Fast Reactor Physics and Computational Methods

Six advanced reactor concepts have been selected for Generation-IV reactors and are being investigated worldwide to meet the challenging goals of effective resource utilization and waste minimization (sustainability), improved safety, enhanced proliferation resistance, and reduced system cost. [1] The six systems are very high temperature reactor (VHTR), sodium-cooled fast reactor (SFR), supercritical water-cooled reactor (SCWR), gascooled fast reactor (GFR), lead-cooled fast reactor (LFR), and molten salt reactor (MSR). Most of the six systems employ a closed fuel cycle to maximize uranium resources and minimize high-level wastes to be sent to a repository. Three of the six are fast reactors (SFR, LFR and GFR), one (SCWR) can be built as a fast reactor, and one (MSR) is described as epithermal. Under the Generation-IV International Forum (GIF) framework, international collaboration on fast reactor designs is proceeding with high priority. [2] Fast reactors have intrinsic nuclear characteristics that offer greatly more efficient use of uranium resources and the ability to burn actinides which are the long-lived component of high-level nuclear wastes. Combination of the increased fission-to-capture ratio and the increased number of neutrons per fission in fast neutron region yields more excess neutrons from Pu fission, which can be captured in U to breed more Pu. While current Generation-III commercial reactors utilize less than one percent of uranium resources, fast reactors can utilize essentially all fissile and fertile isotopes through recycling, except for small losses in processing, resulting in a hundred-fold improvement. Compared to thermal spectrum reactors, fast spectrum reactors are much more efficient in destroying actinides because of higher fission-to-capture reaction ratios. Thus the hazardous transuranics (TRU) elements of used nuclear fuel can be removed from the waste stream and subsequently transmuted to shorterlived fission products in fast reactors while producing about one MW-day (MWD) of energy for every gram. The transmutation of TRU would reduce the long-term environment burden of nuclear energy through significant reduction of released dose and radiotoxicity and efficient utilization of permanent disposal space. Fast reactors for production of fuel and electricity were conceived by Fermi and his team as early as April 26, 1944. [3] The world’s first plutonium-fueled nuclear reactor Clementine was designed and built in 1945-46 and first achieved criticality in late 1946. [4] The Experimental Breeder Reactor I (EBR-I) generated the world’s first useful electricity from nuclear power on December 20, 1951 and demonstrated the breeding principle in 1953. The Experimental Breeder Reactor II (EBR-II) was the first power reactor system in the United States power This paper reviews the fast reactor physics and computational methods. The basic reactor physics specific to fast spectrum reactors are briefly reviewed, focused on fissile material breeding and actinide burning. Design implications and reactivity feedback characteristics are compared between breeder and burner reactors. Some discussions are given to the distinct nuclear characteristics of fast reactors that make the assumptions employed in traditional LWR analysis methods not applicable. Reactor physics analysis codes used for the modeling of fast reactor designs in the U.S. are reviewed. This review covers cross-section generation capabilities, whole-core deterministic (diffusion and transport) and Monte Carlo calculation tools, depletion and fuel cycle analysis codes, perturbation theory codes for reactivity coefficient calculation and cross section sensitivity analysis, and uncertainty analysis codes.