Most neutron emission outside prompt neutron production associated with fission (either induced or spontaneous), is from neutron-heavy isotopes produced as fission products.
One of the more prominent reports of success came from a group at the Georgia Institute of Technology, which observed neutron production.
If a reactor is exactly critical - that is, the neutron production is exactly equal to neutron destruction - then the reactivity is zero.
Since these reactions involve the reactants and products of the primary fusion reaction, it would be difficult to further lower the neutron production by a significant fraction.
In these calculations, the presumption of time invariance requires that neutron production exactly equals neutron loss.
So, "the Germans were the first physicists in the world, with their Leipzig pile L-IV, to achieve positive neutron production."
If is positive, then the core is "supercritical" and the rate of neutron production will grow exponentially until some other effect stops the growth.
The G-III experiment was a small-scale design, but it generated an exceptionally high rate of neutron production.
As neutron production is their main function, most research reactors benefit from reflectors to reduce neutron loss from the core.
If you control the neutron production you can get a nice stable reaction going, as in a nuclear power station.