The development of safe nuclear power focuses basically on the creation of reactors using natural safety principles, which meet the following goals: reduction in the role of internal risk factors and the levels of perturbation that might lead to accidents; and appropriate choice of feedback to ensure reactor stability.Reducing the level of perturbation in reactors basically entails minimizing the margins of reactivity, including the operative margin of reactivity, the burnup component, xenon contamination, and the temperature effect of reactivity.Self-protection of the reactor is ensured mainly by consistent feedback with respect to reactivity (temperature, density, and other coefficients of reactivity). The feedback must be such that any transient process due to any perturbation or combination of perturbations will not lead to rupture of the safety screens and the release of radioactivity into the coolant system. In principle, such well-matched feedback is possible by the appropriate choice of the active-zone construction and materials, the coolant, the fuel composition, and the basic working parameters of the reactor.Current designs of modular high-temperature gas reactors (HTGR) largely satisfy natural safety principles [1]: passive heat release of the residual energy liberation in losses of forced helium circulation (LOFWS-type accident) and negative power and temperature coefficients of reactivity in the working state. In addition, continuous fuel reloading means that the burnup margin of reactivity can be eliminated. However, there is a large margin of reactivity in terms of shutdown cooling (p > 10/~),*The aim of the present work is to show the possibility of self-protection of HTGR relative to any reactive accidents (of TOPWS type) by the choice of fuel reloading with the required combination of temperature coefficients of reactivity.The physical preconditions suggesting the possibility of meeting these goals are: the large range of fuel and graphite temperature variation (-1000-1500~ the temperature dependence of the effective resonant integral of the starting nuclides (uranium and thorium) in a system with double heterogeneity; the use of fissile nuclides with a resonant structure of the cross section in the thermalization region.First, we consider the criterion of optimal fuel reloading in terms of internal safety with respect to TOPWS-type accidents.For the sake of simplicity, attention is confined to the point model of an infinite reactor with spherical packing. Any permissible steady state of the reactor is assumed to be characterized by some mean temperature T of the spherical fuel element and a specified mean energy intensity P. In view of the low density and the small absorption cross section of helium, its influence on the multiplying and scattering properties of the spherical charge may be neglected. Therefore, all possible states of this reactor may be described by the coordinates P and T; at fixed fuel-element energy intensity, its mean temperature may vary in some range with variation in hel...
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