Safety And Power Multiplication Aspects Of Mirror Fusion-Fission Hybrids

K. Noack1, O. Ågren1, J. Källne1, A. Hagnestål1, V. E. Moiseenko2

1Uppsala University, Ångström Laboratory, Division of Electricity, Box 534, SE 751 21 Uppsala, Sweden

2Institute of Plasma Physics, National Science Center “Kharkiv Institute of Physics and Technology”, Akademichna st. 1, 61108 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 Model3/17

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% Bnat

New component: Reactivity modulator (RM)

TABLE 1.

1. Present Neutronic Model4/17

Standard axial dependence of the neutron source:

: Length of the core = 25 m !

1. Present Neutronic Model5/17

Reactivity effect of the „Reactivity modulator“ (RM):

: Reactivity range = 4000 pcm (10-5)

~4000 pcm

# Calculation model: • 2 B4C-annuli at the outer core surface at both ends• Thickness = 1 cm, height = 2.5 m• Boron is 90% enriched in 10B

1. Present Neutronic Model6/17

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/cm3 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 Considerations7/17

Steady-state power amplification:

fus1,c

eff

eff

fus

fisfis P

S

J

)k1(

k1

E

EP

Fission power Fusion powerPAF

Meff*

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:• Pfus (fusion driver)• *

appr (fusion driver)• Meff (fission blanket)

PAF

: The blanket must remain sub-critical in any case!

2. Safety Considerations8/17

Temperature feedback effects at start-up & switch-off:

Effect Δkeff /(keff·ΔT) (10-6 1/K) Δkeff

(pcm)

1) Doppler effect of the fuel -1.05 30% ▬73

2) LBE-coolant density effect -7.4 5% ▬350

3) Axial core expansion ~ 0 0

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, 2004. Data given for a Na-cooled FR with oxide fuel.

: Expected maximal total temperature effect for start-up/switch-off (or „loss of plasma“): ~ ▬/+800 pcm

# Calculation model: • Fuel 400 K 1100 K• LBE, structure 400 K 900 K

2. Safety Considerations9/17

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]4 entire core5 buffer&core&expansion zone

: Expected maximal reactivity effect by radial LBE-voiding: ~ +1500 pcm

~1445 pcm 3%

2. Safety Considerations10/17

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 Considerations11/17

Reactivity effects of water in the coolant loop and in the vacuum chamber:

Cases: 1 – H2O within the core 2 – H2O within core, buffer, exp. zone3 – H2O within buffer, exp. zone 4 – H2O within the vacuum chamber

: • Case 1 must be excluded by design!• All the other „water effects“ are negative.

2. Safety Considerations12/17

*-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 2 1.03

2) Source length compressed to 20 m 1.13

3) Full intensity concentrated at z=0 1.42

13/17

Axial dependence of the specific fission heating:

hfis = 1513 (MeV)

hfis(z)= Fission heating per source neutron emitted at z

2. Safety Considerations

dz)z(S)z(hP fisfis

3. Discussion and Conclusions14/17

With regard to the blanket (A – keff=0.95, B – keff=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 (+800 pcm): Insertion of the RM to fulfill keff ≤ 0.95 (0.97).

# No safety relevant disadvantage of option B compared to A!

1) Response to changes of Pfus.• To reduce thermal shocks the Pfis 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“

+800 pcm Restriction: ≤ ~1000 pcm

# Then, even B is in deep sub-criticality!

3. Discussion and Conclusions15/17

With regard to the blanket (A – keff=0.95, B – keff=0.97):

4) Response to „unprotected“ transients. • Incidence: Driver cannot be shut off on demand.

T- increase insertion of (▬)-reactivityT-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 Conclusions16/17

With regard to the blanket (A – keff=0.95, B – keff=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: <+1500 pcm + cooling down the blanket + 800 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 Conclusions17/17

With regard to the mirror driver:

8) Minimal value as low as possible < Pfus < definite maximal value.

10) Pfus should be supplied gradually tunable and stable.

11) If Pfus 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!