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New generations: The high temperature reactor

Nuclear Monitor Issue: 

(November 21, 1997) After the 1979 Harrisburg (US) accident the nuclear industry faced a serious decline in sales of nuclear power reactors. The 1986 Chernobyl disaster led sales to drop off even more, as several countries decided not to build any new nuclear power plants. Since the early eighties, the industry has been working on development of so-called inherently safe reactors. These new reactors are now being presented as a solution to the greenhouse effect. This first article in a series on new generation reactors looks closer at the HTR, the High Temperature Reactor.

(481.4774) Laka Foundation -The HTR is based on the technology of a gas-cooled graphite-moderated reactor. In the UK, gas-graphite reactors are the main type of reactors used in the production of nuclear electricity; only one water reactor (Sizewell B) is being used. Although the principles of an HTR are based on the gas-graphite reactor, no HTR was developed in the UK. At the German research center Juelich, an HTR, called the 'Arbeidsgemeinschaft Versuchsreactor AVR', was opened in 1967. In this reactor the fuel was embedded in graphite balls. It was a thorium reactor; neutrons from a chain reaction were used to breed fissionable uranium-233 from the non-fissionable thorium-232. Later models dropped the thorium cyclus as there were too many problems with the reprocessing of thorium fuel. Other problems with the AVR were leakages in the helium cooling circuit, a fire with turbine-oil and the production of fuel. In 1985 the Hamm-Uentrop Thorium High Temperature Reactor (THTR) was opened. This DM6 billion (US$ 3.5 billion) reactor faced serious safety problems. In May 1986 (a week after the Chernobyl accident), radioactive gas escaped from the cooling system, after graphite-fuel balls stuck in the fuel inlet. Other problems occured when fuel balls were damaged and with sticking control rods. In 1989 the reactor was permanently closed due to economic and political reasons. The Siemens company studied the HTR-Module, a 200 MW HTR, for the production of process-heat (steam) and electricity. In 1987 Siemens asked for a non-site specific license for building a prototype in the German state of Lower-Saxony. As no possible buyer was found, the license was ultimately denied. After spending some DM2 billion (US$ 1.2 billion) on the HTR, Siemens ended the project in 1991 by selling the technology to China.

In the US an HTR, developed by General Atomics, was opened in 1967 in Peach Bottom. It closed in 1974, the same year when the Fort St. Vrain HTR was opened. This reactor was closed in 1989 due to problems with the cooling system and control rods. General Atomics is still working on the development of a new HTR, but faces financial problems. In 1995 the US House of Representatives stopped subsidizing General Atomics research. Now the company hopes to sell an HTR to Russia for burning weapons plutonium. Smaller HTR projects are being developed elsewhere: the South African utility Eskom is studying the possibilities of HTR reactors, Japan has almost finished building its first HTR research reactor, and China is building a research HTR with the Siemens technology.

The HTR is not only made for the production of electricity. Due to the high temperature of the coolant, the HTR can also be used for the production of heat. This process-heat, f.i. through the production of steam, could be used in chemical industry, paper-mills, city heating and desalination plants. The uranium or thorium fuel is embedded in millimeter-small fuel particles. Some thousands of these particles are put in a graphite ball, approximately 5 centimeters in diameter. The function of the graphite is to moderate the neutrons produced by fission. Without moderation, the fission of uranium or thorium is impossible. The graphite balls are in the reactor vessel and are cooled by helium gas. Cooling by water is impossible as graphite reacts heavily with water. The heated helium is used to drive an electricity producing turbine. After this the heat is used for the production of steam in a steam generator. As the HTR is used both for the production of heat and power it is sometimes called a Cogeneration (or Combined) Heat and Power (CHP) plant. Some concepts are based on a uranium-cycle, using enriched uranium. Other concepts are based on the thorium cycle, using the neutrons from uranium fission to breed uranium-233 from thorium-232. Although there is little experience with thorium technology, a longer future is foreseen, mainly in India (see also WISE NC 461.4577: India; experimental thorium reactor gone critical.), as uranium resources are getting smaller.

