2.5.2.2.3 FMIT-type target option

The original FMIT target shown in Figure 2.5.2-7, is resized for a larger nozzle and flow rates, all else being identical. Vacuum condition of 10¹-10³ Pa will be maintained in the test cell. Alternatively, low-pressure (10⁴ Pa) helium or argon gas may be used. A low-pressure or vacuum test cell environment helps reduce the potential risk of backwall rupture as a result of pressure stress. Because the backwall is exposed to the most severe neutron damage, it will be the life-limiting component of the target assembly.

2.5.2.2.4 Free jet target option

The free jet target assembly, shown in Figure 2.5.2-8, has no "backwall" eliminating the need for backwall replacement. After emerging from the nozzle and interacting with the beam, the jet is collected and its dynamic pressure recovered in a downstream diffuser. Since target assembly structure is at least 10 cm from the edges of the beam footprint, the assembly can be designed for a permanent lifetime, subject to possible nozzle erosion. Because there is no physical boundary between the target and test assemblies, the test cell must be maintained at the same vacuum condition of the target chamber (10^-3 Pa), which may be the most challenging requirement imposed by the free jet option.

Alternatively, the first wall of the high-flux test assembly may serve as the physical boundary to maintain a pressure difference between the target and test cell. In essence, the test assembly first wall acts like the replaceable backwall of the baseline design. This option, however, requires changes to the test assembly dimensions to conform with the free jet target opening. In addition, the impact of temperature difference between the test assembly (up to 600°C for NaK-cooled test module) and the target assembly (< 250°C) requires further analysis.

2.5.2.2.5 Lithium jet parameters

Table 2.5.2-1 summarizes the lithium flow parameters for 40-MeV D+ beam with a gaussian standard deviation of 0.5-MeV. Total beam current is 250 mA. Analysis of beam-on-target interaction and jet thermal response have been presented in [3,4].

Table 2.5.2-1. Lithium jet parameters

2.5.2.3 Lithium loop

2.5.2.3.1 Main loop

A detail flow diagram of the lithium loop is shown in Figure 2.5.2-9. The loop may be conveniently divided into three basic functional systems. The first is the main loop, which circulates the lithium to and from the target assembly, and is illustrated by the heavy line in the figure. Two targets are shown, since the loop must be able to deliver flow to either of the test cells.

Figure 2.5.2-7. FMIT-type target assembly.

Figure 2.5.2-8. Free jet target assembly.

Figure 2.5.2-9. Flow diagram of lithium loop facility.

The major components in this loop are the target quench tanks the surge or overflow tank, the lithium dump tank, the organic dump tank, the main electromagnetic pump, and two heat exchangers. All of the piping and tanks are constructed of austenitic stainless steel (either 304 or 316). Excellent performance of a large SS 304 lithium loop at IFMIF relevant conditions of temperature and flow rate has been previously obtained in support of FMIT development [5]. Additional corrosion/erosion studies of austenitic steels have also shown good long term behavior [6,7]. There are, in addition, a trace heating system (to maintain the temperature throughout the loop above the melting point of the lithium at all times the liquid metal is present in the loop), thermal insulation, valves, electromagnetic flow meters, instrumentation, and connections to vacuum and argon headers. Table 2.5.2-2 lists the the major lithium loop components and the approximate dimensions and lithium volumes. The total lithium inventory is about 21 m³.

Table 2.5.2-2. Approximate main loop component dimensions and lithium volumes

The quench tank transfers and distributes the heat deposited by the beam into a liquid pool to avoid any boiling phenomena and thermal shock to the loop piping. It has been sized to the volumetric flow requirement of up to 110 L/s. During normal operation the surge tank contains only the thermal expansion overflow of the loop, but must be sized to accommodate possible backflow from the quench tank. The dump tank, located at the lowest elevation of the loop, must accommodate all of the lithium in the system. During loop maintenance, and off normal events, the dump valve opens, and a gravity drain empties the loop into the dump tank. The tank must be heavily shielded to permit limited personnel access into the lithium vault during system shutdown.

The lithium from the main loop flows into the shell side of a conventional tube-in-shell heat exchanger, which contains an organic heat transfer fluid on the secondary tube side. A slight positive pressure on the shell side prevents leakage of the organic into the primary side during a tube leak. Leakage of lithium into the organic fluid will be detected by gamma-ray radiation monitors in the organic coolant circuit. The heat exchanger has been sized to remove up to 10 MW of heat, with 30% allowance for tube plugging. The organic fluid will be subject to the high radiation field from the impurities in the lithium. Since the magnitude of this field, as well as its effects on the flow and thermal properties of the organic fluid, are not well known, an organic dump tank has been included to permit change-out of the organic fluid. The primary heat exchanger will have to be shielded, since insoluble activation products will accumulate on the large, cool surfaces inside the HX tank. The organic fluid flows into an organic-water secondary heat exchanger that transfers the heat to the ultimate heat sink.

The main electromagnetic pump must be sized to provide a flow up to 110 L/s to the operating target, as well as 10% of this flow to the second target for removal of decay heat from the target structure, for a total flow requirement of 120 L/s. An Annular Linear Induction Pump (ALIP) has been selected as most appropriate for this application. Two options of this pump design are being considered, a straight-through duct design and a return-duct design. The straight through design has greater efficiency but requires cutting and rewelding of pipe for removal in the event of a pump failure. The return-duct design has both inlet and outlet connections on the same side of the pump, permitting replacement of the stator coils without disconnecting any of the pump connections. A short circuit in the stator coils is the most likely cause of pump failure. The estimated power requirement of the return-duct type of ALIP pump is 200 kW.

Analysis of pump performance shows a Net Positive Suction Head (NPSH) requirement of approximately 0.043 MPa. This is the minimum pressure that must be present at the pump inlet to avoid cavitation. Cavitation would result in instabilities in the jet, as well as greatly accelerated corrosion of the piping in which the cavitation is occurring. Since the system must operate under vacuum, this pressure is provided by the static head of the lithium above the pump inlet. This height requirement is approximately 8.4 m. The current layout of the Target Facilities as will be shown later in Figure 2.5.2-13, provides a head of over 15 m and, therefore, provides significant margin to cover calculational uncertainties and unanticipated flow losses.

Target Facilities 4/17