Reactor core nuclear characteristics

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. 

Criterion 11 - Reactor inherent protection. The reactor core and associated coolant systems shall be designed so that in the power operating range the net effect of the prompt inherent nuclear feedback characteristics tends to compensate for a rapid increase in reactivity. 

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 configuration and other important characteristics of a fast reactor core design are strongly influenced by the nuclear parameters of the fuel and other core materials. It is important, therefore, to have some assessment of the effect of uncertainties in the basic nuclear data. 

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. 

These design objectives were carried over to the work on the power reactor PIUS, basically a pressurized water reactor (PWR) in which the primary system has been rearranged in order to accomplish an efficient protection of the reactor core and the nuclear fuel by means of thermal-hydraulic characteristics, in combination with inherent and passive features, without reliance on operator intervention or proper functioning of any mechanical or electrical equipment. Together with wide operating margins, this should make the plant design and its function, in normal operation as well as in transient and accident situations, much more easily understood and with less requirements on the capabilities and qualifications of the operators. 

By the end of 1994, 92 BWR nuclear power plants with a total electrical capacity of about 79 GWe were in operation in the Western countries and Japan an additional 5 plants with about 5.6 GWe were under construction at this time. Within the borders of the former Soviet Union a particular type of BWR had been built, the so-called RBMK reactor 16 plants of this type with about 17 GWe were operating by the middle of 1993. The characteristic feature of the BWR design - in contrast to the closed, one-phase PWR design - is heat removal from the reactor core by boiling water, i. e. by a mixture of water and steam. As a consequence of this difference in design, the behavior of many radionuclides in the BWR primary system during plant operation differs considerably from that in the primary circuit of a pressurized water reactor. 

This group of characteristics gives basic information on reactor core features, parameters and materials, including the nuclear fuel and moderator. 

The AHTR reactor core consists of coated-particle graphite-matrix fuel cooled with a molten fluoride salt. The fuel is similar to helium-cooled reactor fuel (Fig. 2). The important characteristic of these fuels is that they can operate at very high temperatures with peak temperatures of 4200 C. They are the only practical, demonstrated nuclear fuels capable of producing heat at sufficient temperatures for H2 production. 

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 ... 

Criterion 45. Control of the reactor core The reactor core should have prompt inherent nuclear feedback characteristics to compensate for rapid reactivity insertions. 

The critical requirement for successful integration of the hydrogen production systems with nuclear power plants is the delivery of the heat from the reactor core to the thermochemical plant for hydrogen production under appropriate conditions. Characteristics of the reactor are used to meet these conditions. 

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... 

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

The use of activation methods for the analysis of biological material has been reviewed by Bowen ( 7). In Instrumental Neutron Activation Analysis, the dried sample is placed in the core of a nuclear reactor where it is bombarded with neutrons. Many of the elements present in the sample undergo nuclear reactions of which the most common are the (n, y) type. The products of these reactions are radioactive and decay with the emission of gamma photons of characteristic energy. 

A concept of an evolutionary reactor is pursued with the joint French / German European Pressurized Water Reactor , EPR, a 1525 MW(e) plant with evolutionary steam generating system and innovative double-walled containment [20]. A three years basic design phase as a prerequisite for the beginning of the licensing procedure was finished in 1997. The characteristic feature is a core catcher to restrict a possible core melt to the power plant itself. The joint effort by Germany ind France, however, finds in both countries a situation where no further base load is required. The EPR, confirmed as a future standard in France, is projected to substitute decommissioned nuclear plants. 

Nuclear reactors emit 5 x 10 n/s per each MW of the released power. Another important characteristic of such neutron sources is the maximum flux density, the neutron brightness, inside the core or moderator of the reactor. In research reactors, it may reach 10 n/sxcm. In pulsed reactors, even greater brightness can be obtained -10 n/s x cm in pulse -100 ps. Some typical technical characteristics of a small nuclear reactor are capacity -100 MW, protective shell radius -100 cm, after each act of fission goes out of the shell an average -1 n, which corresponds to the brightness -3 x 10 n/s x cm. ... 

In this chapter, after giving an overview of the embrittlement of Western pressurized water reactor (PWR) reactor pressure vessel (RPV) beltline materials, together with the characteristics of PWR RPVs, such as their general specification, core region materials and the effects of variables on embrittlement, the surveillance database obtained from US, French and Japanese nuclear power plants (NPPs) and those from other countries is presented based on open literature. The surveillance program of each country is also briefiy described. Trends of surveillance data which will be obtained in the near future are described. The possibility of new data from reconstituted and miniature specimen techniques is described. 

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