Frequently Asked Questions about the Safety of the Research Reactor BER II
(last updated 2020)
1. Why did we need a research reactor in Berlin?
The research reactor BER II delivered neutrons for scientific studies. Neutrons are uncharged particles – parts of an atom's nucleus – that are excellently suited to studying materials. Neutrons can be bundled together into a ray, much like the way light can, and then shined onto many different kinds of sample. A neutron source is like a large microscope that researchers can use to study the inner properties of materials. Scientists from many disciplines, such as biologists, chemists, physicists, medical physicists, materials scientists, and even art historians, used our research reactor BER II to find answers to their questions. The research reactor was not used for developing or sampling nuclear power technology.
Research using neutrons delivered insights that scientists cannot obtained using other research methods. Many of the technologies we used today were researched using neutrons, much of which over the decades has been performed at the Berlin research reactor because of its unique experimental conditions.
However, the HZB has changed its scientific profile as part of a strategy process and corresponds to the decision of the Supervisory Board to shut down the neutron source BER II at the end of 2019. For our users, the focus now is on the operation and further development of the photon source BESSY II.
2. What was the difference between the BER II research reactor at Helmholtz-Zentrum Berlin and a nuclear power plant?
Our research reactor BER II in Berlin-Wannsee was not comparable with a nuclear power plant. It was not used to produce heat or electricity. We produced neutrons by nuclear fission for use in research. Neutrons are tiny, electrically neutral particles (see 1).
At Helmholtz Zentrum Berlin, they were needed to understand the deep, inner structure of matter to improve solar cells, for example, or to answer long-standing questions in archaeology, biology and many other fields of research. Given their different purposes, they are built very differently as well. The research reactor worked at low temperature and at normal pressure. A power plant, on the other hand, produces steam at high temperature and pressure for generating electricity. Because this was not the case, our research reactor did not have a reactor pressure vessel as nuclear power plants do. Since we did not use the excess heat, it was released through heat exchangers and cooling towers. To compare, the power of our research reactor was 10 megawatts. The thermal power of an average nuclear power plant is 3,000 to 4,000 megawatts.
3. What fuel elements were used in the research reactor BER II?
BER II worked with so-called LEU, or low-enriched uranium. The concentration of the isotope 235 is lower than 20%.
4. What safety measures existed to monitor operation of the research reactor, or to stop the nuclear fission if necessary?
The research reactor had automatic shutdown systems that would have triggered by any deviation from normal operating conditions. This included failure of instruments and controls, for example. Other systems would have intervened immediately or the reactor would have been shut down. With the control rods, which fall into the reactor core when required, the chain reaction of nuclear fission can be stopped immediately (see also Question 6).
5. How was the cooling system at BER II built?
The research reactor was a so-called "pool-type reactor" or "swimming pool reactor": The core of the reactor was immersed in an open pool of water. The core contained around 7 kilograms of fissile uranium 235, which was needed for nuclear fission. The research reactor consumed 2.5 kilograms of this uranium per year. By comparison, nuclear power plants consume around 1.5 metric tons of fissile uranium per year. The water around the reactor core served as three things: a neutron moderator, meaning it slows down the fast neutrons released during fission, which then sustain the chain reaction; a cooling agent; and a radiation shield.
This design had a crucial advantage: since no pressure vessel was present, the entire pool was under normal pressure. The high temperatures and pressures typical of nuclear power plants did not occur. The water in the pool heated to around 40° Celsius during operation. The heat was dissipated through heat exchangers and pumped in a circuit. No water was released to the outside.
6. What happened if the cooling failed?
There were control rods held suspended above the reactor core by electromagnets so that, if the electricity cutted out, they would fell down under their own weight into the reactor core and thereby shutted the reactor down. Once nuclear fission has stopped, the core required only one minute of active cooling. This has been ensured by the continuation of the pumps, which were also backed up by batteries. After this one minute of active cooling, the natural convection of the water (dissipation of heat through the movement of water molecules) was enough to dissipate the residual heat on its own. That means no pumps were required to dissipate the residual heat. Nevertheless, there were two backup diesel generators should there ever be an electrical power outage.
7. How was the research reactor BER II tested for safety?
The supervisory authority, the Senate Department for Health, Environment and Consumer Protection, regularly tested the safety of the research reactor with the assistance of independent experts. Furthermore, members of the European Atomic Energy Community (EURATOM) and the International Atomic Energy Agency (IAEA) verified the stocks of nuclear fuels every year. Operation of the facility was subject to the strictest safety requirements.
8. Where were the spent fuel elements of BER II stored, and how were they disposed of?
Far fewer fuel elements were spent than in a nuclear power plant. Until transporting, they were kept in a spent fuel pool, where their power droped to below 40 watts. Only then they has been disposed. The transportation of spent fuel elements took place approximately every two or three years in special thick-walled special containers. Until May 2016, the return to the United States was contractually regulated. For the later, until the end of operation (end of 2019) incurred spent fuel elements, the interim storage in Ahaus has been contractually agreed.
9. What measures did and do the operators take against terrorist attacks?
A security service seamlessly guards the entire HZB premises. Unauthorized entering of the campus cannot go unnoticed. The reactor itself and its surroundings are additionally guarded by armed security personnel who are trained and examined according to the relevant regulations. The facility is under tightest monitoring and access control.
