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The New German Research Reactor FRM-11

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The New German Research Reactor FRM-11 CH04AO002 The New German Research Reactor FRM-11 A. Axrnann, K. 1136ning and M. Rottmann Technische Universifilit Mfinchen ZBE FRM-11 Bau, Reaktorstation, D-85747 Garching ABSTRACT A new German high-flux research reactor is presently being built in Garching by the Technical University of Munich. The new reactor, called FRM-11, shall replace the exi- sting "Forschungsreaktor MOnchen" FRM which has been operating very successfully for about 40 years now. The new reactor has been optimized primarily with respect to beam tube applications of slow neutrons, but will also allow to irradiate samples with thermal neutrons. So the FRM-11 has been designed to provide a high flux of thermal neutrons in a large volume outside of the reactor core, where the neutron spectrum can be locally modified by using special spectrum shifters. The goal was further to obtain this high flux at a reactor power being as low as possible since this represents the best choice because of lowest background radiation for the experiments, lowest nuclear risk potential, lowest costs and superior inherent safety features. The essential design feature of the FRM-11 is a very compact reactor core consisting of a single fuel element only, which is cooled by light water and surrounded by a large heavy water moderator tank. The cylindrical fuel element is made up of two concentric tubes, the outer one having a diameter of about 24 cm. A total of 1 1 3 fuel plates is wel- ded between the two tubes. They are all indentical and curved to involute shape so that the cooling channels between them have a constant width of 2.2 mm. Each of the pla- tes is 1. 36 mm thick and structured as a three layers' sandwich with two cladding layers at the surfaces and the fuel zone in between. Since it is not easy at all to provide suffi- cient excess reactivity for such a small core, the new high-density uranium silicide fuel has to be used in combination with highly enriched uranium. Fuel density grading and a ring of burnable poison are provided to flatten the power density profile in the fuel ele- ment. The fuel element is placed in a vertical core channel tube, which separates the light water of the primary cooling circuit (down flow) from the surrounding heavy water moderator tank which has both 250 cm diameter and height. More than 50% of the fast fission neutrons immediately leak out of the small core into the large heavy water moderator tank where they slow down and thermalize to build up a high flux of thermal neutrons. The design values of the FRM-11 are: a reactor power of 20 MW, an unperturbed thermal flux maximum of 8'1 014 WCM2 S in the moderator tank, and a cycle length of about 50 full power days. The average power density in the active zone of the core is about 1. 1 MW/liter. The ratio of thermal flux (outside of the core) to power is highest of all reactors in the world. The moderator tank is placed in the center of a big reactor pool filled with light water. 10 horizontal beam tubes penetrate the concrete of the biological shield of this pool and lead the neutrons to the scattering instruments in the experimental hall of the reac- tor building and in an adjacent neutron guide hall. Some of the beam tube noses are in contact with "spectrum shifters", i.e. with a large "cold source" filled with liquid deuteri- um or with a "hot source" containing graphite of more than 20000 C temperature, but there is also an uranium converter to produce fission neutrons for one beam tube. Further there are two inclined beam tubes and one vertical guide tube. Several vertical channels allow to insert samples to get irradiated in a high thermal neutron flux. The fields of applications of this "multipurpose neutron source" range from fundamental re- search in physics, biophysics and chemistry to applied research (e.g. material scien- ces), medical research and treatments, up to environmental applications (e.g. trace impurities detection by activation analysis) and industrial utilization (as e.g. silicon do- ping and radioisotopes production). The FRM-I I reactor will be controlled by a single hafnium control rod which moves wit- hin the inner tube of the fuel element; at its lower end it is connected with a beryllium follower. It can be decoupled from the control rod drive mechanism to fall down and act as a fast shutdown system. A second, redundant and diverse fast shutdown system is provided by five hafnium shutdown rods in the moderator tank which are fully withdrawn during reactor operation, however. Four of the five rods would suffice to shut down the reactor even if the control rod would totally move out of a fresh fuel element. Additional- ly the "compact core" reactor is characterized by pronounced inherent safety features which would, e.g., make the reactor subcritical under all postulated severe accident conditions. The primary cooling circuit is a virtually closed loop which however is connected with the reactor pool through a strainer located underneath the core; in this way the large pool water reservoir is made use of as a pressurizer of the primary circuit and for the core cooling after shutdown.There are four primary pumps which are all equipped with flywheels and with check valves against reverse flow. They are mounted - together with the heat exchangers to the secondary circuit - in a water-tight "primary cell" of small volume. After reactor shutdown core cooling is still provided by the forced flow of the primary pumps. Three independent battery buffered shutdown pumps are installed, one of which would be sufficient to maintain the forced flow through the fuel element if the primary pumps were not available. The shutdown-pumps suck water from the pool, feed this water via check valves into the collector of the primary circuit and, after having passed the fuel element, back info the pool. Three hours after shutdown the fuel ele- ment can be cooled by natural convection: the pumps are shut ff, with the decreasing pressure in the collector two natural circulation flaps open automatically and the water flow through the fuel element reverses. If the external heat sink were not available, the decay heat could be stored completely in the pool water, no recooling of which would be needed. The FRM-11 reactor building is 30 m high and has the form of a 40 x 40 M2 square in its lower and of an octagon in its upper part. It provides full protection against earthquakes and - a new feature for research reactors - against an air plane crash. For that the ou- ter walls and the roof of the building consist of reinforced concrete of 1.8 m thickness, and the reactor pool is decoupled from the outer building structure such that the pool water would not be lost. The confinement of the reactor building also represents the ultimate barrier against an uncontrolled release of radioactive fission products to the environment in case of an accident. On one side the reactor building is connected with a neutron guide hall which is about 60 x 46 M2 wide and 1 1 m high. Finally, the project status is as follows. While the conceptual design work for the FRM- 11was started by the Technical University of Munich JUM) as early as 1980, the safety analysis report was completed and the application for nuclear licensing of the FRM-11 was submitted to the licensing authority in February 1993. All this has been done by the TUM together with the Siemens company which was nominated to become the ge- neral contractor in June 1994. In April 1996 the project obtained the first partial nuclear licence which covered the general safety concept acceptance, the site opening and the construction of the reactor building. mmediately after that the construction work has been started. A dummy fuel element, which is now subject to hydraulic testing, has be- en fabricated by the company CERCA, France. The final (i.e. third) partial nuclear li- cence is expected for the year 2001 which will be followed by the nuclear start-up, a fifty days full power test run and, finally, routine operation of the research reactor for the benefit of the user community..
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