Reaction rate equals to 0 when Q is negative in depletion

[VictorL]

Hello everyone,

I am confused why the (n,2n) reaction rate of Zr93 is 0 in the depletion. In fact, in my simulation there is the neutron flux, the concentration and the relative cross-section, and the energy is large enough, according to R=Nφσ, the 93Zr(n,2n) reaction rate shouldn’t be 0. Isn’t it?

I searched the information in the chain file and found that for the reaction rate which equals to 0 (not only Zr93(n,2n) reaction), the corresponding value Q is negative. I am not sure if this is the reason?

But it seems not reasonable, because the negative Q is relative to the endothermic reaction, maybe it doesn’t matter to the reaction rate in the depletion?

Maybe somebody knows the real reason for the 0 reaction rate? Why OpenMC does not tally this reaction rate? I am sorry if I’ve asked the stupid question…

[paulromano]

A reaction with a negative Q value needs incident energy from the projectile to be energetically possible. In the case of Zr93(n,2n), the threshold of the reaction is ~6.8 MeV which means unless you have neutrons that are at that energy or higher, the reaction will not happen.