IAEA-SM-360/39

THE NEW GERMAN SOURCE FRM II

W. PETRY Physik Department E13, Technische Universität München, , Germany

Abstract

The new of the Technische Universität München will provide the German needs of 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 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 . 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, 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 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.

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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.

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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.

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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. A particular example of future industrial applications is the silicon doping facilities. Si single crystals up to 8 inch diameter and 50 cm in length can be irradiated in a particular homogeneous field of thermal neutrons, thereby transmuting 31Si to 31P. One facility with an annual capacity of 10 tons P doped Si will be operational with the nuclear start up. Optionally a second facility can be realized at a later stage.

3.2. Experiments in the reactor hall

The instrumentation in the reactor hall benefits from the large opening angles of the beam tubes (>11°) and from the compact biological shielding (radius 4 m). This allows to place several instruments at one beam tube and by means of suitable neutron optics, it should be possible to build some of the most intense instruments. Further, the entrance windows of all beam tubes have been optimized for each particular instrument.

Currently the following neutron scattering instruments – all aiming at a wide spread use in material science – are under construction:

– Cold triple axis spectrometer: The instrument is placed in front of a beam aperture which faces the cold source. Its large entrance window (135 × 80 mm²) with horizontal as well as vertical focusing Bragg monochromators and analysers will provide a particular intense cold triple axis spectrometer with the possibility of using polarized neutrons and strong magnetic fields at the sample position. – Materials diffractometer: This instrument is optimized for the determination of internal stresses and textures. It is fed by a thermal beam and allows by means of a Bragg monochromator a continuous change of the incoming wavelength. This allows measurements under the optimum scattering angle of 90°. The high intensity of the instrument and its precise beam collimators will make it possible to measure internal stresses in large samples at a volume smaller than 1 mm³. This instrument serves mainly the applied research and industrial needs, (see Fig. 3). – Thermal triple axis spectrometer: Similar to the cold triple axis spectrometer this instrument uses a large entrance window and monochromatic vertical and horizontal focusing. A multidetector bank will further optimise the use of the neutrons. Also this instrument can be used in a polarised version and certainly belongs to the most intense instruments of its kind (see Fig. 4). – Single crystal diffractometer for thermal neutrons: is the most used neutron technique in Europe and more than half of all neutron scattering experiments are devoted to diffraction. This single crystal diffractometer uses thermal neutrons in the wavelength range of 0.8-1.5Å. There will be an innovative use of neutron sensitive image plates as a two dimensional multidetector. Typical applications are structure determinations of anorganic single crystals.

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FIG. 3. Scheme of the materials diffractometer.

FIG. 4. Scheme of the thermal triple axis spectrometer.

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– Powder diffractometer: This instrument likewise uses thermal neutrons and focuses on the determination of magnetic structures, localisation of light elements and the determination of thermal movements. The instrument will also use a multidetector. – Single crystal diffractometer for hot neutrons: This diffractometer is fed by the hot source and uses particular short wavelengths in the range 0.3-1.1Å. It aims on the determination of large elementary cells (up to 10.000 Å3) and the position of light elements. The very short wavelength reduces the absorption probability of neutrons and therefore allows the structure determination in crystals containing strongly absorbing materials.

Further, the reactor hall houses three non-scattering instruments, two of them solely for use in material science:

– Positron source: One of the two inclined beam tubes is used for the production of thermalized positrons. By means of a strong absorber at the entrance aperture of the beam tube (Cd) an intense g-radiation is produced. In a Pt converter this g-radiation converts into positron-electron pairs. The positrons thermalize in the Pt converter, are extracted by electromagnetic guide fields from the beam tube to a remoderator outside the biological shielding and then finally led to the experiments. This will be the most intense source of thermalized positrons in the world. Typical applications are the study of defects in metallic and non-metallic samples. – Neutron radiography and tomography: Neutrons penetrate all kind of materials which are opaque for X-rays or light. Combining neutron radiographic with tomographic techniques enables the three dimensional visualisation of complex metallic tools or machinery. Typical examples are: Glue in composite materials, cooling channels in turbine plates, details of engines. One instrument will use cold neutrons of variable wavelengths, while a second instrument will use the fast neutrons of the converter. This extreme change in wavelength results in a rather different absorption cross section of the same elements. Applying both techniques to the same sample will increase the contrast. Crucial for the progress made in this field was the development of very precise two dimensional neutron detectors. The tomography will be mainly used for non destructive materials testing. – Cancer therapy: Similar to other high energy particles, fission neutrons have a well defined stopping power in biological materials. This leads to a huge energy deposit at a well defined penetration depth. For MeV neutrons the maximum energy deposit is limited to some 10 mm near the skin surface. For several years this therapy has been used very successfully for the treatment of cancer near the skin surface at the old FRM. The converter at the FRM II now allows building up an irradiation facility for cancer therapy under clinical conditions, far better defined irradiation conditions and much shorter irradiation times for the patients.

Nuclear and fundamental physics will be served by three projects:

– Cold neutron guide for particle physics: A super mirror guides the cold neutrons to a measuring site of particularly low background. The intense cold neutron beam will be used for experiments devoted to fundamental questions like symmetries in the standard model of particle physics or determination of coupling constants for cosmology and astrophysics. – Ultra cold source of neutrons: Neutrons with a wavelength beyond 100Å are particularly suited for the study of optics with particle beams and for the determination of fundamental constants. Currently the option of an ultra cold neutron (UCN) source placed in a beam tube facing the cold source is being studied. This UCN source would consist of a thin target of solid deuterium. In the pulsed operating mode this solid D2 source is expected to produce UCN densities of about 5x104 n/cm³. This density is considerably higher than that of the world’s best source at the Institute Laue Langevin in Grenoble. The large grain factor will enable new measurements of the elementary properties of the free neutron, such as the electric dipole moment or the neutron lifetime, with strongly improved precision.

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FIG. 5. Floor plan for the instrumentation in the reactor hall and the neutron guide hall. To the left it is indicated how the old FRM will serve as an extension of the neutron guide hall. The reactor and neutron guide hall are 40 m and 80 m wide, respectively.

235 – Production and acceleration of fission fragments: Fission of U produces radio isotopes of medium mass with a considerable excess of neutrons. They are best suited as projectiles on heavy targets in order to produce super heavy nuclei via cold fusion. The “through-going” beam tube will be prepared so that it will house the fission source. Currently it is being studied how best to separate and accelerate the radioisotopes.

3.3. Experiments in the neutron guide hall

The neutron guide hall is fed by six neutron guides, each of which is split into several smaller guides. This allows the positioning of most of the scattering instruments at end positions. All guides are curved to prevent direct sight of the cold source. According to the needs of the different instruments neutron guides are coated with 58Ni or super mirrors. Figure 5 shows the floor plan for the neutron guide hall.

– Neutron resonance spin echo spectrometer: spectroscopy with cold neutrons is the method used to measure slow relaxations in condensed matter on an atomic length scale. Typical examples are reptation in polymer melts, the dynamics of the liquid- glass transition or critical magnetic fluctuations. At the FRM II a new spin echo method will be used, the so-called resonance spin echo. This method is very versatile. It allows for easy measurement of variable scattering angles and for measuring separately coherent and incoherent scattering. A polarising neutron guide will also be used. – Backscattering spectrometer: Also this instrument aims at the detection of slow motions, e.g. diffusion of hydrogen in metals, molecular rotations, tunnelling processes. This instrument will optimize the principle of phase space shifting, i.e. concentrate highest flux with DE/E~10-4 at the sample position. – Time of flight spectrometer: Of all the neutron spectrometers, the time of flight spectrometer has the best signal to noise ratio. Its resolution function is better shaped than any of the other inelastic instrumentation. At , disc choppers made of carbon fibres and magnetic bearing will be used. This allows a higher speed of the choppers than ever before

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realised, thereby increasing the energy resolution. Focusing guides between the choppers concentrate the intensity on a relatively small sample volume of roughly 3 × 2 cm2 - see Fig. 6.

FIG. 6. Scheme of the time-of-flight spectrometer.

– Small angle camera: It is foreseen to move one of two small angle cameras installed at the DIDO reactor at Jülich to the FRM II. This camera has a variable detector-to-sample distance. The maximum distance of the two dimensional detector is 20 m. This gives access to a Q-range from 2 × 10-3 – 0.2Å-1. – Reflectometer for soft matter: Although the FRM II is a continuous source, this reflectometer will work in a time-of-flight modus. It has a horizontal sample geometry for soft matter studies. Further polarisation of the in-coming neutrons and polarisation analysis of the scattered neutrons will be possible. For the first time such a neutron reflectometer will be combined with a small angle detector for the detection of evanescent small angle scattering. – Measuring place for optics with very long wavelength neutrons: A neutron switch prepares a reflected neutron beam with neutrons in a wavelength range from 20-30Å. This beam serves mainly for experiments in the field of neutron optics.

4. PERSPECTIVES

With the two exceptions of the ultra cold neutron source and the fission product accelerator all these instruments are currently under construction. All irradiation facilities will be built by the staff of the FRM II, however most of the scattering instruments are being built by user groups spread throughout Germany. Further instrumentation at the FRM II are planned by the Max Planck Gesellschaft (hard matter reflectometer, neutron resonance spin echo spectrometer) and by the Helmholtz Gesellschaft. Those engaged in the construction and maintenance of the instruments will have privileged access to beam time. Most of the beam time will be distributed subsequent to a peer review of the experimental proposals. As a university facility a considerable fraction of the beam time

8 IAEA-SM-360/39 will be dedicated to the training of students. As a national facility the FRM II will also be the basis for international co-operators of the German user community.

The FRM II will be one of the strongest neutron sources in the beginning of the next century. Applied research and industrial needs will play an important part in the utilisation of this source.

BIBLIOGRAPHY

BÖNING K., DIDIER H.J., HENNINGS U., “Das bauliche Konzept der neuen Neutronenquelle FRM II”, Tagungsbericht, Seite 431-434, Jahrestugung kerntechnkin 1993, Köln. BÖNING K., AXMANN A., BLOMBACH J., “Status and Safety Concept of the New German Research Reactor FRM II”, Proceedings of the International Topical Meeting on Advanced Reactors Safety, Vol. II, Orlando, Florida, 1997. Several contributions in: Jahrestagung Kerntechnik 1999, Fachsitzung: Die neue Forschungsneutronenquelle FRM II.

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