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The RBMK-1500 reactor at the Ignalinsk Atomic Power Plant was designed for fuel of 2% enrichment. In the first unit, after removing all the additional absorbers intended to compensate for the excess reactivity in reactor startup, the depth of fuel burnup is 18-19 MW.day/kg. The mean operational reserve of reactivity is 36 manual control rods.Analysis of the causes of the accident at the Chernobyl Atomic Power Plant has implicated design deficiencies of the control rods and a nonoptimal uranium-graphite ratio, as a result of which the steam reactivity % was (4-5)~ [1]. After the accident, measures to increase reactor safety were taken at all atomic power stations with RBMK reactors, including the Ignalinsk plant [2, 3]. In the first stage, the steam reactivity was reduced to 1/3 by loading 52-54 additional absorbers and increasing the operational reactivity reserve to 55 rods. This eliminated the possibility of uncontrollable increase in reactor power (on the basis of instantaneous neutrons) in he case of dehydration of the active zone.Increasing the number of absorbers in the active zone considerably reduces the depth of fuel burnup and impairs the economic characteristics of the fuel cycle. The depth of fuel burnup is decreased to approximately 14 MW-day/kg, i.e., by 25 %. In addition to direct economic losses due to reduction in fuel burnup, spent-fuel storage becomes a problem, because the accelerated rate of fuel-rod replacement leads to rapid filling of the storage tanks.In the second stage, fuel of enrichment 2.4% was introduced at RBMK-1000 reactors. This restored the design burnup depth and considerably improved the economic characteristics of the fuel cycle. However, in RBMK-1500 reactors, the accompanying increase in graphite temperature prevents increase in the initial enrichment. Calculations show that, despite reduction in the permitted thermal power to 4200 MW, the increase in nonuniformity of energy liberation on transition to fuel of 2.4% enrichment leads to disruption of the operational limits. Hence, other approaches are required to improve the economic performance of RBMK-1500 reactors while maintaining safe operation.In 1987, an intensive search for a more economical means of reducing c%, other than the introduction of additional absorbers, began at the Kurchatovskii Institute Russian Scientific Center (RSC) and the Scientific-Research and Design Institute of Energy Technology (SRDIET). Around 30 different designs of the fuel assembly and fuel and casing materials were considered [4, 5]. The addition of erbium to uranium dioxide proved most promising. Staff at the Ignalinsk plant had the idea od using erbium as the material for the additional absorbers and rods loaded in the fuel assembly. However, subsequent research showed that the only practical approach with a real economic impact (without loss of safety) is to place the erbium in the fuel. Note that non-Russian research on the use of erbium in PWR was published at about the same time (the late 1980s and early 1990s) [6, 7]. PROPERTIES OF ...

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