Natural nuclear fission reactor
- 1.39444413.160833Koordinaten: 1 ° 24 ' S, 13 ° 10' O
The Oklo natural reactor in Mounana, in the Gabonese province of Haut - Ogooué is a uranium deposit located in caused by natural uranium concentration started a nuclear chain reaction. Meanwhile, the remains of a total of 17 natural reactors at Oklo and 30 km away Bangombé have been discovered.
The high uranium concentration was approximately two billion years ago, ie in the era of the Proterozoic. The nuclear reactor was about 500,000 years active and continued during this period, with a thermal output of up to 100 kW, energy in the lower three-digit tera- watt-hour range freely. This corresponds roughly to the amount of energy generated an average nuclear power plant over a period of four years. In the course of a total of about 10 tons of 235U were cleaved nuclear and 238U produced from about 4 tons of 239Pu. Their activities occurred at least 1.5 billion years ago to a halt, as the natural isotopic had so far reduced from 235U at this time that the conditions for self-sustaining fission processes were no longer met.
History
As early as 1956 discussed the possibility of the existence of natural reactors by Paul K. Kuroda (USA). The necessary conditions for the maintenance of a nuclear fission process are:
- The sufficient amount and concentration of fissionable isotopes such as 235U
- Absorb the absence of substances such as lead or cadmium, the free neutrons and so escape from the process.
- The presence of a facilitator of light atoms such as water for decelerating the fast neutrons.
His thesis was controversial because it held a meeting of these conditions in the wild for too improbable.
1972 French physicist found Francis Perrin in the uranium enrichment plant Eurodif in Pierrelatte (France) an anomaly in the isotopic ratio of UF6, which had been recovered from the uranium ore the Oklo mine in Gabon. In particular, the isotope 235U had a lower compared to all other deposits located on the world share. Instead of the usual share of 0.7202 % share of only 0.7171 % 235U ( 0.0031 percentage points less) was measured. Since the isotopic ratio for 235U in natural uranium on the earth in the lunar rocks and also in meteorites found is very accurate in 0.7202 %, this difference was interpreted as a " significant deviation ". There were even later measured in other samples from the Oklo mine even lower 235U shares.
The first attempts to explain attracted former aboveground nuclear weapons experiments a possible cause. However, the short-lived isotopes associated could not be found, so that this theory was discarded. The then hired investigations led to the discovery of the reactor. When the reactor was active before about two billion years ago, was his 235U share about 3 % ( The half-life of 235U is about 704 million years ). Since the reactor is long extinct, missing in his environment, all reaction products of short half -life. Reaction products of extended half-life exist at precisely the isotope ratio, as one would expect from a reactor spent fuel. For example, the isotopic ratio differs with neodymium in the Oklo ore significantly from the global average only 6 % 142nd instead of the usual 27%.
Operation
New research suggests that this natural reactor was moderated by inflow of ( ground) water, which is a cyclic activity moved to yourself: About 30 minutes, the water slowed the neutrons to the required rate of nuclear fusion from. It became heated and vaporized. Without water, the neutrons were too fast for a chain reaction, which meant that they temporarily came to a halt. During this time - about 2 to 2.5 hours - ran water until again was enough available for the chain reaction and the cycle began again.
Research
For modern science natural reactors provide very interesting insights. You can, among other conclusions as to how radioactive substances in the natural spread what with regard to the planning of nuclear repository has great significance. However, writes about the NAGRA that such natural analogues should still be considered not as a 100 percent " evidence," but merely as " evidence " for the behavior of repositories.
The found ratio of nuclides enables a conclusion to it that two billion years ago expired, the nuclear reactions as they are today, and thus sets a possible change of fundamental constants, in particular the fine structure constant, narrow limits.