Oklo reactors

C22-0125. Erom the mass number distribution given in Eigure 22-10 and the belt of stability shown in Eigures 22-5 and 2-20, predict which elements should be found in greatest abundance in the remains of a fission event like that of the Oklo reactor (Box). 

Table 4 summarizes the geographical behavior of fission products at the Oklo reactors compared with the behavior of fission products from neutron-irradiated uranium dioxide. The reactor zones RZ 1-9 are exposed on the ground, while RZ 10-16 are situated below the ground, all quite deep except RZ 15. 

Bros, R., Hidaka, H., Kamei, G. Ohnuki, T. 2001. Retardation of fissiogenic REE in clays surrounding the Oklo reactor 2 (Gabon). In Proceedings of Atomic Energy Research Society of Japan Meeting, Sapporo, September 2001, 932. 

Cowan, G. A., "Migration Paths for Oklo Reactor Products and Application to the Problem of Geological Storage of Nuclear Wastes", IAEA Symp., Paris, Dec. 19-21, 1977... 

Analysis of the natural reactors at Oklo gives valuable information about the migration behaviour of fission products and actinides in the geosphere. Uranium and the lanthanides have been redistributed locally. Plutonium produced in the Oklo reactors did not move during its lifetime from the site of its formation 85-100% of the lanthanides, 75-90% of the Ru and 60-85% of the Tc were retained within the reactor zones. Small amounts of U, lanthanides, Ru and Tc moved with the water over distances of up to 20-50 m. 

Table 2. Neodymium from the Oklo reactor (Zone 2) compared with natural and fission-product neodymium.

We are limited in this modeling process by the accuracy with which measurements can be made and by the accuracy of the fission yields and neutron reaction cross sections which are used to interpret the results. As an example consider the Nd- Nd fission product pair, which has been used as an indicator of thermal neutron fluence because the capture cross section for the former is large and for the latter is small. The thermal cross section for l53Nd has recently been listed as 325 ( 10) barns (20), and more recently as 266 barns (11). Using the 325-barn value we deduce an age of about 2 to 27T billion years from neodymium to uranium ratios in the Oklo reactors, while an age of about 1.8 billion years is obtained using the 266-barn figure. 

One of the more interesting questions about the Oklo reactors is how long they remained critical. The answer is intimately related to the fraction of fissions in 238U, for reasons discussed below, which is in turn related to the concentrations of uranium in the ore and to the amount of water present. If we assume that 1/2 of the radiogenic lead is now missing then a present day uranium concentration of 58% extrapolates to a 70% concentration 1.8 billion years ago. At such a concentration the optimum hydrogen to uranium ratio for criticality is in the range of two to ten as is shown in Fig. 2, while at lower concentration the optimum moves up to values of 5 to 20. Since it seems likely... 

Further analysis and calculations have shown that these natural Oklo reactors (similar conditions occurred at several places) lasted for 10 y. Probably, criticality occurred periodically as the heat from fission boiled away the water so that the chain reaction ceased after a while. After water returned (as the temperature decreased) the chain reaction would... 

The nuclear wastes generated by man-made nuclear reactors are the same as those produced in the Oklo reactors, and the geochemical behavior of the rare earths in the billion years since the chain reactions in the Oklo reactor zones ceased is much more than a curiosity because of the chemical similarity of the trivalent lanthanide... 

An important point for the safety of reactors is the influence of the core temperature on fc, i.e., on the reactivity. Water-moderated reactors can be built to have a negative temperature coefficient an increase in temperature/power will lead to steam bubbles near the fuel elements and to a decrease in moderation. If the reactor has been designed a little undermoderated, a drop in moderation will bring about a drop in reactivity (fc). Such a reactor will be naturally stable against undesired changes in power. One may recall the discussion of the Oklo reactor at the beginning of the chapter. 

Gould, C.R., Sharapov, E.I., and Lamoreaux, S.K., Time variability of a from realistic models of Oklo reactors, Phys. Rev. C, 74,024607, 2006 Petrov, Yu.V, Nazarov, A.I., Onegin, M.S., Petrov, VYu. and Sakhnovsky, E.G., Natural nuclear reactor at Oklo and variation of fundamental constants Computation of neutronics of a fresh core, Phys. Rev. C, 74,064610, 2006. 

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