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 ...
The main reasons for and the results of switching to uranium-erbium fuel in the units of the Lengingrad, Kursk, and Smolensk nuclear power plants are presented. It is shown that uranium-erbium fuel made it possible to regulate the steam coefficient of reactivity, upgrade the control rods, lower the power density in the core, increase the reliability of the fuel assemblies, increase burnup, decrease the volume of spent fuel, and improve the commercial indicators. The prospects for improving the characteristics of uranium-erbium fuel for RBMK-1000 reactors are also presented.Increasing the safety of nuclear power plants and improving the cost-effectiveness of power-generating units is a natural and stable trend in the modern stage of development of nuclear power. Safety analysis of nuclear power plants with RBMK-1000 reactors, performed by experts in this country and abroad, has shown that the improvements adopted after the Chernobyl accident substantially decrease the possibility of anticipated accidents developing into serious accidents. First and foremost, these improvements were directed toward eliminating the drawbacks of the construction of the control rods and the suboptimal uranium-graphite ratio, as a result of which the steam coefficient of reactivity was (4-5)β eff . It was decreased by loading additional boron absorbers and increasing the excess reactivity. The goal was achieved quickly, using means which the industry possessed during the mid-1980s.However, these measures were suboptimal, since at that time it was pointless to talk about the quality of the fuel cycle and certain other parameters, which do not directly determine the flow of accidents. Examples of the negative consequences are:• approximately a 25% decrease of fuel burnup, resulting in a higher fuel component of the intrinsic cost and sharpening of the problem of storing spent fuel, associated with an increase in the reloading rate;• increase of the operational excess reactivity, resulting in the appearance of a useless, for the purpose of controlling the distribution of energy release, part of the control rods (which are completely inserted), decrease of the efficiency of the
The basic results of the measures taken to increase the fuel utilization efficiency of RBMK reactors and the promising directions for developing them are examined. The results of a staged increase of fuel enrichment and the introduction of a consumable absorber (erbium) into the fuel are presented. It is shown that the work performed not only greatly improved the cost-effectiveness of RBMK-1000 but should also increase the reliability of fuel operation, which was reflected in a substantial decrease of fuel assembly failures.Measures to decrease the steam coefficient of reactivity (loading additional absorbers, increasing the operational reactivity excess, and so forth) which were implemented in 1986-1987 to increase the safety of RBMK reactors decreased fuel burnup and the degradation of economic performance substantially. The average fuel burnup decreased by approximately 30% (to 14 MW·days/kg). On the other hand, the problem of storing spent fuel was exacerbated, since the increase of refueling increased the rate of filling of the holding ponds.To increase the fuel utilization efficiency of RBMK-1000, a transition was made from regular enrichment 2% to fuel with enhanced enrichment 2.4%. This made it possible to preserve the fuel burnup at the nominal level and substantially improve the economic performance. A similar approach was impossible in RBMK-1500, since the initial enrichment could not be increased because the maximum temperature of the graphite increased at the same time. Calculations showed that even though the allowed thermal power level decreased to 4200 MW, the increase of the energy release nonuniformity accompanying a transition to 2.4% fuel enrichment could exceed the operational limits.In 1987, the Research and Design Institute of Electrical Technology and the Russian Science Center Kurchatov Institute began to search for an alternative, acceptable, and cost-effective method for lowering the steam coefficient of reactivity. About 30 variants of different structures of fuel assemblies and fuel materials were examined [1]. It was found that the most promising direction was one that allowed a consumable absorber (erbium) to be inserted into the uranium dioxide fuel matrix.The required complex of computational and experimental work was completed quickly. The Bochvar Research Institute of Standardization in Machine Engineering and MSZ JSC developed a technology for fabricating the uraniumerbium fuel, which made it possible to substantiate the possibility of loading in 1995-1996 the first experimental batches of fuel assemblies at the Ignalina nuclear power plant (with 2.4% fuel enrichment and 0.41% erbium content) and the Leningrad nuclear power plant (the fuel enrichment 2.6% and 0.41% erbium content). The results of loading these batches should have
Research is in hand on converting RBMK reactors to fuel containing the consumable absorber erbium in order to improve the safety and economy. Our institutes have performed calculations on various ways of reducing the void reactivity coefficient. Erbium is distinguished from other consumable absorbers in that it has a resonance at 0.47 eV in the absorption cross section of t67Er. The mean temperature of the graphite in the RBMK is 200~ higher than the mean temperature of the water, so if the water is lost, the spectrum shifts towards that resonance, which provides an additional component in the void reactivity effect. Adding erbium as Er203 to the fuel reduces the void reactivity coefficient to a level at which it is not necessary to insert additional absorbers in the core. Also, the consumable absorber in a fresh fuel pin substantially reduces the power and reactivity changes on reloading. This greatly simplifies the loading and also monitoring the core power distribution. With 0.41% erbium by mass, all the units retain the same construction, and the annual saving is 4 million dollars per RBMK-1000 unit on account of the increased burnup.To determine the manor characteristics of uranium-erbium fuel in the design of the fuel pins, experiments were done with pins that contained standard fuel loaded with erbium. The gas release from the fuel is a major parameter governing the pin viability. It is necessary to perform experiments with the pins for burnup up to the design level, as was evident from comparative tests on the fresh and spent fuel under the conditions of reactivity surge, and also in thermal tests.We tested cooled pins containing standard and uranium-erbium fuels under identical conditions and measured the yields of fission products in relation to the extent of burnup, temperature, and decay constant, and we also obtained evidence on the states of the material in the fuel and sheaths after working-life irradiation.Apparatus. The tests were done with a swimming pool-type IW-2M research reactor at a power of 15 MW in two ASU-18/2 irradiators in the RISK-SPRINT testers. The channels were loaded in a core cell of diameter 60 mm formed in the second and third rows of the beryllium reflector. An ASU-18/2 channel consisted of a jacket, a sealed ampule, and a mechanism for moving the ampule vertically in the core. The ampul contained an exposed pin in an aluminum radiator, together with inlet and outlet tubes, thermocouples, and a neutron flux monitor. The gap between the radiator and the ampule wall (100 t~m) provided the necessary temperature in the fuel-pin sheath.We prepared the channel and set up a well-defined gas medium in the ampule and installed the carrier gas system for the fission products as part of a gas-handling and pumping system in the RISK-SPRINT testers. A computerized monitoring and control system measured the temperature and thermal neutron flux density continuously.These experimental pins consisted each of a sheath, a column of fuel, and two steel-zirconium junctions. A tungsten-rhenium ther...
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