Full Submersion Criticality Accident Mitigation in the Carbide LEU-NTR

Transcription

1 Full Submersion Criticality Accident Mitigation in the Carbide LEU-NTR Paolo Venneri, Yonghee Kim Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon, Korea,

2 Nuclear Thermal Rocket Background Full-Submersion Criticality Accident Description Neutronic Effects Mitigation Systems External Internal Proposed Method: Spectral Shift Absorber Previous Work Keys to Successful Implementation Gadolinium Implementation Baseline Core Configurations Poison Location Spectrum Hardening Results Conclusion and Future Work Overview Paolo Venneri - NETS Albuquerque, NM 2

3 Nuclear Thermal Rockets Overview Purpose of Implementing NTRs: Space Travel Requires Minimum Mass Requires most efficient use of propellant Minimization of Transit Times Crew exposure to cosmic radiation Proven Technology Tested and verified in ground tests in the 196 and 197s Use of LEU Fuels is Key to the Future of NTR R&D Enhances proliferation resistance Enable non nuclear weapon states to engage in serious R&D Lower costs of research and development First concrete step towards eventual commercialization Paolo Venneri - NETS Albuquerque, NM 3

4 Typical LEU-NTR Configuration Paolo Venneri - NETS Albuquerque, NM 4

5 Baseline Carbide LEU-NTR Design Parameter SNRE C-LEU-NTR Power (MW) I sp (s) ~94 ~ 9 Thrust (kn) ~ Total System Mass* (kg) Thrust/Weight Coolant Flow Rate in the Fuel Elements (kg/s) U Mass (kg) 59.6 ~ 7 # of Tie Tubes # of Fuel Elements Average Fuel Exit Temperature (K) * Reactor plus shielding Parameter Value Reactor Mass Including Axial Reflector (kg) 216 Number of Fuel Elements 379 Number of Moderator Elements 774 Active Core Radius (m).35 Reflector Thickness (Axial and Radial) (m).2 Active Core Length (m).75 Inner Enrichment Zone Radius (m).154 Radial Power Profile Paolo Venneri - NETS Albuquerque, NM 5

6 Full Submersion Criticality Mission Abort Launch accident whereby the rocket has to be destroyed Unforeseen re-entry into Earth s atmosphere Can and Will Happen Eventually One of the few design basis accidents for space nuclear reactors today UN Space Treaty The design and construction of the nuclear reactor shall ensure that it cannot become critical before reaching the operating orbit during all possible events, including rocket explosion, re-entry, impact on ground or water, submersion in water or water intruding into the core. Principle 3 Sec. 2 Par. E The Accident: Core is flooded with moderating material Water Salt water Wet sand Core is surrounded by reflecting material Water Salt Water Wet Sand Loss of reflector region Control drum stuck in the out position Loss of control drums Loss of the entire reflector Paolo Venneri - NETS Albuquerque, NM 6

7 Full Submersion Criticality Neutronic Effects Neutron Loss Fraction k =h fpep FNL P TNL.5.45 η Thermal fission factor.4.35 f Thermal utilization.3 p Resonance escape probability ε Fast fission factor.1 P FNL Fast non-leakage probability P TNL Thermal non-leakage probability Water Density (g/cm^3) CERMET - Escape CERMET - Capture CERMET - Fission CC - Escape CC - Capture CC - Fission Paolo Venneri - NETS Albuquerque, NM 7

8 Mitigation Systems External Isolate the neutronics of the active core from external changes Maximize the reflector worth Neutron poison on the outside of the active core Limited by the need to have one side of the core essentially bare Un-impeded hydrogen outlet Extreme outlet temperatures Internal Maximize neutron capture with the increased thermalization of the active core Spectral shift absorbers Over-moderation Neutron absorbing wires (NERVA) Limited by the ability to place neutron poisons and availability of space for additional moderator Thermal neutron spectrum Compact core size Objective is to minimize the worth of the water when the core is fully submerged. k water = k empty k flooded Paolo Venneri - NETS Albuquerque, NM 8

9 Several Options (King and El-Genk, 26): Spectral Shift Absorbers Paolo Venneri - NETS Albuquerque, NM 9

10 Keys to Successful Implementation Spectrum difference between submerged and dry conditions Low uranium content requires a thermal spectrum unless additional fissile content is added to the fuel Self shielding of the poison Self shielding reduces the effectiveness of the poison and can result in hot spot in the core 1.E+5 1.E+4 1.E-2 1.E+3 1.E+2 8.E-3 1.E+1 6.E-3 1.E+ 1.E-1 4.E-3 Microscopic Cross-Section (barns) Reactivity loss due to spectrum hardening 1.E+6 1.2E-2 Neutron Flux per Unit Lethargy Effectiveness of the poison is determined by the spectrum difference between the two conditions 1.4E-2 1.E-2 2.E-3 1.E-3.E+ 1.E-9 1.E-8 1.E-7 1.E-6 1.E-5 1.E-4 1.E-3 1.E-2 1.E-1 1.E-4 1.E+ 1.E+1 1.E+2 Energy (MeV) Spectrum Inverse Alpha Paolo Venneri - NETS Albuquerque, NM Gd157 (n,gamma) U235 (n,fission) 1

