ELECTRIC HEATERS FOR THERMOPHYSICAL MODELS OF FUEL ELEMENTS

Abstract

Tubular electric heaters are widely used for scientific investigations in reactor thermophysics and for equipment in a nuclear power plant. Electric heaters equipped with thermocouples and possessing properties close to those of a fuel element ‐ casing temperature, power, heat flux distribution, and so on ‐ are often used to simulate a fuel element. In contrast to direct-heating tubes, the potential of the case of a simulator is zero; this makes it possible to use the simulator in current-c onducting media. Such simulators are used in investigations with liquid-metal coolants, materials engineering experiments, for studying serious accidents, and also on stands for studying the thermohydraulics and emergency regimes of water-cooled reactors. Their number on large stands can reach 2000. In some experiments, electric heaters operate in the channel of a research reactor with nominal parameters, simulating heat release of nuclear fuel in fuel element tubes made of zirconium alloy. Commercial heaters with electric insulation in the form of periclase powder (MgO) with a long lifetime possess low characteristics (T c max = 700°C and q max = 0.15 MW/m 2 ). New technology and materials such as boron nitride have made it possible to achieve high results, for example, high heat fluxes ~3‐5 MW/m 2 . Heater characteristics can also be improved by using, together with the conventional coil as heating elements, rods, tubes, parallel rods, and so on. Many years of experience with such developments have confirmed that they are effective and their characteristics are good in investigations of degraded heat transfer, repeated filling, thermohydraulic stability in parallel channels, and crisis of heat transfer. Simulators of RBMK fuel elements with stainless steel and Zr‐1%Nb alloy cladding up to 7 m long were employed in the experiments [1]. A great deal of experience has been accumulated in fabricating and assembling models of fuel assemblies and technological channels. It has been confirmed that the simulators work well. For topside flooding from the emergency cooling system of the reactor, it has been shown that the geometry not only of the fuel assembly itself but also of the components of the suspension must be adhered to in the model but also the effect of the heat flux from the graphite masonry on the character of the coolant distribution over the cross section of the technological channel must also be replicated [2]. The experimental results showed that sleeved tubular electric heaters are convenient for use in experiments. They