optimal? • First step: destroy 75% of TRU in LWRs • Limited by physics: cross sections of ~ 25% of the isotopes are too small in an LWR neutron spectrum (close to thermal) for destruction • Thermal spectrum systems also destroy a much larger percentage of fuel in a single pass- and virtually all Pu239 • Thermal cross sections of easily fissile isotopes are much larger in thermal spectrum system • Destruction of most TRU is rapid, significantly reducing time for destruction • Easily weaponizable isotopes (Pu239, etc.) quickly eliminated in the first step • The remaining 25% (think “sludge”) must be “incinerated” in a sub-critical assembly for safety • An inexpensive, prolific external neutron source is needed- fusion! • This system does not have any breeding, so no new TRU waste (proliferation hazard) is created in this step (unlike FRs) That is if we have a fusion driver

28. LWR: Uranium Oxide Fuel UOX Spent Fuel (SF) Temporary Storage Direct Disposal Geological Repository Generic Hybrid Cycle Unburned TRU Spent Fuel LWR: Uranium Oxide Fuel Fission-fusion Hybrids TRU in Fertile Matrix Spent Fuel Reprocess Reprocess Geological Repository U, Fission products Fission products New hybrid fuel cycle UT Proposal: IMF-LWRs & Hybrids Sybiotically Cheaper burn ~75% LWR: Uranium Oxide Fuel LWR: Inert Matrix Fuel (IMF) Spent Fuel No Pu239 Trans uranics Reprocess Reprocess Fission products Remaining 25% Fission Fusion Hybrids 50% burn Geological Repository Fission Products +1% TRU Reprocess Hard-to-burn TRU Unburned TRU Fission products

29. Why it is important to destroy the remaining 25% residue • Isotopes which grossly dominate the long lived biohazards of fission waste are in not destroyed in the first LWR step • Pu242 half life 4 x 105 years • Np237 half life 2 x 106 years • Geologic disposal is difficult precisely because these specific isotopes remain for such long times • Example: DOE analysis of Yucca Mountain finds that: these isotopes lead to surface radiation doses much higher than allowed by any other man made source 2-4 x 105 years in future • The fast spectrum incinerator is needed to eliminate these makes acceptable geological isolation much easier to guarantee at long times, which is why geological disposal is technically difficult

30. UT-Hybrid vs Fission-only Cycle Required Reactor fleets for zero net transuranic nuclear waste production from the current ~100 US utility reactors Under our proposal 4-6 new utility-scale hybrid reactors would suffice Waste reprocessing for fast-spectrum reactors will also be reduced by roughly an order of magnitude

31. Reactor Requirements for Waste Transmutation for different schemes Reactors needed to destroy waste from 100 LWRs FR cost = 1.5 LWR, Hybrid = 2 LWR

32. This hybrid based scheme has a major system cost advantage over an FR based scheme • First hybrid based scheme with this advantage (to our knowledge) • This advantage is more than enough to overcome the cost disadvantage of individual hybrid vs an individual FR The system cost advantage may be enough to overcome the other disadvantages of the hybrid: Complexity, stage of development, new failure pathways • We turn to these drawbacks momentarily

33. The new scheme relies heavily on IMF fuel-but many technologies are crucial • Crucial technologies: Fission: • IMF fuels • Commercial scale reprocessing for fast spectrum fuels Fusion: • Qualification of structural materials for 14 MeV neutrons • Advanced divertor technology • First wall technology with adequate tritium retention, erosion, surface evolution, etc. • Very long pulse system reliability (CD, control systems, profile control, etc.) • Tritium breeding / processing at large through-put • Each of these is “crucial”- without it, there is no attractive hybrid • IMF is certainly not the most difficult or expensive technology to develop

34. The challenge of making a hybrid a nearer - term task

35. Generic Hybrids - Serious issues • Many say: • Physics easier, but engineering as bad or worse than pure fusion • Licensing even harder than pure fission: complexity => new safety issues Development of generic hybrids sooner than pure fusion unlikely • Compare generic hybrid to US and EU FR programs • FRs: expect ~15-20 years development before a commercial prototype FR • FRs start with 60 years of experience with many FRs • Fusion technology development is at least several times harder than FR- It is contended that generic hybrids and pure fusion have similar and long time scales.

