Thermonuclear fusion reactors do not work as neutron sources for neutron collectors (v.1.2.66). Maybe this is caused by the following code in "scripts/control/neutron_collector.lua".

Line 397:
local recipe = "ei-fusion-F1-ei-heated-deuterium-F2-ei-heated-tritium-TM-medium-FM-medium"

Lines 403-406:
fuel1 = recipe:match("F1-(.+)-F2-")
fuel2 = recipe:match("F2-(.+)-TM-")
temp_mode = recipe:match("TM-(.+)-FM-")
fuel_mode = recipe:match("FM-(.+)")

Example of actual recipe name: ei_fusion-F1__ei-heated-deuterium-F2__ei-heated-tritium-TM__medium-FM__medium

Deleted (submitted in mistake)

Yes—most likely happened when moving from ei_ to ei- prefix, thanks.

Fixed for 1.2.67.

e: Not entirely working.

How about changing to the following code?

"scripts/control/neutron_collector.lua", Lines 403-406:
fuel1 = recipe:match("F1__(.+)%-F2__")
fuel2 = recipe:match("F2__(.+)%-TM__")
temp_mode = recipe:match("TM__(.+)%-FM__")
fuel_mode = recipe:match("FM__(.+)")

Applied to 1.2.69.