Forschungsreaktor München II

F1

The research reactor Munich II (right) together with his disused predecessor from 1957 (left).

The neutron source Heinz Maier- Leibnitz (after the German nuclear physicist Heinz Maier- Leibnitz, also research reactor Munich II, shortly FRM II ) in Garching near Munich, with a nominal capacity of 20 MW, the most powerful German research reactor. The reactor is operated by the Technical University of Munich as a central scientific institution that is not associated faculty. The neutrons produced are mainly used for basic research in physics, chemistry, biology and materials science.

• 5.1 ultracold neutrons
• 5.2 Cold neutrons
• 5.3 Thermal neutrons
• 5.4 Hot neutrons
• 5.5 fission neutrons
• 5.6 positron source
• 5.7 irradiation facilities

History

The fundamental decision for the construction of a research reactor was prepared in 1985 plans to build a national spallation neutron source failed. 1981 has begun preliminary studies for a compact core for a new source of cash flow, from 1984, project funds are available. 1989 to 1992 was carried out the assessment, most recently by the Science Council, which recommended the construction of the FRM II with high priority.

The decision to build the FRM II was criticized by various parties for different reasons. Since issuing the first partial construction license on 29 April 1996 each permit was challenged in court; all appeals were dismissed last instance. A 2003 referendum initiated by opponents, with a small majority of Garching urged their city council to enter against the commissioning of the reactor, had no lasting effect. After Ausreizen all statutory audit options, the then Federal Environment Minister Jürgen Trittin had ( which the federal supervision of the responsible really for the enforcement of nuclear law Bavaria exercised ) on 2 May 2003, ultimately, the third partial construction permit, which consists of the operating license essentially emerge.

In addition to safety concerns ( radiation exposure or meltdown ), primarily the particular vulnerability due to the proximity ( about 10 miles) was called to Munich Airport. To counter this threat, the reactor hall was built with a 1.8 -meter-thick concrete wall and ceiling. After the decision to build had fallen, the criticism focused on the use of highly enriched and therefore, if it can be isolated from the present U3Si2 connection, weapons- uranium. The currently valid operating permit shall contain the requirement to medium term, towards a yet to be developed fuel that enables even higher by chemical uranium density lower nuclear enrichment. Currently, this particular research in uranium- molybdenum compounds.

The reactor was built by Siemens AG and cost over 400 million euros. He was struck for the first time on 2 March 2004 and reached on 24 August 2004, the rated output of 20 MW. In April 2005 he was formally handed over by Siemens at the Technical University of Munich and subsequently transferred into routine operation.

Buildings

The reactor is located on the campus of the Technical University of Munich in close proximity to the east of its predecessor, the first German research reactor FRM -I ( in operation 1957-2000 ). The under preservation distinctive dome of the FRM -I, known as " Garching Atomic Egg ", is now used partly as an extension of the neutron guide hall of FRM II The area is structurally separated by a solid fence from the rest of the campus. A originally present moat was dismantled.

Structurally, there is the FRM II from the reactor building, which houses the so-called experimental hall, a neutron guide hall and ancillary buildings with offices, workshops and laboratories. The reactor building, which has a square base with edges 42 m long and 30 m high, contains the actual nuclear reactor as well as the lying around it " experimental hall " with various facilities, which are supplied via beam with neutrons. The neutron guide hall, a 55 × 30 square meter cultivation is supplied via neutron guide with neutrons. In the future, " neutron guide hall east " further experiments are also housed in the so-called, which are supplied from the reactor through neutron guides, which are guided by specially provided openings in the outer wall of the reactor building with neutrons.

An additional building, the Industrial User Centre ( IAZ ) on the site of the FRM II is used by the radiochemical industry for the production of radiopharmaceuticals. The main tenant is currently ITM isotope technologies Munich.

Furthermore, other, mostly older buildings are on the premises, which date from the time of FRM -I or the construction phase. These house mainly offices next to a cyclotron and workshops.

Plant safety

The FRM II has, according to the operator the most extensive safety equipment for research reactors worldwide. Featuring permanent surveillance and strict controls particular emphasis was placed on an inherent safety of the reactor: due to the construction of the fuel at the plant is possible disturbances due to the laws of physics by themselves to a stable operating state. These include, inter alia, a negative temperature coefficient of reactivity for both the fuel and the coolant, and a negative local void coefficient. Also, a mixture of light and heavy water in the cooling channel or in the moderator tank, would lead to a physically related shutdown of the reactor.

