36 The ALPHA pump developed a gas leak across the lower shaft seal after 56 hr of intermittent operation and a total of 510 hr at elevated temperature in the MSR-FCL2. Disassembly and inspection of the pump revealed than an O-ring made of Buna-N had failed. The O-ring is part of the shaft seal which was purchased from the Sealol Corporation. The lower shaft seal operates with oil lubricant contacting its top side and gas contacting its lower side. The gas is principally helium containing approximately 0.1% BF3 whose source is the sodium fluoroborate in the pump tank. The dilute BF3 attacked the Buna-N material and produced the gas leak. The split purge flow of helium in the shaft annulus was not operated in this application because it had not been found necessary to the successful operatiou of a similar seal on the LFB pump in the MSR-FCL1. Impurity control and facility operation are also simplified when the split purge is not used. The mechanical design of the lower shaft seal is being changed to (1) replace the present elastomeric static seal with a metallic bellows and (2) to eliminate the press fit of the seal cartridge in the inner bearing housing. The press fit has caused galling of the static sealing surface in the inner bearing housing. In addition, the cross section of the pump shaft will be strengthened in the vicinity of the lower shaft seal and bearing. These improvements in the pump will be accomplished during a future scheduled shutdown. In order to proceed with the test program for the MSR-FCL2, temporary repairs were made to the pump. An O-ring made of Viton, which is more resistant to BF3 than Buna-N, was installed in the present lower shaft seal. â€˜The inner bearing housing was machined and plated with copper to provide a good sealing surface for the seal cartridge. After a cold shakedown test the pump was reinstalled in the MSR-FCL2 facility and is presently supplying 850Â°F salt at the design flow rate of 4 gpm at 4800-rpm shaft speed.
The jet pump system is shown schematically in Fig. 3.7. During normal operation, jet pump A, having a pumping capability of a little greater than 36 gpm, will
take salt and gas from the bottom of the drain tank and discharge it along with its driving salt, Q n ~ into , the inner vessel. It will probably be necessary to install a gas separator in the discharge line of jet pump A. Jet pump B is the primary salt return pump and is designed to have a suction flow that is strongly dependent upon the salt level in the inner vessel. The characteristics of jet pump B are shown in Fig. 3.8, where Q,B is the flow return from the drain tank and includes the driving flow, Q n ~for , jet pump A. During normal operation the salt level, Ho, in the inner vessel will be between 6 and 10 ft above the center line of jet pump B. If the salt level in the inner vessel should drop to about 5 ft below the jet pump center line, the suction flow will go to zero and the pump will be operating very near cavitation. The purpose of having this pump characteristic is to prevent the unwanted and uncontrolled reintroduction of gas from the drain tank into the fuel-salt system should the salt flow from the separator be drastically reduced. The jet pumps have some overcapacity should the flow from the gas
3.7.3 Drain Tank Jet Pump System for MSDR A study was made of a jet pump system to return fuel salt from the drain tank to the fuel-salt system of the MoltenSalt Demonstration Reactor (MSDR). The salt going to the drain tank is part of the effluent from the gas separator used to remove the gaseous fission products from the fuel salt. It is estimated that the flow of fuel salt into the drain tank will be between 27 and 36 gpm when all three fuel-salt pumps and their gas injector-separator systems are operating normally.
Fig. 3.7. Jet pump system for returning salt from drain tank to fuel-salt system in a molten-salt demonstration reactor (MSDR). Schematic of the jet pump drain tank fuel-salt system showing installation of jet pumps A and Band giving elevations and driving flows.