Oxide contamination of the coolant was a major preoperational difficulty. Flow meters were erratic, 6 of the 12 rods could not be raised due to crud — ding on the rod mechanism which broke the surface of the sodium, and the cold trap circuits rapidly became clogged.

The coolant was dumped and cleaned, and the rod mechanisms were modified to avoid breaking the surface. Some of the purification circuit lines proved to be too small in diameter; they were eventually replaced by an external purification circuit. The dirty coolant was principally a quality assurance problem prior to the filling of the primary circuit and was cleared by draining and refilling with clean NaK.

Gas entrainment in the primary system was quickly observed. Expansion tank levels were erratic and, at low power, reactivity changes occurred due to the voidage in the core region. Some of the gas was being entrained through thermocouple and control rod guide tubes that dipped below the surface of the coolant. The entrainment problems in these areas were solved by drilling holes in the thermocouple guide tubes to equalize pressures in­side and out and by installing “hats” on top of the control rod guide tubes, so that they could be supplied with an independent cover gas system. Both of these problems, coolant crudding and gas entrainment, were accentuated by the multiplicity of circuits in the system, since a single modification to a circuit had to be repeated 24 times.