TRANSITION TO FOUR BATCH LOADING SCHEME IN LOVIISA NPP

Transcription

1 TRANSITION TO FOUR BATCH LOADING SCHEME IN LOVIISA NPP S.Saarinen, T. Lahtinen, M. Antila Fortum Nuclear Services Ltd, Espoo Finland ABSTRACT The VVER-440 reactors of Loviisa NPP are operated with 1500 MW th power and reduced core. During recent years a 3-batch loading scheme has been used. Loviisa-1 is currently running with BNFL fuel equilibrium cycle and Loviisa-2 with TVEL fuel equilibrium cycle. Our goal is to move to a 4-batch scheme with TVEL fuel for both reactors. To achieve this goal the U-235 enrichment has to be increased from the current designs used. The fuel to be used in the near future is 4.37 % enriched fuel with six Gd 2 O 3 doped rods. The characteristics and consequences of the core consisting of Gd-fuel are discussed based on our target equilibrium loading pattern. With the 4-batch loading scheme the discharge burnups exceed the current assembly burnup limit with a clear margin. Thus, we also have to prepare an application for the safety authority to increase the assembly average burnup limit from the current 45 MWd/kgU to about 56 MWd/kgU. First Gd-fuel assemblies are loaded into Loviisa-1 core in September 2009 and into Loviisa-2 core in October The reload batch of Loviisa-1 in 2009 consists of 60 Gd-assemblies and 24 non-gd-assemblies plus 12 followers. In this paper some results are presented from Loviisa-1 zero power startup experiments and the first days of power operation. Among other aspects the six Gd 2 O 3 doped pins used in the assembly have an effect on the pin power profile of the assembly during the first half of the cycle. The influence of the changing pin power profile on the outlet temperature measurements is briefly discussed based on expected effect and measurement observations.

2 1. INTRODUCTION In recent years Loviisa-1 and Loviisa-2 have been operated in nearly equilibrium cycle with BNFL and TVEL fuel, respectively. Core loading patterns and core characteristics have been discussed in [1]. In this paper Section 2 describes the new TVEL Gd fuel assembly to be used in Loviisa NPP. Some of the main parameters are shortly compared between old and new TVEL fuel assemblies. In Section 3 we present a four batch loading scheme with Gd fuel. Properties of the Gd loading are compared with regular non-gd loadings. In order to fully utilize the Gd fuel assemblies an increase in the burnup limit set by the safety authority is required. The ongoing burnup limit increase project is discussed in Section 4. Section 5 deals with the modifications required in modeling due to the use of Gd assemblies. Section 6 gives insight to the first Gd loading in Loviisa NPP. We compare measured data from the criticality experiments and first days of power operation. The influence of Gd fuel to outlet temperature measurements is considered based on actual measurements and the expected effect. 2. GD FUEL Some differences exist between TVEL and BNFL fuel types. The differences are considered in more detail in [1]. In Table 1 the new TVEL Gd fuel assembly is compared with the old regular (non-gd) TVEL fuel assembly. Major changes include higher enrichment, increase in the height of fuel column, increase in U-mass, profiling and Gd 2 O 3 doped fuel pins. Layout of fuel pin enrichments is shown in Fig. 1. The assembly consists of % enriched rods, six 4.2 % enriched rods at the corners and six 4.0 % enriched rods with 3.35 % Gd 2 O 3 next to the corner rods. There is no Gd in other rods. In Fig. 2 k value for both TVEL assemblies is plotted. k values are calculated with CASMO-4E [2], [3]. Gd burns out from the assembly roughly at half point of the 1st cycle. Average assembly EOC burnups obtained are usually roughly 15 MWd/kgU (after 1st cycle), 28 MWd/kgU (after

3 2nd cycle) and 40 MWd/kgU (after 3rd cycle). At these burnups the Gd fuel is more reactive than the regular fuel. The higher EOC reactivity follows directly from the higher enrichment since there is practically no Gd left. The excess reactivity is enough to enable four batch loading scheme. In Section 3 a four batch scheme will be presented. Gd is required in the fresh fuel assemblies to ensure subcriticality with the current fuel handling systems. The amount of Gd 2 O 3 has not yet been thoroughly optimized. Requirement for the minimum amount of Gd can be deduced from the subcriticality requirements. However, deciding the optimal amount of Gd for fuel loading purposes is a more complicated task. Preliminary studies indicate that lower Gd 2 O 3 concentration may be favorable. Table 1. Some main parameters of new (Gd) and old (non-gd) TVEL fuel types. In Gd fuel there are six Gd 2 O 3 doped pins. old (non-gd) new (Gd) Fuel rod outer radius, cm Bundle pitch, cm height of the fuel column (fixed), cm U-mass, kg enrichment, % profiled no yes Gd203, % EQUILIBRIUM LOADING Equilibrium loading pattern for Gd fuel is shown in Fig. 3. Fuel followers are 4.0 % enriched without profiling and Gd. The behavior of some key parameters as a function of cycle energy is presented in Figs Calculated values of the parameters are shown for target Gd equilibrium loading, Loviisa-1 cycle 32 (mostly BNFL fuel), Loviisa-2 cycle 29 (mostly TVEL fuel) and Loviisa-1 cycle 33 (first cycle with Gd assemblies). The first Gd loading is described in more detail in section 6. In Fig. 4 boron concentration at full power conditions is shown. BOC boron concentration is significantly lower (about 400 ppm) when Gd assemblies are loaded into the core. The boron concentration at the first Gd loading is already very close to the equilibrium loading. With both

