Helium, purification

Commercially pure (< 99.997%) helium is shipped directiy from helium-purification plants located near the natural-gas supply to bulk users and secondary distribution points throughout the world. Commercially pure argon is produced at many large air-separation plants and is transported to bulk users up to several hundred kilometers away by tmck, by railcar, and occasionally by dedicated gas pipeline (see Pipelines). Normally, only cmde grades of neon, krypton, and xenon are produced at air-separation plants. These are shipped to a central purification faciUty from which the pure materials, as well as smaller quantities and special grades of helium and argon, are then distributed. Radon is not distributed commercially. 

Cmde helium (containing 50—70% helium, associated hydrogen and neon, 1—3% methane, and the balance nitrogen) can easily be obtained by minor enhancements to the nitrogen rejection unit, particularly with natural gases containing 0.5% or more helium. For example, by operating the double-column condenser in a partial condensation mode, a stream of uncondensed vapor at about 50% helium concentration can be obtained. This cmde helium stream can be fed directly to helium purification and Hquefaction units. 

Helium Purification and Liquefaction. HeHum, which is the lowest-boiling gas, has only 1 degree K difference between its normal boiling point (4.2 K) and its critical temperature (5.2 K), and has no classical triple point (26,27). It exhibits a phase transition at its lambda line (miming from 2.18 K at 5.03 kPa (0.73 psia) to 1.76 K at 3.01 MPa (437 psia)) below which it exhibits superfluid properties (27). 

The fuel for the Peach Bottom reactor consisted of a uranium-thorium dicarbide kernel, overcoated with pyrolytic carbon and silicon carbide which were dispersed in carbon compacts (see Section 5), and encased in graphite sleeves [37]. There were 804 fuel elements oriented vertically in the reactor core. Helium coolant flowed upward through the tricusp-shaped coolant channels between the fuel elements. A small helium purge stream was diverted through the top of each element and flowed downward through the element to purge any fission products leaking from the fuel compacts to the helium purification system. The Peach... 

Helium Purification system well Top reflector Thermal barrier... 

Two concepts of a He - He intermediate heat exchanger for a heat rating of 125 - 170 MW have been selected. For both, a 10 MW test plant has been operated in the KVK loop verifying the operation of reformers with convective helium. A 10 MW decay heat removal system cooler, hot gas ducts including insulation and liner, hot gas valves, and a steam generator were other components of the KVK loop. Furthermore, a helium purification system was operated in a bypass of the main system. Starting in 1982, the KVK facility was operated for 18,400 h with approx. 7000 h above 900 C [28]. Hot gas duct with internal insulation was operated at temperatures up to 950 °C. The KVK experimental loop has demonstrated reliability and availability even of newly developed components. 

Helium Purification High Temperature Adsorber Module Oxidizer-Cooler Module Dryer Module... 

The components of the fuel handling system, the helium purification system and other auxiliary systems will be also domestically made. 

Helium turbine with gas seals, oil seals, bearings, cooling systems, insulation Helium compressor with gas seals, oil seals, bearings Helium/helium heat exchangers/recuperators, coolers Hot gas ducts with valves, butter fly valves, bellows, bends Helium purification Coatings in helium atmosphere... 

In order to gain experience, the helium purification-system of the fossil-fired EVO plant was designed in accordance with the requirements for a nuclear heated plant, except without provision for the removal of radioactive impurities. The design througlq>ut was 100 kg/h and the required cleanliness was < 1 ppm for any substance. The flow scheme of the system is given in Fig. 30. 

The overall performance of the helium purification system was judged satisfactory. Since no oxidation bed for the removal of was provided, no statement on the removal of that type of impurity can be made. 

Fig. 31 shows the flow scheme of the continously operating helium purification system of the HHV-plant. 

As a whole, the helium purification system proved to be a reliable and well-functioning system. Although the content of impurities in the helium to be purified was higher than specified and expected, the purification system lowered each of the gaseous impurities to values <0.1 ppmv. This latter value is below the lower detection limit of the highly sensitive gas chromotograph used, which means that the actual function was better than specified. 

Excellent performance of the hot gas ducting, of the turbomachinery cooling, of the helium purification system (except the oil aerosole separation), and of the instrumentation and regulation. 

The component test loop (KVK) was provided for testing the main helium components of a nuclear process heat reactor concept (PNP) at 950 °C (max. 1050 °C for short periods). The main test components were helium to helium heat exchangers, hot gas ducts, hot gas valves, bellows, steam generators, a helium purification system, helium blowers and their acoustic emissions, helium leakage, control, and other miscellaneous systems and components. The overall operation time of the KVK amounts to more than 20,000 hours at 950 C or even higher temperatures. Its main operating parameters were ... 

The AVR is provided with a helium purification system for removing radioactive and non-radioactive impurities. It consists of three sections ... 

The KVK is provided with a redundant helium purification system. In additon to its classical layout, it is additionally equipped with an injection/doping system for injecting such substances as CO, CH4 and H2O into the helium circuit in order to adjust a detined helium atmosphere. This is very important for helium systems operating above 850 °C because the metallic materials applied in high temperature helium systems are very sensitive to the chemical attack by helium impurities (i.e., inner oxidation or carburization, decarburization or formation of stable oxide layers on the surfaces). The material tests have indicated that only a very deHned narrow band of a permissible helium atmosphere composition can be tolerated and the permissible narrow band becomes smaller with increasing temperatures. In order to test the helium components in the KVK under realistic conditions, it was necessary to operate with a controlled helium atmosphere, i.e., with controlled impurities in the helium. 

The helium-purification plant for EVA-II has layout similar to that for KVK, except that no injection/doping device has been provided. The operational experiences correspond with those of the KVK-plant. 

It could be demonstrated in the material test stands as well as in the large test facilities (KVK, EVA-II EVO, HHV) that the permissible helium atmosphere can be orderly controlled and adjusted by means of the helium purification systems and at very high helium temperatures by the addtional provision of injection systems. 

The helium purification system purifies the helium primary coolant, removing fission products released from defective fuel particles, tritium and chemical impurities (HjO, CO, COj, N2, CHJ. Additionally, the helium purification system provided for helium supply (to the power module) and for helium pressure control. Two-fold redundant helium purification systems are provided for each power module. 

The helium purification system is installed for the purification of the primary coolant to specified values by removing chemical impurities (H2O, CO, CO2, N2, CH4), the supply... 

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