Appl. Phys. A 74 [Suppl.], S40–S42 (2002) / Digital Object Identiﬁer (DOI) 10.1007/s003390201756 Applied Physics A Materials Science & Processing Design of neutron-guide systems at the Australian replacement research reactor S.J. Kennedy 1, ∗ , B.A. Hunter 1 , F. Mezei 2 , L. Rosta 3 1 Neutron Scattering Group, Australian Nuclear Science and Technology Organisation, Menai, NSW 2234, Australia 2 BENSC, Hahn-Meitner-Institut, Glienicker Strasse 100, 14109 Berlin, Germany 3 Budapest Neutron Centre, KFKI, Budapest, Hungary Received: 18 July 2001/Accepted: 12 March 2002 –  Springer-Verlag 2002 Abstract. Australia’s new research reactor will cater for neutron-beam science and radioisotope production from the end of 2005, with an unperturbed neutron ﬂux of 4 × 10 14 n/cm 2 /s 1 , a liquid-deuterium cold-neutron source and a capacity for 18 neutron-beam instruments. The de- sign of supermirror reﬂecting guides was optimized to deliver maximum neutron ﬂux to neutron-beam instruments at the re- actor face and in the neutron-guide hall. Optimization of the neutron-transport system used coupled Monte Carlo simula- tions of the neutron source and the neutron-transport system. The design of the neutron-transport system is discussed, including key performance issues and simulations of ﬂux pro- ﬁles and spectra. PACS: 28.20.Cz; 28.50.Dr; 03.75.Be Neutron beam science began in Australia with the commis- sioning of the HIFAR research reactor at the Lucas Heights Research Laboratories in 1958. Since then HIFAR has op- erated with HEU fuel generating 10 MW thermal power and a thermal neutron ﬂux of 1 × 10 14 n/cm 2 /s and providing neutrons for science, radioisotope production and neutron transmutation doped (NTD) silicon. ANSTO will replace HI- FAR with a new reactor by the end of 2005. The new reactor will operate with LEU fuel at ∼ 20 MW thermal power and a thermal neutron ﬂux (unperturbed) of 4 × 10 14 n/cm 2 /s, with improved capabilities for neutron beam research and for the production of radioisotopes for pharmaceutical, scientiﬁc and industrial use. The scientiﬁc capabilities of the neutron beams were planned in consultation with representatives from academia, industry and government research laboratories to cater for the scientiﬁc priorities of the Australian research community well into the 21 st century [1]. The reactor and associated infrastructure, with the ex- ception of the neutron beam instruments, is being built to ANSTO’s speciﬁcations by INVAP, SE (Argentina)and sub- contractors. St. Petersburg Nuclear Physics Institute (Russia) ∗ Corresponding author. (Fax: +61-2/9717-3606, E-mail: sjk@ansto.gov.au) and Mirrotron (Hungary) are involved in the cold neutron source and neutron guide systems respectively. The cold neu- tron source will be a vertical liquid deuterium thermosyphon operating at 24 K. It will be ∼ 20 litres in volume, re-entrant in the direction of the cold neutron guides, located as near as practicable to the peak thermal neutron ﬂux and supported by a 5-kW liquid helium cooling plant. The neutron beam facility, illustrated in Fig. 1, will be largely based around a neutron guide hall served by two ther- mal and two cold neutron guides. This ensures adequate space for a range of neutron beam instruments with low background radiation. Capacity beyond the initial suite of neutron beam instruments will be achieved by building neutron guides on extra cold and thermal beam lines that terminate at the reac- tor face. The design allows replacement of the thermal and cold neutron beams opposite the neutron guide hall in the re- actor hall with neutron guides to transport beams to a second guide hall and provision for later installation of a hot neutron source. More information on the facility can be found in [2] and at the website [3]. 1 Modelling the neutron-transport system In order to develop performance speciﬁcations for the neu- tron beam facility for the tendering process it was necessary to calculate absolute neutron beam ﬂux at critical points in the system that could be veriﬁed during commissioning of the reactor. This entailed completion of conceptual design of re- actor core, moderator vessel, cold neutron source (CNS) and neutron transport system before selection of contractor. It was clear from preliminary calculations of CNS char- acteristics and of neutron beam coupling to the guide system that ﬂux calculations must accurately model the actual guide system in three dimensions including segmentation and re- ﬂectivity curves and that the transport calculations should couple directly to source input. A survey of known neutron beam transport programs [4–6] in January 1998 revealed that none could meet our requirement for absolute ﬂux predic- tions in our facility. The MCNP program [7] was used to model ﬁssion process in core and to calculate ﬂux crossing