of the uranium undergoes fission, it develops fissures and the sodium enters the voids. It extracts an important fission product, caesium-137, and hence becomes intensely radioactive. The void above the uranium collects fission gases, mainly krypton-85. Clusters of the pins inside hexagonal stainless steel jackets 234 cm (92 inches) long are assembled honeycomb-like; each unit has about 4.5 kg (10 lbs) of uranium. Altogether, the core contains about 308 kg (680 lbs) of uranium fuel, and this part is called the driver. The EBR-II core can accommodate as many as 65 experimental sub-assemblies for irradiation and operational reliability tests, fueled with a variety of metallic and ceramic fuels—the oxides, carbides, or nitrides of uranium and plutonium, and metallic fuel alloys such as uranium-plutonium-zirconium fuel. Other sub-assembly positions may contain structural-material experiments. Passive safety The pool-type reactor design of the EBR-II provides passive safety: the reactor core, its fuel handling equipment, and many other systems of the reactor are submerged under molten sodium. By providing a fluid which readily conducts heat from the fuel to the coolant, and which operates at relatively low temperatures, the EBR-II takes maximum advantage of expansion of the coolant, fuel, and structure during off-normal events which increase temperatures. The expansion of the fuel and structure in an off-normal situation causes the system to shut down even without human operator intervention. In April 1986, two special tests were performed on the EBR-II, in which the main primary cooling pumps were shut off with the reactor at full power (62.5 megawatts, thermal). By not allowing the normal shutdown systems to interfere, the reactor power dropped to near zero within about 300 seconds. No damage to the fuel or the reactor resulted. The same day, this demonstration was followed by another important test. With the reactor again at full power, flow in the secondary cooling system was stopped. This test caused the temperature to increase, since there was nowhere for the reactor heat to go. As the primary (reactor) cooling system became hotter, the fuel, sodium coolant, and structure expanded, and the reactor shut down. This test showed that it will shut down using inherent features such as thermal expansion, even if the ability to remove heat from the primary cooling system is lost. EBR-II is now defueled. The EBR-II shutdown activity also includes the treatment of its discharged spent fuel using an electrometallurgical fuel treatment process in the