VHTR

VHTR-based nuclear electricity and hydrogen generation process. 

Comprehensive experience and know-how in design, construction, operation, and maintenance acquired through three decades of the HTTR project are the basis for the full-scale commercial VHTR reactor design used by the GTHTR300 system variants as shown in Figure 4.24. To meet economical and safety requirements of the commercial... 

Alternative reactor types are possible for the VHTR. China s HTR-10 [35] and South Africa s pebble bed modular reactor (PBMR) [41] adopted major elements of pebble bed reactor design including fuel element from the past German experience. The fuel cycles might be thorium- or plutonium-based or potentially use mixed oxide (MOX) fuel. 

JAEA s plan for nuclear hydrogen production with the VHTR. 

Hittner, D., The European programme of development of HTR/VHTR technology, in Proc. of the 2nd International Topical Meeting on High Temperature Reactor Technology, Beijing, September 22-24,2004. 

Massive hydrogen production using VHTR is necessary to support growing demand of hydrogen as a fuel as well as a chemical feedstock. 

Most of the material research is in collaboration with Generation-IV VHTR research activities. Properties of the nuclear graphite, the metallic material such as A617 and 9CrlMo, the ceramic composite are under investigation by measuring specimens. 

Kim, D-O., et al. (2008), Dynamic Analysis Models and Analysis Examples for Stacked Graphite Fuel Blocks of a VHTR Using a Commercial Structural Analysis Code , Trans. Korean Nuc. Soc. Autumn Mtg., Pyeongchang, Korea, 30-31 October. 

Kim, Y-W., et al. (2008), Development of a Coupling Process Heat Exchanger Between a VHTR and a Sulphur-iodine Hydrogen Production System , Proc. of HTR2008, Washington, DC, USA, 28 September-1 October. 

Figure 1 Overall HTE hydrogen production efficiencies for the VHTR/recuperated direct Brayton cycle, as a function of per-cell operating voltage...

Hydrogen generation is considered a promising application for VHTR. Simple thermodynamics show that the high temperature heat they can provide can lead to significant increase in efficiency when compared to low temperature processes, such as alkaline electrolysis coupled to a pressurised or boiling water reactor. 

Sulphur-based cycles, such as the sulphur-iodine cycle, take full advantage of the range of temperatures at which the VHTR provides heat. In particular, the oxygen generation step (sulphur trioxide decomposition) requires very high temperature heat (typically T > 800°C), which can only be... 

General Atomics (GA) and the Commissariat a Ytnergie Atomique (CEA) have been working on sulphur-iodine cycle flow-sheeting for several years, leading to sometimes differing efficiency estimates. They have undertaken to understand and reconcile these differences, and have come to consider in more detail the effect of the VHTR characteristics on the optimisation of the sulphur-iodine flow sheet. This paper will present the outcome of these studies, and stress the interplay between nuclear reactor and chemical process. 

This paper discusses in particular the elements of coupling an S-I process to a VHTR, and how the parameters that describe this coupling can affect the energy consumption of the cycle. 

In general, there are two alternatives for coupling the S-I process to a VHTR. In the first, secondary helium is initially used to supply heat to the MT/HT1 sulphuric acid decomposition steps. LT helium heat is then consumed by the HI decomposition section before the helium return to the intermediate heat exchanger. In the second alternative, heat integration between the chemical process steps allows for helium heat supply solely to the MT/HT sulphuric acid decomposition steps. Residual process LT heat recovered there is utilised in the HI decomposition section. 

In 2006, GA participated in a study conducted by the Savannah River National Laboratory (Summers, 2006). The S-I process was coupled to a VHTR with a required helium return temperature near 600°C. To efficiently match temperature requirements with available heat, a design was developed to supply HI decomposition section energy with recovered heat from the sulphuric acid decomposition section. For the purposes of comparison and analysis in this paper, the GA flow sheets will refer to this design, and CEA flow sheets will refer to a design in which helium supplies heat to both acid decomposition sections. CEA uses ProSimPlus for flow sheet analysis, and GA uses Aspen Plus . A previous study (Buckingham, 2008) showed that the two process simulators give similar calculated results when the same unit operations and stream compositions are modelled, although different thermodynamic models are used for the calculations. 

The proposed flow sheet produces hydrogen at a pressure of 10 bar according to the chosen pressure of the electrolysis cell. Compared to the electrolysis of water, whose pressure of hydrogen product is about 1 bar, the power requirement to raise the pressure up to 10 bar is not negligible. The electric power requirement to do such work is about 9.8 kj/mole H2. With this output condition, the new heat requirement for VHTR-powered water electrolysis becomes 881 kj/mole H2 With a total equivalent heat requirement of 680 kj/mole H2, the proposed HyS process flow sheet compares favourably to VHTR-powered water electrolysis. 

The CEA launched in 2001 an integrated programme to compare the most promising way to produce hydrogen using the high temperature heat available from a VHTR. In order to develop its own expertise on thermochemical cycle assessment, CEA has chosen to develop a scientific approach based on data acquisition (development of devoted devices and specific analytical methods) and modelling (physical models, flow sheet analysis, systemic approach). 

Furthermore, HEEP will consider several reactor concepts including water reactors such as PWR and PHWR for the lower temperature range, the very high temperature reactors (VHTR), fast breeder reactors (FBR) and molten-salt cooled reactors for the high temperature range, and super-critical water reactor (SCWR) capable of output temperatures up to around 625°C for the medium range of temperature. 

Such a solution is obviously less efficient than the one including a VHTR since it only provides 1.9 kg H2/s instead of 2.1 kg H2/s in the VHTR case (DOE, 2006), but the difference is not that important. Anyhow, it is small enough to make it possible for the cost of hydrogen in this heat + electricity option to turn out to be competitive as less stringent requirements will be put on the reactor and other systems such as ... 

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. 

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