The Neutronics of Heavy Ion Fusion Chambers

1. The Neutronics of Heavy Ion Fusion Chambers Jeff Latkowski and Susana Reyes 15th Heavy Ion Inertial Fusion Symposium Princeton, NJ June 9, 2004 Work performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.

2. Neutron transport affects a varietyof components and raises a set of issues First Structural Wall FinalFocusMagnets Hohlraum Liquidblanket Capsule Blanket Radius

3. Neutron transport affects a varietyof components and raises a set of issues Neutron energymultiplication Isochoric heating Neutron activation Radiation damage

4. Neutron transport affects a varietyof components and raises a set of issues Output spectra Neutron energymultiplication Boosts power above (10-20% fusion output typical) Isochoric heating Neutron activation Radiation damage

5. Neutron transport affects a varietyof components and raises a set of issues Neutron energymultiplication Affects liquid break-up & chamber clearing Isochoric heating Imparts stresses to the first structural wall & other components Neutron activation Radiation damage

6. Neutron transport affects a varietyof components and raises a set of issues Neutron energymultiplication Isochoric heating Tritium breeding Transmutation products Neutron activation Radiological safety Waste management Radiation damage

7. Neutron transport affects a varietyof components and raises a set of issues Neutron energymultiplication Isochoric heating Neutron activation Component performance Maintenance requirements Radiation damage Component lifetime Waste management

8. In selection of a liquid, thetritium breeding ratio is a key issue • TBR of ~1.1 is needed to cover uncertainties and losses: • Decay • Leakage • 3-D effects • Nuclear data • Don’t want to be>>1.1, but relatively easy to reduce theTBR

9. Liquid thicknesses are selected to provide adequate protection to the first structural wall • Lifetime limits (in displacements per atom) for structural materials of interest: • SS304: 25 dpa(0.83 dpa/y) • ODS-FS: 200 dpa(6.7 dpa/y) • 304-PCA might be a path to 100 dpa

10. Reweldability is desirable but probably not achievable for wall at reasonable thicknesses • Reweldability limit (issue is cracking) is 1 appm He • Welding to 10 appm He may be possible with stress modified welding technique

11. The waste disposal rating (WDR) is a measureof the level of activation within a component • WDR is calculated as the ratio of concentration to a concentration limit, summed over all radionuclides • WDR <1 indicates disposal via shallow land burial possible WDR for SS304 first structural wall after 30 FPY

12. 93Nb(n,g) 94Nb = 6.5 98Mo(n,g) 99Mo  99Tc = 95 191Ir(n,g) 192sIr = 15 The waste disposal rating does not uniformly decrease with increasing liquid thickness

13. The neutron spectrum changes considerablyin its magnitude but only a little in its shape

14. The waste disposal rating is a measureof the level of activation within a component • WDR peaks after ~10 years of irradiation • Beyond the peak, 98Mo depletion occurs and WDR begins to fall • 93Nb depletion occurs even earlier WDR during irradiation for SS304 wall behind 30 cm of flinabe

15. Significant burn-up but high levels of activation Liquid thick enough that burn-up drops Liquid thick enough that initial activation reduced The waste disposal rating does not uniformly decrease with increasing liquid thickness

16. Radiation damage to and transmutationof the first structural wall • An important issue for fusion is the ratio of the He production rate to the displacements per atom (dpa) rate: • Fission-based neutron sources do not produce nearly as much helium as is produced in a fusion system (~0.1 appm He/dpa) • For a dry wall fusion system ~10 appm He produced for each dpa • Leaves fission-based neutron sources as inappropriate tools for study of dry wall fusion neutron damage • The use of thick-liquids, however, significantly increases opportunities for the use of currently available fission-based neutron sources  main advantage is not a change in the He/dpa ratio, but in the reduction of the dpa rate, which allows accelerated damage testing

17. “Dry wall” HYLIFE-II: 7.4 appm He/dpa HYLIFE-II w/ 60 cm Flibe: 5.4 appm He/dpa An important issue is the ratioof He production rate to dpa rate

18. It is possible to alter He/dpa ratioin existing irradiation facilities • Greenwood & Graczyk report enhanced He production from 55Fe in ferritic materials  can isotopically enrich samples (expensive) • Longest et al. began use of Hf shields in HFIR to achieve 14 appm He/dpa  gives desired ratio, but reduces overall damage & transmutation rates • Investigation of other dopants or other means to alter the He/dpa ratio is warranted

19. Summary • There are a variety of neutronics issues that must be considered for heavy ion fusion systems: • Neutron interactions in the target • Neutron activation & transmutation reactions • Isochoric heating • Radiation damage • The various technical issues strongly affect important areas of power plant operation: • Chamber clearing • Radiological safety • Component reliability & performance • Waste management • Economics