Belgian Reactor 1 - BR1

> What is reactor BR1 used for?
> Future activities and services
> Technical specifications


BR1 reactor fornt viewBR1 is the first Belgian reactor. It was critical for the first time on 11 May 11 1956. A critical reactor is a reactor in which an itself maintaining chain reaction occurs. The critical state is a normal state of activity of an operational reactor. BR1 is an air-cooled reactor with graphite as moderator. It is a flexible instrument for fundamental research and training.

After the start-up period, BR1 was mainly used for research in reactor and neutron physics. Until after the start-up of BR2 in 1964, BR1 was also used for the production of radioisotopes for medical applications. The reactor worked continuously, 24 hours a day, 7 days a week.

Two main fans, each consuming 0.8 MW, were necessary to guarantee a sufficient cooling of the reactor. The reactor's thermal power was 4 MW. Nowadays the reactor works on request of the experimenters, for at most 8 hours a day, at a maximum power of 700 kW (short periods up to 1 MW are also possible). The cooling for this reduced power can be guaranteed by a smaller auxiliary fan.

Discover all about BR1's history in the brochure "50 years BR1", published in 2006 on the occasion of the 50th anniversary of BR1.

What is the BR1 reactor used for?


This is a radiology technique similar to X-ray photography: a neutron beam is used instead of an X-ray beam. Neutrography "screens" an object with thermal neutrons. Because some elements (like hydrogen) easily absorb neutrons and others (like for instance aluminium) not or nearly, an image will appear, in which different materials can be distinguished, just like with X-ray diagnostics. Since neutrons are not ionising, they cannot form an image on a photographic plate: so, one has to convert the neutrons first into ionising radiation.

To this end, one generally uses gadolinium that strongly absorbs the neutrons and by that generates beta rays, to which a photographic plate is sensitive. However, the image is very different from an X-ray image. You can, for example, easily distinguish synthetic materials (containing quite a lot of hydrogen), because they turn black on the picture. It is less easy to distinguish several metals. In the case of X-rays, the density of the material determines how much radiation will be absorbed: heavier elements will absorb more than light elements.

Calibration and validation


The instruments used to measure radiation (neutrons, gammas, etc.) need to be calibrated beforehand in reference neutron or photon fields of which the characteristics are well-known. BR1 offers such reference fields used for this application. More information can be found in 'Our Services'.

Neutron Activation Analysis (NAA)

Neutron Activation Analysis is a non-destructive analytical technique that allows determining the composition of a sample. If a sample is irradiated with neutrons, its elements will be activated. By measuring the gamma rays, which are characteristic for each chemical element it is possible to precisely determine the composition of the sample.

For a number of elements, the activation analysis is more sensitive than a chemical analysis and it is used for research, industry, archaeology and criminology. The biggest advantage of this technique with regard to chemical techniques is the non-destructive character: the sample remains in its original form and if necessary, it can be measured/analysed again or it can be subjected to other studies.

Training and Education

Theoretical training on nuclear reactor theory for students (BNEN, Belgian and foreign universities) and engineers (nuclear industry) are complemented with practical sessions in the control room of BR1. These sessions include reactor start-up, calibration of control rods, measurements of temperature coefficients, etc, depending on the specific training program. For more information is referred to the Academy website.

Production of Neutron Transmutation Doped (NTD) Silicon

Despite its relatively low neutron fluxes, the BR1 reactor is a valuable neutron source for Si NTD. Thanks to its large reactor core (4.7 m diameter, 4.9 m length), in-core irradiations are characterised by low neutron flux gradients. The massive graphite moderator results in a high degree of thermalisation in most irradiation positions.

Because of its relatively low neutrons fluxes, BR1 is particularly suited for production of high-resistivity Si. More information can be found in 'Our Services'.

Future activities and services

The continued existence of the BR1 reactor is ensured because there are no budgetary or safety issues. The exploitation costs are quite low: we do not have costs for new fuel, because with the actual exploitation regime, we are able to run the reactor for many more years with the present fuel. The electricity costs are quite low too, since we work with reduced power.

Also, the personnel expenses are limited. An exploitation team consists of 5 persons: the exploitation engineer, one adjunct, one pilot and two operators. This team is responsible for the exploitation of the reactor on request of our clients (load/unload an experiment, start of the reactor, measurements ...) and they also carry out the periodical controls and maintenance of the reactor and the related installations, so that the reactor is ready at all times for a safe use.

We try to guarantee our different internal and external clients the highest possible flexibility, to offer them stable irradiation conditions and to support them in the development of new experimental devices.

More information about these services can be found in the section 'Services'.

BR1 technical specifications

Nuclear Fuel

The fuel is natural metallic uranium (approximately 25 ton). The uranium is originating from the former Belgian Congo (now Democratic Republic of Congo) where the uranium reserves have played an important role in the development of the nuclear sector in Belgium (see our History Brochure).

A remarkable fact: the current fuel in BR1 is still the original one. After more than 50 years of working, the burn-up of 235U is only a few% (burn-up: quantity of burnt-out fissile material in comparison with the quantity of fissile material of the fresh nuclear fuel). 


The moderator of the reactor is graphite (carbon). A moderator is needed to slow down the energetic fission neutrons. There are 14,500 graphite blocks in the reactor (approximately 500 ton). The reactor is surrounded by a concrete construction of about 2 m thickness. This construction mainly serves as a biological shield against radiation; thanks to this shield it is possible to perform experiments on and around the reactor without exposure to irradiation doses.


BR1 irradiation channel

There are 829 channels (dim. 50 x 50 mm) for nuclear fuel, of which only 569 are loaded. Next to these fuel channels, there are about 70 channels, intended for experimental purposes. These channels are of various dimensions: rectangular ones of 10 x 10 cm2, 18 x 18 cm2, 24 x 24 cm2, and round ones of 8 cm in diameter.

Moreover, the reactor has 2 thermal columns: these are places where the graphite further extends up to the concrete and where specific experimental settings can be installed. In order to load or unload specific samples during the working of the reactor, BR1 is equipped with a number of pneumatic sample dispensers.


BR1 is cooled with air. This occurs through a forced convection using a fan and the removal of warm air via the chimney. There are of course filters in the cooling circuit to prevent for example dust particles from getting in the reactor or to prevent the emission of radioactive material via the chimney.


The reactor is controlled with control rods. These are aluminium tubes containing cadmium and graphite . They are operated using electric engines in vertical channels. In total there are 18 such rods: 6 security rods, 5 pairs of control rods and 2 precision regulation rods.

Control room

BR1 control roomThe reactor is operated by the pilot. Once the reactor has reached the desired power, it can automatically be controlled. This makes it possible to maintain the power level with a precision of 0.2%. Just like the reactor and the nuclear fuel, a part of the equipment in the control room is still original and operational. Evidently through the years the necessary modernisation has been carried out, especially with regard to the electronic nuclear measuring devices and the possibility of data acquisition via pc.


The safety of the reactor operation is followed-up and ensured in various ways. There is for example a sniffler system that measures the released radioactivity in case one of the fuel rods would be damaged. In the control room the temperature of various components is constantly monitored via thermocouples. The maximum temperature of uranium may not exceed 250 °C, graphite cannot exceed 104 °C. The power that can be generated is limited by means of alarm levels on the nuclear measuring devices. The reactor and the reactor building are constantly in underpressure, i.e. the pressure in the building is lower than outside to prevent radioactive leaks. The reactor has a negative temperature coefficient. This means that, if the power of the reactor rises, the reactivity will go down as a result of which, if the pilot does not take action, the power of the reactor will automatically decline.

Contact: Vittiglio GuidoBart Van Houdt