Comparison of U-Pu and Th-U cycles in MSR

Transcription

1 WIR SCHAFFEN WISSEN HEUTE FÜR MORGEN Jiri Krepel :: Advanced Nuclear System Group :: Paul Scherrer Institut Comparison of U-Pu and Th-U cycles in MSR ThEC 2018 conference October 2018, Brussels, Belgium

2 Aim: objective comparison of the U-Pu & Th-U Purely from neutronic perspective URANUS THOR / Page 2

3 General properties of 238 U and 232 Th More is better Less is better 238 U 232 Th reference Abundance on Earth 2-4 times less Decay half-life 4.5 bn years 14 bn years Known reserves in ,718,400 tonnes 6,355,000 tonnes So far mined 2,797,435 tonnes??? Direct availability enrichment waste 1,188,273 tonnes??? Karlsruhe nuclide card Fissile by thermal neutrons no no Any nuclear data library Minimal neutron energy for fission Share of direct fission Transmutable to Intermediate product decay half-life Fissile brother 1.5MeV 2.1 MeV (up to 10% of all fission) (up to 3% of all fission) 239 Pu via 239 Np fission/critical-energy-threshold-energy-for-fission/ Own calculations 233 U via 233 Pa Karlsruhe nuclide card Karlsruhe nuclide card 2.4 days 27 days 235 U (0.7% of natural U) No, must rely on strangers Page 3

4 General properties of 239 Pu and 233 U More is better Less is better 239 Pu 233 U reference Natural abundance Decay half-life and decay product Annual production Produced until 2017 Separated until years, 235 U years, 228 Th cca 70 tonnes of LWR Pu??? 1,300 tonnes of LWR Pu??? 400 tonnes of LWR Pu (130 tonnes recycled) no no ~2 tonnes Karlsruhe nuclide card Fission probability in fast and (thermal) reactors 83% (62%) 90% Capture probability in thermal and (fast) reactors 38% (17%) 10% Produced neutrons per fission (ν) Excess neutrons ν-2 (-2 for fission and breeding) Own calculations Own calculations Own calculations Own calculations Page 4

5 Comparison of U-Pu & Th-U in MSR MS cooled not really MSR Page 5

6 Two studies & two compared characteristics 1. Comparison of U-Pu and Th-U equilibrium cycle in 16 different reactors at infinite lattice level 2. Comparison of MSR concept options based on 5 salts and 6 moderators and 8 salts in unmoderated core. I. How good is the equilibrium cycle performance. II. How to reach the equilibrium cycle. Page 6

7 Equilibrium cycle = repetitive recycling (EQL0D) When fuel cycle parameters: power, reprocessing scheme, feed composition, etc. are fixed, reactor will converge to equilibrium state / fuel composition. The composition depends on feed type 238 U or 232 Th and on the reactor spectrum. The spectrum is determined by scattering materials. Equilibrium composition and spectrum determine the multiplication factor. Equilibrium is inherent core state. (composition, spectrum, reactivity) [Hombourger et al., EQL0D paper submitted to Ann. Nucl. Energy, 2018] Equilibrium reactivity indicates neutron efficiency of the core and its capability for breeding in closed fuel cycle. Page 7

8 Assumptions: 1) infinite lattice. 2) neglected fission products. 3) design as is, no additional optimization. 4) ENDF/B-VII.0 8 thermal and 8 fast reactors were compared in both U-Pu and Th-U cycles. Equilibrium cycle comparison for 16 reactors Křepel, J., Losa, E., Closed U-Pu and Th-U cycle in sixteen selected reactors: evaluation of major equilibrium features. Submitted to Ann. Nucl. Energy. Page 8

9 Infinite multiplication factor Equilibrium multiplication factor (reactivity) Better performance: Th-U in thermal and U-Pu in fast spectra. Graphite moderated MSR only in Th-U. Fluorides fast MSFR possible in both cycles (equal performance). Chlorides fast MSR max. reactivity in both cycles. B&B possible for U-Pu with react. gain option with react. gain option nominal value Th-U nominal value U-Pu Th-U cycle U-Pu cycle Page 9

10 Additional insight: excess reactivity break-down For better understanding of the equilibrium reactivity excess, it was decomposed this way: 2 R R R Th Th F n,2n 2R 232 total Th F n,2n R R other _ Ac structural C C R 2R 232 total Th F n,2n Available neutrons Bonus from fertile Parasitic captures It was derived from neutron balance equation: k inf R R total P total F 2R R total C total n,2n total Rn,2n total total total Th other _ Ac structural Using four assumptions: 1) RP vrf 2) RC RC RC RC 232 total Th 3) Rn,2 n Rn,2 n (main fertile ~ 90% of all (n,2n) reactions) Th Th total Th 4) R R R R (equilibrium of non-th Ac production and destruction) C n, 2n F F 232 Page 10