11 Nutron Flux per Unit Lethargy Reference Cores Carbide LEU-NTR Configuration Dry Flooded k-eff std dev k-eff std dev Water Worth Void Fraction Fuel Fraction # ME # FE 235 U Mass (g) Inverse Alpha (2:1) Bulls-Eye (1:1) E-2 Dry and Submerged Neutron Spectrums 1.4E-2 1.2E-2 1.E-2 8.E-3 6.E-3 4.E-3 2.E-3.E+ 1.E-9 1.E-8 1.E-7 1.E-6 1.E-5 1.E-4 1.E-3 1.E-2 1.E-1 1.E+ 1.E+1 1.E+2 Neutron Energy (MeV) Bulls-Eye Submerged Bulls-Eye Dry 1/α Submerged 1/α Dry Paolo Venneri - NETS Albuquerque, NM 11

12 Gadolinia (Gadolinium Oxide) Gd 2 O 3 High melting temperature Relatively easy to implement Fuel Matrix Additions of minute quantities should adversely affect the performance of the fuel not Poison was added while ensuring that the fissile density remained constant Inner and Outer Tie Tubes Oxide dispersion strengthened zirconium alloy Improve Zircaloy properties high temperature material Current research in Russia and South Korea to develop accident tolerant cladding for light water reactors Poison in the structural material will not have an adverse effect on the strength as a function of burnup Burnup after a Mars mission (less than 2 hours of operating time) is less than.1 atom % Have different shelf-shielding characteristics Poison Location Fuel Matrix Inner Tie Tube Outer Tie Tube Paolo Venneri - NETS Albuquerque, NM 12

13 Neutron Flux per Unit Lethargy Spectrum Hardening Moderator Removal Neutron Flux per Unit Lethargy 1.4E-2 1.2E-2 1.E-2 8.E-3 6.E-3 4.E-3 2.E-3 Inverse Alpha (2:1) Spectrum Changes Bulls-Eye (1:1) Spectrum Changes 1.4E-2 1.2E-2 1.E-2 8.E-3 6.E-3 4.E-3 2.E-3.E+ 1.E-9 1.E-8 1.E-7 1.E-6 1.E-5 1.E-4 1.E-3 1.E-2 1.E-1 1.E+ 1.E+1 1.E+2 Neutron Energy (MeV) Mod Thick =.44 cm Mod Thick =.354 cm Mod Thick =.268 cm Mod Thick =.182 cm.e+ 1.E-9 1.E-8 1.E-7 1.E-6 1.E-5 1.E-4 1.E-3 1.E-2 1.E-1 1.E+ 1.E+1 1.E+2 Neutron Energy (MeV) Mod Thick =.44 cm Mod Thick =.354 cm Mod Thick =.268 cm Mod Thick =.182 cm Paolo Venneri - NETS Albuquerque, NM 13

14 k-eff loss due to Gd loading Reactivity Loss Due to Gd2O3 Loading Bulls-Eye (1:1) k-eff loss due to Gd loading k-eff loss due to Gd loading Bullseye - Poison in Inner Tie Tube - Gd Loss k-effective Bullseye - Poison in OuterTie Tube - Gd Loss k-effective Linear increase in k-eff loss shows that there is relatively little self-shielding. Spectrum hardening has the expected result of reducing the reactivity penalty due to the introduction of Gd. There is relatively little difference between the different poison locations The fuel has the hardest local spectrum, resulting in the lowest reactivity penalty Bullseye - Poison in Fuel Element - Gd Loss k-effective Paolo Venneri - NETS Albuquerque, NM 14

15 k-eff loss due to Gd loading k-eff loss due to Gd loading k-eff loss due to Gd loading Reactivity Loss Due to Gd2O3 Loading Inverse Alpha (2:1) IA - Poison in Fuel Element - Gd Loss k-effective Similar behavior as the Bulls-Eye except with a larger penalty due to the softer spectrum. IA - Poison in Inner Tie Tube - Gd Loss k-effective IA - Poison in OuterTie Tube - Gd Loss k-effective Paolo Venneri - NETS Albuquerque, NM 15

16 Total k-eff loss (Gd + Mod loss) Total k-eff loss (Gd + Mod loss) Total k-eff loss (Gd + Mod loss) Total Reactivity Loss Bulls-Eye (1:1) Total loss due to the addition of Gd and spectrum hardening. Penalty due to spectrum hardening dominates the reactivity loss Bullseye - Poison in Fuel Element - Total Loss k-effective Bullseye - Poison in Inner Tie Tube - Total Loss k-effective Bullseye - Poison in Outer Tie Tube - Total Loss k-effective Paolo Venneri - NETS Albuquerque, NM 16

