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Physics and Technology of Spallation Neutron Sources CH9900035 to o CO a PAUL SCHERRER INSTITUT PSI Bericht Nr. 98-06 m August 1998 53 ISSN 1019-0643 0. Solid State Research at Large Facilities Physics and Technology of Spallation Neutron Sources G. S. Bauer Paul Scherrer Institut CH - 5232 Villigen PSI Telefon 056 310 21 11 30-49 Telefax 056 310 21 99 PSI-Bericht Nr. 98-06 Physics and Technology of Spallation Neutron Sources G.S. Bauer Spallation Neutron Source Division Paul Scherrer Institut CH-5232 Villigen PSI, Switzerland [email protected] Lecture notes of a course given at the 1998 Frederic Jolliot Summer School in Cadarache, France, August 16-27, 1998 intended for publication, in revised form, in a Special Issue of Nuclear Instruments and Methods A. Paul Scherrer Institut Solid State Research at Large Facilities August 1998 Physics and Technology of Spallation Neutron Sources1 G.S. Bauer Spallation Neutron Source Division Paul Scherrer Institut CH-5232 Villigen PSI, Switzerland [email protected] Abstract Next to fission and fusion, spallation is an efficient process for releasing neutrons from nuclei. Unlike the other two reactions, it is an endothermal process and can, therefore, not be used per se in energy generation. In order to sustain a spallation reaction, an energetic beam of particles, most commonly protons, must be supplied onto a heavy target. Spallation can, however, play an important role as a source of neutrons whose flux can be easily controlled via the driving beam. Up to a few GeV of energy, the neutron production is roughly proportional to the beam power. Although sophisticated Monte Carlo codes exist to compute all aspects of a spallation facility, many features can be understood on the basis of simple physics arguments. Technically a spallation facility is very demanding, not only because a reliable and economic accelerator of high power is needed to drive the reaction, but also, and in particular, because high levels of radiation and heat are generated in the target which are difficult to cope with. Radiation effects in a spallation environment are different from those commonly encountered in a reactor and are probably even more temperature dependent than the latter because of the high gas production rate. A commonly favored solution is the use of molten heavy metal targets. While radiation damage is not a problem in this case, except for the container, a number of other issues are discussed. Spallation, on the other hand, can 1. Introduction generate high neutron densities and hard spectra, but has its own technological Using medium energy particle bombardment of issues. In this context the experience that heavy metals to release neutrons is not a new can be gained from research spallation idea. In fact, before large deposits of Uranium sources may become very valuable. are were discovered in the US, a project called MTA was well advanced, which was designed in the field of research neutron sources to provide enough neutrons to convert 238U into reactors have more or less reached their Plutonium for the nuclear weapons program. limits of thermal flux that can be The project was given up when breeding in generated with existing fuel technology fission reactors became a more economic due to power density problems in the alternative. The recent revival of interest in core. A perspective for progress is seen in spallation-based neutron sources has the spallation reaction for two reasons: essentially two reasons: - the lower heat release per neutron and • fission reactor technology is essentially the increased flexibility in materials neutron-poor, i.e. only few excess neutrons choices, which might help to push time can be made available either for fertile- average fluxes to higher values than in fissile conversion or for nuclear fission reactors transmutation towards waste management. Furthermore, fast reactors, i.e. those with a - the possibility of imposing a time hard neutron spectrum, or desired for many structure on the neutron flux, which can advanced technologies are difficult, because lead to several orders of magnitude only metallic coolants can be used. higher flux in the pulse than in the time average. Methods are being developed 1 Lecture notes of a course given at the 1998 Frederic Jolliot Summer School in Cadarache, France; intended for publication, in revised form, in a Special Issue of Nuclear Instruments and Methods A. U:\BQ30\NIM-a\PTS_rew_1h.doc, 16.09.98 in the utilization of pulsed sources which essentially make use of the peak flux during most of the period between pulses by exploiting the energy-dependent flight time of neutrons from the moderator to detectors located several tens of meters away. The principal design of a research spallation neutron source for neutron scattering is shown in Fig. 1-1. A medium energy proton beam is injected into a target in which fast neutron are generated. In order to provide neutrons useful EShielding; for scattering experiments, they must be slowed down to the