Some General Dynamics Modules

Several weeks ago I received a packet containing seven analog computer modules made by General Dynamics which were used in the control of a nuclear power plant. These modules are shown below in more detail (the pictures were taken by my friend Joachim Wagner).

There are five double height modules containing amplifiers, a logarithmic integrator, a modulator and a demodulator as well as two single height modules containing a power supply and a bistable trip circuit (see below).

DEMODULATOR LP:

LOG INTEG. B:

AMPLIFIER D:

AMPLIFIER S:

MODULATOR P:

Two half height modules, a bistable trip and a power supply:

POWER SUPPLY N080001:

BISTABLE TRIP A-25:

The bistable trip shown above has an interesting marking on its front plate. It reads "periodic scram" - the term "scram" has been coined by Enrico Fermi who built the first nuclear reactor. This device had control rods which were lowered by ropes into the reactor core and thus allowed to control the chain reaction by removing more or less neutrons depending on the position of the control rod.

To perform a shutdown of the reactor all control rods would have been lowered into the core effectively stopping the chain reaction in an instant. As a safety measure, Enrico Fermi placed several men armed with axes on top of the reactor who had to cut the ropes on which the control rods were hanging. This would have instantly stopped the chain recation.

Enrico Fermi called these people "Safety Cut Rope Axe Man" or "SCRAM", for short. Even today the term "SCRAM" means a fast shutdown of a nuclear reactor by inserting the control rods. This activity seems to have been the purpose of this bistable trip module.

The power supply module obviously has faced disaster - it has been completely destroyed by excessive heat, maybe caused by a short circuit. The electrolytic capacitors have cracked open, the insulation on the wires has been destroyed by heat and even power resistors have changed color and have developed cracks, etc.

Post scriptum: After quite some years a trace concerning the history of these modules has shown up by private mail. It seems as if these modules (or modules like these) were used in the reactors of the University of Virginia. I got the following mail from the former operations supervisor of the reactor, Mr. Paul Benneche: "The type of modules that are in your pictures were used in the control consoles of both of the reactors at the University of Virginia, now decommissioned (2005). We sold many of the modules as surplus. I still have some here. They are in fact part of analog circuitry that did everything from powering and reading the output of the detectors used to monitor the reactor as well as control the safety control system, shutting the reactor down if the power increased too fast or went up too high. I am not aware of any other reactors using the same electronics."