1. Two Billion-Year-Old Natural Fission Reactors
: Natural Analogue Study for High-level Radioactive Waste
20억 년 前의 천연 원자로
: 고준위방사성폐기물의 안정적 처분을 위한
自然類似現象
Chang, Ho Wan
The National Academy of Sciences, Republic of Korea

2. Contents Ⅰ. Introduction Ⅱ. Some Surprising Prediction
: Natural Nuclear Fission Reactor
Ⅲ. Uranium Ore Formation Process
Ⅳ. OKLO Phenomenon
Ⅴ. Natural analogue study
Ⅵ. Conclusion

3. Ⅰ. Introduction - The Fission Process
-Fission is the splitting of an atomic nucleus. The easiest nuclei to split are very heavy nuclei like 235U and 239Pu which if they absorb a small sub-atomic particle like a neutron can split into two fission fragments (or fission products) and produce 2 or 3 neutrons.

4. - Enrico Fermi :Italian-American physicist
Fermi was awarded the Nobel Prize for physics in 1938 for the production of transuranic elements by neutron irradiation.
The First Man Made World's First Fission Reactors in the December 2, 1942.
- Paul Kuroda : Japanese-American chemist
Less than 15 years after the first man made fission reactor scientists were thinking about the possibility of naturally occurring nuclear reactors.

5. Ⅱ. Some Surprising Prediction : Natural Nuclear Fission Reactor
Professor Paul Kuroda ( ) studied natural and artificial radioactivity after he became an Assistant Professor of Chemistry at the University of Arkansas in In the scientific community Kuroda is perhaps best known for having predicted, in 1956, that self-sustaining nuclear chain reactions could have occurred naturally in Earth's geologic history.
Professor Paul Kuroda determined the detailed requirements for any likely
natural reactors
- Approximate age range for a natural reactor
- The U concentration
- 235U/238U ratio requirements
- The natural reactor shape requirements.

6. On 25 September 1972, the French Atomic Energy Commission
announced the discovery of evidence that a natural nuclear
reactor had occurred at OKLO in the Republic of Gabon, Africa

7. How Nature Beat Humans to the Punch.
Oklo Reactors
How Nature Beat Humans to the Punch.
by David Monagham
What these people fail to realize is that nature has already learned the trick for creating energy from radioactive materials long before man ever appeared on this planet. In the area now known as Gabon Africa almost 2 billion years ago, natural forces made the only known naturally occurring nuclear reactor referred to as the Oklo Reactors..

8. The Oklo mine site looking east
Remains of zone 2

9. Description:  A natural nuclear fission reactor is a uranium deposit where analysis of isotope ratios has shown that self-sustaining nuclear chain reactions have occurred. The existence of this phenomenon was discovered in 1972 at Oklo in Gabon, Africa, by French physicist Francis Perrin. In May 1972 at the Pierrelatte uranium enrichment facility in France, routine mass spectrometry comparing UF6 samples from the Oklo Mine, located in Gabon, Central Africa, showed a discrepancy in the amount of the U isotope.
Normally the concentration is 0.720%; these samples had only 0.717% a significant difference. This discrepancy required explanation, as all uranium handling facilities must meticulously account for all fissionable isotopes to assure that none are diverted for weapons purposes.
Thus the French Commissariat de l‘energie atomique (CEA) began an investigation.
647 pages

10. -The Oklo natural fossil reactors were discovered on June by a French analyst (Bougzigues) whilst working at the Pierrelatte nuclear fuel processing plant. 238 U = % 235 U = % → av % ( %) 234 U = % 235 U / 238 U = → Large quantities of ancient (no longer radioactive) fission product waste embedded in the natural U ore, confirming that natural nuclear fission reactions had taken place at Oklo some 2000 million years ago.

11. Ⅲ. Uranium Ore- Forming Processes

12. - Chemical reaction types in aqueous solution
1. Acid- Base reaction
- Acid : proton donors Base : proton acceptors
2. Redox reaction
-Oxidation : election loss
-Reduction : election gain
3. Solid phase interaction
- Dissolution-Precipitation Adsorption-ion exchange
Redox reaction  
The valence state of an element can significantly affect its geochemical behavior. The mobility of uranium element will depend strongly on the whether the environment is reducing or oxidizing.

16. Fig. Schematic representation of a roll-front deposit where a crescent-shaped uranium ore body forms at the dynamic reaction front in the sandstone aquifer between oxidized and reduced lithologies.

