Sheet 1 9th International Conference on WWER Fuel Performance, Modelling and Experimental Support 17-24 september 2011, Helena Resort, Burgas CORE DESIGNS FOR NEW VVER REACTORS AND OPERATIONAL EXPERIENCE OF IMMEDIATE PROTOTYPES I.N. Vasilchenko, V.A.Mokhov, S.B.Ryzhov OKB “GIDROPRESS”, Podolsk, Russia Introduction An intensive development of nuclear power engineering is under way in Russia both due to the commissioning of new power units of already existing designs and due to the improvement of VVER reactor designs for the units to be constructed in the XXI century. The design of the reactor core and its components is also involved into the process of improvement. The reactor cores for new VVER reactors under construction and in elaboration have a considerable common invariable part, its parameters being chosen during the development of the commercial VVER-1000. They are the core and fuel assembly cross-section dimensions, appropriate FA shape and core arrangement. They are an important part of the whole VVER technology which has taken a significant place in the world nuclear power engineering. All further modifications are directed firstly at the adaptation to the requirements for the new reactor designs, secondly, to the achievement of the competitive economic indices. The designs of the cores for new VVER reactors based on the evolutionary principle respectively take into account all the previous improvements. Besides, these designs are developed with regard for the capability to implement the targets specified in Terms of Reference for the new reactor plants. It refers, for example, to the justification of a high fuel burnup and reactor load-follow operation. The paper covers the recent improvements analyzed in order to implement the enhanced core performances. AES-2006 reactor core design is considered from the point of view of its application and improvement in the planned VVER-TOI project and of the possibilities of using the basic engineering solutions for the cores with spectral control. The discussion of several types of mixing grids considered in the paper involves a preliminary assessment of their efficiency and the information on their introduction into pilot operation at the VVER-1000 Units. Special attention is given to the results of the operation of immediate prototypes (TVS-2 and TVS-2М) that corroborate the reliability of the design both with regard for the core geometrical stability and fuel cladding tightness. 1. Main parameters of reactor cores for the new VVER reactors A comparison of the characteristics of the reactor cores for new reactor designs are given in Table 1. It is evident from the Table that in case of a significant rise in thermal power the average linear heat rate of the fuel rod actually practically does not change mainly due to the increase in the reactor core height. The minor differences in other parameters result from the design optimization, which is described below. Table 1 – Main parameters of V-320, AES-2006 and VVER-TOI reactor plants No. Parameter VVER-1000 AES-2006 VVER-TOI 1 Availability factor ~0,82 0,92 0,93 2 Thermal output, MW 3000 ÷3120 3200 3300 3 Average core inlet/outlet temperature: Tinlet, °С Тoutlet,°С 289 298,1 297,6 Sheet 2 321 329,5 329,1 4 Core inlet/outlet pressure: P outlet, MPa 15,7 16,2 16,2 5 Number of fuel assemblies, pcs 163 163 163 6 Number of fuel rods, pcs 311÷312 312 313 7 Core height, mm 3530÷3680 3730 3730 8 Average linear heat rate, W/cm 167 168 173 9 Maximum linear heat rate 448 420 420 10 Number of RCCA, pcs 61 Up to 121 94 11 Mass of fuel in the core, kg 85950 87065 87344 12 Total number of absorber rods in the core 18х61 18х121 18х79+ 16х15 2. Fuel assembly designs for the new VVER reactors The changes in the parameters of the new VVER reactors brought about some changes in FA characteristics. The new VVER fuel assembly characteristics given in Table 2 show the succession in the designs developed which allows using the considerable positive experience in FA operation as the reference one. The design measures on fuel loading increase in FAs were taken to extend the fuel cycle. Since the fuel enrichment is limited to 5% and it is necessary to maintain, if possible, the water-to-uranium ratio the only way for the fuel cycle extension is to increase the content of uranium dioxide in a fuel assembly. Table 2 – Comparative parameters of fuel assemblies № Parameter VVER-1000 VVER-1200 VVER-TOI 1 Fuel assembly type TVS-2, (2M) TVS-2006 TVS-TOI 2 Cladding temperature, t clad °C 352 - 355 355 355 3 Fuel rod diameter/ spacer grid pitch, mm 9,1/12,75 9,1/12,75 9,1/12,75 4 Central tube yes no no 5 