36. Hybrid: Interacting fusion - fission systems Fusion driver: • Complexity/availability • A fission core is a collection of simple identical rods with coolant flowing past- compare that to a tokamak/stellarator • Internal maintenance, with TF coils and vacuum vessel, • “Like disassembling a ship in a bottle” • Damage from 14 MeV neutrons is greater than fission neutrons • Helium generation => more material degradation • No engineering experience long pulse devices with fusion spectrum • Fast Fission spectrum reactors have 60 years of experience in many devices Fission blanket is connected to fusion driver: • Mechanically=> new failure modes, difficult to certify safety in unlikely accident scenarios • Magnetically=> coolant flow impeded via MHD • Electro-magnetically=> plasma disruptions cause mechanical EM loads

37. A B Replaceable Fusion Driver Concept • Due to SXD, the whole CFNS is small enough to fit inside fission blanket • CFNS driver to last about 1-2 full power years • It can be replaced by another CFNS driver and refurbished away from hybrid • CFNS driver itself is small fraction of cost, so a spare is affordable

38. A B Replaceable Fusion Driver Concept • Pull CFNS driver A out to service bay once every 1-2 years or so - at the same time when fission blanket maintenance is usually done • Refurbish driver A in service bay - much easier than in-situ repairs

39. A B Replaceable Fusion Driver Concept • Put driver B into fission blanket • This can coincide with fission blanket maintenance • Use driver B while driver A is being repaired

40. A new design concept: addresses all these issues in one stroke Replaceable fusion driver • Driver replaced roughly yearly while fuel rods reshuffled (neutron damage, availability) • Damaged driver refurbished in remote maintenance bay (maintenance, availability) • Fission blanket is outside TF coils (coolant MHD drastically reduced) • Fission blanket is electro-magnetically shielded from plasma transients by TF coils (disruption effects greatly enhanced) • Fission blanket is physically separate from fusion driver (complexity, development time, safety) We shall now spell out each one

41. Issue: Neutron Damage and Availability • Driver is only exposed to about one year of damage • In traditional hybrids/fusion reactors, components must be exposed for about 3-4 years because the replacement time is so long • Fusion driver walls would be exposed to ~ 1 Mwyr/m2 (~ 30 dpa/300 appm He)- about what present fusion materials are expected to be able to handle • This is much less than the ~ 6 MWyr/m2 requirement for a CTF to test pure fusion DEMO components • With expected CTF availability of ~ 0.3, testing cycle times are • 3 years for hybrid CFNS • 20 years for DEMO • Many testing cycles are possible for CFNS in 10-20 years, leading to much higher availability growth than the one cycle possible for CTF leading to DEMO The availability/ neutron damage issues are tremendously ameliorated

42. Issue: Maintenance of highly radioactive driver • Driver is removed as a unit relatively quickly • Refurbishment of a “spent” driver is done relatively slowly in a remote maintenance bay • Rapid inspection/replacement of components of the “ship in a bottle” method- which we don’t know how to do- is avoided Credible inspection/maintenance improves the credibility of high availability

43. MHD coolant effects • Fission blanket power density is ~ 1 1/2 orders of magnitude higher than pure fusion- MHD drag on coolant could be a show-stopper for a hybrid • Magnetic field outside the TF coils is only from PF, and is almost exactly vertical- aligns almost perfectly with the coolant flow direction MHD drag effects reduced by orders of magnitude from standard hybrid configuration

44. Electromagnetic disruption effects on blanket • The L/R time of the fairly thick, highly conducting TF is ~ 1 second (even with substantial holes to let neutrons through) • Disruptions as fast as ~ 1 ms • TF slows down EM transients in the fission blanket which arise in the plasma by orders of magnitude Eddy currents and forces are reduced orders of magnitude

45. Physical separation of Driver and fission blanket • Failures that arise inside the complex fusion driver do not directly affect the fission blanket • The fission blanket can consist of conventional fuel rods • Much of fission FR technology can be used by hybrid- speeding development • Licensing safety analysis is tremendously simplified • The fusion driver has no fission products/TRU in it • It can be designed/developed/tested/qualified into a reliable unit without having fission hazards which would tremendously slow development

46. New Hybrid versus Generic Hybrid • The new hybrid is technologically much more credible Together with the advantages of the IMF-hybrid fuel cycle, the new hybrid emerges as a potentially attractive and credible endeavor

47. Hybrid in the context of fusion reasearch - an intermediate milestone • Development of a hybrid would tremendously advance fusion technology - hybrid-fusion are symbiotic The performance/cost of a hybrid improve as the physics and technology improves towards the requirements of pure fusion The requirements for initial operation of this hybrid are low enough that a substantially quicker application of fusion may become credible