Corresponding author: Sergey B. Vygovsky (vigovskii@mail.ru)
Academic editor: Yury Korovin
This article presents the results of research, that were focused on determining the optimal parameters of the extension of (reactor life-time) reactor fuel cycle in order to reduce the total operating costs of nuclear power plants during the transition from 12-month reactor fuel cycle to 18-month fuel cycle.
The relevance of the research is related to the fact that, in recent years, there is a transition at all operating nuclear power plants VVER-1000 (1200) from 12-month reactor fuel cycle to extended 18-month fuel cycle. At the same time, represent the interests to solve the problem of conservation the extension of reactor life-time while reducing the number of loaded fuel assemblies with fresh fuel assemblies, which would reduce the total operating, and fuel costs. Search for solutions of this problem is associated with mandatory implementation of all requirements for the safe operation of the reactor and the reduction of the maximum fast neutron fluence on the reactor vessel in comparison with its value at the operating nuclear power plants.
In the present work, with using the program PROSTOR software complex researched the neutron-physical characteristics of the core at the nominal parameters of the VVER-1200 reactor through the implementation of various fuel cycle strategies. The article developed various schemes of fuel-reloading for an 18-month fuel cycle with a different number of fuel assemblies. The article carries out a comparative analysis of the main parameters in the core for fuel-reloading schemes options of an 18- and 12-month fuel cycle with each other. Determine the minimum amount of fuel assemblies and provide the necessary duration of the reactor life-time for 18-month fuel cycle with using the extension of reactor life-time by reducing the power at the end of the reactor cycle to 70% of the nominal power. In the article, the arrangements of fuel assemblies were developed to provide limitations of local power by volume of the core, which reduce the fluence of fast neutrons on the reactor vessel in comparison with the projected value of the fluence. This article shows that the 18-month fuel cycle for the VVER-1200 reactor is more economical than the 12-month fuel cycle. These studies were carried out for the VVER-1200 reactor at the power of 100% of the nominal.
The purpose of this research was to determine the reactor refuelings with corresponding to fuel-reloading schemes for the 18-month fuel cycle at the basis of use constructions of fuel assemblies in VVER-1200 reactors (
It was necessary to determine the minimum number of fuel assemblies to provide the extension of reactor life-time for the one-an-a-half-year long fuel cycle when using elongation reactor life-time, by reducing the power at the end of the campaign to 70% of the nominal power, and such an arrangement of fuel assemblies, which will provide local power limitations on the core volume. Additional purpose of this research was the examination of the possibility of reduction of neutron fluence on the reactor vessel as compared with the value of fluence currently observed on operated VVER-1200 NPPs (
The fuel cycle of nuclear power plants with a VVER reactor touches many aspects related to reactor physics, technologies of uranium mining, enrichment, fabrication of fuel rods (fuel pins) and fuel assemblies (bundles) for loading the reactor core, their transportation, fuel storage and re-processing. Accent was placed in the present research of the possibility of reducing the total operating costs of NPP for extended fuel campaigns during the transition from the 12-month fuel cycle to the 18-month due to a decrease in the number of fresh loaded fuel assemblies (
A formula was suggested in (
where α is the fraction of operating costs during the period of scheduled repairs relative to the costs during power operation of the power unit, α > 1 due to dose commitments during scheduled repairs; b0 is the value of the fuel cost coefficient relative to general operating costs (b0 < 1); Nrel is the number of fuel-reloading operations for the case of continuous operation of the power unit with 18-month fuel cycle (Nrel = 60); Nrel0 is the number of fuel-reloading operations for continuous operation of the power unit with 12-month fuel cycle (Nrel0 = 40); NFA is the standard number of fresh fuel assemblies used during reloading for 18-month fuel cycle; NFA0 is the standard number of fresh fuel assemblies used during reloading for 12-month fuel irradiation cycle; Mfuel is the mass of fuel in one fuel assembly the projected; Mfuel0 is the mass of fuel in one fuel assembly for operated NPPs with VVER-1200; n% is the enrichment of uranium-235 