I thought they use steel rods. I remember reading that from a book. I attached the book info here. i got that book from my departmental library. constant-thinker, you may like to read up to know more.
Nuclear Politics - The History of Nuclear Power in Britain
Harmondsworth: Pelican, 1986 Soft Cover. Very Good/No Jacket. First Edition. Trade Pb. 200pp. Slight reading crease, o/w as new. Follows the British atomic power industry from its birth in the shadow of the atom bomb up to the Windscale/Sellafield investigation (1977) and subsequent reported concerns, and the Sizewell inquiry. (Environment, Technology, Nuclear, Atomic, Economics, Politics, Energy)