[MNM]

Hi. English is not my first language, so please understand if there will be grammar mistakes.

I need to compare two types of nuclear fuel: (U,Pu)O2 and (U,Pu)N. First is a typical MOX fuel, the second one is similar to MOX, but includes nitrogen instead of oxygen. I've made some calculations (FBR with lead coolant) and get these results:

1. Delayed neutron fraction - beta, is higher for nitride fuel than oxide. Anyone knows why? I try to discover it. Has it something to do with reactor's size? For nitride fuel, the core is smaller because of high density.

2. Due to thermal expansion, reactivity drops. In both fuels. But in oxide fuel it dropped more. I need to know why.

3. In neutron spectrum, oxide fuels has more low energy neutrons than nitride fuel. I have no idea why.

Thanks in advance and sorry for my english.