4 Gd loadings the boron curve is still monotonously decreasing i.e. the highest boron concentration is obtained at BOC. Maximum linear heat rate is shown in Fig. 5. So far linear heat rate has been one of the limiting parameters at BOC. The limit is 325 W/cm for fresh fuel. The limit is decreasing with burnup. With Gd equilibrium load, however, the maximum linear heat rate is significantly lower and, thus, the linear heat rate margin is higher. Therefore, in the future one does not have to be so worried about the linear heat rate. Subchannel outlet temperature has also been a limiting parameter. The limit is 325 C. Maximum subchannel outlet temperature is shown in Fig. 6. Contrary to the previous behavior the maximum temperature with Gd loadings is obtained at the middle of the cycle and not at the beginning of the cycle as before. In the Gd equilibrium loading the maximum subchannel outlet temperature is higher than in the other loadings. Radial form factor Kq and 3D form factor Kv are shown in Figs. 7 and 8, respectively. With Gd these parameters are somewhat lower than without it. Gd reduces power in fresh assemblies and thus the power distribution is more flat. 4. THE INCREASE OF BURNUP LIMIT From Fig. 3 it can be seen that the maximum assembly burnup achieved is 53 MWd/kgU. Current assembly burnup limit is 45 MWd/kgU. No other burnup limits exist i.e. there is no limit for rod or pellet burnup. Clearly, a higher burnup limit is required to enable the four batch loading presented in Fig. 3. Taking into account changes in cycle lengths and asymmetries in power distributions and consequently in burnup distribution, a burnup limit of 56 MWd/kgU would have to be licensed to enable the use of four batch loading scheme such as presented in Fig. 3. Fortum has ongoing project to prepare suitable documents to apply for a higher burnup limit from the safety authority. In the application main emphasis is given to demonstrate that: collapsing of the fuel rod cladding does not happen the adequate limitation of fission gas release is satisfied

5 fuel assemblies and rods have enough margin for growth in the reactor and assembly, respectively the limit for number of damaged rods in a class 2 accidents is not exceeded First Gd assemblies were loaded in September The fourth cycle of these Gd assemblies will start at Thus the license for higher burnup is required by The application is set to be sent to the safety authority in INFLUENCE TO MODELING In the new fixed Gd fuel assemblies the fuel column extends six cm deeper at the lower end than in the old fixed fuel assemblies and fuel followers (both old and new). Differing fuel lengths at the lower end cannot be directly modeled in the current version of HEXBU-3D [4], [5]. HEXBU- 3D is the code used in Loviisa to calculate the 3D reactor physical characteristics. The problem is solved using some effective parameters in HEXBU-3D in the case of mixed cores. For first Gd load, Gd assemblies are modeled as 244 cm high i.e. 6 cm too short. Thus, the lower end of each assembly is at the same level. However, this causes an error in the calculated axial power distribution and total power produced in the Gd assemblies. In HEXBU-3D boundary conditions are dealt using two group albedo formalism. The error in axial distribution is corrected by tuning α 11 (albedo for reflection from fast to fast group) at the lower boundary. Power of Gd assemblies is adjusted by increasing energy production cross section εσ f for these assemblies. Individual effective parameters are required for each loading until all fixed assemblies in the reactor are of the increased height. However, the difference in the position of the lower end remains between fixed and follower assemblies in equilibrium loading. In the modeling the followers are extended to the same level at the lower end. The error caused by this is corrected by tuning upper and lower axial albedo boundary conditions. These modifications should modify the theoretical flux distribution so that it is very close to the "correct" one. One should also keep in mind that after the calculation of the theoretical distribution it is fitted to neutron flux and assembly outlet temperature measurements in Loviisa

