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The New German Neutron Source Frm Ii IAEA-SM-360/39 THE NEW GERMAN NEUTRON SOURCE FRM II W. PETRY Physik Department E13, Technische Universität München, Garching, Germany Abstract The new research reactor of the Technische Universität München will provide the German needs of neutrons for the beginning of the next century. High intensities of thermal neutrons are provided by a particular densely packed core of highly enriched uranium and are extracted by 12 beam apertures. The low thermal power of 20 MW allows the positioning of a D2 cold source at a maximum of the thermal neutron flux in the D2O moderator. This cold source is seen by 3 beam tubes, one of which feeds a neutron guide hall. A graphite hot source shifts the thermal spectrum to shorter wavelengths with a maximum of the Maxwellian distribution at l~0.5Å. At the outer limit of the moderator a Uranium converter is installed to provide fission neutrons of MeV energy for use in cancer therapy and fast neutron tomography. Pneumatic and hydraulic rabbit irradiation systems, a silicon transmutation facility, irradiation positions in the control rod and outside the moderator tank serve the needs for radio nuclei, radiopharmaceutical, neutron activation analysis and other industrial applications. The instrumentation, worked out largely by the German user community, is currently under construction and aims on a multidisciplinary use for basic and applied research as well as industrial needs. Both the introduction of new techniques and the considerable progress in the performance of standard techniques is envisaged. Being a national neutron source the FRM II will also be the basis for international co-operation of the German neutron user community. 1. INTRODUCTION In October 1957 after only 11 months of construction the Forschungsreaktor München (FRM) went into operation. It was built by the Technische Universität München and was Germany’s first nuclear facility. More than 40 years later and in close proximity to the FRM the Technische Universität München is now building a new neutron source, the FRM II. Its construction is in progress and nuclear operation is foreseen for the year 2001. As a modern high flux source located in the centre of the university campus, it will serve the needs of education, research and industrial applications. The instrumentation conceived largely by user groups wide spread throughout Germany, is currently under construction and aims to provide innovative techniques and neutron beams of the highest intensity at the sample position. 2. THE CONTINUOUS HIGH FLUX NEUTRON SOURCE FRM II To provide highest intensity of thermal neutrons at the beam apertures with a minimum of nuclear inventory, the FRM II has one compact core, which will only be operated at 20 MW thermal power. The compact core has a diameter of 24 cm with a height of 70 cm and consists of 113 curved fuel plates, each of which is 1.36 mm thick (meat thickness 0.6 mm). The meat of the plates consists 235 of highly enriched U of 93% (HEU) in the chemical form of U3Si2 with a maximum density of r = 3 g/cm³. The compactness of the core guarantees a high leakage of fast neutrons into the D2O moderator. The back flow of the thermal neutrons into the core is essential for maintaining the chain reaction but is also responsible for a peak in thermal power at the outer diameter of the fuel element. For this purpose the outer one third of the fuel element’s diameter contains a HEU density of only r = 1.5 g/cm³. Cooling of the fuel element itself is provided by a light water circuit. At a distance of roughly 12 cm from the outer side of the fuel element, there is a maximum build up of the thermal neutron density (unperturbed maximum thermal neutron flux ³8 × 1014n/cm²s). Including the loss of reactivity due to the beam tubes and irradiation facilities, the calculated lifetime of one fuel element is 52 (±1.5) days. The use of five fuel elements per year is foreseen. 1 IAEA-SM-360/39 Neutron Fluxes in the FRM-II 1015 thermal beam tube (1 cm D 2 O in front of SR-5) 1014 Cold Source /s/Å] 13 2 10 Hot Source 1012 Neutron Flux [n/cm 1011 1010 0.1 1 10 100 Wave Length [Å] FIG. 1. Spectral fluxes for the different sources at a reference point at the surface of the sources. For thermal neutrons a point in front of a typical beam window has been taken. The values are representative but do not represent the geometry of the particular beam tubes. Due to the moderate thermal power of 20 MW the cold source (cylindrical shape, re-entrance hole, about 30 l liquid D2) can be located in the maximum of the thermal flux, thus providing a flux of cold neutrons in the same order as the cold sources of the HFR at the Institute Laue Langevin. The cold source is slightly under-moderated, thereby