Neutron Transport

Design of a reactor core requires detailed prediction of the balance between neutron production and neutron loss. The rates of neutron production, transport and absorption are key information not only for core design and analysis but also for thermal–hydraulic, heat–mass transfer, accident scenarios and radioactivity release estimates. After neutrons are born in fission reactions, they move through the reactor core and undergo collisions of various types (absorption and scattering). There are two main absorption processes which may occur, radiative capture and fission. In fission, the target isotope splits and releases additional neutrons. In radiative capture, the neutron is parasitically absorbed and does not contribute to sustaining the chain reaction. In scattering collisions (elastic or inelastic) neutrons change their energy, spatial position and direction of motion in a process known as slowing down. In general, the interaction of neutrons with nuclei in medium may be considered as neutrons being transferred or transported from one location to another, from one energy to another and from one direction of motion to another. A schematic diagram of the various paths for a neutron born in a thermal reactor is depicted in Fig. 8-1. The details of neutron interactions as well as the concept of the sustained (and controlled) chain reaction are described in the succeeding sections.

When we have carried out the indicator experiments that proved that barium was present, I wrote some personal letters to Lise Meitner, telling her of our results. In my letter of 19 December I wrote: … The thing is there is something so odd about the ‘radium isotopes’ that for the moment we don’t want to to tell anyone but you. The half-lives of three isotopes are pretty accurately determined; they can be separated from all elements except barium; all reactions are correct. Except for one – unless there are some very weird accidental circumstances involved: the fractionation doesn’t work. Our Ra isotopes behave like Ba… Strassmann and I agree that for time being nobody should know but you. Perhaps you can put forward some fantastic explanation… Otto Hahn (1879–1968)