The HTR is often presented as an inherently safe reactor. The term inherently safe suggests that absolutely no accidents can happen. This is of course not true; one can never exclude an accident. The IAEA recommends not to use this term, they prefer the use of 'next generation reactors'. In the most common reactors, water reactors, the danger of a large release of radioactivity exists if the fuel elements melt. This can happen when the water cooling fails, for instance, due to a leak in the coolant circuit. Due to residual heat, the fuel- elements would melt and release the fissionproducts. In the HTR the fuel particles are enclosed in graphite balls that cannot melt. But graphite can burn, a property that caused the serious 1957 accident at the UK Windscale plutonium producing reactor. The burning of graphite in the 1986 Chernobyl disaster extended the release of radioactivity and made it difficult to fight the fire, as graphite also reacts heavily with water. In the HTR a fire could occur when air comes into the reactor, for instance through an external explosion or an accident with an airplane. Water can enter an HTR when leaks occur in the steam generator. Research is being done to give the graphite balls a corrosion resistent layer. Graphite is damaged when temperatures reach 1600 degrees Celcius. To keep the temperature under this critical 1600 degrees, the release of residual heat from the reactor must be high in case of a loss of coolant accident. Therefore the HTR lacks the safety containment that is used in a light water reactor building. A safety containment would have an isolating effect on the reactor. But the function of a safety containment is to keep radioactivity inside the reactor building in the event of an accident, as well as give protection from forces from outside. In 1988 the US safety authority Nuclear Regulatory Commission (NRC) doubted the safety characteristics of the HTR: the improvement in safety by the use of graphity would be followed by a decrease in safety due to the absence of a containment.

In a fission reactor uranium-238 is formed into plutonium-239 when it captures a neutron. Plutonium-239 can be used in nuclear weapons. But other plutonium isotopes are also bred which are unsuitable in nuclear weapons. It is the amounts of plutonium-239 and other plutonium isotopes that make the plutonium more or less suitable for nuclear weapons. The plutonium produced in gas-graphite reactors is especially of a high weapons quality. Therefore the HTR can be misused for the production of weapons material. Some HTR concepts make use of higher enriched uranium, that can also be used in nuclear weapons. In the thorium cyclus based HTR, uranium-233 is produced. This uranium-233 is of weapons quality, just like plutonium-239. The choice for a thorium cyclus means also a choice for reprocessing. The uranium-233 must be extracted from spent fuel for the production of new uranium-233 fuel. This means, in addition to the environmental risks of reprocessing, the production of pure uranium-233 with the risks of misuse and theft. The possibility to load and unload an HTR during electricity production makes the control or misuse more difficult than with a water reactor, which must be shut down for fuel to be unloaded.

The HTR is a reactor with which there has been very little experience worldwide. Therefore a very big financial investment is required before production of this reactor can be economical. Investments are not only needed for the reactor but also for a fuel production and reprocessing infra-structure. Nowadays an HTR cannot compete with a gas-fueled Cogeneration plant. Altough the fuel costs would be lower than in a gas plant, the main costs will come with building the reactor. According to a Dutch study gas prices would have to rise to three times their current level before an HTR (in serial production) could become competitive.

The choice of the nuclear industry to develop an HTR looks to be an attempt to show the public a 'safe' reactor that cannot melt. But complete safety can never be assured because of the possibility of graphite burning, the lack of a containment, etc. With the HTR, proliferation risks will increase. The environmental pollution from reprocessing will continue in the thorium concept. And off course the uranium or thorium mining will destroy mining areas, contaminating environment and people with radioactivity. The argument to fight the greenhouse effect with a safe non carbon dioxide producing reactor is false. The HTR will produce radioactive waste, in volume even more than a water reactor due to the radioactive graphite balls. This will enlarge the amount of waste that has to be stored for millions of years. Like the greenhouse effect, another worldwide environmental problem will be created.


  • Greenpeace press release, 11 July 1996
  • Laka foundation fact-sheets; 4 July 1996 and 16 January 1997
  • WISE News Communique 455.4509: Dutch dreams: A nuclear reactor in every town. ,12 July 1996
  • Energy Research Foundation ECN (NL), Incogen pre-feasability study - Nuclear Cogeneration based on HTR Technology, September 1997.

Contact: Laka Foundation, Ketelhuisplein 43, NL-1054 RD Amsterdam. The Netherlands
Tel: +31-20-6168294; Fax: +31-20-6892179.

OECD/IAEA meeting on HTR. From 10 to 14 two meetings on the HTR took place at the Energy Research Foundation (ECN) in The Netherlands. The IAEA Technical Committee Meeting dealt with the future prospects of the HTR. A second meeting (of the OECD Nuclear Energy Agency) discussed the technical aspects of research on the HTR. According to the discussions at the IAEA meeting a market share of the HTR is not foreseen in the next decade: the HTR is still in a developing stage and can not compete with other energy sources. Only one small chance for an economically realizable HTR in the next decade, is foreseen in South Africa, were it would compete with coal plants. The Dutch Energy Research Foundation presented at the meetings its research report on the 'Incogen', INherently safe Nuclear COGENeration. Whether the research on the HTR at the institute would continue was doubted when dutch government ended in 1996 a finance program for new generation reactors. Now the ECN is looking for cooperation with other European institutes and companies. Under coordination of Framatome and with finances from the European Union new research has started.
Laka Foundation, 19 November 1997

See also Reactor grade PU also suitable for N-weapons [NC 483-4.4799] for important information on this subject.