Improvements in Scram (or nuclear Trip)
In 3-Mile Island, the scram involved pumping water into
reactor core that was heating too fast -- started with resin blockage of
transfer tube (pipe transferring river water to be filtered through resin reads,
used for coolant) and followed by total loss of coolant water.
Now coolant water would be self-contained inside concrete external
wall, with extra make-up water also held internally.
Scram (or trip) has been improved
- Water streams by gravity on reactor core.
- More importantly, scram turns off electric power to control-rod motors
holding them against springs. Hence rods drop both by their weight and
springs driving them, making reactor sub-critical (about 4 seconds).
- In nuclear submarines, motors can run in reverse for a few
second, thus driving the rods down more quickly, while allowing
restart to take place more quickly -- important for warship.