Physical particularities of nuclear reactors using heavy moderators of neutrons
- PDF / 1,899,048 Bytes
- 11 Pages / 612 x 792 pts (letter) Page_size
- 88 Downloads / 244 Views
CAL AND EXPERIMENTAL PHYSICS OF NUCLEAR REACTORS
Physical Particularities of Nuclear Reactors Using Heavy Moderators of Neutrons G. G. Kulikov* and A. N. Shmelev National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe sh. 31, Moscow, 115409 Russia *e-mail: [email protected] Received December 12, 2014
Abstract—In nuclear reactors, thermal neutron spectra are formed using moderators with small atomic weights. For fast reactors, inserting such moderators in the core may create problems since they efficiently decelerate the neutrons. In order to form an intermediate neutron spectrum, it is preferable to employ neutron moderators with sufficiently large atomic weights, using 233U as a fissile nuclide and 232Th and 231Pa as fertile ones. The aim of the work is to investigate the properties of heavy neutron moderators and to assess their advantages. The analysis employs the JENDL-4.0 nuclear data library and the SCALE program package for simulating the variation of fuel composition caused by irradiation in the reactor. The following main results are obtained. By using heavy moderators with small neutron moderation steps, one is able to (1) increase the rate of resonance capture, so that the amount of fertile material in the fuel may be reduced while maintaining the breeding factor of the core; (2) use the vacant space for improving the fuel-element properties by adding inert, strong, and thermally conductive materials and by implementing dispersive fuel elements in which the fissile material is self-replenished and neutron multiplication remains stable during the process of fuel burnup; and (3) employ mixtures of different fertile materials with resonance capture cross sections in order to increase the resonance-lattice density and the probability of resonance neutron capture leading to formation of fissile material. The general conclusion is that, by forming an intermediate neutron spectrum with heavy neutron moderators, one can use the fuel more efficiently and improve nuclear safety. Keywords: neutron moderation step, intermediate neutron spectrum, wide and narrow resonances, capture and fission cross sections DOI: 10.1134/S1063778816090076
1. INTRODUCTION Nuclear reactors are known to have different physical and technical characteristics and, in particular, different spectra of neutrons involved in the fission chain reaction (FCR). A reactor may operate with thermal neutrons, fast neutrons, or intermediate neutrons (resonance-neutron reactors). In a thermal reactor, the thermal neutron spectrum is formed using neutron moderators in the form of materials with small atomic weights and weak neutron absorption (such as H2O, D2O, 9Be, and 12С). In a fast reactor, the fast neutron spectrum in the core is formed by materials that are free of nuclides of small atomic weight. The term “heavy neutron moderators” reflects their basic difference from those with low atomic weights. It was coined soon after the initial studies on neutron physics in the 1940s [1, 2]. Thus, natural lead was use
Data Loading...
