a conventional steam turbine. This series of cooling loops is intended to isolate the sodium coolant and any radioactivity it contains as close to the reactor as possible, making maintenance on the rest of the plant simpler. In overall design terms, the loop-type reactor is similar to a light water reactor with an additional loop for the sodium. In light water designs, the coolant running through the core normally drives the steam generators directly. This contrasts with the pool-type design which is also used for sodium-cooled reactors. The pool design consists of two bowl-like chambers, one inside the other. The sodium is pumped between the two spaces to remove heat from the core, and water coolant loops remove heat from the space between the two bowls. The two designs have their advantages and disadvantages, with the loop designs generally being somewhat smaller and thus less expensive, while the pool designs better isolate the sodium from the environment, and their large sodium mass is an effective guard against thermal transients. The Fermi design had three primary coolant loops, each entering the reactor above the core at 120 degree separations. The sodium then flowed down around the core into a tank area at the bottom of the reactor. On top of the tank area was a plate with many holes drilled in it. The sodium flowed up from the tank through the holes in the plate which spread the flow over the volume of the core and the surrounding radial blanket. The outlets were located just above the core area. At the top of the reactor was a large plug that could be rotated to provide access to the top of the core and blanket, allowing fuel to be added or removed. Fuel was produced in long rods which were then bundled together into larger assemblies. The fuel was enriched to 25.6% 235, compared to perhaps 3 to 5% in a typical light water reactor. Criticality required about 1734 kg of fuel, and could be achieved wth as few as 91 and as many as 111 assemblies, typically using around 96. The reactor core was offset within the reactor chamber, with a drum sitting on one side. This drum was connected by a tube leading upward to the top of the reactor area, allowing fuel to be inserted or removed from the reactor without opening it. A large claw-like arm above the