how to set a if condition in chemical reaction flow?

Hi,

This is a starter of COMSOL.

I'm calculating a chemical recation with the solution flowing through a thick porous media in one dimention x.

I want to set a condition to define: only when the flow arrives position x, the reaction occurs, else the the reaction rate r(t)=0.

the following is my setting:

if(t*u(t,x)>0,r(t),0),

where t is time, u(t,x) is the velocity field,r(t) is the reaction rate,

but it fails.

Thank you for your help.