Research reactors

The Research Reactor Geesthacht 1 (FRG-1) was finally shut-down on 28 June 2010. An application for decommissioning has been filed.

As opposed to nuclear power plants, research reactors do not serve to produce electricity but to produce neutrons (neutron source). Principally, the produced neutrons are used for various purposes in the technological and medical field.

For example, in research reactors:

• The behaviour of new materials is scientifically and industrially analysed,
• Medical applications in radiation therapy are carried out,
• Special radioactive isotopes for medical diagnostics and therapy are produced, and
• Students and the (junior) staff working in nuclear technology are trained and further educated.

Neutrons produced in the reactor core during the fission of uranium, typically have energies of some mega-electron volts and are not very suitable for experimental purposes. They need to be slowed first.

A moderator serves to slow the neutrons. Water or graphite are frequently used for this. Through the elastic collisions with the single atoms in the moderator, neutrons loose part of their kinetic energy.

When leaving the moderator, the energy of the single neutrons is in the area of some milli-electron volts up to about 100 milli-electron volts (the energy of the slowed neutrons is thus about 1 billion times lower than the energy of the original fast neutrons). These are referred to as thermal neutrons and used for experimental purposes.

Differences to power reactor (nuclear power plant)

Compared with a nuclear power plant, the capacity of a research reactor is generally clearly lower. Accordingly smaller is the amount of nuclear fuel used and the radioactive waste volume produced. From this results an accordingly much lower risk potential, as compared with a nuclear power plant.

Research reactors differ partially considerably from each other as to:

• Design,
• Thermal power,
• Nuclear fuel used,
• Radioactive inventory as well as
• Site (e.g. central location in a city or in a suburb), and
• Mode of operation.

Research reactors in Germany

In Germany, altogether six research reactors are currently in operation. They include:

• 1 large pool reactor: FRM II in Garching near Munich with a thermal output of 20 megawatts (MW).

  In pool reactors, the reactor core is positioned in a pool filled with water. The water has several functions here; among others, it serves as coolant for the reactor core and as moderator for the neutrons. From the core, the neutrons are transported via beam tubes to the single experiment stations. These reactors are especially suitable for manifold research applications and for radiation therapies.

• 1 TRIGA Mark II reactor in Mainz with a thermal output of 100 kilowatts.

  In terms of design, a TRIGA reactor (Training, Research, Isotopes, General Atomic) is among the small pool reactors. Its special characteristic are the fuel elements consisting of a homogeneous mixture of fuel (uranium) and moderator (zirconium hydride). This results in favourable safety features that are typical for TRIGA: The TRIGA has a prompt negative temperature coefficient of reactivity. That means that the number of neutrons in the reactor core rapidly decreases with increasing temperature of the reactor core. In the case of a capacity increase this has a self-stabilising effect and enables a relatively simple safety instrumentation and control of the reactor. TRIGA reactors are especially suitable for the production of isotopes and neutron activation analyses.

• 4 smaller training reactors, the so-called homogenous zero-power reactors: AKR-2 in Dresden with a thermal output of 2 watts and 3 SURs (Siemens training reactors) in Stuttgart, Furtwangen and Ulm with a thermal output of 100 milliwatts (mW) each.

  The core of these training reactors consists of cylindrical polyethylene disks in which the uranium fuel is evenly distributed. On account of the low thermal output they do not require cooling. The homogenous zero-power reactors are especially suitable for training and further educational purposes.

Example: FRM II research neutron source in Garching near Munich

The newest research reactor in Germany and at the same time the one with the highest neutron number is the Heinz Maier-Leibnitz research neutron source (FRM II) at the Technische Universität München (TUM). The reactor went into routine operation in 2005, so that the users could start with first experiments.

The FRM-II is a pool reactor. Light water (normal water) serves as coolant for the core and heavy water (as opposed to the light water, the hydrogen atoms here are replaced by their heavier isotope, deuterium) serves as moderator to slow the fast neutrons. The reactor core consists of one single fuel element in which 113 single fuel plates with compacted uranium fuel are arranged.

Due to its unique design, the reactor produces very many neutrons with a comparatively low thermal output of 20 megawatts. The number of thermal neutrons reaches 800 trillion (2 x 10¹⁴) per square centimetre and per second. With such a high flux density of thermal neutrons, the FRM II is among the leading high-flux research reactors in the world and enables high-performance experimentation techniques. Given that the high flux density of neutrons also clearly decreases the irradiation time of the single samples, also neutron-intensive research projects can be carried out at the FRM II.

Licensing and supervision

Although the research reactors have a comparatively low risk potential, they are basically subject to the same requirements for the licensing and supervisory procedure as the nuclear power plants. Generally, the rules and regulations developed for nuclear power plants are applied here, but are graded correspondingly depending on the respective risk potential of given research reactor installation.