Core Designs of Modern VVER Projects I. Vasilchenko, S. Kushmanov, V. Vjalitsyn, R. Vasilchenko JSC OKB "GIDROPRESS", Podolsk, Russian Federation 1. Introduction actor vessel. Number of CPS RCCA during evo- lutionary designing varied from 49 pcs. (V-302) and 61pcs. (V-320) to 121 pcs. (AES - 2006) and Semi-centennial anniversary of VVER safe op- 94 pcs. (VVER-TOI). The requirements for safety eration was commemorated at NovovoronegNPP (temperature of repeated criticality) and for fuel (NvNPP) in 2014 The VVER history has included composition (possibility of application of MOX fuel) several generations of designs. The basic of them called for this change. And possibility of increase are the following: in the number of drives to 121 pcs. is realized al- ● generation of small and medium power reac- ready in designs of Tianwan NPP, Bushehr NPP, tors V-1, V-2, V-3М, V-189, V-213; Kudankulam NPP. For VVER-TOI 94 CPS RCCAs ● the fi rst (and the single) reactor with 1000 MW are chosen considering the experience of previ- of type V-187 (NvNPP, Unit 5) with jacketed ous designs. For arrangement of suffi cient num- fuel assemblies; ber of ICIS indicators (54 pcs.) in 15 controllable ● generation of large power reactors V-302 (V- cells of protective tube unit, in addition to those 338), V-320 (V-428, V-412, V-446) with jacket- uncontrollable, channels for ICIS indicators are in- free fuel assemblies; stalled. With this purpose CPS RCCAs contain not 18 absorbing rods but 16 absorber rods (fi g. 1) in ● designs of the 21-st century: AES - 2006, these cells, and one of the respective fuel assem- VVER-TOI. bly guiding tubes, (GT) happened to get free , can Reference operational experience is one of the also be used for arrangement of the ICIS indicator. most attractive features at the modern market of For measurement of coolant temperature at nuclear - hazardous objects. Due to the initial de- the FA outlet one of GT of fuel assemblies is per- sign solutions successfully arrived at, designs of forated in the section between fuel rod bundle and VVER reactor plants of 21-st century have inher- the top nozzle. This has enabled to arrange 313 ited them and, respectively, a positive operational fuel rods instead of 312 in the bundle. experience both of installations as a whole and of The design of fuel assemblies of new projects their separate components. follows basic design solutions of TVS - 2М, having From the viewpoint of infl uences on fuel oper- an extensive positive operational experience /1/. ation such design solutions of reactor are success- The basic element, providing stability of shape, is ful as the elliptic bottom of the core barrel providing a welded skeleton of GT and spacing grids (SG) preliminary distribution of fl owrate at the core inlet; A distinctive feature of fuel assemblies of designs protective tubes of PTU preventing, together with of AES - 2006 and VVER-TOI is fuel stack height a shell, FA top nozzle, from fl ow-induced effect on extended by 50 mm in comparison with TVS - 2М. CPS RCCA; design of CPS RCCA itself and the Respectively, the height of fuel assembly top noz- system of its damping. Application of the 6-facet zle is reduced as compared with TVS - 2М. The shape of fuel assemblies provides more uniform mechanical interaction of fuel assemblies and, re- spectively, their mutual supporting infl uence. Be- sides, PTU structure and its fastening enables to adjust holding-down of fuel assemblies in the reac- tor considering their radiation growth. 2. Reference Feature of Core New Designs Starting from plants V-302 and V-320, cores con- CPS RCCA CPS RCCA tain 163 fuel assemblies. This number is governed with 18 ARs with 16 ARs by the dimensions of fuel assemblies and the re- Figure 1. 191 design of other components is considerably uni- the unsealed reactor. From the graph it follows, fi ed and technologically developed. Application of that from the very beginning of implementation of zirconium materials only within the core space that such fuel assemblies forces of motion of control have low neutron absorption together with other rods were considerably decreased that has re- specifi ed properties makes possible to use this de- moved the comment on reactor scram time. sign to reactor plants of Generation 3 +. That fact, that PTU before refuelling occupies Operational experience of TVS - 2М that is higher position than after refuelling proves absence given below enables to use it as reference for new of “contraction” of fuel assemblies as a result of projects because of similarity in designs and op- bowing or creeping of GT, and natural radiation erational conditions. growth of GT