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Fast Neutron Reactor

March 18, 2013

Introduction to fast neutron reactor

Fast neutron reactor can be breeder that produce more nuclear fuel than it consume, by using an intermediate fertile nucleus such as 238U or 232Th. Fast neutron reactors are a different technology from those considered so far. They generate power from plutonium by much more fully utilizing the uranium-238 in the reactor fuel assembly, instead of needing just the fissile U-235 isotope used in most reactors. If they are designed to produce more plutonium they consume, they are called Fast Breeder Reactors (FBR). If they are net consumers of plutonium, they are sometimes called ‘burners’. For many years, the focus was on the potential of this kind of reactor to produce more fuel than they consume, but with low uranium prices and the need to dispose of plutonium from military weapons stockpiles, the main short-term interest is in their role as incinerators. More than two neutrons per fission are produced in fast neutron 239Pu reactors. One of this neutrons can be used to maintain the chain reaction and the others can create further 239 Pu via neutron absorbtion on 238 U. If the probability for this to happen is sufficiently close to unity, the 239 Pu destroyed by fission can be replaced by a 239 Pu created by radiative capture an 238 U. The final result is that 238 U is the effective fuel to the reactor.

principle behind making of Fast neutron Reactor

The following remarks are in order. Consider for definiteness, a breeder with the fertile 238U. The fissile nucleus is 239 Pu for two reasons. Fristly, it is produced in neutron absorption by 238 U, which leads to a closed cycle. secondly it is produced abundantly in nuclear technologies, whereas one can only rely on the natural resourses of 235 U. In order for a fertile capture of a neutron to produce an appreciable amount of the fissile 239 Pu inside the fuel, the probability for this capture must not be too small compared to the probability that the various nuclei in the medium undergo fission. This probability depends both on the amounts of 239 Pu , 235 U and 238 U, and on the physical design of the fuel elements. It can be calculated in terms of the amount of various nuclides and of the capture and fission cross sections of respectively, 238 U and the pair 239Pu- 235 U.

Precautions to be taken in Fast neutron Reactor

  • One must avoid the slowing down of neutrons through the presence of light nuclei. In particular, water cannot be used as a coolant.
  • Since fission cross sections are much smaller than the thermal neutrons, so it is more difficult to reach the critical regime.
  • Great care must be taken concerning the mechanical damage caused by the fast neutrons to the structures and construction materials.
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