The subject says most of it but when any nuclear reactor i make on either SSP or SMP completes its cycle i get the following error (This is the error log from SSP, i don't have the one from SMP at the momment.)
At this point the server crashes, though the server runs fine until the reactor is loaded in render distance. After someone logs near the reactor, they stay connected for a few seconds and this error happens. During this time connected the player is unable to interact with the world.