An arithmetic combinator set to subtract used fuel cells from fresh fuel cells, outputting the result as fresh fuel cells, acts as a memory cell. It is wired input to output, and then to the two inserters for the reactor. The output inserter is set to read hand contents, pulse mode. The input inserter is stack limited to 1, and set to read hand contents, pulse mode, and to enable if and only if all received signals are zero.
Mode of operation is:
Starting from all signals are zero, nothing in the reactor, but material available to be picked up:
Tick 1: Signals zero. Input inserter triggers and tries to pick up material.
Tick N (inserter swing time): Input inserter grabs 1 nuclear fuel cell. This will output a signal of 1 fresh fuel cell onto the circuit wire.
Tick N+1: Everything on the circuit receives the 1 fresh signal and processes it. The input inserter will disable, but will complete its already-started swing. The arithmetic combinator will output 1 on the fresh fuel cell channel. This will hold and keep the input inserter disabled.
Tick ~12000: The reactor ejects the used-up fuel cell. Output inserter will put a signal of 1 used-up fuel cell to the network.
Tick ~12000 + 1: The arithmetic combinator subtracts the used-up fuel cell signal from the fresh fuel cell signal, resulting in a 0 value for the fresh fuel cell signal.
Tick ~12000 + 2: The arithmetic combinator has no input signals, and outputs a zero, which it will maintain until an inserter sends a new signal onto the net. The input inserter sees no signals on the network, and enables, restarting the cycle.
The reactors will not operate continuously with this -- there is a minimum downtime of one inserter swing every 200 seconds. This can be minimized by using fast inserters operating from chests, which will require 26 ticks. Allowing for the additional two ticks of overhead from the circuits, this gets you 99.77% uptime.
It is resilient against fuel interruptions because the null case is that it will pick up fuel. As such it it will always autostart following a disruption or when initially built. Further, a backup in removing the fuel cells will simply cause it to stop inserting new ones, without breaking the logic. Once the clog is cleared the reactor will resume normal operations.
In addition to this, any other desired logic -- for example, steam tank measurements -- can be used to generate a blocking condition to prevent loading. Because the memory cell cares only about the two kinds of fuel cells, it is possible to put any blocking signal (except those) onto the same circuit. As well, this can be used to synchronize multiple reactors by wiring their input inserters into the same circuit, but only with the enable/disable and stack-size-1 options set.
The physical arrangement of the inserters and combinator can be altered at will, as long as the circuit connections remain logically identical. Colour of circuit wire is irrelevant.
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