This game was developed on the heels of the Three Mile Island nuclear disaster in Pennsylvania.
The object of the game is to be the player with the least amount of radiation (identified in millirems or "Mr") when the nuclear reactor is brought to a cold shutdown.
The players, in some ways, represent water being cycled through the "cooling loops" of the nuclear power plant in an effort to reduce their radioactivity.
Each player starts out with 180 Mr of radiation identified by Mr cards in 10, 20, 50 and 100 Mr values. Card hands are kept secret - an important feature of the game. Radiation is reduced by 100 Mr with each completed circuit of the Cooling Tower loop, the longest of the three loops. While traveling through the loops, the player lands on Operation spaces and Mr spaces. The Operational spaces allow players to draw operation cards which might help to reduce their millirems, cause them to lose a turn, or increase/decrease the temp in the reactor core. Mr spaces add radiation to the player's hand.
Player's may also draw operation valve cards or other cards that might help them to effect a cold shutdown on the reactor. Valve cards prevent other players from moving around the loop. The challenge is in knowing when to shut down the reactor based on what you know about the other players' hands. Of course, the possibility also exists that the core temperature will rise to a meltdown point, which will also end the game.