meV range by moderators Figure 1-1: Principal arrangement of target placed near the target. Good neutronic moderators and reflector in a spallation coupling between these moderators and the neutron source for neutron scattering target is, of course, an important design goal. In order to increase this coupling, the moderators are surrounded by a reflector, whose task is to scatter as many of the Neutrons as a tool Neutrons as a medium neutrons that would otherwise escape the moderators back into them. Neutrons to be Research Energy supply used in the scattering experiments are transported outwards through the shielding by J self sustaining beam tubes viewing the moderators in a way Research chain reaction Nuclear power reactors stations that minimizes high energy and fast neutron as new: Electricity well as gamma ray contamination in the beam. Basic- Particle- Energy-supply Depending on the type of neutron source, the knowledge accelerators Natl. resources moderators will be laid out differently in order to Understanding of management achieve the kind of optimization desired by the nature w Protons Treatment of- Technological users: For maximum time average flux the life 'SpallationN nuclear waste development time of the neutrons must be long, meaning driven reaction ) that neutron absorption in the moderators and Neutrons NeutrononsSs ^ T the reflector (and in the target as well) should Spallation- Driven reactors, be minimized. For this reason heavy water is neutron sources Spallation facilities commonly used as moderator and reflector Ell (ESS) Common technologies CEA alike. For very short pulses the moderators will USA (SNS) (accelerators, targets) Japan Japan (JHF) beam tosses, activation Sweden be surrounded by absorbing material to avoid CERN Japan (JNSC) heat removal, radiation damage slow neutron return from the reflector, and, for CH (SINQ) USA the intermediate case of intense pulses with moderately good time structure, one will try to minimize moderation in the reflector material Figure 1-2: Neutrons in science and economy but not kill neutrons by placing absorbers around the moderator. So, spallation neutron As it was true in the development of reactors, sources offer a wide variety of choices and, of where research facilities provided most of the course, requite different technical solutions for information needed to develop reliable different optimization criteria. A very important nuclear power stations, a similar symbiosis parameter in this context is the moderator might also be in the interest of research temperature, which affects the spectral spallation sources and new accelerator distribution as well as the pulse shape over a driven devices. This is shown schematically large energy range. in Fig. 1.2. This present paper will first review some of the physics issues of neutron generations and moderation in order to provide a basic understanding of the design decisions involved and, after giving an example 1.29 MeV for 235U-fission and 1.33 MeV for each for a modern continuous and a pulsed 239Pu fission representing temperatures of research neutron source, discuss some 1.14 • 108 K and 1.18 • 108 K, respectively. technical issues related to the further The mean energy of the neutron in the development of spallation neutron sources. distribution (2.1) is Most of these issues are relevant also in accelerator driven devices for nuclear power (2.2) E=3/2ET applications. A calculated spectrum of fission neutrons is shown in Fig. 2-1 [1]. The energy deposited 2. Neutrons from fission and in the reactor core comes mainly from the fission fragments. A break down is given in spallation Table 2-1. The physics of spallation is being treated in Table 2-1: Energy and range of reaction detail in different lectures of this school. Here products in nuclear fission we give only a phenomenological description useful to visualize the underlying phenomena Reaction Energy Range in reactor core and to develop a feeling for the influence of the product (MeV) materials various parameters that govern the process. fission fragmts. 167±5 very short (< 1mm) neutrons 5 medium (tens of cm) prompt is 6±1 long (10 cm -*• some m) 2.1 The fission process p-particles 8 ±1.5 short (mm to cm) decay ys 6±1 long (10 cm* some m) Since fission is an extremely well studied neutrinos 12±2.5 infinite process, it may be worthwhile to point out 204 some of the similarities and dissimilarities between fission and spallation: 1 I l I ' ' • "-'1 ' I u.i^ ! " ' 1 The most common fission reactions are the Iff r ones resulting from the capture of a thermal or iff2 r • fast neutron in a so called "fissionable" 8 o Iff3 r • 0 nucleus. A compound nucleus forms, which is : • o 0 II * 0 unstable against deformation. According to the 3 i2 10"4 : o Fission ° "droplet" model, it breaks apart when the : • o • o 6 * Spallation 800 KleVp+ Coulomb repulsion between the two halves of io- — • o - 5t 0 (W Target) 0 the deformed nucleus equals the restoring ff6 n ; LI.HHII—' • ' • '•" I , IM.,,1 .
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