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Msg  33 of 41  at  10/7/2018 4:50:33 AM  by

Mooser


Some Pebble bed reactors, China plus one in S. Africa

 

HTR-10

China's HTR-10, a 10 MWt high-temperature gas-cooled experimental reactor at the Institute of Nuclear & New Energy Technology (INET) at Tsinghua University north of Beijing started up in 2000 and reached full power in 2003. It has its fuel as a 'pebble bed' (27,000 elements) of oxide fuel with average burn-up of 80 GWday/t U. Each pebble fuel element has 5g of uranium enriched to 17% in around 8300 TRISO-coated particles. The reactor operates at 700°C (potentially 900°C) and has broad research purposes. Eventually it will be coupled to a gas turbine, but meanwhile it has been driving a steam turbine.

In 2004, the small HTR-10 reactor was subject to an extreme test of its safety when the helium circulator was deliberately shut off without the reactor being shut down. The temperature increased steadily, but the physics of the fuel meant that the reaction progressively diminished and eventually died away over three hours. At this stage a balance between decay heat in the core and heat dissipation through the steel reactor wall was achieved, the temperature never exceeded a safe 1600°C, and there was no fuel failure. This was one of six safety demonstration tests conducted then. The high surface area relative to volume, and the low power density in the core, will also be features of the full-scale units (which are nevertheless much smaller than most light water types.)

HTR-PM

Construction of a larger version of the HTR-10, China's HTR-PM, was approved in principle in November 2005, with preparation for first concrete in mid-2011 and full construction start in December 2012. It is also based on the German HTR-Modul design of 200 MWt. Originally envisaged as a single 200 MWe (450 MWt) unit, this will now have twin reactors, each of 250 MWt driving a single 210 MWe steam turbine.* Each reactor has a single steam generator with 19 elements (665 tubes). The fuel as 60 mm diameter pebbles is 8.5% enriched (520,000 elements in the two reactors) giving 90 GWd/t discharge burn-up. Core outlet temperature is 750ºC for the helium, steam temperature is 566°C and core inlet temperature is 250°C. It has a thermal efficiency of 40%. Core height is 11 metres, diameter 3 m in a 25 m high, 5.7 m diameter reactor vessel. There are two independent reactivity control systems: the primary one consists of 24 control rods in the side graphite reflector, the secondary one of six channels for small absorber spheres falling by gravity, also in the side reflector. Pebbles are released into the top of the core one by one with the reactor operating. They are correspondingly removed from the bottom, broken ones are separated, the burn-up is measured, and spent fuel elements are screened out and transferred to storage.

* The size was reduced to 250 MWt from earlier 458 MWt modules in order to retain the same core configuration as the prototype HTR-10 and avoid moving to an annular design like South Africa's PBMR (see section on PBMR below).

China Huaneng Group, one of China's major generators, is the lead organization involved in the demonstration unit with 47.5% share; China Nuclear Engineering & Construction (CNEC) has a 32.5% stake and Tsinghua University's INET 20% – it being the main R&D contributor. Projected cost is US$ 430 million (but later units falling to US$1500/kW with generating cost about 5 ¢/kWh). Fuel pebbles were loaded in September 2017 and start-up is expected in 2018. The HTR-PM rationale is both eventually to replace conventional reactor technology for power, and also to provide for future hydrogen production. INET is in charge of R&D, and was aiming to increase the size of the 250 MWt module and also utilize thorium in the fuel.

The 210 MWe Shidaowan HTR-PM DPP demonstration plant at Rongcheng in Shandong province is to pave the way for commercial 600 MWe reactor units (6x250 MWt, total 655 MWe) with a single turbine, also using the steam cycle at 43.7% thermal efficiency. Plant operating lifetime is envisaged as 40 years with 85% load factor. The capital cost per kW is expected to be 75% of the small HTR-PM, and for subsequent units, 50%. Meanwhile CNEC is promoting the technology for HTR-PM 600 plants using six 250 MWt modules. Eventually a series of HTRs, possibly with Brayton cycle directly driving the gas turbines, would be factory-built and widely installed throughout China.

Performance of both this and South Africa's PBMR design includes great flexibility in loads (40-100%) without loss of thermal efficiency, and with rapid change in power settings. Power density in the core is about one-tenth of that in a light water reactor, and if coolant circulation ceases the fuel will survive initial high temperatures while the reactor shuts itself down – giving inherent safety. Power control is by varying the coolant pressure, and hence flow.



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