Gas-cooled reactors have been studied in various countries since the start of the nuclear power programme (Methnani, 2003; Mitchell et al., 2002). Future generation plants will benefit from this experience. In this section, attention is focussed on the high temperature thermal systems, in the following section, fast spectrum systems will be considered. The early gas reactors were natural uranium fuelled, graphite moderated and air cooled and used for military operations. Following on, in the UK, Magnox plants incorporated pressurised carbon dioxide cooling followed by advanced gas reactors with enriched uranium oxide fuel and higher pressure carbon dioxide as coolant.

High temperature gas-cooled reactor (HTGR) concepts have been studied in parallel with the carbon dioxide-cooled plants. Early experimental and prototype reactors included Dragon, AVR and Peach Bottom. The Dragon reactor operated at Winfrith and incorporated helium cooling and ceramic-coated particle fuel. This reactor included highly enriched uranium-thorium carbide fuel particles. The coolant operating outlet temperature was 750°C and much useful information on helium-based HTGR systems arose from the early Dragon programme. The AVR system operated in Julich in Germany. It had a higher temperature of 950°C and used 100,000 coated fuel spheres. This was the concept that is currently being considered for the Pebble Bed Modular Reactor (PBMR) design. In this design, the fuel spheres move downwards in the reactor core within a graphite reflector vessel. The first HTGR in the US was Peach Bottom Unit 1, rated at 40 MWe. Several fuel designs have been developed to overcome problems with cracked fuel.

Two main types of HTGR designs have emerged over the last 2 decades, through the operation of several prototypes. The German thorium high-temperature reactor (THTR — 300) was of a pebble bed type; The US Fort St. Vrain design was of the prismatic design.

Power ratings were raised to 300 MWe and there were various design features including a pre-stressed concrete reactor pressure vessel and a more advanced coated fuel particle design known as TRISO.

More recent designs have incorporated reduced power density, reduced overall power and more passive systems. The general atomics modular high temperature gas reactor (MHTGR) was rated at 350-450 MWt and the German HTR series design was rated at 200-300 MWt. These system designs were more modular. The direct cycle MHTGR design, utilising advanced gas turbine and high temperature turbine technology, could yield efficiencies up to 50%.

The IAEA has co-ordinated several safety-related research projects on the physics, heat removal aspects and fuel and fission product behaviour of HTGRs. A latest activity is concerned with benchmarking core physics and thermal-hydraulic methods against experimental data in order to evaluate HTGR performance.

The European Commission has recently supported a network R&D activity to address the major design issues associated with the core physics and fuel cycle, and the material and components issues. The project is also concerned with the safety and licensing issues associated with the HTGR design.

In respect of their reactor physics, HTGRs have a relatively low power density compared with light water reactors, of the order of 2-3MWm_3. They include a large volume of graphite as moderator that also implies a relatively large core size. The core is usually annular to give a flat radial power distribution. HTGRs typically include a central graphite reflector and radial and axial reflectors, and are designed such that the inner reflectors that absorb a large fluence are replaceable. HTGRs exhibit good neutron economy due to the low absorption of the graphite and negligible absorption by the helium coolant. Another desirable feature is a negative reactivity core temperature coefficient that increases in magnitude at higher burn-up and lower fuel enrichment.

In current PBMR designs, the control rods for both operation and safety purposes are situated outside the reflector region in order to limit exposure at high temperature. This means that they have reduced worth, which tends to imply smaller diameter annular cores are designed. The fuel inventory is relatively low due to the use of low enriched fuel, which means that safety is not compromised. The power can also be effectively managed by varying the helium inventory and taking advantage of the negative temperature coefficient in the 25-100% power range.

HTGR core physics tools have been validated by comparison with the HTR-10 reactor in China, the high temperature test reactor (HTTR) reactor in Japan and the Proteus critical facility in Switzerland. Reactor physics methods have been applied utilising methods ranging from detailed Monte Carlo methods to combinations of cell transport and core diffusion models. Benchmarks have shown that some of these codes predicted the core criticality loading to a good level of accuracy. Thus, there are adequate methods available for reactor physics calculations for low-enriched gas-cooled reactors.

Regarding their thermal design, the characteristics features of HTGRs include low power density, high core thermal capacity with very high core outlet temperatures as high as 950°C, much higher than other reactor types. Other geometric features include a large height to diameter annular core with a steel pressure vessel, which enable decay heat removal under normal and abnormal conditions.

Modern designs utilise helium gas enabling a direct Brayton cycle to improve thermal efficiency and economics. The coolant circuit is based on gas at high pressure in the core, moving upwards to a gas plenum, cooling the external reflector regions and the upper core structures before entering the core flowing downwards. The gas then exits at temperatures in the range 800-950°C. Efficiencies of up to 50% are the target. More ambitious future designs have even higher temperatures as described below.

The power conversion unit converts the core thermal energy into mechanical and then electrical energy by means of various engineering components designed to achieve high efficiency. The gas turbine is connected to the generator, turbo-compressors to pressurise the helium, pre-cooler, inter-cooler and recuperator.

Different HTGR designers have proposed different direct and also indirect cycle designs. In the former case, the reactor vessel is connected by a cross-duct to the power conversion unit. In the latter case, primary and secondary circuits are interfaced by an intermediate heat exchanger (IHX). An advantage of the latter is to include an additional barrier against radioactive contamination of the turbo machinery. There have been considerable advances in turbo-machinery technology that have been achieved in parallel with the development of the Brayton cycle.

Below are briefly described some of the currently proposed designs of high-temperature thermal reactors. These are listed in Table 12.3.

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