Hello.
Somebody has written about a reaction ring. I think it's one of the factors which has influence in chattering (or fluttering). It deals with the concept of "blowdown" in a PSV. Depend on how you adjest the blowdown ring you can modify the pressure for close in two ways: more early o more...
You can have a look at german code KTA-3504: Electrical Drives of the Safety System in Nuclear Power Plants.
Here you can find the basic features for and actuator.
Visit this page and click in "KTA 2000" there you will down load free the code KTA-3504.
http://www.kta-gs.de/
http://www.kta-gs.de/