Martin Grossbeck

Abstract:

NASA has teamed with the DOE to develop Fission Surface Power technology to support flight power systems for lunar outposts and later missions to Mars. The Fission Surface Power System recommended as the initial baseline design includes a liquid metal primary coolant system that transfers heat to two intermediate liquid metal heat transfer loops. Each intermediate loop transfers heat to two Stirling engines. Both the primary and intermediate loops will use sodium-potassium (NaK) as the liquid metal coolant, and the primary loop will operate at temperatures exceeding 600A°C. The alloy selected for the heat exchangers and piping is AISI Type 316L stainless steel. The interaction between the high-temperature coolant and the stainless steel welds remains an uncertainty for the projected eight year life of the system, and the goal of this research is to select an optimum weld configuration for the heat exchanger tube to sheet welds. Gas tungsten arc and electron beam weld specimens in several candidate configurations have been made for testing. Liquid metal transfer, especially nickel leaching, is a concern in a high-temperature liquid metal loop. A compact natural circulation loop has been designed to operate at 600A°C with a complement of 64 weld specimens positioned in a furnace. The loop is contained in an argon glove box so that the NaK can be handled safely, especially in the case of leakage, which has been experienced during operation. NaK flow velocity has been measured by pulse heating a spot in the loop and measuring the temperature rise along the loop, and was determined to be 0.2-0.4 cm/s. Specimens will not be discharged prior to six months exposure with subsequent discharges at one and two years. It is anticipated to extrapolate behavior to the goal of eight years operation by specimen mass gain and microscopy analysis. Key Words Welding, space reactor, stainless steel, NaK