EBR-II, designed in 1955 under the EBR-I shadow, is the major United States irradiation facility today. It is a pool type reactor designed for 60 MWt with a metallic uranium alloy fuel but using oxide and carbide fuels in irradiation testing. It now uses oxide fuel for driver assemblies.

The core is a two-plenum design, a high-pressure inner plenum and a low-pressure outer plenum for the blankets. Emergency cooling in the 26-ft diameter tank is provided by natural circulation with an elevated IHX in the tank. The tank is doubly contained, has no fill line, penetrations, or drain lines. When natural circulation is insufficient, there is a small EM pump to augment the cooling and there are added tank coolers to reject heat by air blast coolers. These reject 10-20 kW continuously but could manage 250 kW per cooler if needed in an emergency. Also, to maintain adequate cooling, the reactor protective system does not scram the reactor flow when the reactor itself scrams. The thermal stresses inherent in this operation are accepted.

The reactor cover is held by flexible clamps and the reactor plug has a hold­down system comprising screwed obstructions designed to take 75 psig. The reactor vault has six steel girders embedded in concrete and is designed for 3001b of TNT. The concrete is reinforced and anchored to the containment.

The pool system was chosen here to take advantage of the simpler tank design and avoid the complex nozzles and pipes of a loop system. The pool also enjoys reliable natural circulation and a large heat capacity, although it has considerable problems associated with the head fabrication which is a vast, complex and very costly item.

Figure 4.39 shows a cutaway diagram of a vertical section of EBR-II.