Helium-cooled reactor

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... 

In the past 20 years, several advanced versions of the LWR, collectively called ALWRs, have been designed, but only one type has been built the advanced boiling water reactor (ABWR), which was built in Japan. New versions of light-water reactors are now under review for safety certification by the U.S. Nuclear Regulatory Commission (USNRC). It is expected that a high-temperature helium-cooled reactor, if built in South Africa, would become of interest to U.S. utilities and would also be reviewed by the USNRC for certification. 

The SMR process can be coupled to a high-temperature helium-cooled reactor, such as the MHR. The MHR can function as the heat source operating at about 850°C, to replace the natural gas burning. The high operating temperature can enable the process to take place at about 80 percent efficiency. This approach (which might be called N [nu-... 

Similar to Helium-Cooled Reactors Prismatic or Pebble-Bed Fuel)... 

VGR 50 - helium cooled reactor with pebble bed core was intended for electricity production and radiation of polyethylene tubes. To this effect circulation of spherical fuel elements around a closed path was provided. 

An anticipated difference between the ATHR and helium-cooled reactors is the coolant void coefficient of reactivity, since the relevant nuclear cross sections for molten salts are larger than those for helium. The void coefficient corresponds to the amount of reactivity that is added or subtracted by complete removal of the coolant. Since initial AHTR calculations indicated that the void coefficient could be positive or negative depending on the precise design of the core, the focus of the physics analysis effort was to characterize this effect more carefully. 

The production of tritium will depend upon the final choice of salts. If the AHTR uses nonlithium molten salts, the total tritium production will be less than for gas-cooled reactors and there will be a much lower tritium level in the coolant. If molten-salts with LiF are used, the tritium production will be significantly higher than for helium-cooled reactors but similar tothat for the Canadian Deuterium... 

MHR thermal discharge to the environment is low, due to the system s high efficient The GT-MHR is free of the emissions associated with burning fossil fuels Radioactive emissions ifom helium-cooled reactor plants are lower than those fi om comparably sized coal-fired plants... 

The THTR 300 prototype nuclear power plant in Hamm (Westphalia) with a graphitemoderated and helium-cooled reactor was shut-down for a scheduled revision after an operation time equivalent to 423 days of full-load operation. About one year later, the decision on decommissioning was taken by the federal and state authorities and the shareholders of HKG, the plant operator. 

While the AHTR uses the same graphite-matrix coated-particle fuel as helium-cooled reactors, there will ultimately be differences in fuel requirements. Five potential differences have been identified but not yet been quantified. 

The accident analysis indicates a peak AHTR fuel temperature of 1200 C imder loss of forced circulation accident conditions. The coolant boils at -1400 C. These peak temperatures are significantly less than those predicted for traditional gas-cooled reactors. As a consequence, the high-temperature accident performance requirements for AHTR fuel are likely to be less rigorous than those for helium-cooled reactors. 

As a consequence of the better heat transfer and heat transport properties of liquids compared with gases, the normal peak operating fuel temperature in an AHTR is expected to be lower than in helium-cooled reactors for heat delivered at the same temperatures to the power cycle or thermochemical hydrogen production plant. There are four effects. 

If one compares a helium-cooled and a molten-salt-cooled high-temperature reactor, a helium cooled reactor (the GT-MHR) with a peak temperature of 850 C delivers its average heat at the same temperature as a molten-salt-cooled AHTR with a peak coolant temperature of 750 C. This implies that for any given peak temperature, the AHTR will have substantially higher efficiency that the gas-cooled reactor with the same peak temperatures. Alternatively, for the same efficiency the AHTR can operate at lower peak temperatures. 

For helium-cooled reactors, the fuel quality requirements depend upon the safety strategy. If the fuel is to be the primary barrier to prevent release of radionuclides to the environment under accident conditions, there are stringent fuel reliability requirements. Under such circumstances, a low-failure-fraction fuel, only about 1 particle in -100,000, is required to meet normal operation or accident conditions and still meet the regulatory requirements. The most mobile radioactive species are Ag-1 0m, Cs, I, and Sr. The controlling isotopes for site-boundary release are Cs and I while Ag-110/w tends to controls the maintenance dose (Moormann 2001 International Atomic Energy Agency 1997). 

The preconceptual AHTR designs have assumed fuel power densities (8.3 watts/em ) similar to those of traditional helium-cooled reactors. However, the heat transfer capabilities of the molten salt coolant are superior to those of helium. As a consequence, the peak fuel temperatures during normal operation are 100 to 200 C lower than for a comparable gas-cooled reactor. Economic incentives to reduce the reactor core size and thus lower plant capital cost and refueling times are substantial. As such, there are strong economic incentives to increase fuel power densities, which will, in turn, increase the thermal gradient between the centerline fuel temperature and the coolant channel. 

However, strong economie ineentives exist to operate the fuel at higher power densities than in helium-cooled reactors and more demanding requirements may be placed on the fuel assembly geometry. 

The AHTR reactor core consists of coated-particle graphite-matrix fuel cooled with a molten fluoride salt. The fuel is similar to helium-cooled reactor fuel (Fig. 2). The important characteristic of these fuels is that they can operate at very high temperatures with peak temperatures of 4200 C. They are the only practical, demonstrated nuclear fuels capable of producing heat at sufficient temperatures for H2 production. 

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