RESULTS OF NEUTRONICS CALCULATIONS: • The ERANOS 2.2n core-physics code has been used with JEFF3.1 library • The uranium composition (~25 wt% 235U) 4.5 Ga ago provided k∞~1.4 • A homogenous U and Th mix (35%U-65%Th) provided k∞ ~1.02 -> 4.5 Ga ago the geo-reactor was critical in a shell from the inner radius to a location with approximately 65%Th-35%U • Region of criticality then started a contraction driven by decay of 235U and U-Th stratification (Figures 1 and 2) • Without breeding the inner layer (U, no Th) switched off ~2.5 Gy ago (235U<7%) • However, breeding of 235U might have been possible (Figures 1 and 3):

238U (n,γ) 239U 239Np 239Pu 235U

• 235U has “short” half life. The georeactor could still operate today with 3 TW [3] power if the criticality region was a few hundreds meters shell. Production of 235U would be enough to balance 235U decay

0

0.05

0.1

0.15

0.2

0.25

0.3

-4.5 -4 -3.5 -3 -2.5 -2 -1.5

U2

35

en

rich

me

nt

[wt%

]

TIme (Ga)

INTRODUCTION: • Geo-reactors have been suspected to occur in large uranium deposits [1 ]. • They were for example found in the earth crust in Oklo, Gabon [2 ]. • The feasibility of a nuclear fission reactor at the Earth center was proposed as an energy source based on the fissile inventory, helium isotope ratio in volcanic gases and geomagnetism variations [3]. • Recently, geo-reactors were suggested in the core-mantel boundary [4]. However the later is a source of controversies. • The feasibility of the geo-reactor is revisited here with emphasis on the role of thorium.

GEO-REACTOR EVIDENCES:

The proto-Earth geo-reactor: a thorium reactor? Claude Degueldre1, Carlo Fiorina2

1LNM/LES, NES , PSI & University of Geneva, 2LRS, NES, PSI, Switzerland

REFERENCES: 1. P. Kuroda, On the nuclear physical stability of the uranium minerals, J. Chem. Phys. 4 (1956) 781-782 2. M. Neuilly, et al, Evidence of Early Spontaneous Chain Reaction found in Gabon Mine,excerpts from press conference regarding

Geological and Mineral Documentation published by Commissariat a l'Energie Atomique, 1972 3. J. Herndon, Feasibility of a nuclear fission reactor at the center of the Earth as the energy source for the geomagnetic field. J.

Geomagnet. Geoelectr., 45 (1993) 423-437 4. R. de Meijer, W. van Westrenen, The feasibility and implications of nuclear georeactors in Earth‘s core-mantle boundary region, South

African J. Sci. 104 (2008) 111- 118. 5. J. Herndon, Nuclear georeactor generation of the Earth's geomagnetic field. Current Science, 93 (2007) 1485-1487 6. A.M. Dziewonski, D.L. Anderson, Preliminary reference Earth model, Phys. Earth Planet. Interiors, 25 (1981) 297-356 7. H. Staudigel, F. Albarède, J. Blichert-Toft, J. Edmond, B. McDonough, S.B. Jacobsen, R. Keeling, C.H. Langmuir, R.L. Nielsen, T. Plank, R.

Rudnick, H.F. Shaw, S. Shirey, J. Veizer, W. White, Geochemical Earth Reference Model (GERM): description of the initiative, Chem. Geol., 145 (1998) 153-159

Geo-reactor [4]

• Based on the fissile inventory in proto-earth period (P: -4.5 Ga): U and Th analysis in Abee enstatite chondrite

232Th 235U 238U 244Pu Total mass

T1/2 (Ga) 14.05 0.70 4.47 0.08 M0 (1017 kg) 3.15 0.00587 0.80 - 3.95

Ab0 (%) 100 0.73 99.27 - MP (1017 kg) 3.94 0.52 1.62 0.012 6.09

AbP (%) 100 24.3 75.7 100

• Based on the 3He/4He ratio in various basalts samples Vs air [3]. 3H is generated by actinide triple fissions: 235U + 1n → 92Kr + 141Cs + 3H + 2 1n

3H decays according to 3H → 3He + ϐ- + 1 ṽ

• Based on Xe isotope ratios from various geo-system samples [4]

0.1

1

10

100

128Xe 129Xe 130Xe 131Xe 132Xe 134Xe 136Xe

Rela

tive a

bu

md

an

cy

(%)

Air

Mantel mix

Fission

Spent fuel

• Based on the geomagnetism: the North Magnetic Pole has recently moved at a rapid rate toward Siberia. Herndon suggests, intermittently disrupt the stability of georeactor geomagnetic field [5]

GEO-REACTOR CONDITIONS: Based on the fissile proto-earth inventory and on the element stratification following

PERM [6] and GERM [7] approaches and coupling chemical/physical potentials (redox and gravitational potentials) for uranium, thorium and fission products

Oxidizing

Reducing Uranium

Thorium

Fission Prod.

CONCLUSIONS: • The geo-reactor concept could resolve specific questions such as:

isotopes ratios (He, Xe, Kr, ..) elemental concentrations (REE pattern) magnetic field translation and reversal heat generation from fission (≤ 3 TW)

• Thorium mainly acts as a neutron poison, since the generated 233U has negligible half-life in geological terms • Thorium and uranium stratifications, together with U-235 decay, have driven the geo-reactor evolution • The geo-reactor may still be operating today thanks to the breeding of 235U from 238U • Additional experimental investigations are required to confirm such results (e.g. Earth antineutrino tomography)

As calculated by Herndon [3] for two geo-reactor powers

U4+ + 2SiO32- U(SiO3)2

USi2 U0 + 2Si0

6O2- 6O2- 12e- 12e-

g g

With g=0

With g≠0

~10 km

~20 km

~15 km

~1 mW/kg

0 mW/kg

Criticality threshold

Fig. 1: 235U enrichment for two different georeactor specific powers

Fig. 2: Example of isotopic evolution in the georeactor (staring with 35%U-65%Th)

Fig. 3: Neutron spectrum in the georeactor (35%U-65%Th, -4.5 Ga)

1.E-4

1.E-3

1.E-2

1.E-1

1.E+0

1.E+1

1.E+2

1.E+3

1.E+4

-4.5 -3 -1.5

Ma

ss

[k

g]

Time [Ga]

Pu239 U233

U235 U238

TH232

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.E+02 1.E+04 1.E+06 1.E+08

No

rmalized

neu

tro

n f

lux

Energy (eV)

standard FRs

Georeactor