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Oklo- ANuclear Reactor1800 MillionYearsAgoby Dr. S.Eklund Director General

Ascientific conferen ceabout theso-called OKLO- phenomenon will take placein GaboninWest Africa from 2327June this year. Theconference isarrangedjointly by theInternationalAtomic Energy Agency, theGaboneseautho rities and theFrenchAtomicEnergy Commission (CEA). As theOK LO phenomenon, named after a uranium mine inGabon,doesnot seemto be known outside specialist circles,it may be of interest toreporton a few detailsabout this discovery and the interesting conclusions that havebeen drawnaboutwhat happened during ashortperiodof 1005000 00years,in theearth'sdevelopment 1 800million yearsago.Natural uranium contains 0.7202% ofuranium-235, the fissionable isotope contained innuclear fuel. Until June 1972 this concentration hadbeen found to be thesame for alluranium thatha dbeen discovereduntil that time,regardless of theplaceof discovery,andthesameconcentration was encountered in the uranium of which traceshave been foundon themoon. In the Frenchgaseousdiffusion plant fo r enrichment of the isotope-235 atPierrelatte, regular measurem entsarecarried out on theconcentration of uranium-235 inthe uranium with which theplantis supplied. This is done primarily to ensu re that thecontained uranium really is of natural origin anddoesnot come from aplantwheresome fraction of the uranium-235 has already beenused. In the course of one suchisotopicanalysis it wasfound that theuranium-23 5 concentration was slightly less than whatonewould expect from natural uranium, name ly 0.71 71 % instead of thevalueust mentioned.If thescientistsat Pierrelatte had not madeapointof beingsocareful theymighthaveoverlooke d this result. How ever, they carried out aseriesof checksan ddiscoveredthat itwasnotaque stion ofmeasurement error, but that they were dealingwith uranium whichha dacomposition differing from thatof "normal" uranium, withadditional variationsbetween different samples. It wassoon established that theuranium ha dcome from Gabon or more precisely from a mine, OKLO, in thevicinity of Francevil le in south-east Gabon.Thegeo logical formation in which theuraniferous deposits beingexploiteda t Oklo occur isknown as the Francevillian. This is asedimentary seriescontained within themiddlepre-Cambrianbasin which covers anarea of about 35 000 squa re kilome tres in Gabon.It overliesacrysta lline basealong thecontact with whichseveral uraniferous lodes have beenfound. The uranium in theselodes may havebeen derived from leaching of the basem entrocks. Dating measurem ents indicate an age of 1 740 20million years. Theexploitationof the Oklo deposit is carried o ut by a Gabonesefirm, La Compagnie des Minesd'Uranium deFranceville (COMUF) , with theparticipation of French capital.As early as thebeginning of August 1972theidea wasput forward that thedepletion inuranium-235 had beencausedby a chain reaction along time ag o, and in Septem ber 1 972this theory wasannounced togetherwithpreliminary research resultsboth in the FrenchAcademy of Scienceand at theGeneral Conferenceof theIAEA.

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Standingat the f oo t of the Oklo mineinGabon,th e Di rector General f ou r t h f rom r ight) looksat the siteof the so-calledOklo Phenomenon, which hev is i ted in preparation of the Juneconference on thissub ject .