These active safety features come as five magnetically suspended on springs shutdown rods of hafnium, which are shot immediately in the vicinity of the fuel irregularities in the operation and the reactor trip ( scram ). Even in case of loss of the control rod of the four five shutdown rods would be sufficient to safely shut down the reactor.

Especially after the attacks of 11 September 2001 calculations were performed again to confirm the safety of the FRM II in terms of the crash faster military aircraft, large transport aircraft and a kerosene fire. Before granting an operating license was by independent experts, a large number of possible accidents investigated, so that the safety of the plant was eventually occupied by the competent authority.

With regard to concerns about an increased radiation dose in the environment of the FRM II, measurements and calculations for the inhabited environment, an additional effective radiation dose is less than 0.01 % of exposure to natural radioactivity. And the ventilation system of the FRM II is a closed system in which the air is cleaned by the filter. Only a small fraction is emitted to the environment; this is also filtered, measured delivery and documented. About the website of the State Department for the Environment, this can be tracked online. The high safety requirements for the reactor are the reason that the TU Munich on the Garching campus as the only German university next to the University of the Bundeswehr Munich maintains an independent university fire department.

Nuclear Engineering and cooling

The reactor concept follows the basic ideas that were first implemented in 1970 at the 55 MW high flux reactor of the Institute Laue -Langevin (ILL ) in Grenoble. Innovative is at the FRM II in particular the use of a denser uranium compound. This compound was originally developed to high - to convert existing research reactors without disproportionate loss of performance of low- enriched uranium. At FRM II allows the combination of high chemical uranium density with a high nuclear enrichment a particularly compact reactor core and thus a particularly high ratio of neutron flux to thermal performance. Like all other high-performance research reactors and the FRM II is thus operated with highly enriched uranium.

In contrast to most other reactors of the FRM II comes with a single focal element must be replaced after a cycle time from the current 60 days. The fuel zone of the element is about 70 cm high and contains 8 kg of fissile 235U. The uranium is present as Uransilicid -aluminum dispersion fuel. In the fuel element 113 each 1.36 mm thick fuel plates are involute curved. Between the wrapped in an Al -Fe- Ni- alloy fuel plates wide gaps the refrigerant flows in 2.2 mm. Outwardly it less dense fuel is used than in the interior (uranium density of 1.5 g / cc instead of 3.0 g / cc) to avoid by higher neutron flux and, consequently, higher densities induced gap thermal spikes. The plates have a fuel for research reactors typical fuel rod cladding of 0.38 mm in thickness and are therefore designed so that the cleavage products remaining in the fuel. The fuel itself has a thickness of 0.60 mm.

The fuel assembly is housed in a filled with heavy water moderator tank. Heavy water is distinguished from ordinary water by a significantly lower absorption of neutrons at only slightly worse moderation behavior. The fuel element is cooled by light water. With maximum capacity of 20 MW, the cooling water is heated as from 37 ° C to a maximum of 53 ° C. The knob controls the reactor with a fuel element is present in the control rod of hafnium with beryllium follower. The control rod is connected by a magnetic coupling with the drive. This is achieved, the control rod is pressed by both the force of gravity and by the flow of cooling water in its lower end position and the reactor off it immediately.

The moderator tank is located in the 700 m³ reactor basin, which is filled with deionized water. Due to the enclosed design can thus only a small Cherenkov radiation can be observed outside of the moderator tank at the FRM II.

Neutron statistics

The arrangement described above requires that 72.5 % of the produced neutrons leave the fission zone with the light-water area and so to find the maximum of the neutron flux is not in the fuel element itself but outside it, 12 cm from the surface of the fuel in the moderator tank. In this area, forming some of the beamlines, which is not directly point to the core, but past him. Advantage of this technique is a particularly pure spectrum, which is very little disturbed by intermediate and fast neutrons. The gamma radiation in the beam is significantly reduced. The neutron flux here is about 800 trillion neutrons per second per square centimeter ( 8 × 1014 n/cm2s ). Due to the numerous obstructions in the moderator, this flow rate is reduced to an average of about 80 % of this value. To the actual locations at the end of experiment, the neutron guide the flux density is still up to 1010 N/cm2 S. These densities are comparable to those of the ILL reactor. In the flow maximum value of the moderator tank, further elements are housed: The cold source provides particularly long-wavelength neutrons, the hot spring is shorter-wavelength neutrons. One at the edge of the moderator tank mounted, extendable converter plate produces fast fission neutrons to the medical irradiation apparatus (equivalent to a temperature of about 10 billion Kelvin).