11 Isotope-wise break-down of v-bar Isotope-wise break-down of v-bar Available neutrons do not differ between reactors. Available neutrons hypothetical reactivity U Othrer Ac Cm Am Pu 240Pu 239Pu 2 ν = 2.5 in Th-U cycle U-Pu cycle ν = 2.9 in U-Pu cycle Hypothetical reactivity excess: pcm in Th-U pcm in U-Pu cycle (2/3) Th Othrer Ac Pu 237Np U 234U 233U Th-U cycle Page 11

12 232 Th fission and (n,2n) reactions relative to the total fission rate (%) 238 U fission and (n,2n) reactions relative to the total fission rate (%) Bonus from fertile strongly differ between reactors. Bonus from fertile material R R (n,2n) (n,f) 2R Th Th F n,2n 2R 232 total Th F n,2n It is up to 4 % in Th-U cycle 6 3 U238 It is up to 15 % in U-Pu cycle Bonus reactivity 3 in fast spectrum: pcm in Th-U pcm in U-Pu 0 cycle (2/5) (n,2n) (n,f) Th232 Page 12

13 Isotope-wise break-down of parasitic capture rate relative to the total fission rate (%) Isotope-wise break-down of parasitic capture rate relative to the total fission rate (%) Parasitic captures strongly differ between reactors. Parasitic neutron captures by actinides R R other _ Ac C 232 total Th F 2Rn,2n Higher Ac Cm Am Pu 240Pu 239Np 239Pu 20% in fast & up to 40 % in thermal Th-U cycle. 30% in fast & up to 120 % in thermal U-Pu cycle U-Pu cycle Higher Ac all Pu 237Np U 234U 233Pa 233U Reactivity loss in Th-U is much lower (1/3 in thermal, (2/3 in fast case) Th-U cycle Page 13

14 Reactivity (PCM) Reactivity (PCM) Th-U: lower ν and lower parasitic capture of 233 U, breeding also in thermal spectrum. U-Pu: higher ν and higher parasitic capture of 239 Pu, better performance in fast spectra. Th-U high efficiency. U-Pu high economy. Overall excess reactivity R R Th Th F 2Rn,2n 232 total Th F 2Rn,2n R Hypoth. ρ 239Pu 240Pu Pu Am Cm 239Np Other Ac Structural Excess ρ U-Pu cycle Hypoth. ρ 233U 234U U 237Np Pu 233Pa Other Ac Structural Excess ρ Th-U cycle R other _ Ac structural C C 232 total Th RF 2Rn,2n Page 14

15 Ac density in the core (kg/m 3 ) or migration area (cm 2 ) Ac density in the core (kg/m 3 ) or migration area (cm 2 ) Comparable M for both cycles; it differs between reactors. MSFR: small M caused by Li and F scattering and mass. Graphite based MSR higher M caused by low fuel density. MCFR: highest M, absence of scattering resonances, lower density than MSFR. Migration area M (determines with k inf the core size) Ac density 239Pu density Migration area Ac density 233U density Migration area Pu density in the core (kg/m 3 ) U-Pu cycle 233 U density in the core (kg/m 3 ) Th-U cycle Page 15

16 Other salts and moderators? 5 fluoride salts were analyzed with 6 selected moderators. Equilibrium k inf is presented as a function of salt share and channel radius. FLi salt is neutronically the best. Good results for Be, BeO, and D2O; however, they are not compatible with the salt without cladding (SiC..?). Hydrogen based moderators ZrH and H 2 O not applicable for closed cycle. Graphite is not the best moderator, but the only one directly compatible with salt. [Hombourger, B.A., Ph.D. Thesis. EPFL Lausanne, Switzerland.] Page 16

17 Neutronics impact of cladding LiF salt combined with Be and D 2 O moderators was selected to analyze the impact of cladding: Hastelloy, SS316, and SiC. LiF-ThF 4 Only SiC seems to have acceptable low parasitic neutron capture. From purely neutronics perspective we can design Heavy Water Boiling Thermal Thorium MSR HWB-TT-MSR Page 17

18 Reactivity (PCM) Other salts without moderator? 8 salts were evaluated: FLi, FLiBe, FLiNa, FNaBe, FNaK, NaCl (nat), Na 37 Cl, Ac 37 Cl. 32% AcCl 3 32% AcCl 3 100% AcCl 3 4 options in Th-U (reasonable melting point and reactivity): FLi, FLiNa, FNaK, Na 37 Cl. Na 37 Cl provides the highest excess in Th-U of pcm. FLi is best fluoride salt with 6000 pcm Hyp. rho 233U 234U 235-9U 237Np Pu 233Pa Other Ac FPs Salt Excess ρ Th-U cycle Page 18

