Nuclear Engineering and Design 237 (2007) 1158–1163 Pu-breeding feasibility in PWR J.F.A. Delbeke a, ∗ , G. Janssens-Maenhout b , P. Peerani b a University of Gent, Faculty of Engineering TW08, c.a. Institute for the Protection and the Security of the Citizen, Joint Research Centre Ispra, Via E. Fermi, 1, I-21020 ISPRA, Italy b Institute for the Protection and the Security of the Citizen, Joint Research Centre Ispra, Via E. Fermi, 1, I-21020 ISPRA, Italy Received 22 February 2006; received in revised form 17 December 2006; accepted 17 December 2006 Abstract This study addresses the issue of alternative pathways for breeding plutonium in a 900 MWe three loop thermal pressurized water reactor (PWR), either fueled with uranium fuel (3.5% U-235) or with mixed fuel (20% MOX). During the operation of a nuclear reactor the in-core neutron flux and the ex-core neutron flux are monitored with flux detectors. At the places where those detectors operate, the guide thimbles and the vessel wall, respectively, the neutron flux can be used to irradiate material samples. This paper investigates whether it would be possible to produce plutonium by breeding it at the walls of a PWR vessel and/or in the guide thimbles. The neutron flux in the reactor and the corresponding multi-group spectra are estimated with Monte Carlo simulations for different positions at the vessel wall of a PWR operating with either UO 2 or MOX. Then the irradiation of fresh uranium samples at the vessel wall and in the guide thimbles are calculated and the isotopic composition of the irradiated samples are determined. The minimum irradiation period and the necessary minimum amount of fresh uranium to breed different grades of plutonium are derived. © 2007 Elsevier B.V. All rights reserved. PACS: 89.30.Gg; 28.41.Bm 1. Introduction: frequent refueling for breeding plutonium Under the Non-Proliferation Treaty and the Additional Proto- col of the International Atomic Energy Agency (IAEA), a set of safeguards measures exist to monitor the peaceful use of nuclear materials and nuclear technologies. Nuclear reactors, and in par- ticular light water reactors (LWR), are safeguarded by counting and identifying individual fuel assemblies. In addition, to pre- vent the undeclared removal and replacement of fuel assemblies the IAEA makes use of surveillance and sealing to guard against undeclared activities. Irradiation histories are also monitored to generate a nuclear materials account. The IAEA has specified various significant quantities of plutonium (Pu) isotopes for the purposes establishing detection thresholds. The classification is commonly based on the irradiation history of the plutonium, although the IAEA (2002) considers all Pu with a Pu-238 con- tent less than 80% as direct use material. With the high rate of spontaneous fission (1020–1030 neutrons/g s) Pu-240 in large ∗ Corresponding author. E-mail address: jochen.delbeke@jrc.it (J.F.A. Delbeke). concentrations could initiate a premature chain reaction in a critical mass as indicated by Carson (1993). Therefore differ- ent compositions of Pu are commonly distinguished in order of diminishing appropriateness for non peaceful applications. Definitions for each of them are given in Pellaud (2002) and indicated in Table 1. In reactors the build-up of the isotope Pu-240 can be strictly controlled by the irradiation time. This is easily done in reactors that are refueled online, for instance in heavy water reactors (e.g. CANDU type) and graphite moderated reactors (e.g. RBMK). For commercial LWR’s, the premature withdrawal of spent fuel assemblies from the reactor core can limit the Pu-240 content but for each withdrawal the reactor needs to be shut down and opened. There exist also other ways to breed plutonium with a con- trolled irradiation time at PWR’s and this paper will focus on two: the use of the neutron flux in the guide thimbles and use of the neutron flux located at the outer vessel wall: (1) During the operation of a nuclear reactor the flux in the core is monitored by flux detectors. These detectors are miniature fission chambers that are filled with highly enriched uranium 0029-5493/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2006.12.006