Before revision: 239Pu can also be used for the core, which means once the system is up and running, it can be refuelled using the 239Pu it creates, with enough left over to fee into other reactors as well.
After revision: 239Pu can also be used for the core, which means once the system is up and running, it can be refuelled using the 239Pu it creates, with enough left over to feed into other reactors as well.