17 Total k-eff loss (Gd + Mod loss) Total k-eff loss (Gd + Mod loss) Total Reactivity Loss Inverse Alpha (2:1) Total k-eff loss (Gd + Mod loss) IA - Poison in Fuel Element - Total Loss k-effective Reactivity loss due to spectrum hardening is less pronounced due to the over-moderated nature of the infinite inverse-alpha lattice IA - Poison in Inner Tie Tube - Total Loss k-effective IA - Poison in Outer Tie Tube - Total Loss k-effective Paolo Venneri - NETS Albuquerque, NM 17

18 flood k-eff flood k-eff flood k-eff Water Worth Bulls-Eye (1:1) As the dry spectrum is hardened, the effect of the Gd becomes more pronounced. Moderator reduction has a stronger effect on the dry core than on the submerged core Reduces the effect of the Gd during submersion Bullseye - Poison in Fuel Element - Flood Δk.25.2 Bullseye - Poison in Inner Tie Tube - Flood Δk.25.2 Bullseye - Poison in OuterTie Tube - Flood Δk Paolo Venneri - NETS Albuquerque, NM 18

19 flood k-eff flood k-eff flood k-eff Water Worth Inverse Alpha (2:1) IA - Poison in Fuel Element - Flood Δk Same behavior as the Bulls-Eye configuration Initial water worth is much smaller than the Bulls-Eye configuration The over moderated spectrum significantly reduces the water worth Both configurations can reduce the water worth by about 25%. IA - Poison in Inner Tie Tube - Flood Δk IA - Poison in OuterTie Tube - Flood Δk Paolo Venneri - NETS Albuquerque, NM 19

20 k-effective Effect on Burnup Time (hours) Gd2O4 Core loading = 4.5 g baseline Paolo Venneri - NETS Albuquerque, NM 2

21 Result Summary Mod Thick (cm) Bulls-Eye Configuration (1:1) Inner Tie Tube Outer Tie Tube Fuel Element Water Worth Loss/Gd(g) Gd Loss/Gd(g) Water Worth Loss/Gd(g) Gd Loss/Gd(g) Water Worth Loss/Gd(g) Gd Loss/Gd(g) Mod Thick (cm) Inverse Alpha Configuration (2:1) Inner Tie Tube Outer Tie Tube Fuel Element Water Worth Loss/Gd(g) Gd Loss/Gd(g) Water Worth Loss/Gd(g) Gd Loss/Gd(g) Water Worth Loss/Gd(g) Gd Loss/Gd(g) Paolo Venneri - NETS Albuquerque, NM 21

22 Effects of Neutron Spectrum Hardening Moderator Replacement with Tie Tube Material Reduces: Gd Reactivity Loss Flooded and Dry k-effectives Spectrum Change Lattice Configurations Bulls-Eye Significant reduction in neutron absorption due to over moderation Increased value of poison contribution to reducing the water worth Inverse Alpha Neutron absorption due to over moderation is significant Larger void fraction results in a notable spectrum change with water submersion, despite an initially softer spectrum Fraction of Neutrons Lost to Absorption Clean Flood Flood + poison Inverse Alpha Bulls-Eye Paolo Venneri - NETS Albuquerque, NM 22

23 Neutron Flux per Unit Lethargy Spectrum Change With Submersion Neutron Flux per Unit Lethargy Inverse Alpha (2:1) Spectrum Changes Bulls-Eye (1:1) - Spectrum Changes 1.6E-2 1.4E-2 1.2E-2 1.E-2 1.6E-2 1.4E-2 1.2E-2 1.E-2 8.E-3 8.E-3 6.E-3 6.E-3 4.E-3 2.E-3.E+ 1.E-9 1.E-7 1.E-5 1.E-3 1.E-1 1.E+1 Neutron Energy (MeV) 4.E-3 2.E-3.E+ 1.E-9 1.E-7 1.E-5 1.E-3 1.E-1 1.E+1 Neutron Energy (MeV) Mod Thick =.44 cm Mod Thick =.182 cm Mod Thick =.44 cm Mod Thick =.182 cm Mod Thick =.44 cm Mod Thick =.182 cm Mod Thick =.44 cm Mod Thick =.182 cm Paolo Venneri - NETS Albuquerque, NM 23

24 Spectral shift absorbers can be implemented There is a notable benefit. Conclusion While neutron spectrum hardening does reduce the poison penalty, the penalty from a harder spectrum is significantly larger. Acceptable reduction in water worth is seen even without spectrum hardening. We can increase the amount Gadolinia in the fuel to further reduce the water worth due to the rapid burn up of the poison during initial operation. Over moderation provides a large negative reactivity insertion with submersion Having an over-moderated active core has a larger effect than the poison loading on the total water worth. Paolo Venneri - NETS Albuquerque, NM 24

25 Optimizing the implementation of spectral shift absorbers in the over-moderated core Further increase the Gadolinia loading More detailed burnup analysis is required to verify the burnup behavior of the poison Radial power profile flattening Spectrum hardening in discrete locations Reduce poison reactivity loss and increase poison effectiveness Reduce reflector size Mass of the reflector is non-negligible Explore options for reducing axial neutron streaming from the core Will reduce the water worth significantly Options for fuel element coolant design Russian twisted ribbon fuel design Future Work Paolo Venneri - NETS Albuquerque, NM 25