**3.1 Dry/wet critical experiment**

In April of 1961, before EBR-II was used as a power producing or breeding facility, it underwent a series of zero power experiments (designated as less than 1 kW of power) before the primary system was filled with sodium [8–10]. To perform the dry critical experiment, fuel and blanket assemblies that would be used for normal operations were loaded into the core in a similar configuration to when sodium would be added. For this, additional driver assemblies (~87 driver assemblies compared with ~56 driver assemblies for a sodium filled core) were required achieve criticality since the lack of sodium increased neutron leakage in the core. These experiments were able to take place while construction work was being performed elsewhere in the plant.

The basis of these experiments was twofold. The first was used to determine the performance of the system without sodium, which allowed them to subsequently identify sodium effects on system neutronics. The second gathered operational data to determine if modifications or improvements were required prior to adding sodium. To gather this information, four major experiments were conducted. The first was to determine the strength of the neutron source and the neutron detector responses to ensure an adequate relationship between the two. The second was an approach to critical to verify the ability to insert assemblies and determine the dry critical mass. The dry critical mass could then be compared with the wet critical mass to determine the total reactivity worth of the sodium. The third aspect examined the neutron flux distribution and fission distribution throughout the core and provided a power calibration. The final aspect that was examined was a series of reactivity measurements. This included seven measurements ranging from the total worth of the control rods, individual control rods, to the dry isothermal temperature coefficient of reactivity.
