8.1.2 Experience of thorium use in experimental and power reactors

During the pioneering years of nuclear energy, 1950-1970, with great enthusiasm and regardless of the costs, a large number of possible avenues for energy production with thorium were investigated, not only in the USA and USSR, but also in Europe and, to some extent, in Asia.[14] For example, it is remarkable that the thorium-based Elk River (1963) and Peach Bottom (1967) reactors were started only a few years after the ‘founding fathers’ of the two main reactor families of today, based on uranium fuel, PWR Shippingport (1957) and BWR Dresden (1960). It is also remarkable that breeder demonstration was performed at Shippingport in the late 1970s and early 1980s using a U-233/thorium cycle.3 The conversion ratio1 reached 1.0139. This was the only US demonstration programme using U-233 as the fissile seed material. Although this demonstration was successful from the standpoint that slightly more U-233 was bred than consumed, success was only achieved at the high cost of a sophisticated core design, and by sacrificing reactor performance.

From that time on, a significant amount of experience on thorium-based fuel in experimental and power reactors has accumulated. An exhaustive list of these reactors is provided Table 8.1 (however, this table does not include experimental reactors in which thorium fuels have also been tested, such as CIRUS in India, KUCA in Japan, MARIUS in France, etc.).

Within the framework of this chapter, it is not possible to provide details on all of these reactors and, instead, the reader is directed to references 3 to 6. Here we shall focus on high-temperature gas-cooled reactors (HTRs) since, as is seen in Table 8.1, thorium fuel was mainly developed for this type of reactor.

In the US, during the 1960s and 1970s, the use of a HEU (highly enriched uranium)-thorium fuel cycle was demonstrated at the Peach Bottom and subsequently, Fort Saint Vrain HTRs. Both reactors used prismatic block type fuel elements containing either fissile or fertile fuel. The fissile fuel consisted of HEU dicarbide, the fertile fuel was thorium dicarbide. Both fuels were in the form of

Country Name Type Power (MW) Startup date Fuel Note USA Indian Point 1 PWR 265 1962 Th02-U02 Power includes 104MWe from oil-fired superheater Elk River BWR 22 e 1964 Th02-U02 Power includes 5MWe from coal-fired superheater. Thorium loaded in the first core only Shippingport PWR 60 e 1957 Th02-U02 Used both U-235 and plutonium as the initial fissile material. Successfully demonstrated thermal breeding using the ‘seed/blanket’ concept (Th/U-233) Peach Bottom HTR 40e 1967 ThC2-UC2 Coated particle fuel in prismatic graphite blocs — Th/HEU Fort St Vrain HTR 330 1976 ThC2-UC2 Coated particle fuel in prismatic graphite blocs — Th/HEU MSRE MSR 1°,h 1965 ThF4-UF4 Operated with U-233 fuel since October 1968. No electricity production UK Dragon HTR 20th 1964 ThC2-UC2 Coated particle fuel. No electricity production. Many types of fuel irradiated Germany AVR HTR 15 e 1967 ThC2-UC2 Coated particle fuel in pebbles. Maximum burn-up achieved: 150 GWd/t — TH/HEU THTR HTR 300 e 1985 ThC2-UC2 Coated particle fuel in pebbles. Maximum burn-up achieved: 150 GWd/t — Th/HEU Lingen BWR 60 e 1968 Th/Pu Th/Pu was only loaded in some fuel test elements India Kakrapar (KAPS) 1-2 PHWR 200 e 1993/95 U02-Th02 Fuel: 19-element bundles. 500kg of Th loaded Kaiga 1-2 PHWR 200 e 2000/03 U02-Th03 Fuel: 19-element bundles. Th is used only for power flattening Rajasthan (RAPS) 3-4 PHWR 200 e 2000 U02-Th04 Fuel: 19-element bundles. Th is used only for power flattening KAMINI Neutron source 30 Kwe U-233 Experimental reactor used for neutron radiography

carbon-coated particles, the fissile particles being somewhat smaller than the fertile ones.

In the UK, the first HTR demonstration known as Dragon operated between 1966 and 1975. Various types of fuel elements including thorium with a 10:1 Th/U (HEU) ratio were irradiated.

In Germany, two pebble bed type HTRs were operated. The first one, AVR, was a prototype pebble bed reactor that mainly used a HEU/thorium cycle. The fuel consisted of billiard ball-sized fuel elements. A commercial version, the THTR — 300, a 300 MWe thorium/HEU fuelled HTR, started operation in 1985. It was permanently shut down in 1989 largely for political reasons although high operational costs and an operational incident in 1986 that resulted in the release of radioactive materials are often mentioned as the grounds for shutdown.