Core Nuclear Characteristics

Nuclear calculations are based on nuclear data selected from the best current sources of information fhroughout the nuclear industry and on mathematical computer codes developed by General Electric for fhe BWR. 

In a BWR, two reactivity coefficients are of primary importance the fuel Doppler coefficient and the moderator density reactivity coefficient. The moderator density reactivity coefficient may be broken into two components that due to temperature and that due to steam voids. 

As in all light water-moderated and low-enrichment reactors, the fuel Doppler reactivity coefficient is negative and prompt in its effect, opposing reactor power transients. When reactor power increases, UO2 temperature increases with minimum time delay, resulting in higher neutron absorption by resonance capture in the U-238. 

During normal plant operations, the steam void component of the moderator density reactivity coefficients is of prime importance. The steam void component is large and negative at all power levels. At full rated power, the steam voids are equivalent to approximately 3% reactivity. 

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L.F. Donovan et al, Heat and Mass Transfer Characteristics of an Axial Flow Liquid Core Nuclear Rocket Employing Radiation Heat Transfer , NASA TND 4127 (1967)... 

In the core design of large FBRs, it is essential to improve the prediction accuracy of nuclear characteristics from the viewpoint of both reducing construction cost and insuring plant reliability. Extensive work is being performed in this context to accumulate and evaluate many results of reactor physics experiments in FBR field. As a part of the effort to develop a standard data base for large FBR core nuclear design, the physical consistency of JUPITER experiment and analysis was evaluated by full use of sensitivity analysis, effect of different nuclear data Ubraries and q>pUcation of most-detailed analytical tools. 

The reactor core is designed so its nuclear characteristics do not contribute to a divergent power transient. The reactor is designed so there is no tendency for divergent oscillation of any operating characteristics considering the interaction of the reactor with other appropriate plant systems. 

The effect of clustering was also Investigated, in a core arrangement suitable for the proposed heavy y ter operation of EBWR. Into each of thirty-six, 4 x 4 in. clusters positioned to match the central region of the EBWR core were placed 28 fuel pins, spaced in a triangular pattern first at 3/4 in., next at 3/8 in. Several nuclear characteristics of these two core arrangements are compared in the table. 

Studies of the nuclear characteristics of DjQHiO- moderated lattices have recently been extended to include measurements of reactivity and flux perturbations. These experiments provided nuclear data on relatively simple configurations for testing theoretical methods. The basic lattices consisted of 1.206-cm-diameter aluminum-clad 2.46%-enrlched UOi fuel rods on a square pitch of 1.511 cm. Moderator compositions varied from 0 to 72% B,0.to provide a wide range of neutron spectrum. Boric acid was added to keep the core radius constant (61.11 cm). 

Reactor physics PNC developed a nuclear design analyds method which consists of nuclear data and reactor constants, calculation models, computer codes and methods for interpolation and extrapolation (Bondarenko-type 26 group constants, computer codes for 2D or 3D diffusion calculations, etc.). In addition, PNC performed a fiill size mockup test (the MOZART project) in the ZEBRA t critical test fa< ty at Winfidth in the UK, and a partial mockup test at the FCA (JAERI) in Japan, to help understand the nuclear characteristics of the Monju core and confirm the validity and accuracy of the nuclear dedgn. 

The fuel loading for a reactor operating on plutonium recycle will in general contain a mixture of standard enriched uranium oxide fuel elements and elements containing a mixture of plutonium and uranium oxides. When the fuel is recycled in the same reactor in which it was produced, the amount available would mean that some one third of the core would be composed of plutonium elements. On the other hand, it may be advantageous to load a whole core with plutonium elements only, since this would permit the lattice to be redesigned to take advantage of the nuclear characteristics of plutonium. The more important distinctions between uranium and plutonium fuel are summarized briefly below ... 

Since fabrication problems and the associated cost of pressure vessels capable of operating at 2000 psi increase rapidly for diameters above 12 ft, and since the effect of larger diameters on the nuclear characteristics of the two-region reactors is relatively small, 12 ft has been taken as the limiting diameter value. Actually, in most of the calculations discussed here, the inside diameter of the pressure vessel has been held at 10 ft and the core diameter allowed to vary over the range of 3 to 9 ft. 

Pu- are considered. Fuel is removed and processed at a rate required to maintain a specified poison level. The reactor consists of a core region in which plutonium is burned and of a blanket region containing uranium and plutonium. Under equilibrium conditions the net rate of production of plutonium in the blanket is eipial to the plutonium consumption in the core, in Table 2-11 are given [2D] some of the nuclear characteristics for... 

Fig. 2-1,5. Nuclear characteristics of a 60-M v (heat) two-region breeder during initial operating period. Core diameter = 4 ft, pressure vessel diameter = 9 ft, 280°C, solution core.

Nuclear Characteristics of Two-Region, Homogeneous, Molten Fluoride-Salt Reactors Fup led with Core diameter 8 ft. Total power 600 Mw (heat). 

Tlhis paper describes the physical and radiochemical characteristics of selected debris from the Kiwi Transient Nuclear Test (TNT) (6, 7). This transient test was conducted in Nevada by the Los Alamos Scientific Laboratory (LASL), and produced approximately 3 X 1020 fissions (1). Zero time was 1059 PST on 12 January 1965. About 5% of the reactor core was vaporized, and some 68% was converted to a cloud of particulate. The measured maximum temperature was 4250°K. (7). Large pieces of fuel rods were recovered near ground zero. 

Future nuclear reactors are expected to be further progressed in terms of safety and reliability, proliferation resistance and physical protection, economics, sustainability (GIF, 2002). One of the most promising nuclear reactor concepts of the next generation (Gen-IV) is the VHTR. Characteristic features are a helium-cooled, graphite-moderated thermal neutron spectrum reactor core with a reference thermal power production of 400-600 MW. Coolant outlet temperatures of 900-1 000°C or higher are ideally suited for a wide spectrum of high temperature process heat applications. 

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