Browsing by Author "Ahmed, Rizwan"
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Article Citation Count: 6Assessment of criticality and burn up behavior of candu reactors with nuclear waste trans uranium fuel(Pergamon-elsevier Science Ltd, 2012) Şahin, Sümer; Ahmed, Rizwan; Khan, Mohammad Javed; Department of Mechanical EngineeringLarge quantities of nuclear waste plutonium and minor actinides (MAs) have been accumulated in the civilian light water reactors (LWRs) and CANDU reactors. These trans uranium (TRU) elements are all fissionable, and thus can be considered as fissile fuel materials in form of mixed fuel with thorium or naturanium in the latter. CANDU fuel compacts made of tristructural-isotropic (TRISO) type pellets would withstand very high burn ups without fuel change. As carbide fuels allow higher fissile material density than oxide fuels, following fuel compositions have been selected for investigations: (1) 90% nat-UC + 10% TRUC, (2) 70% nat-UC + 30% TRUC and (3) 50% nat-UC + 50% TRUC. Higher TRUC charge leads to longer power plant operation periods without fuel change. The behavior of the criticality k(infinity) and the burn up values of the reactor have been pursued by full power operation for > similar to 12 years. For these selected fuel compositions, the reactor criticality starts by k(infinity) = 1.4443, 1.4872 and 1.5238, where corresponding reactor operation times and burn up values have been calculated as 2.8 years, 8 years and 12.5 years, and 62, 430 MW.D/MT, 176,000 and 280,000 MW.D/MT, with fuel consumption rates of similar to 16, 5.68 and 3.57 g/MW.D respectively. These high burn ups would reduce the nuclear waste mass per unit energy output drastically. The study has show clearly that TRU in form of TRISO fuel pellets will provide sufficient criticality as well as reasonable burn up for CANDU reactors in order to justify their consideration as alternative fuel. (c) 2012 Elsevier Ltd. All rights reserved.Article Citation Count: 25Fissile fuel breeding and minor actinide transmutation in the life engine(Elsevier Science Sa, 2011) Şahin, Sümer; Khan, Mohammad Javed; Ahmed, Rizwan; Department of Mechanical EngineeringProgress on The National Ignition Facility (NIF) brings fusion a viable energy source in foreseeable future. Energy multiplication in a fusion-fission (hybrid) reactor could lead earlier market penetration of fusion energy for commercial utilization. Originally, scientists at the Lawrence Livermore National Laboratory (LLNL) have worked out a hybrid reactor design concept; the so-called Laser Inertial Confinement Fusion-Fission Energy (LIFE) engine, which has consisted of a spherical fusion chamber of similar to 5 m diameter, surrounded by a multi-layered blanket with a beryllium multiplier zone after the first wall. However, earlier work had indicated extreme power peaks at immediate vicinity of the first wall of a hybrid assembly, if a beryllium multiplier is used. Hence, in the current work, the beryllium multiplier zone has been removed in order to mitigate fission power peaks at the vicinity of the first wall as a result of neutron moderation on beryllium. Furthermore, minor actinides (MA) will cause significant neutron multiplication under fusion neutron irradiation so that an extra beryllium multiplier will not be needed. Present work has made following modifications on the LLNL design of the original (LIFE) engine: Omission of beryllium multiplier. TRISO fuel has been suspended as micro-size particles in Flibe coolant in lieu of being dissolved in uranium salt or imbedded carbon matrix in macro-size pebbles. Carbide fuel is used. Fissionable fuel charge is kept lower than in the LLNL (LIFE) engine. The modified (LIFE) engine is kept similar to the LLNL design to a great degree in order to allow mutual feedback between two geographically separated teams towards a more advanced and improved design under consideration of totally independent views. The first wall is made of ODS (2 cm) and followed by a Li17Pbg3 zone (2 cm), acting as neutron multiplier, tritium breeding and front coolant zone. It is separated by an ODS layer (2 cm) from the Flibe molten salt zone (50 cm), containing MA as fissionable fuel. A 3rd ODS layer (2 cm) separates the molten salt zone on the right side from the graphite reflector (30 cm). Calculations have been conducted for a fusion driver power of 500 MWth in S-8-P-3 approximation using 238-neutron groups. Minor actinides (MA) out of the nuclear waste of LWRs are used as fissile carbide fuel in TRISO particles with volume fractions of 0,2,3,4 and 5% have been dispersed homogenously in the Flibe coolant. For these cases, tritium breeding at startup is calculated as TBR= 1.134, 1.286, 1.387, 1.52 and 1.67, respectively. In the course of plant operation, TBR and fissile neutron multiplication factor decrease gradually. For a self-sustained reactor, TBR > 1.05 can be kept for all cases over 8 years. Higher fissionable fuel content in the molten salt leads also to higher blanket energy multiplication, namely M = 3.3, 4.6, 6.15 and 8.1 with 2, 3, 4 and 5% TRISO volume fraction at start up, respectively. For all investigated cases, fissile burn up exceeds 400000 MW D/MT. Major damage mechanisms have been calculated as DPA = 50 and He = 176 appm per year. This implies a replacement of the first wall every 3 years. (C) 2011 Elsevier B.V. All rights reserved.