IMPLEMENTATION AND OPERATION EXPERIENCE OF URANIUM FUEL ASSEMBLIES WITH BURNABLE ABSORBER AND CLUSTER CONTROL RODS AT THE IGNALINA NPP

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1 IMPLEMENTATION AND OPERATION EXPERIENCE OF URANIUM FUEL ASSEMBLIES WITH BURNABLE ABSORBER AND CLUSTER CONTROL RODS AT THE IGNALINA NPP Saulius Stravinskas, Nikolajus Lebedevichius, Aleksej Tarasov, Georgy Krivoshein Engineer, Nuclear Safety Department Ignalina NPP, Lithuania 1. INTRODUCTION At the present time there is 1 unit operating at Ignalina NPP Unit 2, which is equipped with RBMK-1500 reactor of design thermal power 4800 MW and allowed thermal power 4200 MW. Following the accident at the unit 4 of the Chernobyl NPP the top priority technical measures were developed and implemented to improve RBMK s safety within the framework of these measures stage-by-stage reduction of the positive void reactivity coefficient were realized. First two basic stages on its reduction were: increasing the operational reactivity margin up to manual control rods, as a result void reactivity coefficient has reduce down to +1.7 β eff ; implementation 53 additional absorber rods into the reactor core, as a result, void reactivity coefficient has reduce down to no more than +1.0 β eff. Implementation of these measures resulted in the decrease of fuel burnup down to MW day/kgu (design value MW day/kgu). Implementation of 2.4 % enrichment fuel, which began at the power units with reactors RBMK-1000, was impossible for RBMK-1500 because of significant growth of fuel channel power in reloading process. The following stage on reduction of void reactivity coefficient was the implementation of 2.4 % enrichment fuel with burnable absorber at RBMK-1500 reactors. The given action was very important for the further existence of RBMK reactors, because it allows not only increase their safety, but also considerably improves technical and economic characteristics. The different type of burnable absorbers were considered to be used, namely, boron, dysprosium, hafnium and erbium. Based on the results obtained it was specified that the most efficient neutron flux flattening between fuel channels could be achieved when either erbium or boron was used. The advantage of erbium is in decrease of the average void reactivity coefficient of three times comparing to boron.

2 The optimal weight content of erbium in the new fuel was selected on the basis of the following conditions: the unloaded fuel burn-up is not less 22 MW day/kg when there are no additional absorbers in the core; the void reactivity coefficient is not higher +1.0β eff ; the maximum power level of a fresh fuel assembly with 2.4% enrichment does not exceed the corresponding power level of a fuel assembly with 2.0% enrichment. The first condition defines the upper limit of initial weight content of erbium which is equal to 0.48%. The other two conditions define the lower boundary 0.4%. On the basis of the calculations 0.41% was selected as the value of the erbium optimal weight content. As a result of the big amount of the researches executed by Kurchatov institute, RDIPE and other research organizations since 1992, it was determined, that increase of fuel enrichment in the RBMK-1500 fuel assembly up to 2.4 % with addition of erbium allows to receive the following advantages: to improve the reactor safety (reducing void reactivity factor); to increase the unloaded fuel burn-up; to decrease the fuel consumption; to unload from the reactor almost all additional absorbers. In years the loading of the first experimental batch of 150 assemblies with uranium-erbium fuel was performed at INPP Unit 2. It has been shown that the neutronic-physical reactor characteristics (void reactivity factor first) were improved, the limits of normal and safe operation was not exceeded. In particular: the void reactivity coefficient was decreased from 0.8 β eff to 0.5 β eff. After thus experimental check of correctness of researches the step-by-step U-Er fuel implementation was begun (by batches of fuel assemblies) during which the gradual unloading of an additional absorbers from the core is taken place. The following action for nuclear safety improvement of RBMK-1500 reactors was replacement of old 2091 design control rods with the modernized 2477 design control rods, with 7 m displacer and 6.8 m absorber. The new control rods 2477 design has a special absorber skirt at the bottom of absorbing section of the rod, which covers the telescopic joint between absorber and graphite displacer. Replacement of rods results in increase of reactors nuclear safety as consequence of reduction of the control and protection system (CPS) cooling circuit voiding effect. Results of calculation researches have allowed determining, that full replacement of the manual control rods (MCR) and the local scram rods (LSR) will result in reduction of CPS cooling circuit voiding effect at the operational state from β eff down to value of 2 β eff.

