Safety And Power Multiplication Aspects Of Mirror Fusion-Fission Hybrids K. Noack 1, O. Ågren 1, J. Källne 1, A. Hagnestål 1, V. E. Moiseenko 2 1 Uppsala University, Ångström Laboratory, Division of Electricity, Box 534, SE Uppsala, Sweden 2 Institute of Plasma Physics, National Science Center “Kharkiv Institute of Physics and Technology”, Akademichna st. 1, Kharkiv, Ukraine FUNFI workshop, Varenna, Italy, September 12-15, 2011  Articel:Annals of Nuclear Energy 38, 578 (2011)

CONTENT 1.Present Neutronic Model 2.Safety Considerations 3.Discussion and Conclusions 2/17

1. Present Neutronic Model 3/17 Modified radial structure: LBE-cooling loop T-breeding Thickness decreased 20 wt% of Li-6 New component: Shield (60:40) vol% steel&water Steel with 1.75 wt% B nat New component: Reactivity modulator (RM) Two hybrid options: A B k eff Core thickness (cm) Fission power (GW) Fusion power (MW) TABLE 1.

1. Present Neutronic Model 4/17 Standard axial dependence of the neutron source:  : Length of the core = 25 m !

1. Present Neutronic Model 5/17 Reactivity effect of the „Reactivity modulator“ (RM):  : Reactivity range = 4000 pcm (10 -5 ) ~4000 pcm # Calculation model: 2 B 4 C-annuli at the outer core surface at both ends Thickness = 1 cm, height = 2.5 m Boron is 90% enriched in 10 B

1. Present Neutronic Model 6/17 Disadvantage & Advantages:  Disadvantage: Reactor technology has no experience with such long systems.  Advantages: 1) Highly efficient utilization of the neutron source. 2) First wall problem is considerably mitigated. 3) The shielding of the magnetic coils is a fission shielding problem. 4) The vertical installation could enable natural circulation of the LBE-coolant. See talk O12 of H. Anglart, this workshop. 5) The long system implies a small leakage and hence a relatively small effect of the thermo-structural expansion. 6) Low average fission power density of 76 W/cm 3 and low average linear pin power of 80 W/cm. 7) Low radial peaking factor of 1.15 and of 1.30 over the whole core.

Reactivity feedback effects! 2. Safety Considerations 7/17 Steady-state power amplification: Fission power Fusion power PAF M eff  * appr ● Demand:The generation of the fission power must be manageable in any case to prevent the system from damage!  : Three possibilities to control the fission power: P fus (fusion driver)  * appr (fusion driver) M eff (fission blanket) PAF  : The blanket must remain sub-critical in any case!

2. Safety Considerations 8/17 Temperature feedback effects at start-up & switch-off: EffectΔk eff /(k eff ·ΔT) ( /K) Δk eff (pcm) 1) Doppler effect of the fuel-1.05  30%▬73 2) LBE-coolant density effect -7.4  5%▬350 3) Axial core expansion ~ 00 4) Radial expansion of fuel pins 0.4 (from Ref. 12 * ) 19 5) Radial core expansion -6.8 (from Ref. 12) ▬320(?) * [12] W. M. Stacey, Nuclear Reactor Physics, Data given for a Na-cooled FR with oxide fuel.  :Expected maximal total temperature effect for start-up/switch-off (or „loss of plasma“):~ ▬/ pcm # Calculation model: Fuel400 K  1100 K LBE, structure400 K  900 K

2. Safety Considerations 9/17 Coolant void effects− Voided radial areas within the core: # Calculation model: LBE-voided radial core areas (cm) 1[115 < r < 122] 2[113 < r < 124] 3[111 < r < 126] 4entire core 5buffer&core&expansion zone  : Expected maximal reactivity effect by radial LBE-voiding: ~ pcm ~1445 pcm  3%

2. Safety Considerations 10/17 Coolant void effects− Loss of LBE-coolant: # Calculation model: vertically installed hybrid united volume of buffer, core, exp. zone different LBE-levels  : Loss of the LBE-coolant results in a negative reactivity effect!

2. Safety Considerations 11/17 Reactivity effects of water in the coolant loop and in the vacuum chamber: Cases:1 – H 2 O within the core2 – H 2 O within core, buffer, exp. zone 3 – H 2 O within buffer, exp. zone 4 – H 2 O within the vacuum chamber  :Case 1 must be excluded by design! All the other „water effects“ are negative.

2. Safety Considerations 12/17  * -Effect of the axial distribution of the neutron source: Standard axial dependence of the neutron source # Calculation model: Deformations of the axial dependence. Deformation of the n-Source Ratio of fission heatings (def./stand.) 1) Peak height reduced by factor ) Source length compressed to 20 m1.13 3) Full intensity concentrated at z=01.42

13/17 Axial dependence of the specific fission heating: h fis = 1513 (MeV) h fis (z)= Fission heating per source neutron emitted at z 2. Safety Considerations

3. Discussion and Conclusions 14/17 With regard to the blanket (A – k eff =0.95, B – k eff =0.97): 3)Responses to start-up and switch-off at the beginning of the cycle. Start-up (▬800 pcm):  Withdrawal of the RM to meet the nominal criticality in the operation state. Switch-off ( pcm):  Insertion of the RM to fulfill k eff ≤ 0.95 (0.97). # No safety relevant disadvantage of option B compared to A! 1)Response to changes of P fus. To reduce thermal shocks the P fis should respond gradually. #In this respect, option B is not worse than A! 2)Response to inadvertant insertion of ( + )-reactivity. Worst case: „Inflow of cold LBE“ + „Ejection of the inserted RM“ pcm  Restriction: ≤ ~1000 pcm # Then, even B is in deep sub-criticality!

3. Discussion and Conclusions 15/17 With regard to the blanket (A – k eff =0.95, B – k eff =0.97): 4)Response to „unprotected“ transients. Incidence: Driver cannot be shut off on demand.  T- increase  insertion of (▬)-reactivity  T-increase is slowed down. # In this respect, option B is more advantageous than A!  Further reduction of the PAF by completely inserting the RM. # In this respect, option B is more advantageous than A! Our position: The shut-off of the driver definitely takes place after a minimal delay!  Quantitative estimates of possible core damage need dynamic calculations!

3. Discussion and Conclusions 16/17 With regard to the blanket (A – k eff =0.95, B – k eff =0.97): 6)Response to filling the LBE-coolant loop with water: Incidence: For example, intended misuse.  Negative effects, provided that buffer, core and exp. zone form a united volume! # No safety relevant difference between both hybrid options! 5)Response to coolant void effects. Loss of coolant:  negative effect. Voided areas within the core:  < pcm + cooling down the blanket  pcm The RM could be used to compensate reactivity. # Both hybrid options remain sub-critical! < 2300 pcm 7)Comparison of the hybrid options A and B: The study revealed that option B does not exhibit substantial disadvantages with regard to safety!

3. Discussion and Conclusions 17/17 With regard to the mirror driver: 8)Minimal value as low as possible < P fus < definite maximal value. 10) P fus should be supplied gradually tunable and stable. 11)If P fus is fluctuating, the frequencies should be clearly above 10 Hz. 12)The probability of plasma collapses must be minimal. 13)The neutron source should have the axial peaks at stable positions. In case of fluctuations, the frequency range should be clearly above 10 Hz. 9)The driver must be equipped with several redundant, quick shut-off techniques.

Thank you for your attention!