of fuel the projected; n0% is the enrichment ofuranium-235 of fuel used on operated NPPs; Тeff is the duration of the fuel campaign of the reactor at the base power with the parameters of the 18-month fuel cycle for operated NPPs with VVER-1200; Тeff0 is the duration of the fuel campaign of the reactor at the base power with the parameters of the 12-month fuel cycle for operated NPPs with VVER-1200; DT0 is the calendar period with taking into account the duration of scheduled repairs for the 12-month fuel cycle (Тcal0 + ∆Тrep, days); ∆Т is the calendar period with taking into account the duration of scheduled repairs for the 18-month fuel cycle (Тcal + ∆Тrep, days); ∆Тrep is the duration of scheduled repairs, DTrep ≈ 32 days; and the ratio Teff/DT characterizes the NPP installed capacity utilization factor (
The above formula differs from common formula in references (
Life time of vessel of reactor at base power is constant and unchanging value for different fuel cycles. The projected operability period of reactor vessel is equal to 60 – 70 calendar years for different operating conditions (
For the projected fuel load, the average enrichment of loaded fuel is equal for NPP with VVER-1200 to 4.68% for the 18-month fuel cycle. Mass of uranium metal in each fuel assembly amounts is approximately the same value as that for 12-month fuel cycle. Alternative options with excellent fuel-reloading schemes (with loading 67 fresh fuel assemblies instead of 72) were examined. Fuel-reloading schemes differed from each other with respect to the arrangement of fuel assemblies on the periphery of the reactor core. In one case, residence of spent fuel assemblies of the third-year and two fresh fuel assemblies were placed on the core periphery, and in the second case, residence only spent fuel assemblies of the second-year and third-year were placed on the core periphery (
Calculations of neutronics parameters for different options of refueling fuel-reloading schemes were performed using PROSTOR software complex, which is certified by the supervisory authorities of the Russian Federation and demonstrating documented characteristics at the level of standard service software supporting operation of equipment on NPPs with VVER-1200 reactors (
The option of using 67 fuel assemblies, with arrangement on the reactor core periphery of two fresh fuel assemblies and spent fuel assemblies of the third-year on the reactor core periphery was chosen. The option of using only spent fuel assemblies on the core periphery was rejected because of incompatibility with thermal engineering safety requirements for the reactor core.
In this article presents the results of comparison of main parameters of the reactor core for stationary refuelings, which were calculated for the projected data for 12-month, for 18-month cycles and for the selected alternative fuel-reloading schemes with using 67 fuel assemblies during loading fresh fuel. The following parameters were considered in the analysis of reactor core safety and estimated given operating costs:
DNBRmin – The minimum departure from nuclear boiling ratio;
max (QL – 0.98⋅ QLlim) – The maximum difference between the calculated-measured and limiting values of linear fuel rod per unit length (must be < 0);
KQmax – The maximum irregularity factor of a power of fuel assembly power over the reactor core;
KVmax – The maximum irregularity factor of power release over the core volume;
average fuel burnup values during fuel irradiation campaign for peripheral fuel assemblies (for the purpose of comparison of the values of fast neutron fluence on the reactor vessel for different options);
average values of the fuel burnout for the discharged fuel;
average values of the fuel burnout per campaign in peripheral fuel assemblies (to compare values of fluence of fast neutrons on the reactor vessel different options);
the maximum (average) values burnup for the discharged fuel;
the maximum burnup values of fuel in fuel rod in the discharged fuel;
the maximum burnup values for fuel pellet in fuel rods in the discharged fuel.
Determining estimated variation of fast neutron fluence on reactor vessel for different core loading options is required for performing comprehensive comparison. Straightforward methodology for estimating this variation suggested in (
For a full comparison, it is necessary to make assessments of changes the fluence of fast neutrons on the reactor vessel for different options. in (
where
The condition for ensuring lifetime of reactor vessel equipment for the projected is following:
Taking into consideration the 60-degree symmetry of the reflector surrounding the core it is sufficient to verify satisfaction of the above inequation for peripheral fuel assemblies within the segment.