17. Ⅳ. OKLO Phenomenon :
Natural Nuclear Fission Reactor

18. Time constraint for the occurrence of uranium deposits and natural nuclear fission reactors in the Paleoproterozoic Franceville Basin (Gabon). F. Gauthier-Lafaye, GSA Memoirs, 2006, 198, p

19. Natural nuclear fission reactors 1. Nuclear reactor zones 2
Natural nuclear fission reactors 1. Nuclear reactor zones 2. Sandstone 3. Uranium ore layer 4. Granite

20. Yellow is Uranium Oxide and White is recrystallized Quartz

21. - Radiogenic Isotopes
Uranium-235 has a half-life of million years ; uranium-238, 4.468×109 years
Graph showing the retrospective amounts of U-238 and U-235. Due to the shorter half-life of U-235, 2 billion years ago the percentage of U-235 was higher than today.
Uranium-235 / uranium-238 in the Earth’s crust over time. The x-axis is in units of millions of years. When the Gabon natural nuclear reactors operated about 2 billion years ago, the Earth’s crust contained approximately 3.68% uranium-235.

22. Ce diagramme montre l’abondance isotopique naturelle (normale) du neodyme, ainsi que celle du site modifiee par les isotopes du neodyme produits par la fission de 235U. Le Nd typique contient 27 % de 142Nd ; celui de Oklo en contient moins de 6 %, et contient davantage de 143Nd. La composition correspond a celle du produit de la fission du 235U.
Ce diagramme montre l’abondance isotopique naturelle (normale) du ruthenium, ainsi que celle du site modifiee par les isotopes du ruthenium produits par la fission de 235U. La forte concentration de 99Ru(27-30 %, contre 12,7 % typiquement).Ce surplus peut s’expliquer par la desintegration du 99Tc (produit de fission) en 99Ru.

23. Oxygenic Photosynthesis
Laurence A. Coogan, Jay T. Cullen, GSA Today ,2009,V19,
Issue 10, p. 4-10
“Did natural reactors form as a consequence of the emergence of oxygenic photosynthesis during the Archean?,”
Current models suggest that organisms that can perform oxygenic photosynthesis first took hold in isolated marine and freshwater basins, producing local oxygen oases. It is possible that these fission products provided a negative feedback, helping to limit the proliferation of the cyanobacteria in the Archean environment.

24. F. Gauthier-Lafaye, GSA Memoirs 2006, v. 198, p. 157-167
“Time constraint for the occurrence of uranium deposits and natural nuclear fission reactors in the Paleoproterozoic Franceville Basin (Gabon)”
Thus, oxygen in the atmosphere was probably the main factor controlling the occurrence of natural nuclear fission reactions. This conclusion is in agreement with earlier suggestions that oxygen contents in atmosphere increased during a “transition phase” some 2450–2100 m.y. ago.

25. Fig. Simplified schematic timeline of photosynthesis and the oxygenation of the atmosphere and oceans through Earths history. GOE = Great Oxidation Event, ~2.4 bya. Three main phases are evident: (1) an early anoxic world in which there was very low oxygen and was dominated by anoxygenic photosynthesis; (2) an intermediate low-oxygen world in which significant contributions to photosynthesis were made by both oxygenic and anoxygenic phototrophs and (3) the modern oxic world in which oxygenic photosynthesis predominates. Not until this later stage did complex plants and animals evolve.

27. Cartoon showing a possible mechanism by which oxygenic photosynthesis could lead to formation of natural fission reactors. Uraninite weathered out of igneous and metamorphic rocks is transported to isolated basins and deposited in shallow water environments, providing a ready source of U as soon as the waters become oxidizing. Photolytically produced H2O2 rains out of the atmosphere and oxidizes the uppermost water column, reducing the concentration of electron donors required by anoxygenic photosynthesizers such as H2S and Fe2+. This provides the selective pressure required for the emergence of oxygenic photosynthesis due to the abundance of H2O as an alternative electron donor.