100 neutrons produced in the core, reach, as already mentioned, about 72.5 in heavy water, of which approximately 34.8%, corresponding to about 25.2% of the originally present neutrons back of D2O in the fuel zone back are reflected. These neutrons are fast or epithermal. In H2O, they are then decelerated along with the remaining 27.5 already there neutrons to thermal energies. By absorbing this go around 18.4 neutrons lost, partly in the fuel, resulting in 22.2 new fission neutrons. The remaining 34.3 neutrons generated by fission neutrons 47.4 new - the rest is lost in other absorption processes.

18.3 % of the original neutron diffuse than thermal neutrons from D2O back into the fuel zone. They lead to cleavage to 30.5 new neutrons.

A total of about 1.54 × 1018 neutrons per second are produced in the FRM II in normal operation.

Cooling

The FRM II is operated with three cooling circuits. The primary system uses the pool water and a flow rate of about 1000 registered m³ / h, or about 280 l / s, corresponding to a speed of 17 m / s in the 2.2 mm wide cooling channel between the fuel plates. The secondary system is a closed water circuit. The tertiary system consists of wet cooling units through which the heat is dissipated to the atmosphere. In addition to the 20 MW thermal power of the core are about 4 MW of operating components dissipate.

In the primary cooling circuit of four pumps provide the necessary throughput, of which two pumps are combined to a string. Already three pumps are sufficient to cool the core at full rated output sufficient. In the case of a reactor shutdown three RHR pumps are switched on, the pumps pool water through the core. These pumps are turned off again three hours after the shutdown, it is sufficient for the natural convection for removing the residual heat of the core. Already one of these pumps is sufficient for safe decay heat removal. In addition, the pumps are connected to an emergency diesel generator, so that even a complete power failure can be bridged. Even in the hypothetical scenario of a failure of all three pumps of the core would not melt because of insufficient residual heat would be available. In addition, the system is designed so that in the event of a failure of all the pumps, the pool water could absorb a complete decay heat of the fuel, without boiling.

In the secondary circuit in addition to the heat of the primary cooling circuit and the other operating components of the waste heat is injected.

Use

The FRM II is optimized for neutron scattering experiments on radiant tubes and neutron guides. In addition, there are facilities for material irradiations, medical radiation and nuclear physics experiments.

The experimental facilities are not operated from the FRM II itself, but by various departments of the Technical University Munich and other universities and research institutions that maintain for this purpose, field offices on the site of the FRM II. Represented institutions are the Max Planck Society, the Leibniz Association and the Helmholtz Society. The latter represents the largest field office with the Jülich Centre for Neutron Science at the Research Centre Jülich with more than 30 employees. About 2 /3 of the measuring time of each instrument are visiting scientists from around the world. A total of 30 % of the capacity for commercial use are provided.

The instruments at the FRM II are largely spectrometer and diffractometer and cover a wide range of applications, both in terms of research as well as industrial use:

With respect to the pure element and isotope analysis exists alongside the classical neutron activation analysis, a tool for prompting gamma activation analysis ( PGAA ). Conventional irradiation facilities are available inside the moderator tank with different neutron fluxes and spectra are available. They are a prerequisite for neutron activation analysis, are also used for the production of radioactive sources, for example for medical treatment in the form of radiopharmaceuticals. Also density measurements are possible. The largest radiation means is that is converted to silicon doping in silicon by neutron capture and subsequent beta decay in phosphorus.

Two radiography and tomography systems use the high penetration capability of neutrons through matter for the screening of technical static and moving objects. Both 2D images can ( radiography ) and complete three-dimensional reconstructions of the internal structure are made are made. In combination with the prompting gamma Aktivierunsanalyse this internal structure can also be broken down by isotopes.

Another irradiation device is medical irradiation facility is irradiated in the fast fission neutrons using tumor tissue. In this neutron therapy is not the better-known boron neutron capture therapy, whose effect is based on the induced by neutron capture in boron decay, but a form of therapy, whose effect is based on the recoil of light scattered by hydrogen protons neutrons.

In materials science and catalysis there are ways to structure analysis and structure determination. In addition can be performed at multi-component alloys with the available instruments at the FRM II phase analyzes. Stresses and textures can be analyzed with or without load. This is used for example in the residual stress analysis in manufacturing technology, component manufacturing and the development of materials and texture determination after rolling and forming processes. With regard to the life sciences there are ways to state determination of organic compounds and to study the dynamics of complex molecules. Even structures and bonds in organic compounds ( for single crystals ) can be analyzed.