occurs. It qualitatively differs from operation of previous fuel assemblies having insuf- fi cient rigid structure when PTU after refuelling oc- 3. Operational Experience of the cupied higher position than before refuelling due to Prototype “fresh” more rigid fuel assemblies. Moreover, forces of CPS RCCA exercising at the end of cycle, at least, were not increased in comparison with those Pilot-commercial operation of TVS - 2М started measured at the beginning of cycle. The given in 2006 at Balakovo NPP -1. Further, it was intro- graph also demonstrates that change in number of duced at all units of this plant and also at Rostov loaded fuel assemblies from 54 up to 66 in transi- NPP. To provide compatibility with fuel assemblies tion to 18-month fuel cycle did not bring a notice- of a previous design with the reduced core height, able infl uence on the character of change of friction modifi cation of TVS - 2М with the blankets contain- forces during CPS RCCA motion. Respectively, fric- ing natural uranium was used in transition load- tion force does not depend on fuel cycle with fuel ings. After transition to the cores having the same assemblies being operated. Formation of the core height, transition of the power units to uprated is provided without limitations that were earlier im- power to 104% as well as to 18-month fuel cycle of posed in connection with changing in the shape of operation was provided on the basis of this design. fuel assemblies. To be more precise, CPS RCCA It has became feasible due to TVS - 2М stability of can be installed in any fuel assembly determined shape demonstrated from the very beginning. by physical calculation of loading. There is not also Fig. 2 shows typical graph of change of CPS a necessity of inter-sector shuffl ing of fuel assem- RCCA exercising force and also PTU position in blies. Retrofi tting of PTU tablets did not change fric- tion force of CPS RCCA either. All this is a demonstration of margin of stability in shape of Number of make-up FA TVS - 2М fuel assembly. This circumstance has The entire core of TVS-2 become a convincing proof of necessary transition to fuel Transition to 18-month fuel cycle assemblies of TVS - 2М type Friction force, kgf at power units and since 2014 the very transition has started at two units of Tianwan NPP. The same transition is planned to realize at Bushehr NPP and Kudankulam NPP. Retrofi tting 3mm Development and imple- mentation of TVS - 2М have provided fulfi llment of top pri- ority safety requirement: reli- ability of reactor scram. It has been achieved without loss of fuel utilization economy, PTU position, mm namely, on the basis of zirconi- um structural components (Э- Figure 2. 110 for SG and Э-635 for GT). 192 Table 1. Design VVER-1000 AES-2006 VVER-TOI V-320 104% (MIR-1200) Average enrichment in 4,736 / 4,730 4,71 4,78 235U, % Number of FA of make-up, 66 / 67 72-73 73 / 72 pcs. Length of fuel cycle, 510 / 515 505 511 / 506 eff.days Average burn-up of FA, 48,5 / 49,9 * 47,5 49,4 / 49,5 MWD/kg U 2.1. Main Parameters of Fuel Cycles oped and undergone the pilot operation. Not less effective measure is rigorous arrangement of the Table 1 gives the basic design parameters of repair work pertained to formation of particles. 18-month equilibrium fuel cycle for various designs. The crucial role in counteraction to failures The fuel cycle of new designs is seen to develop due to clad corrosion belongs to application of at practical operation of fuel in installations V-320. corrosion resistant alloys. They are alloy Э -110. (Considering that burn-up of 55 MWD/kg U is actu- Nevertheless, activities are under way on imple- ally achieved). mentation of modifi ed alloys Э -110м for fuel rod claddings and Э -635м for GT. 2.2. Stability to Impact of Damaging Factors FA forming. The data of measurements of drop time and CPS RCCA motion forces, forces of mo- tion of fuel assemblies and position of PTU, show, The concept of zero fuel failure formulates the ba- that no changes in shape occur. To improve op- sic damaging factors. Counter factors of TVS-2M erating conditions of fuel assemblies axial forces are given below. are essentially reduced due to application of thin- One of the major factors is clad fretting-wear. ner spring wire and PTU pressing forces providing Numerous tests of FA with spacing from 240 mm necessary margins on non-lifting of fuel assem- up to 340 mm, used in the structure of VVER fuel blies during operation. assemblies show signifi cant margins of dynamic Fuel rod heat rate. VVER –1000 reactor is accelerations critical for occurrence of fretting- a powered reactor. Nevertheless, the statistics wear.