Number of absorber rods in RCCA, pcs 18 18 18 or 16 6 Maximum width across flats, mm 235,1 235,1 235,1 7 Number of channels, pcs 19 19 18 8 Mass of fuel in a FA, kg 505-527 534 536 9 Pellet dimensions douterxdch, mm 7,57х1,4; 7,6х1,2 7,6х1,2 7,6х1,2 3. Description of the main parameters and properties of fuel assemblies As the operating experience of the TVS-2(2M) with a rigid welded skeleton is being gained, lesser grounds remain to consider its alternatives in the new designs (Fig. 1). The experience has shown that this design has design and process parameters close to the optimal ones. First and foremost, it is the arrangement of the bundle. The spacer grid pitch is 340 mm. In respect to the flow turbulization there is a sufficient number of the spacer grids in a VVER-1000 FA to ensure the power level of 104-107% without mixing grids. From the point of view of rigidity the quantity of the spacer grids with the above mentioned pitch is sufficient to guarantee the stability of the fuel assembly shape and the reliability of the fuel rod spacing. Besides, the process of bundle assembling has been mastered at the Manufacturer’s. The thermo-mechanics of the components is also acceptable. As a result of development of the cell shape, the pressure loss coefficient was obtained that does not significantly influence the reactor core flow rate. Sheet 3 Thus, a sufficiently good balance between the mentioned factors has been achieved. The other designers of alternative FA designs for VVER-1000 reactors are expected to come to a similar bundle arrangement. The FA top and bottom nozzles have also been proven with respect to their design and technology. The latest embodiment of these components is cast. With the obvious external difference the nature of their design remains unchanged (Fig. 2). Nowadays, the introduction of TVS-2 (2M) top and bottom nozzles into the TVSA design corroborates the recognized quality of these components. The fuel assembly design developed for AES-2006 Project is presented in Ref. /1/. All the engineering solutions of TVS-2M design were embodied in it. Some distinctive features are related with the elongation of the core and the fuel rod by 50 mm and the installation of an additional off-centre tube to locate the in-core instrumentation sensors (similar to V-392 plant design). The first alteration brought about mounting one more spacer grid (13 pcs. total), the second change caused some asymmetry in the fuel bundle arrangement. A large scope of work was performed to ensure VVER fuel licensing. Extensive studies and experimental justification were implemented for the FAs of TVS-2 and TVSA designs. The fuel rods of the standard design have been in regular operation within the TVSA for 6 years. A 5-year burnt-up TVSA fuel assembly from Kalinin NPP was studied in SSC RIAR, ref. /2/. Special in-pile test programs for refabricated fuel rods CYCLE, LOCA and RIA were performed in MIR reactor, and also oxidation experiments and high burn-up fuel thermostability tests were performed in hot cells /3-7/. On the basis of Fig.1- FA general view these FA studies the SSC RIAR specialists /2/ made a conclusion on the retention of operability in VVER fuel rods with burn-up up to 72 MWd/kgU. The fivefold margin for the cladding oxide film thickness and the eightfold margin of the cladding hydride content after 6 years of operation were determined experimentally. The design requirements for the gas pressure inside the cladding and for cladding deformation due to the in-service Fig. 2 – TVS-2M bottom nozzle made of a casting dimensional changes in the pellets are not exceeded. As a result of the tests performed on VVER fresh and spent fuel rods data were obtained that were needed to confirm meeting the requirements of the new regulatory document NP-082-07. The in-pile tests in the MIR reactor and the bench tests (in the hot cells) were performed to study the spent fuel under LOCA. Acceptable thermal stability of the fresh and spent fuel rods under LOCA was confirmed, the data for software verification were obtained. Fuel rod tests under the load-follow operating conditions were realized at power level of 100-40-100% /7/. Sheet 4 The impulse tests of VVER fresh and spent fuel rods resulted in an experimental data array for fuel with a burnup to 67 MWD/kgU, which is now being analyzed for justification of average radial fuel enthalpy in RIA used as an acceptance criterion. In view of the considered possibility to uprate the power of VVER-1000 reactor core up to 110% of nominal, the task of determination of the fact of coolant boiling on the cladding surfaces is quite acute.