The positron source opens up another range of applications, mainly in the surface and defect analysis. For example, a near-surface element analysis is performed or the surface morphology can be determined. Through an analysis of defects lattice defects can be determined in crystals.

Ultra-cold neutrons

At FRM II of the construction of a plant for the production of ultracold neutrons ( UCN ) is planned. Frozen deuterium () neutrons are cooled down up to an energy of about 250 neV. It will be used primarily to study fundamental properties of the neutron.

Cold neutrons

Approximately 50% of the experiments at the FRM II using cold neutrons, neutrons with an average energy of less than 5 meV. The cold source is about 16 l of liquid, about 25 K cold deuterium -filled additional moderator, who is placed in the main water tank of the FRM II. To compensate for the heating by heat conduction, gamma radiation and neutron bursts, the cold source has its own cooling circuit. Deuterium area is covered with a protective gas to prevent the contact between deuterium and air also in case of malfunctions. In the cold source, the flux density of cold neutrons is about 9.1 × 1013 n/cm2s. The following experiments are working with cold neutrons:

Thermal neutrons

Thermal neutrons have an average energy of about 25 meV, corresponding to the temperature of the moderator.

Hot neutron

The hot neutrons originate from the hot source ( ~ 2400 ° C, Moderator: 14 kg graphite). They are mainly used for structural studies of condensed matter. These neutrons have an energy between 0.1 eV and 1 eV. The Hot Spring is located in the moderator tank near the river maximum. The heating of the graphite is carried out by gamma radiation, less by neutrons from the reactor core. The source is isolated from the environment by a double-walled Zircaloy - container with embedded insulating felt, so that the temperature on the outside is only about 100 ° C. The final temperature results from the thermal balance between heating and heat loss to the environment.

Fission neutrons

The beam converter system (CS ) for producing the fission neutrons consists of two together 498 g containing plates by thermal neutron capture and subsequent cleavage fast fission neutrons (energy: 0.1 MeV - 10 MeV) 235U produce. The plates are located at the edge of the moderator tank and have a rated power of 80 kW. They can be pulled out of the neutron field as needed to an unnecessary erosion ( loss of cleavage) prevent the uranium.

Positron source

The positron source NEPOMUC (Neutron Induced Positron Source Munich ) is the world's most powerful source of monoenergetic positrons (as of 3/2008). It produces about 9 × 108 moderated positrons per second. Generating positron thermal neutrons are captured in cadmium, whereby hard gamma radiation is produced up to the maximum of 9 MeV energy. By absorption of gamma radiation in the platinum film ( antimatter ) and electrons (matter ) are generated by positron pair production. In platinum positrons primarily generated are moderated to ambient temperature and can be emitted into vacuum by diffusion to the film surface. The thus moderated positrons are accelerated to an energy of 1 keV and guided magnetically. About a beam switch of the monoenergetic positron beam passes to various experiments: The positron source is operated by the Technical University of Munich itself.

Irradiation facilities

The above-mentioned experiments, the irradiation facilities are inside the moderator tank for the production of radioactive isotopes for neutron activation analysis (NAA ) or for neutron transmutation doping of silicon. The thus-obtained doped silicon is doped very homogeneous.

Reportable incidents and other incidents

At FRM II, there have been 15 reportable events, including one in the category " Hasten " and 14 in the category " normal ". Meantime, the events are distributed as follows: 2004: 1; 2005: 1; 2006: 3; 2007: 1; 2008: 1; 2009: 5; 2010: 2; 2011: 1 to accidents involving the release of radioactivity so far did not come all the events have been grouped into the INES 0 category.

Further non- reportable events, which provoked a national media attention:

• In 2006, iron oxide deposits of a few nanometers thickness (in the media as " rust ") is found in the reactor pool, the causes could not previously be answered and which were not resolved (as of January 2012). However, several independent reports concluded from an impairment of the safety of the reactor, so that no further action was taken.
• In November 2012, the reactor was an unscheduled shut down because one of the limit for the emission of the radioactive isotope C14 exceeded threatened. Contrary to reports stating otherwise, the limit that is only one-fifth as permitted under the Radiation Protection Ordinance exemption limit for license- free handling at the FRM II was not achieved. After clarification of the cause, a modified purification procedure of heavy water, the reactor was re- started up and operated until the end of the year without further incident as planned.