2 Responses

1. First of all there is no way in a well designed nuclear reactor for loss of water.
   Fast neutrons can’t assist in the process of fission. Because it has only a less cross section & so it can’t be attracted easily by other nuclei for fission.
   If those fast neutrons leak outside,(in strange case of water loss)
   –they will cause annihilations
   –cause cancer.
   –cause certain radiation disorders

   So only moderated neutrons(thermal neutrons) are utilised for a fission reaction.They are named ‘thermal’ as this level of kinetic energy is similar to the average kinetic energy of the molecules in a room-temperature gas. After a number of collisions with nuclei, neutrons arrive at this energy level, provided that they are not absorbed.

   Thermal neutrons have a much larger effective cross-section than fast neutrons, and can therefore be absorbed more easily by any atomic nuclei that they collide with, creating a heavier – and often unstable – isotope of the chemical element as a result.

2. I disagree with the previous answer. Fast neutrons do contribute to fission. It’s not attraction it’s collision that causes the release of energy. It is the energy they lose in the slowing down that leads to major heat development. If some of the moderator is lost, due to a leak etc, if the amount is small then there will be little effect, due to the surge volume of the pressurizer. If the amount is large, then peak central temperature limit violations will occur, due to loss of coolant and this will lead to critical heat flux, causing swelling of the fuel modules, causing complete loss of flow to the fuel. This in turn will result in even higer PCT and failure of the fuel element. This is what we call melt down, and it’s bad mkay.