17th Symposium of AER Yalta, Ukraine, September 24-29, 2007 Extension of VVER-440 fuel cycles using improved FA design Pavel Mikoláš Jiří Švarný ŠKODA JS a.s. Orlík 266, Plzeň Czech Republic

ABSTRACT Practically full five years cycle has been achieved at NPP Dukovany in the last years. There are two principal means how it could be achieved. First, it is necessary to use fuel assemblies with higher fuel enrichment and second, to use fuel loading with very low leakage. Both these conditions are fulfilled at NPP Dukovany at this time.

ABSTRACT (CONT.) However, the efficiency of fuel cycle can be improved by increasing the fuel residence time in the core up to six years. There are at least two ways how this goal could be achieved. The simplest way is to increase enrichment in fuel. There exists a limit, which is 5.0 w% of U 235. Taking into account some uncertainty, the calculation maximum is 4.95 w% of U 235. The second way is to change fuel assembly design. There are several possibilities, which seem to be suitable from the neutron – physical point of view. The first one is higher mass content of uranium in a fuel assembly. The next possibility is to enlarge pin pitch. The last possibility is to “omit” FA shroud. This is practically unrealistic; anyway, some other structural parts must be introduced.

ABSTRACT (CONT.) The basic characteristics of these cycles for up-rated power are presented showing that the possibilities of fuel assemblies with this improved design in enlargement of fuel cycles are very promising.

1. INTRODUCTION As stated in Abstract, the efficiency of fuel cycle can be improved if six years cycles would be applied. There are at least two ways how this goal could be achieved. The simplest way is to increase enrichment in fuel. There exists a limit, which is 5.0 w% of U 235. Taking into account some uncertainty, calculation maximum is 4.95 w% of U 235. The second possibility is to change fuel assembly design. In this paper, both possibilities are checked in different range individually.

2. FUEL ASSEMBLY WITH HIGHER ENRICHMENT As it has been stated in introduction, we will suppose enrichment of 4.95 w% of U 235 in some of fuel pins in a fuel assembly. The other characteristics (it means excluding enrichment) are the same as for fuel assembly of Gd-2 or Gd-2M for up-rated power. It seems to be clear that this maximal enrichment can not be applied in all fuel pins because in this case pin power non-uniformity would be very high (more than 1.15), which would be a problem from the point of view of pin power factor in the core.

Therefore, enrichment is lower in some pins and one from possible solutions is shown in Fig. VIIa, where the maximum (1,072) has been found in pin No. 17 according to Fig. II. at FA burn-up of [MWd/tU]. K inf values of some similar designs are shown in Graph 1K (also with zoom for the beginning of burn-up process) and values of maximum of Fdh (Kr) in Graph 1P (three digits in name identify lower enrichment in FA). 2. FUL ASSEMBLY WITH HIGHER ENRICHMENT (CONT.)

Fuel assemblies (according to Fig. VIIa) have been loaded into core in transient end equilibrium cycles of Dukovany NPP for up-rated power (cycles 27 th to 34 th of Unit III) and basic characteristics of these cycles are shown in Tables 1-4, where in column 1 is cycle number, in column 2 are values for base calculation (WFA of enrichment 4.38 w% of U 235 and CFA of 4.25 w% of U 235 ), in column 3 for the variant with WFAs according to Fig. VIIa and CAs acc. to Fig. III, in column 4 for the variant with WFAs according to Fig. VIIa and CAs according to Fig. VI and in column 5 for the variant with WFAs according to Fig. VIIa (but with higher Gd 2 O 3 content [5.0 w%]) and CAs according to Fig. III. It is seen that fuel cycles prolongation has been achieved, but this prolongation may not be sufficient for six-year cycle.

3. NEW FUEL ASSEMBLY DESIGN The other possibility how to extend the fuel cycle (and how potentially achieve six year cycle) is the application of fuel assembly with an improved design. There are several possibilities, which seem to be suitable from the neutron – physical point of view. The first one is higher mass content of uranium in a fuel assembly. This can be achieved by two ways: First, to remove central hole in fuel pellet and second, to enlarge fuel pellet diameter. In our checking, both conditions were applied together. Other characteristics are the same as for FA for up-rated power (Gd-2M).

The next possibility is to enlarge pin pitch. It has been applied too. The “last” possibility is to “omit” FA shroud. This is practically not realistic, in any way, some other structural parts must be introduced in a FA; in our simplified model, thickness of FA shroud was lowered in two steps, namely from 1.5 mm on 1.0 mm and then on 0.5 mm. K inf values for 5 different FAs are shown in Graph 7 (also with zoom for the beginning of burn-up process), 3. NEW FUEL ASSEMBLY DESIGN (CONT.)

where G - means FA according to Fig. VI, Gbezd - FA of the some type as in Fig. VI, but fuel pellet is without central hole and with higher diameter, G124 - as previous, but fuel pin pitch is 12.4 mm (instead of 12.3 mm), G as previous, but FA shroud thickness is only 1.0 mm G as previous, but FA shroud thickness is only 0.5 mm. The basic characteristics of these cycles for up-rated power are shown in Tables 5-7 showing that possibilities of fuel assemblies with this improved design in enlargement of fuel cycles are very promising. All FAs features were calculated by WIMS8 code [1] and core calculations by MOBY-DICK code [2].

