During nuclear fission energy (warmth) and fast neutrons are set free. The nominal thermal power being constant (FRM II's thermal nominal power is 20MW) the middle of the power density is the bigger the smaller the nuclear volume.

The smaller the nuclear fuel element the more fast neutrons leave the uranium zone. Out of 100 fast neutrons 72,5 neutrons leave the compact core at once, getting into the moderator tank.
Out of these 72,5 neutrons 25,2 neutrons are reflected back into the uranium zone at once by means of the heavy water D₂O.
The other neutron stay in the D₂O or moderator tank, get moderated und build up the high thermal neutron flux density because of the very low absorption of the heavy water D₂O.
18,3 thermal neutrons diffuse back into the fuel element, get absorbed and contribute to the nuclear fission by means of the emission of 30,5 new fission neutrons.
The remaining approx. 27,5+25,2 fast neutrons in the uranium zone get moderated by the light cooling water H₂O and lead to 47,3 new fission neutrons.
Together with the 22,2 new fission neutrons from nuclear fission by neutron of middle energies 100 new fission neutrons are totally produced. This means that the nominal thermal power is constant.