19 Reactivity (PCM) Other salts without moderator? 8 salts were evaluated: FLi, FLiBe, FLiNa, FNaBe, FNaK, NaCl (nat), Na 37 Cl, Ac 37 Cl. 32% AcCl 3 32% AcCl 3 100% AcCl 3 5 options in U-Pu (reasonable melting point and reactivity): Fli, FLiNa, FNaK, NaCl (nat), Na 37 Cl. Na 37 Cl provides the highest overall excess of pcm. FLi, FLiNa, FNaK have similar performance of ~6000pcm, PuF 3 solubility is the PuF 3 solubility? Hyp. rho 239Pu 240Pu 241-2Pu Cm Am 239Np Other Ac FPs Salt Excess ρ U-Pu cycle major limiting issue. Page 19

20 Relative fuel radiotoxicity of Th-U and U-Pu cycles Th-U closed cycle provides in general lower radiotoxicity. Nonetheless, Th-U has secondary radiotoxicity peak but U-Pu not. It is predominantly caused by 238 Pu decay chain. 238 Pu-> 234 U-> 230 Th-> This result is exclusively valid for cores with equal salt volume, burnup and treatment! Th-U better U-Pu better Page 20

21 Using 1m hastelloy reflector core size was estimated for single-fluid designs. It was compared with classical fast reactors Minimal core size estimate in equilibrium cycle and MSBR (ORNL design, 13% salt). MSFR in Th-U (4) is compact. MSFR in U-Pu (5) is slightly bigger. MCFR in U-Pu (6) is comparable to MSFR in U-Pu (5). MCFR in Th-U (7) is quite big. Page 21

22 Breed and Burn cycle B&B cycle is possible only with 37 Cl salts (high reactivity excess according to ENDF/B VII.0). Fresh Fuel Solid fuel reactor Spent Fuel The B&B cycle in liquid fuel reactor substantially differs from solid fuel. Fresh Fuel Liquid fuel reactor Spent Fuel Single-fluid layout of the core may be bulky (transparent cores). Illustration of burnup distribution. The fuel is in reality mixed. Multi-fluid layout can possibly reduce the size and increase the final burnup. Fresh Fuel Mulit-Liquid fuel reactor Spent Fuel Hombourger, B., Křepel, J., Mikityuk, K., Pautz, A., On the Feasibility of Breed-and-Burn Fuel Cycles in Molten Salt Reactors, in: Proceedings of FR17. Yekaterinburg, Russian Federation. Page 22

23 Hombourger, B. et al., Fuel Cycle Analysis of a Molten Salt Reactor for Breed-and-Burn Mode. ICAPP France. Breed and Burn cycle Th-U versus U-Pu B&B is practically not possible in Th-U cycle. It is only possible in mixed U-Pu & Th-U cycle. The performance increases in U-Pu cycle with growing UCl 3 share in the core. [FPs % share in the salt] Page 23

24 Transition to equilibrium cycle 5 major existing fissile materials to start the cycle Material RG_Pu LEU HEU U233 WG_Pu Fissile isotope(s) Fissile isotope share Availability 239 Pu, 241 Pu ~60% medium 235 U 1-20% high 235 U 21-95% high 233 U % low 239 Pu >93% medium Proliferation risk medium medium high high high Page 24

25 RG_Pu and LEU as initial fuel load Both RG_Pu and LEU are very natural option to start the U-Pu cycle. Fuel composition - initial cycles (10% 235 U equivalent) U-Pu 235 U 238 U 10% 235 U enriched U U-Pu LWR Pu vector 238 U MOX fuel (+MA) Starting Th-U cycle with LEU induces 238 U presence in the core. Th-U 235 U 238 U 232 Th Th+20% 235 U enriched U Starting Th-U cycle with RG_Pu, LEU or their mixture introduces strong perturbation. Pu and 235 & 238 U are not presented in the salt at equilibrium Th-U cycle. Th-U LWR Pu vector 232 Th Th+Pu (+MA) Page 25

26 Transition to Th-U cycle in MSFR and U-Pu MCFR Equilibrium: MSFR Th-U MCFR U-Pu MCFR Th-U (PuCl 3 ) (ThCl 4 ) (UCl 3 ) Page 26

27 Summary of neutronics comparison U-Pu cycle Th-U cycle Reserves of 238 U and 232 Th: no argument for preference, we are lucky to have both. Features of 238 U and 232 Th: slightly better (direct fission, etc.) Features of 239 Pu and 233 U: higher ν, higher capture lower ν, lower capture Thermal spectrum capability: no yes Fast spectrum capability: yes yes Breed and burn capability: yes no Radiotoxicity at equal conditions: initially higher lower Core size in chlorides (MCFR): smaller substantially bigger Core size in fluorides (MSFR): slightly bigger smaller Transition to eql. In MSFR: smooth (if possible) challenging Transition to eql. In MCFR: smooth smooth Page 27

28 Wir schaffen Wissen heute für morgen Thank you for your attention. Page 28