4. FUEL CYCLES WITH 4.75W% ENRICHMENT FUEL (QO3) Application of the higher fuel enrichment (WFA enriched on 4.75w%, see FA type QO3 with 3.35w% Gd 2 O 3 and CFA enriched on 4.38w%) was realized for power up-rated design (1444 MWt) of NPP Dukovany. Optimization of low leakage fuel cycle sequences was provided by program OPAL_B [3] on the 3D level core burn up modeled by macrocode MOBY- DICK. Nearly 5.5 year cycle (see Fig. VIII) was reached by optimization, which is in Table 8 compared with original 5 years fuel cycle for up-rated power of NPP Dukovany (WFA enriched on 4.38w%, see FA type QS3 or Gd2M with 3.35w% Gd 2 O 3 and CFA enriched on 4.25w%). From Table 8 is seen that average burn up of six years FA will be lower than MWd/tU.

4. FUEL CYCLES WITH 4.75W% ENRICHMENT FUEL (QO3) (CONT.) During 5 years cycle we have loaded 12 fresh FAs in average in each cycle and during 6 years cycle we have loaded 10 fresh FA. From this it follows that each 6 years FA should (approximately) brought excess of reactivity by 20% higher compared to FA of 5 years cycle. That represents excess by 65 FPD in 326 FPD length of 5 years cycle. According Table 2 we have for QO5 FAs excess 27 FPD, which represents 8.3% increase in fuel cycle length. This agrees with decreasing number of FA by 9%. Additional excess cycle length (or reactivity) can be achieved for example by loading of FA of new design QN1. Combination of improvement QO5 + QN1 potentially can assure pure 6 years fuel cycle.

4. DISCUSSION The values shown in Tables cannot be supposed to be quite real because pin power non-uniformity is too high. Different loadings have been found as it is shown in Table 8 and Fig.VIII. Pin power non-uniformity is still relatively big; it could be lowered by optimization, because we have still excess of reactivity (positive residual boric acid concentration).

4. DISCUSSION (CONT.) Loading with lower number of fresh FA has been found instead of simple enlargement of the fuel cycle. As stated above, although not yet proved, application of FAs with higher enrichment and improved design (see Tables 5-8) could lead to full six years cycle. So, it seems to be clear that FA have still potential in sense of the fuel cycle economy.

5. CONCLUSION Although only preliminary analyses described in the paper have been performed, it can be concluded that temporary design of VVER-440 FA is not optimal (as it is well known) and it exists a great potential how to increase FA reactivity, which is a necessary condition in achieving full six years loading strategy. (Potential in lowering core neutron leakage is practically exhausted as is also the possibility in reducing neutron absorption in construction parts of a FA.)

6 CONCLUSIONS (CONT.) Design change (it means FA of “KARKAS” type) seems to be more encouraging than an attempt still to increase fuel enrichment. Of course, both possibilities (or their combination) must be proved on very well designed loading strategies.

REFERENCES [1]Coll.: WIMS - A Modular Scheme for Neutronics Calculations, User Guide for Version 8, ANSWERS/WIMS(99)9, Winfrith, 1999 [2]Krýsl, V.: MOBY-DICK Users Manual, Report of ŠKODA JS a.s. No.: Ae10068/Dok, Rev. 3, Plzeň 2005, (In Czech) [3] Švarný, J.: OPAL_B The In Core Fuel Cycle Management System Development, Proceedings of the 13 th Symposium of AER, Dresden, Germany, September 2003

Fig. I Calculation scheme of „asymptotic“ fuel assembly

central tube n.n % U 235 Fig.II Numeration of fuel rods

4.4 % U % U % Gd 2 O 3 central tube 3.6 % U 235 Gd Fig.III Fuel enrichment [w%U 235 ] in fuel rods of Russian design of FA „Gd-2“ (average enrichment w%U 235 )

Fig.VIII The odd and even equilibrium loadings of 5.5 years fuel cycle (Numbers represent residence time [years] of FA in core)

Fig.VIII The odd and even equilibrium loadings of 5.5 years fuel cycle (cont.) (Numbers represent residence time [years] of FA in core)

Table 1 Cycles length with FAs with higher enrichment Tables Table 1 Cycles length with FAs with higher enrichment

Table 2 Difference (profit) in cycles length [FPD] with FAs with higher fuelenrichment in relation with base variant (base [Gd2M])

Table 3 Residual boric acid concentration after power stretch-out at fuel cycle length found for FAs with higher fuel enrichment

Table 4 Maximum Fdh (K r ) in cycles with FAs with higher fuel enrichment (value in bracket gives burn-up [in FPD] where this maximum occurs) * means that value shown has been found in more consecutive time steps [ in FPD]; the first from these steps is marked

Table 5 Cycles length with different FAs types Table 5 Cycles length with different FAs types

Table 6 Difference (profit) in cycles length with different FAs types in relation with base variant (base [Gd2M])

Table 7 Maximum Fdh (K r ) in cycles wit different FAs types (value in bracket gives burn-up[in FPD], where this maximum occurs) * means that value shown has been found in more consecutive time steps [ in FPD]; the first from these steps is marked

Table 8 Comparison of design fuel cycle for Gd2M FA (QS3) loadings with upupgraded fuel cycle with 4.75w% FA (QO3) loadings (both for uprated power 1444 MWt)

Table 8 (continuation) Comparison of design fuel cycle for Gd2M FA (QS3) loadings with upgraded fuel cycle with 4.75w% FA (QO3) loadings (both for up rated power 1444 MWt) – reactivity coefficients.