Figures
Map for the selected alternative fuel-reloading for the 18-month fuel cycle (67 fuel assemblies, 3200 MW)
Map for the design fuel load for the 18-month fuel cycle (72 fuel assemblies, 3200 MW)
The examined options ensured the local power limitations on the core volume within the volume of the reactor core and the minimum departure from nuclear boiling ratio
Dependence of
Time-dependence of
Time-dependence of max (
Time-dependence of
Figure
From the above results, Figure
Dependence on the given value of dose commitments of gevin cost coefficient for the 18-month fuel cycle relative to the costs for 12-month cycle for the project option of fuel-reloading option and fuel-reloading scheme
Dependence on the given value of dose commitments of given cost coefficient for the 18-month fuel cycle relative to the costs for 12-month cycle for the alternative option of fuel-reloading option and fuel-reloading scheme
For the 18-month fuel cycle, all considered refueling options provided a noticeable decrease in the fluence values as compared with the 12-month cycle. More than that, the value of fluence for the alternative refueling option is lower than that for the projected option in the most vulnerable place of reactor vessel (Table
The maximum values of fuel burnup in fuel assemblies and in fuel rods for all the examined options are presented in Table
Distribution of average values of power generation for fuel assemblies in the peripheral layer within the reactor core segment 60° for the projected and alternative options of stationary refueling, which determines the fast neutron fluence on the reactor vessel
Angle from zero line at which the center of the next peripheral fuel assembly is positioned, qi,° | 0 | 7.59 | 16.10 | 25.29 | 34.72 | 44.84 | 52.41 | |
Power generation during the campaign of the peripheral fuel assembly relative to its fuel mass, ∆ |
Projected fuel-reloading scheme, 3200 MW (42 FAs) | 18.95 | 10.40 | 10.71 | 23.45 | 23.45 | 10.71 | 6.29 |
Projected fuel-reloading scheme, 3200 MW (72 FAs) | 12.11 | 6.82 | 11.41 | 19.88 | 19.89 | 11.42 | 6.98 | |
Alternative fuel-reloading scheme, 3200 MW (67 FAs) | 11.66 | 6.06 | 9.93 | 19.97 | 19.97 | 9.93 | 6.05 |
Values of fuel burnup in fuel assemblies and in fuel rods and the extended of reactor campaign for different options of stationary refueling
|
|
|
|
Average burnup for discharged fuel, MW⋅day / kg U | 57.94 | 50.40 | 52.86 |
Maximum burnup (averaged for fuel assembly) of discharged fuel, MW⋅day / kg U | 64.71 | 58.21 | 58.63 |
Maximum burnup in fuel rod in the discharged fuel, MW⋅day / kg U | 71.19 | 65.20 | 65.70 |
Maximum burnup for fuel pellet in fuel rod discharged fuel from the core, MW⋅day / kg U | 78.93 | 70.13 | 70.13 |
Duration of operation of the refueling for the case of extended reactor campaign with reactor power reduced to 70%, eff. days | 367 | 539 | 528 |
Conducted research allowed to determine an alternative (projected scheme) fuel-reloading scheme and to form the stationary refuelings. For found schemes of the fuel-reloading and refuelings of 18-month fuel cycle with regard to all main characteristics of the reactor core show advantages as refers to the following:
Average fuel burnup for the discharged fuel;
Equability of the fuel burnup for the discharged fuel;
Value of fast neutron fluence in the most vulnerable places of reactor vessel;
Value of given operating costs with maximum dispersion of its separate components.
Thermal engineering safety of reactor core was substantiated For found the fuel-reloading schemes and stationary refuelings. All main parameters characterizing reactor core safety are within the limits of permissible values according to the data provided by the Chief Designer.
It is important to note that the suggested solution can be implemented already at the present moment on the basis of existing project of operated fuel assemblies.
* Russian text published: Izvestiya vuzov. Yadernaya Energetika (ISSN 0204-3327), 2018, n. 3, pp. 113-124.