extending its usable wavelength range to shorter wavelengths. Further, by placing a graphite cylinder in the center of the thermal flux, the cylinder heats up to 2400°C and will serve as a hot source with a flux maximum at l~0.5Å. A comparison of the spectral neutron fluxes from the cold and hot source and from the thermal moderator is given in Fig. 1. Fission neutrons of some MeV energy for the needs of cancer therapy and tomography are produced by means of a HEU converter plate located at the outer diameter of the moderator. One side of this converter plate faces the D20 moderator, from which thermal neutrons diffuse into the converter and induce fission reactions. The other side of the converter faces the beam tube, thereby allowing the extraction of the fast MeV neutrons. Altogether 12 beam tubes face the thermal moderator (see Fig. 2), three of which face the cold source. The largest of these three beam tubes feeds six neutron guides, which serve the neutron guide hall. The hot source and the converter are faced by one beam tube each. All the remaining 7 beam tubes face the thermal moderator. One of these transverses the whole D2O moderator and concrete shielding, thereby giving access to both sides of the biological shielding. This “through-going” beam tube is designed for the production and extraction of fission products. Two further beam tubes – not shown in Fig. 2 – are inclined. All beam tubes are placed tangential to the core, thereby facing the volume of maximum thermalized neutron flux and thus prohibiting direct sight to the hot neutrons which leak out of the core into the moderator. This is the most important measure in reducing the background at the position of the instruments. Curved neutron guides, which avoid the direct sight of the source, and advanced composite shielding materials are other important measures used in reducing the background. 2 IAEA-SM-360/39 FIG. 2. Horizontal cross section of the reactor at the height of the fuel element. The compact core with one control rod in its centre and five safety rods near the core are visible. Both hot and cold sources are located in the maximum of thermal flux. !0 beam tubes are shown, one is “through- going”, a particularly large beam tube feeding the neutron guide hall, and another beam tube faces the Uranium converter. The heavy water moderator is surrounded by light water and outer concrete shielding. 3. INSTRUMENTATION AT THE FRM II As the main German source providing neutrons for the beginning of the next century the FRM II is designed for a broad spectrum of applications. There are different ways of classifying these applications. One scheme could be the irradiation and beam tube facilities, another possibility would be a classification according to basic research, applied research, and industrial production. However, most instruments serve several types of applications. Therefore we shall adopt a more pragmatic procedure: – Irradiation facilities near to the core, – Experiments in the reactor hall, – Experiments in the neutron guide hall. Subsequently, there is a short description of the first generation of this instrumentation. The reader should be aware that not all instruments listed will be fully operational at the beginning of nuclear operation, deliberately not all beam lines and neutron guides will be booked out from the beginning; and finally, being a research reactor many of the instruments are subject to continuous changes and developments. 3 IAEA-SM-360/39 3.1. Irradiation facilities near to the core The wide spread use of radio isotopes demands a great variety in its production. Several irradiation facilities positioned in and outside the moderator tank with fluxes between 5 × 102 n/cm²s and 4 × 1014 n/cm²s with possible irradiation times from seconds to many days and sample sizes from mg to Si single crystals of 8 inch diameter and a length of 50 cm are foreseen. A rapid pneumatic irradiation system with a transport time from the irradiation to the measuring site of the order of 300 ms allows the spectroscopy of very short lived activities like 20F or 28Al. A pneumatic rabbit system enables the transport of irradiated samples within 5 to 10 seconds to the measuring site. Longer irradiation times over weeks will be realised by means of a hydraulic rabbit system. Due to its high flux of fast neutrons the position of the central control rod in the center of the fuel element is ideally suited for the production of the positron emitter 58Co. Additional positions for the irradiation of large samples are foreseen outside the moderator tank. Typical examples would be the neutron activation analysis of large liquid volumes or particular pure scintillator materials for the detection of neutrino oscillations.
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