Because of the reactor core characteristics of fast breeder reactors (FBRs), liquid sodium must be used for the reactor core cooling. Small amounts of oxygen and hydrogen are dissolved in the FBR sodium coolant. To decrease those impurity concentrations, cold traps (CTs) are installed in the primary and secondary systems. CTs are a purification apparatus universally used in FBR sodium systems to maintain the oxygen and hydrogen concentration levels in sodium within acceptable limits. CTs can capture oxygen and hydrogen as sodium compounds by packing materials with stainless steel wires using precipitation phenomenon (crystallization) by decreasing the inlet sodium temperature below the saturation temperature. Oxygen and hydrogen are precipitated as Na2O and NaH.
We have been developing an evaluation method of the impurity capturing capacity in CTs. In this study, we investigated the impurity capturing capacity of CTs through three-dimensional computational fluid dynamics (CFD) analysis. We also used CFD analysis to evaluate the time variation of the pressure loss of packing material. The amount of precipitation was decided from the temperature, concentration and velocity distributions through the CFD analysis.
First, a uniform wire mesh was used for the capturing capacity analysis in the CTs. Precipitation speed was higher for the hydrogen sodium compound than for the oxygen sodium compound. As sodium flowed from the outer to the inner side of the packing material, the hydrogen sodium compound was precipitated near the outer surface of the CTs and that blocked the sodium flow at the outer surface. As a result, if a lot of the hydrogen flowed into the CTs, the inner side of the wire mesh could not be used effectively. Second, we also proposed non-uniform wire meshed shapes and analyzed their capturing capacity in the CTs. The shape of the wire mesh was changed in the radial direction. A coarse meshed wire was placed on the outer side of the packing material and a closely packed meshed wire was placed on the inner side. Our CFD results showed that the most effective impurity capturing capacity of hydrogen sodium compound could be obtained by changing the precipitation speed and wire shape in the radial direction.