3 2. URANIUM FUEL ASSEMBLIES WITH BURNABLE ABSORBER The order of fuel assemblies loading and additional absorbers unloading was selected to maintain the reactor safety parameters within the allowed limits. The data for some basic parameters are submitted in the table 1: void and power reactivity factors, power distribution first radial-azimuth harmonic development period. The results shows that during new fuel loading void reactivity factor was reduced to value less than 1β eff and is supported at a level of β eff. The absolute value of power reactivity factor has increased. The increase of the power distribution first radial-azimuth harmonic development period means the stabilization of power distribution fields in the reactor. The process of U-Er fuel loading into reactor, accompanied by an additional absorbers unloading, is submitted on the Figure 3 as dependence of loaded fuel assemblies amount loaded versus reactors burn-up starting from the beginning of process. Use of U-Er fuel has allowed to increase considerably fuel assemblies burn-up. The average fuel assemblies burn-up in the reactor core increased (Figure 4). On the same diagram the burn-up of unloaded fuel is shown for which the tendency to increase also is well visible. As the result the core refueling rate has decreased considerably: from FA/eff.days up to FA/eff.days. Based upon the experience results of 2.4% enriched fuel assemblies with 0.41% of Erbium loading, the implementation of 2.6% enriched fuel assemblies with 0.5% of Erbium has started at the end of The use of U-Er fuel of such composition allows the following: to increase the burn-up depth in comparison with 2.4% enriched fuel assemblies by more than 10%, i.e. up to value 26 MWdays/kgU; to reduce the void reactivity effect till 0.4β eff ; Investigation results performed by Kurchatov Institute, RDIPE and other research institutions demonstrated that the fresh 2.6% enriched fuel assemblies by its multiplication features are similar to the 2.4% enriched fuel assemblies; i.e. during the re-fuelling 2.6% enriched fuel assemblies causes the same power distribution changes in the surrounding channels. The 2.6% enriched fuel assembly design is the same like 2.4% enriched fuel assemblies. The option of shared loading was accepted at INPP, in particular: 200 fuel assemblies with 2.6% enrichment and 100 enriched 2%. After the loading of 100 fuel assemblies with enrichment 2.6% and 25 fuel assemblies with 2% enrichment the void reactivity coefficient became 0.72 β eff. As far as this value exceeded the value set by the re-fuelling program, the loading of the next lot of fuel assemblies with 2% enrichment was missed.

4 The average fuel assemblies burn-up for unit 2 increased up to 47 % in comparison with initial value. At the present time Ignalina NPP has started to implement the 2.8 % enrichment fuel assembles with 0.6 % burnable absorber erbium. Researches made by Kurchatov Institute, and other research institutions have shown, that joint loading 200 fuel assemblies with 2.8% enrichment and 500 enriched 2.6% will allow besides unloading 4 staying additional absorbers (AA) to carry out such actions, as a replacement of 28 rods design 2091 and 2477 on CCR and to load two parties by 20 fuel assemblies with 2.0 % enrichment in everyone. In particular load as results of research shows that the void reactivity coefficient will not exceed 0.8 β eff at the operational state. Up to the present time at INPP Unit 2 the share of uranium-erbium fuel is approximately 96% (7% FA 2.4%+0.41%Er; 77% FA 2.6%+0.5%Er and 12% FA 2.8%+0.6%Er). The average fuel assemblies burn-up increased up to 52 % in comparison with initial value. 3. CLUSTER CONTROL RODS The cardinal decision of a problem regarding essential reduction of positive CPS cooling circuit voiding effect up to safe value less than 1 β eff, connected to reduction the quantity of water in CPS channels up to minimally possible level is implementation of the cluster control rods (CCR). Positive results of experimental batch of four cluster control rods, which were implemented into INPP Unit 2 in 2004, have formed the basis of decision to continue the CCR implementation. At the present time there are 28 cluster control rods in INPP Unit 2. Using CCR the water quantity on a site of reactor core is reduced up to 3 liters per one channel, and will allow to reduce positive CPS cooling circuit voiding effect from 2 β eff down to 1.7 β eff in operational state. Reduction of water quantity in the reactor core allows increasing technical and economic parameters of the reactor due to increase of depth fuel burn up. Basic difference between CCR and regular CPS rods, including emergency protection rods, is that it s working body moves not in the CPS channel, but in its additional channel-sleeve, which is motionlessly established in the channel (figure 1). The internal cavity of the sleeve is hermetic from external cooling water, which circulates in the limited ring gap. As CCR working body moves in its own directing "dry" cavity of a sleeve, it allows to improve CPS regulation effectiveness reliability and life time in comparison with ordinary INPP rods design 2091 and 2477 because of: Increases of high-speed efficiency in emergency operation (more than twice); Overlappings by an absorber of all height of an reactor core;

