4.2.1 Overview

The technologies of the high temperature gas-cooled reactors (HTGRs) are based on the designs and operating experiences with Magnox reactors and Advanced Gas-cooled Reactors (AGRs). In AGRs, the outlet coolant temperature could not be elevated due to chemical reaction of the CO2 coolant with the graphite structures. Thus, helium gas having high chemical stability is adopted as the coolant of the HTGRs, which enables high reactor outlet coolant temperature. Various gas-cooled reactors are compared in Table 4.5 [18—20]. Since metals could not be used as the fuel clad under the high temperature condition, coated particle fuel using ceramics coating was developed [21]. The coated particle fuel consists of spherical fuel kernels coated with pyrolytic carbon (PyC) and SiC. Utilization of the helium

Table 4.5 Compassion of various gas-cooled reactors Magnox reactor (COo cooled reactor) Advanced gas—cooled reactor (AGR) High-temperature gas-cooled reactor (HTGR) Gas-cooled fast reactor PWR as reference Reactor Tokai NPP unit 1 Hinkley Point-B THTR-300 Conceptual design by JAEA Ooi NPP unit 1 etc (587 MWt) [19] (1,500 MWt) [19] (750 MWt) [19] (2,400 MWt) [20] (3,411 MWt) [21] Moderator Graphite Graphite Graphite Light water Coolant C02 gas C02 gas Helium gas Helium gas Light water Fuel Metal natural uranium (with Magnox alloy clad) UOo (with stainless steel clad) Coated particle fuel Pin type nitride fuel UOo (with Ziocaloy-4 clad) Reactor outlet coolant temperature (°С) 390 665 750 850 320 Power density (W/cm3) 0.8 2.7 6 100 Bumup (GWd/t) 3.6 18 100 150 44

Graphite block Graphite sleeve lAPP’ 60,0m hei8ht)

Coated fuel particle (Multiple coating by ceramics)

Block type HTGR

HTTR. GTMHR

‘uel kerne!

Helium gat as coolant

Pebble ball fuel (App. 60mm diameter)

Fig. 4.15 Fuel of HTGRs gas coolant and development of the coated particle fuel have enabled high reactor outlet coolant temperatures of nearly 1,000 °C to be reached. One characteristic of the HTGRs in terms of reactor physics is that the conversion ratio of fissile nuclei can be high due to the small absorption cross sections of the helium gas coolant and the graphite moderator. The high performance of the coated particle fuel against release of FPs, as well as the high conversion ratio, enables high burnup more than 100GWd/t.

The HTGRs are categorized as the pebble-bed type and the block type according to the fuel configuration. In the pebble bed type HTGRs, coated particle fuels are mixed with graphite powder. The mixture is formed as spherical fuel balls, each with a diameter of 6 cm. The reactor core is formed by disorderly piling up many fuel balls. The unique characteristic of pebble bed type HTGRs is the capability for continuous refueling during operation. The coolant flows in gaps around the fuel balls. The block type HTGRs use hexagonal block type fuel. The reactor core is formed by piling up blocks in the axial direction. Refueling is carried out by the refueling machine during shutdown period. Fuel configurations of those HTGRs are shown in Fig. 4.15. The specifications of constructed HTGRs are summarized in Table 4.6 [22].

The pebble bed type HTGRs, namely, the Arbeitsgemeinschaft Versuchsreaktor (AVR) [23] and Thorium Hochtemperature Reaktor (THTR-300) [24] were constructed in Germany. The THTR-300 was a prototype power reactor. The Hochtemperature Reaktor 10 MW (HTR-10) [25] was constructed in China as an experimental pebble bed type HTGR. In the Republic of South Africa, construction of the Pebble Bed Modular Reactor (PBMR) [26], which is a modular type HTGR with an annular core, was planned.

Table 4.6 Constructed HTGRs Reactor Dragon Peach bottom AVR Fort St. Vrain THTR-300 HTTR HTR-10 Country UK (OECD) USA Germany USA Germany Japan China Operation period 1964-1976 1966-1974 1966-1988 1974-1989 1983-1989 1998- 2000- Reactor power (MWt/MWe) 20/- 144/42 46/15 842/342 750/308 30/- 10/2.5 Core diameter (m) 1.1 2.8 3.0 6.0 5.6 2.3 1.8 Core height (m) 1.6 2.3 2.5 4.8 6.0 2.9 2.0 Average core power density (W/cm3) 14 8.3 2.5 6.3 6 2.5 2 Fuel kernel uo2 (Th, U)C2 (Th, U)C2 (Th, U)C2 (Th, U)C2 uo2 uo2 (Zr, U)C uo2 ThC2 (Th, U)C Fuel type Rod Rod Pebble ball Block (Multi-hole) Pebble ball Block (Pin-in-block) Pebble ball Core inlet coolant temperature (°С) 350 340 270 408 250 395 250 Reactor outlet coolant Temperature (°С) 750 725 950 785 750 850/950 700 Coolant pressure (MPa) 2 2.4 1.1 4.8 4 4 3

The block type HTGRs, namely, Fort St. Vrain (FSV) [27] in the US and the High Temperature Engineering Test Reactor (HTTR) in Japan were constructed. The HTTR has a thermal power of 30 MW and reactor outlet coolant temperature of 950 °C [28—30]. Its fuel type is the so-called pin-in-block type which is composed of fuel rods and a hexagonal graphite block (Fig. 4.15). The fuel rods are composed of fuel compacts loaded in a graphite sleeve. The fuel compacts are formed by mixing coated particle fuel with graphite powder. The major specifica­tions of the HTTR are summarized in Table 4.7.

Since the reactor outlet coolant of LWRs is around 300 °C, their utilization is limited, namely, for electric power generation. On the other hand, helium gas having high chemical stability is used as the coolant for HTGRs and high reactor outlet temperature around 1,000 °C is possible, which enables high generating efficiency and utilization of HTGRs as a heat source particularly in the chemical industry. Thus, one of the HTGR advantages is its multi-purpose utilizations of nuclear energy.

Here, the core design of the HTTR is presented as an example of HTGRs. Also, the annular core design is described since it is noteworthy from the viewpoint of inherent safety.