28. - Behavior of fission products at the Oklo deposits
Element
Comments
Eh-pH Predictions
25℃
200 ℃
Strontium
Probable local redistribution
Not applicable
Yttrium
Most retained
Retention
Zirconium
Most retained; some local redistribution
Some migration
Niobium
Molybdenum
Most migrated
Migration
Technetium
Local redistribution
Migration (?)
Ruthenium
Minor migration
Iodine
Cesium
(?; perhaps locally)
REE
Lead
Variable migration
Retention or local redistribution
Thorium
Uranium
Some local redistribution
Plutonium

30. - Mobilization and retardation of
fission products at various rock types

31. - The history of the Oklo fossil reactors
can be divided into four stages:
1. U mobilization phase: Commenced ~3500 million years ago.
2. U ore/reactor formation: Started ~2800 million years ago.
3. Reactor operation: Commenced 2000 million years ago
(for about a million years).
4. Waste movement: The last 2000 million years.

33. - The Fission Process
1. The fact that sufficient 235U relative to 238U was present in the reactors is related to the difference in the decay rates of these two isotopes.
2. At Oklo the relative abundance of 235U was 3% 2,000 million years ago. This is one of the major reasons why nuclear fission started.
3.Natural fission reactors cannot form today because there is insufficient 235U in natural U.
4. The reactor zones themselves were centimetre to metre thick layers of highly enriched U, buried within the U ore.

34. 4. Each reactor operated on an intermittent basis for a period ranging from a few years to hundreds of thousands of years. The total time period over which the reactors operated is thought to be about a million years. 5. The requirements for a natural reactor. Besides a natural enrichment of 235U compared to 238U, a natural reactor requires 4 other important parameters to be satisfied: - A high overall concentration of U. - A low concentration of neutron absorbers. - A high concentration of a moderator. - A minimum or critical size to sustain the fission reactions.

35. - Natural radioactive waste containment
Isotopic studies of Oklo fossil reactors show that the
reactor parameters can be determined and that most of the fission product elements were retained within the reactor zones for 2000 Million years. There is evidence of mobile fission products being retained close to reactor zones in the iron and clays even in a highly porous environment.
2. The Oklo fossil demonstrates many of the features of what has become known as the ‘multiple barrier’ concept of radioactive waste disposal. This means building into the storage medium and repository as many barriers to the loss of radioactive wastes as possible.

36. Retention of Fission Products at Oklo

38. Ⅴ. Natural Analogue Study
Chemical analogues for the nuclides
Oklo phenomenon in Gabon : Natural Fission Reactors
Basalt glass (1x107y) for devitrification of borosilicate
glass (1.5x105y)
Iron meteorite (2x104y) for corrosion of canister (1x103y)
Igneous carbonate rocks or ultramafic rock
for hyperalkaline groundwater
Saline water interaction

39. - Possible chemical analogues for the long-lived nuclides present in HLW

40. Fig.　Basic concept of geological disposal of high-level radioactive waste (HLW) in Japan

41. The basic concept of geological disposal of high-level radioactive waste in Japan is to construct a multi-barrier system in a stable geological environment. Combining carefully selected engineered barriers with a suitable geological environment to form multiple layers of protection defines the "multi-barrier concept". This system ensures the safety of human being

42. JAEA’s R&D activities

44. Ⅵ. Conclusion
1. Isotopic studies of Oklo fossil reactors show that the reactor parameters can be determined and that most of the fission product elements were retained within the reactor zones for 2000 Million years. There is evidence of mobile fission products being retained close to reactor zones in the iron and clays even in a highly porous environment.
오크로의 천연 원자로에서는 대부분의 핵분열 생성물이 20억년 동안이나 핵분열 반응권 내에 계속 머물러 있었음을 보여주었다. 공극률이 높아 지하수가 유동할 수 있는 사암과 같은 지질환경이더라도, 생성물들은 핵분열 반응권의 부근에 있는 철(유화철)이나 석회질 점토광물 내에 흡착되어 있었으며, 20억년 동안 수m 내외의 움직임만을 보여주었다.
2. The Oklo fossil demonstrates many of the features of what has become known as the ‘multiple barrier’ concept of radioactive waste disposal. This means building into the storage medium and repository as many barriers to the loss of radioactive wastes as possible.( IAEA-TECDOC-1109)
고준위 방사성폐기물의 핵종이 전혀 이동을 하지 않고 있기 위해서는 오크로 원자로와 유사한 지질학적 특성을 가진 지역을 인공적으로 만들면 될 것이다. 인위적으로 이러한 지역을 만들기 위해 ‘다중 방벽(multi-barrier)의 개념을 도입하고 있다. 오크로의 원자로와 같은 자연유사현상을 적극 활용한 지질학적 부지환경을 건설 하도록 권고하고 있다.( IAEA-TECDOC-1109)

45. Thank You !