What causes nuclear fission in a collision between two neutrons?

[adjacent]

What will happen if we collide two neutrons?
As far as I know,since they have no charge, there won't be any repulsive forces. So one neutron will go through the other.
Is this true? What about nuclear fission? Why does the neutron split the nucleus when there are no forces present?

 

[UltrafastPED]

You must also consider the "strong nuclear" force: http://en.wikipedia.org/wiki/Nuclear_force

Also see "Neutron Scattering": http://en.wikipedia.org/wiki/Neutron_scattering

 

[QuantumPion]

Neutrons can scatter off each other but the probability is low. I don't think it's ever been accurately measured. My google-fu came up with this paper of interest: Link