Helium-cooled

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

The molecular constants that describe the stnicture of a molecule can be measured using many optical teclmiques described in section A3.5.1 as long as the resolution is sufficient to separate the rovibrational states [110. 111 and 112]. Absorption spectroscopy is difficult with ions in the gas phase, hence many ion species have been first studied by matrix isolation methods [113], in which the IR spectrum is observed for ions trapped witliin a frozen noble gas on a liquid-helium cooled surface. The measured frequencies may be shifted as much as 1 % from gas phase values because of the weak interaction witli the matrix. 

Optothemial spectroscopy is a bolonietric method that monitors the energy in a stream of molecules rather than in the light beam. A well collimated molecular beam is directed toward a liquid helium cooled bolometer. There will be energy... 

G. Melese and R. Kat2, Thermal andFlow Design of Helium-Cooled Reactors, American Nuclear Society, La Grange Park, lU., 1984. 

The Arbeitsgemeinschaft Versuchsreaktor (AVR) and Thorium High-Temperature Reactor (THTR-300) were both helium-cooled reactors of the pebble-bed design [29,42,43]. The major design parameters of the AVR and THTR are shown in Table 10. Construction started on the AVR in 1961 and full power operation at 15MW(e) commenced in May 1967. The core of the AVR consisted of approximately 100,000 spherical pebble type fuel elements (see Section 5). The pebble bed was surrounded by a cylindrical graphite reflector and structural carbon... 

Superconductors have the ability to levitate vehicles with embedded magnets. This picture shows an experimental zero-friction train in Japan, built to use helium-cooled metal superconductors. 

The advantages of measuring at very low temperatures are well established [4], Because of the increased speed of data collection, it now becomes feasible to consider the use of liquid helium as a cryogen. A prototype open-flow helium cooling device using mainly off-the-shelf components has been developed [3],... 

By carrying out photolyses in liquid nitrogen- or liquid helium-cooled infrared cells using a special low-temperature apparatus (see Figure 4.2), one is often able to obtain direct spectroscopic evidence for intermediates of photochemical reactions. In this section we will briefly review how low-temperature techniques have been used to observe intermediates in type I cleavage reactions. 

SiO or SiS are generated when O2 or H2S are passed over heated Si at about 1500 K in an AI2O3 furnace. The high temperature gases are condensed on a helium cooled copper mirror together with an excess of argon. 

Last, we must remember that cold surfaces adsorb gases. Should a small leak to the atmosphere be present, air will condense at helium-cooled surfaces. If air contains a certain amount of He, which does not condense at the walls, the pressure in the vacuum space raises and the thermal isolation is lost. 

The HTTR is an experimental helium-cooled 30 MW(t) reactor. The HTTR is not designed for electrical power production, but its high temperature process heat capability makes it worthy of inclusion here. Construction started in March 1991 [47] and first criticality is expected in 1998 [48]. The prismatic graphite core of the HTTR is contained in a steel pressure vessel 13.3 m in height and 5.5 m in diameter. The reactor outlet coolant temperature is 850°C under normal rated operation and 950°C under high temperature test operation. The HTTR has a primary helium coolant loop with an intermediate helium-helium heat exchanger and a pressurized water cooler in parallel. The reactor is thus capable of providing... 

Future nuclear reactors are expected to be further progressed in terms of safety and reliability, proliferation resistance and physical protection, economics, sustainability (GIF, 2002). One of the most promising nuclear reactor concepts of the next generation (Gen-IV) is the VHTR. Characteristic features are a helium-cooled, graphite-moderated thermal neutron spectrum reactor core with a reference thermal power production of 400-600 MW. Coolant outlet temperatures of 900-1 000°C or higher are ideally suited for a wide spectrum of high temperature process heat applications. 

Leads and fault current-limiters Current leads and fault current-limiters are the first large scale commercial applications of Bi-based HTSs. Current leads significantly improve efficiency of liquid helium-cooled magnets. 

The function of a current lead is to improve the efficiency of liquid helium-cooled superconducting electromagnets. The current is fed to the coils in three stages. The first comprises cooled copper leads (the resistivity of a metal falls with falling temperature) which are joined to the HTS leads, connected in turn to the low temperature superconductor (e.g. Nb3Sn). The heat management is such... 

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