5 Exceptions of hydrodynamical loadings (which took place for rods with mobile mechanical graphite displacer). Cluster control rod is intended for work in manual and automatic modes to control reactivity. Design of cluster control rod is shown in figure 2. The sleeve in length of ~16.5 m is permanently implemented in the CPS channel and fastens by means of a press flange on the head of the CPS channel. Cooling of a sleeve is carried out by water of CPS cooling circuit, circulating in a ring backlash between the CPS channel and a sleeve. It allows to reduce quantity of water on a site of reactor core in calculation on one CPS channel with CCR up to 3 liters, and it provides essential reduction of CPS void reactivity coefficient. The sleeve has 12 "dry" channels (diameter 10 mm everyone), in regular intervals placed on peripheries of a sleeve and intended for placement and moving to them of absorbing elements of CCR working body. In the central cavity of a sleeve in its bottom part the support intended for restriction of an output of working body from a reactor core at breakage servo-driver tape. CCR sleeve carries out function motionless displacement "superfluous" water from the CPS channel on a site of reactor core and simultaneously is directing for moving of working body in it. The CCR implementation process is accompanied by the experimental check of the reactors neutron-physical characteristics. The void and power reactivity factors, stability of radial-azimuth power distribution fields, reactor subcriticality and other parameters are measured. In the intervals between measurements the neutronphysical reactors characteristics are monitored using STEPAN code (Kurchatov institute). The experimental data received are compared with the predicted calculated data produced by Kurchatov institute before the loading of the next CCR part. 4. CONCLUSION Implementation of uranium-erbium fuel in a combination with the replacement of 2091 design control rods on 2477 design control rods and implementation of cluster control rods at the Ignalina NPP justified expectations: the opportunity of substantial increase of fuel burn-up depth, unloading of additional absorbers (AA), reductions of fuel expenditure at simultaneous improvement of reactors safety characteristics and their maintenance in the earlier established limits. The implementation of uranium-erbium fuel, allowed not only considerably improve technical and economic INPP reactor characteristics, such as: the significant increase of fuel burn-up; additional absorbers unloading; decrease of reloading rate; improvement of the reactor safety characteristics and keeping them within the established limits;

6 reduction of reactivity effect during CPS cooling circuit voiding, but also it allowed to replace regular control rods design 2091 on rods design 2477 and cluster control rods. Figure 1. Regular s control rods and CCR alignment 1a 1b 2a 2b 3a 3b regular rod design upper position regular rod design bottom position regular rod design upper position regular rod design bottom position cluster control rod design upper position cluster control rod design bottom position

7 Figure 2. Cluster control design a absorbing element on top position; b absorbing element on bottom position; c absorbing element at breakage of servo-driver tape 1 tag 2 CPS channel 3 sleeve 4 absorbing element 5 support

8 Figure 3. Average burnup of fuel assemblies in Unit 2 at Ignalina INPP versus effective time of reactor operation Starting uloading FA 2,6% +0,5%Er unloaded fuel assemblies fuel assemblies in reactor Eav FA unloaded, MW*days Starting loading FA 2,4% +0,41%Er Starting loading FA 2,6% +0,5%Er Starting loading FA 2,8% +0,6%Er Eav FA in reactor, MW*days Starting unloading FA 2,4% +0,41%Er г г г г г г г г г г. Time, eff. days Figure 4. Amount of U-Er fuel assemblies and addition absorbers in Unit 2 at Ignaina NPP versus effective time of reactor operation 1800 N FAE y. replacement 24 LSD CR with 2477 design FA 2.4%+0.41%Er FA 2.6%+0.5%Er FA 2.8%+0.6%Er Void reactivity coefficient 1997 y. replacement 24 MCR with 2477 design 41 AA 1999 y. replacement 24 MCR with 2477 design 5 AA 0 AA 6 AA 8 AA 2002 y. replacement 31 MCR with 2477 design 53 AA 1998 y. Starting loading FA 2.6%+0,5%Er AA replacement 33 AA 24 MCR Starting loading FA 2.8%+0,6%Er with design de e.t. =Е now start e.t. -Е 45 AA e.t y y y y y. 4 AA Void reactivity coefficient, β eff Time, eff. days

9 U-Er FA. 2.4%/2.6%/2.8% Table 1. Basic reactor parameters α ϕ, β eff α w, 10-4 β eff / MW τ 01, min Unit > / / / / / / / / / > / / / >20 931/ >20 815/ / / >20 542/ / >20 460/ / /1357/ /1310/ /1274/