How is the HZB research reactor structured?
In 1958 the Berlin Experimental Reactor BER I was put into operation at the Helmholtz-Zentrum Berlin. It was one of the first reactors in Germany. It was shut down in 1971 and was superseded at the end of 1973 by BER II delivering 5 MW. From 1985 to 1989 BER II was expanded to 10 MW and was fitted with better experimental facilities. Like most research reactors it is a so called swimming pool reactor.
The name is suggested by its design: the reactor’s core (essentially the fuel elements and control rods) is suspended in an open pool of water. This water functions as a moderator, coolant, and radiation shield in equal measure. The layer of water over the reactor core absorbs the radiation to such an extreme degree that there need be no time limitations for the personnel on or near the facility. This design has two great advantages: the reactor allows ease of access, and the entire so called primary coolant circuit, i.e. in the pool of water, is under normal pressure. In other words there are none of the high temperatures and pressures needed in a nuclear power station.
The water in the reactor well heats up to a temperature of about 40 °C during operations. The heat generated in BER II is drawn off by three water circuits connected in series via heat exchangers. The first of these water circuits flows through the reactor core and is completely immersed in the reactor well. The heat drawn out of the reactor core is ultimately released into the atmosphere through the cooling towers.
The HZB research reactor generates a thermal power of 10 MW. It is therefore a few hundred times less than the usual thermal power of about 3000 to 4000 MW generated by a nuclear power station. Correspondingly smaller is therefore the size of the facility and the quantity of uranium.
Research reactor BER II
The research reactor’s building consists of three halls. At the top is the reactor hall enclosing the open pool of water. All work on the reactor core is performed from this hall, e.g. the rearrangement of fuel elements. A control room projects into the reactor hall with a partitioning window.
Under the reactor hall are the experimental hall and the adjoining trial hall. There several beam holes provide the experimental setups with neutrons that have passed through the pool and concrete shielding after leaving the reactor core. The scientists working here are protected from the radiation with shielding installed around the experiments.
In the halls of the research reactor a slightly lower air pressure is generated than the surrounding atmospheric. This small difference in pressure is maintained with a ventilation and air conditioning system. The air from this system is examined for radioactivity, if necessary filtered, and then released to the atmosphere through the flue.
The reactor core consists of thirty to forty fuel elements. Each of these is made up of twenty three thin plates, each containing a uranium-aluminium compound sealed in aluminium.
The reactor core is enclosed in a beryllium reflector that serves to raise the neutron intensity.
The cold source is a layer of very cold hydrogen about ten centimetres thick applied to the edge of the reactor core. This is used to generate particularly slow neutrons that open up special fields for scientific research. Moreover slow neutrons can pass through neutron guide tubes to more remote experimental setups and so need not be contained within the confines of a single hall. The HZB research reactor has its own trial hall that is supplied by five neutron guide tubes.