6 on-line system. Thus, possible small inaccuracies in the theoretical flux distribution are corrected by the fit to the measurements. 6. FIRST GD LOADING Loviisa-1 runs currently in 33 rd cycle with the first Gd assemblies in Loviisa. The reload batch consisted of 84 fixed and 12 follower assemblies. The 84 fixed assemblies were: 60 new Gd assemblies, 9 old TVEL assemblies and 15 BNFL assemblies. These were the last BNFL assemblies to be loaded at Loviisa NPP. The 12 followers loaded were provided by TVEL. From this point onwards TVEL assemblies will be loaded for both Loviisa-1 and Loviisa-2. Calculated critical boron at hot zero power conditions was 9.06 g/kg whereas measured value ( ) was 9.09 g/kg. On at 99.7 % power after 4 FPD operation the calculated and measured critical boron concentrations were 5.82 g/kg ( ppm) and 5.74 g/kg ( ppm), respectively. For the TVEL Gd fuel relative pin powers are somewhat higher at the center of the assembly than with the regular TVEL fuel. Pin powers are compared in Fig. 9. In the Gd assembly the pin powers are 5-9 % higher at the center than in the regular assembly. In Fig. 10 the effect of burnup to the pin powers of regular assembly is shown. In the fresh assembly the pin powers are 2-3 % higher at the center than in the burned assembly. The effect of Gd to the pin powers at the center is 2-3 times higher than the effect of burnup. The difference in the pin power distributions may have an effect on the interpretation of the assembly outlet temperature measurements because there is some indication that the outlet flow is not fully mixed before the thermocouple. The effect has been studied using so called K TC -modeling. K TC gives the ratio between measured and average enthalpy rise of an assembly. K TC -modeling is based on CFD-calculations and tries to describe the effect of pin powers and subchannel flows to the thermocouple measurement. Based on this model the subchannels at the centre of the assembly have stronger influence on the measurement. So far, all assemblies are interpreted similarly i.e. assembly outlet flow is assumed to be fully mixed at the thermocouple. The K TC -model has not been taken into use because some uncertainties still remain. We also want to investigate the behavior in the case of Gd assemblies.

7 In Figs. 11 and 12 assembly powers calculated with HEXBU-3D are compared with assembly powers deduced from assembly outlet temperature measurements. In Fig. 11 comparison is done for Loviisa-1 cycle 33 (first Gd loading) and in Fig. 12 for Loviisa-1 cycle 32 (regular loading with BNFL fuel and no Gd). From Fig. 11 it can be seen that the deviation between measured and calculated assembly power differs in Gd assemblies and their neighbors. In cycle 32 the deviation between measured and calculated assembly power is also different in fresh assemblies and their neighbors. The fresh assemblies (in cycle 32) are mostly in the same positions as the Gd assemblies in cycle 33. With fresh Gd assemblies (in cycle 33) the difference is now some 2 % larger than with fresh regular assemblies previously (in cycle 32). 7. CONCLUSIONS With Gd the behavior of the limiting parameters changes slightly. Maximum linear heat rate remains lower and should no longer limit operation on steady state. Maximum subchannel outlet temperature increases and poses more stringent requirements than before on core reload design and operation. First Gd-fuel assemblies were loaded into Loviisa-1 core in September Agreement in critical boron concentration on both zero and full power conditions indicates that the reactivity effect of Gd is modeled with good accuracy. Comparing cycle 33 to cycle 32 it can be estimated that the deviation between measurements and calculation in Gd assemblies is about 2 % larger than in fresh assemblies in cycle 32. Two things have changed in the fresh assemblies from cycle 32 to cycle 33. First, in cycle 32 fresh assemblies were BNFL assemblies and in cycle 33 they are TVEL assemblies. Secondly, fresh TVEL assemblies in cycle 33 have six Gd 2 O 3 doped pins. In BNFL fuel the flow/bumper grid at the top of the fuel is more open and does not mix the coolant flow as much as the flow/bumper grid does in TVEL fuel. Thus, in BNFL fuel the subchannels at the center have stronger weight on the thermocouple measurement than those at the center in TVEL fuel. This probably explains a large portion of the observed 2 % effect caused by TVEL Gd assemblies. The rest of the effect can be explained by the change in pin power distribution caused by the Gd 2 O 3 doped pins.

8 In general, the 2 % change in the deviation between HEXBU-3D calculations and thermocouple measurements can be considered rather small. Next year we get additional evidence on the pure effect of Gd from Loviisa REFERENCES 1. Antila M., Lahtinen T., Recent Core Design and Operating Experience in Loviisa NPP, 17 th AER Symposium on VVER Reactor Physics and Reactor Safety. Yalta, Crimea, Ukraine, September Rhodes J., Edenius M., CASMO-4: A Fuel Assembly Burnup Program, User's Manual, Studsvik Scandpower Report SSP-01/400 Rev 4, Rhodes J., Smith K., Edenius M., CASMO-4E: Extended Capability CASMO-4, User's Manual, Studsvik Scandpower Report SSP-01/401 Rev 2, Kaloinen, E.: New Version of the HEXBU-3D Code. 2 nd Symposium of the AER for Investigating Neutron Physics and Thermohydraulics Problems of Reactor Safety. Paks, Hungary, September Kaloinen, E., Teräsvirta, R., Siltanen, P.: HEXBU-3D, a three-dimensional PWR-simulator program for hexagonal fuel assemblies. Technical Research Centre of Finland, 7/1981, June 1981.

9 Figure 1. Gd fuel assembly used in Loviisa NPP. Enrichment of the fuel rods is given in the figure. The six 4.0 % enriched rods next to the corner rods also include 3.35 % of Gd 2 O 3. There is no Gd in other rods TVEL 4.37% + Gd TVEL 4.0% k [-] Burnup [MWd/kgU]

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