10. What protection did the reactor building offer?
The research reactor was built according to the prevailing safety regulations. These did not call for additional building containment. The roof of the building was a brick-lined steel skeleton construction which was lined gas-tight inside with steel plates. The hall guaranted closure of all materials used and arising in operation. The hall was under slightly negative pressure, so that if a crack should ever form, air could not flow outwards, but would be drawn inwards instead.
11. What danger existed if the research reactor lies beneath the flight route of Berlin Brandenburg International airport?
Aircraft taking off from Berlin Brandenburg airport "Willy Brandt" that would fly over Wannsee close to BER II would have already reached an altitude of around 1.5 to 2 kilometres. As above any nuclear facility in Germany, there was a flight restriction above BER II to approximately 700 metres altitude. This flight restriction could therefore be respected.
12. How was the research reactor operated?
The research reactor operated in the usual mode: 3 weeks experimental operation alternated with one week maintenance. All activities at BER II took place in close consultation and after approval by the competent authorities that it seeked the opinion of independent experts.
By the end of 2019 the research reactor BER II has been operated in this mode, after which it was switched off according to the decision of the Supervisory Board.
13. Why there was renovation work at BER II?
From October 2010 to March 2012 alterations were planned many years in advance and were not related to the vetting discussed in the media (stress test). Some parts and components are routinely replaced. This is necessary as soon as a certain amount of neutrons are flowed through by it. The scientists at the HZB have also used the reconstruction period to improve the scientific instruments and to renew the neutron guides so that up to five times more neutrons reach the instruments. For science, this is a huge improvement that the BER II holds internationally competitive.
In 2014, there was a further extended shutdown, during which a sealing weld was removed, which was located in the area of the partition wall between the two halves of the reactor pool. In 2010, damaged areas were discovered in this weld seam, which have been carefully observed since then. Although it was not a safety-relevant component, it was nevertheless decided in 2013 to remove the weld seam without replacement.
During this break in operation, the new high field magnet was also finally assembled and installed at its final operating position in the neutron guide hall. The high field magnet generated a 26 Tesla strong magnetic field, which was unique in the world in connection with neutron research. Only in Berlin could samples be irradiated with neutrons under the influence of such a high magnetic field. The high field magnet was inaugurated on May 7, 2015 and handed over to the scientific community.
14. What was the meaning of the BER II stress test carried out in 2011?
Also for the BER II a so-called stress test was carried out 2011. This refers to analyzes of how a reactor behaves in certain scenarios - such as power failure, flood, earthquake. Such scenarios were considered for the research reactor BER II. This was followed by an evaluation of the competent authorities. In this, the experts of the plant have granted a high degree of robustness. The BER II has passed the by the Senate coordinated stress test successfully.
15. Why did a disaster control plan exist for BER II?
Operation of the research reactor resulted unavoidably in radioactive materials. We have taken many measures to ensure these remain contained during every operating phase of the research reactor. Nevertheless, the Berlin and Brandenburg authorities have drafted a detailed disaster control plan to allow quick, efficient and effective action to be taken even in an extreme, hypothetical situation that can only be caused by extreme impact from outside. This disaster control plan defined the tasks for all authorities and bodies involved in the event of any fear that radioactive materials may be released into the environment. HZB distributed a pamphlet every five years informing the population around the research reactor about this disaster control plan. This was regulation, as stipulated in § 53 of the Radiation Protection Ordinance (Strahlenschutzverordnung). The last information pamphlet distributed was in the end of 2014.
16. Why is the state collection centre for radioactive waste on the premises of HZB in Wannsee?
Every German federal state is under obligation by the German Atomic Energy Act to set up a state collection centre for processing and storing the radioactive waste originating in its area. The Land of Berlin has entrusted Helmholtz Zentrum Berlin to operate the state collection centre for radioactive waste, ZRA (Zentralstelle für radioaktive Abfälle). The ZRA houses low-level to intermediate-level radioactive materials that arise primarily from medicine, industry, research and teaching. No fuel elements from the research reactor are stored at the ZRA (see also question 8).
17. Was it possible to visit the research reactor?
To guarantee safe operation, the operating personnel had to be allowed to work in concentration and undisturbed. The research reactor BER II could therefore only be visited by appointment. Those wishing to take a visit had to go through security clearances by the responsible offices.
However, on Open House Day, we allowed interested visitors to take guided tours through the experimental halls around the research reactor. Our scientists and reactor experts were be there on these days to answer questions about the facility and the safety measures.
18. How long did the research reactor was still operating?
The Supervisory Board of HZB has decided that the operation of BER II and research with neutrons will be discontinued as of 31.12.2019. In recent years, the HZB has sharpened and reoriented its scientific profile: our research focuses on energy materials - with a focus on thin-film technologies. This includes the operation and further development of the photon source BESSY II in Berlin-Adlershof and the construction of new, central laboratories for the analysis and synthesis of energy materials at the Wannsee site. The objectives of the HZB 2020+ strategy process are already guidelines for our research and reach far beyond 2020.