Safety and Environmental Assessment for Laser IFE

1. Safety and Environmental Assessment for Laser IFE S. Reyes, J. F. Latkowski Lawrence Livermore National Laboratory Laser IFE Meeting November 13-14, 2001 Crowne Plaza Hotel, Pleasanton, CA Work performed under the auspices of the U. S. Department of Energy by University of California Lawrence Livermore National Laboratory under Contract W-7405-Eng-48

2. SOMBRERO LLNL is addressing critical issues for laser IFE safety and environment • Safety and environmental attractiveness is a key requirement for the success of laser IFE • We are using the SOMBRERO laser driven conceptual design to perform safety and environmental analyses • Design uses a low activation material (C/C composite) for chamber structures • Xe gas atmosphere used to protect FW from target emissions • Blanket consists of a moving bed of solid Li2O particles flowing in a He carrier gas through the chamber

3. We have recently performed analyses to address key safety issues • Oxidation of graphite chamber structures may be a concern in case of air ingress event (reaction rate seems to be significant even at T  1000 ºC) • passive safety feature should be easy to implement (inert gas released from tank by rupture disk failure when a differential pressure is reached) • protective coatings for C/C composites (Si-B-C coatings) • alternative materials for FW and/or blanket structures • The activation products for Xe are the main contributors to accident dose, possible solutions are: • removal of I and Cs by the chamber vacuum system • alternative gas such as Kr • Other than Xe, the most important contributor to accident dose is the HTO • tritium trapped in FW/blanket may be greater than 1 kg (need experimental data) • simple modifications in the confinement building material would enhance HTO condensation on walls, reducing off-site dose

4. We are currently assessing waste management options for laser IFE • The importance of safe handing of radioactive waste for the protection of human health and the environment has long been recognized • Traditionally, studies for IFE have used Waste Disposal Rating (WDR) as the preferred waste management index • WDR < 1 means that radioactive material qualifies for shallow land burial • Even in the case of low activation materials, there are concerns about space limitations and negative public perception of large volumes of waste • There is a growing international motivation to develop a fusion waste management system that maximizes the amount of fusion materials that can be unconditionally cleared or recycled

5. The concept of “clearance” allows for new waste management options • The IAEA has proposed clearance levels for radioactive waste below which regulatory control may be relinquished • Clearance implies that radiation hazards associated are trivial so that the final destination of materials (recycle, reuse, surface disposal) is not known in advance • We have implemented the calculation of Clearance Indexes (CI’s) using current IAEA clearance limits • The CI for a material containing n different nuclides is calculated as: CI =  An/Ln where Anis the activity due to the nuclide and Lnis the clearance limit for the nuclide • CI£ 1 means it is possible to clear the material

6. We have calculated Clearance Indexes for the SOMBRERO design • We have upgraded the neutron activation code ACAB to include a new subroutine for calculation of CI’s • A detailed 3-D model of SOMBRERO has been used for neutron transport calculations • We have used ACAB to simulate activation of materials after 30 years-operation • Results have been obtained for WDR’s and CI’s for every plant component and for every isotope SOMBRERO 3-D model for neutronics analysis

7. Open solid angle Neutron dumps Building 0.25% 4.39E-1 4.86E-4 5% 4.72E-1 1.20E-2 Results show that SOMBRERO meets criteria for shallow land burial • The first wall and blanket easily meet waste disposal rating criterion of WDR < 1 • The concrete building also meets criterion, although it represents a volume of  105 m3 • Due to their exposure to line-of-sight neutrons, the neutron dumps have the highest WDR ( 0.5) • However, the neutron dumps are unimportant from the waste volume perspective (they are ~ 0.1% of the building shell volume)

8. We have obtained CI’s for every component for up to 100 years of cooling • Neither component qualifies for clearance, however the concrete building has the lowest index • Ability to clear the building would be highly beneficial because of the enormous waste volume that it represents

9. Calculations have been performed for alternative building materials and thickness • Concrete composition should avoid critical isotopes from waste point of view (long-lived nuclides) • Increasing the thickness of the walls reduces clearance index but increases total plant cost

10. Laser IFE presents unique challenges to the development of structural materials • All materials in a fusion power plant are subject to S&E considerations • Ideally, a material would be cleared unconditionally following its lifetime in the power plant • If clearing is not possible then recycling is the second option • criterion for remote recycling is the contact dose rate (CDR), which must be < 20 mSv/h following 50 years of radioactive decay • hands-on recycling would require a CDR below 25 mSv/h • If a material is not eligible for recycling the next best option is disposal via shallow land burial, which is evaluated using the WDR • Finally, a material must meet the accident safety criterion (dose < 10 mSv) so that it does not result in an unacceptably high accident dose

11. We intend to provide S&E guidance for target and chamber designers • We have performed analyses in order to provide guidance for selection of chamber and target materials from a S&E perspective • CDR and WDR results have been obtained for all naturally occurring elements from lithium to bismuthat the SOMBRERO chamber wall • using the elemental values in the table, one can derive an approximate value for an actual material • the table does not include results for accident dosesgiven that these are accident- and material-dependent • Target materials have been evaluated in a similar manner • CDR criterion for dose is that recycling equipment could withstand 30 Mgy • WDR is evaluated in the same way as for chamber wall • accident dose is evaluated using a dose limit of 5 mSv (the other 5 mSv is allocated to tritium releases) and release fraction calculated with MELCOR

12. Environmental indices resulting from neutron activation of potential IFE chamber materials

13. Safety and environmental results for potential target materials • C=failed contact dose rate criterion (dose < 30 MGy for 30 years,continuous exposure) • W=failed waste management criterion (WDR < 1) • A=failed accident dose criterion (early dose < 5 mSv for 0.3% release) • P=passed all criteria

14. Conclusions (I) • There are key safety issues for laser IFE • Oxidation of chamber structures during accidents • Activation of gas Xenon • Tritium retention in FW and blanket • Although all the components in SOMBRERO qualify for shallow land burial, this option is not very attractive given the enormous waste volumes involved • Clearance of the confinement building would greatly reduce the total volume of waste to be buried, allowing for recycling or reusing of the material

15. Conclusions (II) • We have generated S&E results for all naturally occurring elements from lithium to bismuth to provide guidance to chamber and target designers • For chamber structures one can derive an approximate value for an actual material using the elemental values and densities • For target materials, the most limiting criterion is the CDR, however, the list can be increased either by: • increasing the limit through use of radiation hardened components or through periodic change-out of failed components • decreasing the CDR value of each elementby extending the radioactive decay time

16. Future work • As part of LLNL chamber work, we will continue to address safety and environmental issues for laser IFE • Alternative chamber concepts call for modifications in the baseline SOMBRERO design, which will have to be re-assessed from the safety and environment perspective • As an example, magnetic deflection schemes for wall protection from ion damage will require new neutron activation calculations for the magnets • Still much work to do on target fabrication facility safety