CNRS Université Bordeaux

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uranium (U/Pu) and thorium (Th/U) fuel cycles

The uranium (or 238U/239Pu) fuel cycle starts with a neutron radiative capture by the seed nucleus 238U. The fissile isotope of plutonium 239Pu is generated after a mean time of 2.4 days via a sequence of two radioactive beta decays. The present nuclear power plants use the U/Pu fuel cycle.

In the alternative thorium (or 232Th/233U) fuel cycle, the natural thorium isotope is the seed nucleus, and 233U is the fissile one. The mean delay for to the radioactive decays is 27 days, due to the half-life of the intermediate protactinium isotope 233Pa.

As the seed nucleus has a lower mass, the thorium cycle is expected to produce much less amount of minor actinides, which are the most radiotoxic part of the nuclear waste for several thousands of years.

The thorium based fuel cycle is expected to have a breeding power for a wide range of neutron energy. The important parameter for the breeding potential of the cycle is the ratio of capture to fission cross sections, also named “alpha yield ratio”.

During the past years, the ACEN group has performed numerous experiments relevant for the thorium fuel cycle: neutron capture and fission of 232Th and fission of 233U have been studied in direct measurements with fast neutrons, neutron capture and fission cross section of <233>Pa have been determined via the surrogate method.