Scram is a game designed by Chris Crawford for the Atari 800 and released by the Atari Program Exchange. Written in Atari BASIC, Scram utilized differential equations to simulate reactor behavior. In the game, the player controlled the valves and switches of a nuclear reactor directly with the joystick. Occasionally, earthquakes would occur and the player would analyze the heat readings and dispatch repair crews to the affected area of the plant.
The game display showed a schematic-like representation of a light water reactor, typical of nuclear reactors in use in the United States at that time. The reactor core was on the left of the screen, with the primary coolant loop to its immediate right. Further right was the secondary cooling loop, and finally the tertiary cooling loop and its associated cooling tower.
The user interacted with the game by moving the joystick, which made a cursor jump from one "hot spot" to another on the screen, each one controlling one part of the reactor systems. There were hot spots for the control rods, cooling pumps and valves. The user could experiment with the reactor systems by moving the joystick up and down, operating the equipment. It was possible to simulate a meltdown by shutting off the primary cooling pumps and withdrawing the control rods all the way.
The game had several skill levels, which controlled the frequency of earthquakes and the "obviousness" of the damage. In the event of an earthquake the screen would shake and a breaking sound would be heard if there was damage. The user then had to watch the on-screen displays to try to isolate where the problem was.
Source: Wikipedia, "http://en.wikipedia.org/wiki/Scram_%28computer_game%29," available under the CC-BY-SA License.