The thorough geologicaland mineralogical investigations which have been carried outsincethen indicate that a total of about 500 tonso f uraniumexhibit abnormally lowconcentrations of uranium-2 35 , with anaverageconce ntration of 0.62%. A high totalconcentration of uranium in a mineral vein corresponds to a substantial reduction in theuranium-235conce ntration if the vein is not very limited, in which case thede pletion islesspronoun ced. In one case,asamplehad a uranium-2 35 concentration of only 0.296%.It wasconcludedby various methods that theuranium in OKLO isabout 1 800millionyears old. As thehalf-life of uranium-235 isaboutseven times shorter than that ofuranium-238,namely 700million years, thismeans that theuranium-2 37 concentrationabout 2 000million yearsago wasmu ch higher relative to that of uranium-238 over3%insteadof the 0.7% which hashithertoalways (with the exception o f O KLO ) been found tobe thecase. Thelocal high con centration of uranium, an "enrichment" to over3 %,theabsenceof strongly neutron absorbing materials and thepresenceof wateras amoderator - these four circumstances led, 1 800million yearsago,tochain reactionsincertain parts of themine, in whicha total energyof about 10 000 MW years wasprobablygenerated overaperiod of some hundredso f thousandsof years.The detailed investigations which have been made of the OKLO phenomenon are aninteresting example of how nuclear physics methods are now being used in geology andgeochronology. For age determination, use is mad e, inter alia, of the uranium-leadmethod,which isbased on the fact that the radioactive decays of uranium-238 and 235 lead totw o isotopeso f lead. Thechain reaction in uranium gives rise to fission products whicha rehighly radioactive in a mo dern power rea ctor, but w hich at OK LO havede cayed in thecourse of millions of years tostablenon radioactive products, whichcan beidentified massspectrome trically on thebasis of their isotopic com position.

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One conclusion whichcan be drawn from thesem easurementss that therearevery fewfission products from plutonium. Sincethehalf-life ofplutonium is 24 00 0 years thisindicatesthat thechain reaction proceeded very slowly,sothat theplutonium whichwasformed in the OKLO reactor during thereaction decayed beforeit could take part inthe fission process. Moreover, consideration of heat transport from the sites where thereaction occurred leads to theconclusion that the reaction must have lastedatleastW O0 00 years.Another fundamental question ishowastableequilibrium couldhave been m aintained attheplaces whe re the reaction occu rred for a time as long as this. It is one thing to im aginethat under certain circumstancescriticality was achievedwithinacertain areaand anotherthing to imagine whatcontrol mechan isms existed,and not only regulated the reactionmomentari ly but alsofunctioned for thehundredsof thousandsof years we areco nsideringhere.Theassum ption which appears to be best supported by the results of the investigations isthat therea ctivity wasregulatedboth through thepresenceof neutron-absorbing materialswhich were gradually used up (thesame method is employed in our present power reactors)and through variations of theamountof water in theuranium orewith thepowerdevelopment in the reaction.Thanks to fortunate circumstances, when the OK LO phenom enon was discovered, althoughabout half of theuranium hadalready been mined, thesitesof thechain reactions whichhadtaken placecouldstill beidentified. If thediscoveryha dbeen made40yearsag o, before thediscovery of the uranium fission in 1938, it would probably have been considered merelyasascientific curiosity. How ever, the extensive knowledge which ha s been gained of thephysicsandchem istry of uranium fission wasmean while available,and it wasthereforepossible to embark upon an analysis of the existing fission products. One has to admire theFrench scientists represe nting different disciplinesboth withinandoutside the CEA, who,together with the Gabonese authorities,carried out anextraordinari ly comprehensiveinvestigation insuchashort time, and thereby arrivedat afirst interpretation of thetextonan interesting page in the world's history.Exploitation of thedepo sit will continue (abou t 500 tonso f uranium ayear). It is verymuch to behoped that,despite inconveniencesto the normal operation of themine arisingout o f the preservation of these fossil reactors, at leastmajor parts of theareaswhere thereactor sites are to be found wil l be preserved. Because of formal and financialdifficulties after the end of the war it was not considered possibletoconserveas a historicmonument thereactor which was started up in Chicagoin 1942 with natural uraniumin agraphite mode rator. Itsnatural predecessor,the fossil reactor inOKLO,should notsuffer thesame fate.TheAfrican and the South American continents were separatedno more thanaboutonehundred million yearsago. It will beinteresting to seewhetheracorresponding discoverywill bemadein future in someof Brazil 's uranium mines. Theauthor of theselineshadtheopportunity to visit the OKLO minea fewmonths ago. It is agreat experience to visittheplacewhere Natureitselfstarted aprocess which hom osapiensmanagedto reproduceonly 1 800million years laterIt isperhaps a ppropriate toemphasizethat,in this time ofconcern regarding the commercialexpansion of nuclearpower, thereis no risk that the OKLO process wil l recur in any othermine. The concentration of U-235 has, in the course ofmillions of years,decreased too far.4