Dysprosium as a resonance absorber and its effect on the coolant void reactivity in Advanced Heavy Water Reactor (AHWR) Umasankari Kannan a, * , S. Ganesan b,1 a Reactor Physics Design Division, Hall No. 1, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India b Reactor Physics Design Division, A-5-18, Central Complex, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India article info Article history: Received 16 June 2009 Received in revised form 5 October 2009 Accepted 15 October 2009 Available online 17 November 2009 abstract Dysprosium has been used as a slow neutron absorber in the fuel assembly of Advanced Heavy Water Reactor (AHWR) to achieve a negative coolant void reactivity. Dysprosium as occurring in nature has as many as seven isotopes namely, 156 Dy, 158 Dy, 160 Dy, 161 Dy, 162 Dy, 163 Dy, and 164 Dy. Of these, the iso- tope 164 Dy has the largest absorption cross section for thermal neutrons. In the past, nuclear data libraries used in our studies have considered only 164 Dy isotope and this was sufficient for performing foil activa- tion studies of Dy. The other isotopes of Dy have significant resonances and could affect the design. The treatment of the dysprosium isotopes with resonance tabulations is required for a more accurate estima- tion of the lattice characteristics like the coolant void reactivity. The use of resonance tabulations for the dysprosium isotopes and its effect on the coolant void reactivity of AHWR fuel cluster has been studied in this paper. Also, the treatment of the stand-alone structural rod with dysprosium as burnable absorber having resonance tabulations has been done for the first time. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The AHWR is a 920 MWth, vertical pressure tube type thorium- based reactor cooled by boiling light water and moderated by hea- vy water (Sinha and Kakodkar, 2006). One of the most important safety features of Advanced Heavy Water Reactor (AHWR) is to achieve a negative void coefficient of reactivity (Kakodkar, 1998). This has been achieved in the cluster design of AHWR by using dys- prosium, which is a slow burning absorber. A composite cluster designated D5 has been developed for AHWR (Kumar, 2000) which has a central displacer region using dysprosium as burnable absor- ber. AHWR uses boiling light water as coolant and the presence of dysprosium alters the flux profile upon voiding thereby making the void coefficient of reactivity negative (Kumar et al., 1999). In an earlier work, we had studied the sensitivity of the nuclear data for isotopes of the thorium fuel cycle and its influence on the lattice parameters of the AHWR D5 cluster (Kumar et al., 2002). The lattice design then was based on using both void tubes and dysprosium for achieving a negative coolant void reactivity. Since that work, the fuel cluster has undergone a major change by way of removing the void tubes, decreasing the lattice pitch and other structural changes (Umasankari et al., 2005). In this work we have studied the modified fuel cluster focusing on the dysprosium iso- topes, the modeling of the displacer rods containing dysprosium and its influence on the coolant void reactivity of the AHWR D5 cluster. It was of utmost importance to generate the relevant nuclear data libraries for dysprosium as it could have an influence on the physics design of the AHWR fuel cluster. The problem was system- atically approached. The various aspects of generation and use of the dysprosium data for AHWR design simulations include:  Checking the availability of the nuclear data evaluations for the isotopes of dysprosium.  Assessment of the evaluated nuclear data libraries of dyspro- sium isotopes.  Better approach to explicitly model the AHWR lattice which uses dysprosium.  Studies on the resonance treatment for the isotopes of dyspro- sium in relation to the modeling of the displacer using the WIMS code for the AHWR D5 fuel cluster. 2. Status of nuclear data for dysprosium When any material is chosen for potential reactor application, it would be worthwhile to study the availability of basic nuclear data, its basic characteristics such as the presence of resonances, the ex- tent of the resonance regions and their status. Dysprosium has been used for reactor applications to achieve low coolant void reac- tivity in a CANDU PHWR, in their CANFLEX design and the Koreans too have done some work in this respect (Min et al., 1995). It may 0306-4549/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.anucene.2009.10.012 * Corresponding author. Tel.: +91 22 25595305; fax: +91 22 25505151. E-mail addresses: uma_k@barc.gov.in (U. Kannan), ganesan@barc.gov.in (S. Ganesan). 1 Tel.: +91 22 25595002; fax: +91 22 25505151. Annals of Nuclear Energy 37 (2010) 270–276 Contents lists available at ScienceDirect Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene