Dukovany NPP - Fuel Cycle Development
Josef Bajgl
CEZ Inc., Dukovany NPP
Czech Republic
ABSTRACT
There is described history and possible future of Dukovany NPP fuel cycle developmentin this paper. There are listed main characteristics important to make decision about future of this fuel cycle. The influence of Dukovany NPP Long Term Operation Project (LTO) to the internal fuel cycle is briefly discussed.
1. DNPP FUEL cycle HISTORY
The operation of Dukovany NPP started in 1985 with 3-years fuel cycle with mean discharged burn-up about 30 MWd/kgU. This strategy was called “out-in” fuel strategy, because fresh fuel assemblies were put on the core border. One of disadvantages of this strategy was high reactor pressure vessel (RPV) dose, which caused quite fast irradiation damage of RPV. There was decided to implement Low Leakage Loading Pattern strategy (L3P, so called “in-out”). It was successful and in the 2-nd half of 90-ties this strategy was implemented. It was supported by the fact, that there were made some changes in fuel assembly construction (Zr spacer grids in place of stainless steel one, less thickness of the shroud and other small changes). These changes were finished by new fuel assembly design with profiled enrichment (mean enrichment 3.82%).These assemblies allowed us to reach full 4-years fuel cycle with mean discharged burn-up about 40 MWd/kgU.
The fuel cycle strategy was developed more very quickly. There were made some analyses and their conclusion was that it was possible to implement more economic 5-years fuel cycle. Then we prepared implementation of Gd-1 fuel assemblies (enr. 4.38%, 5-years cycle of working fuel assemblies and 4-years cycle with fuel part of control rod). This fuel has been used since the year 2003. But, fuel assemblies development went on and improvement of pin pitch (then water uranium ratio) allowed to reach the same property with less enrichment(4.25%). This fuel is known as Gd-2 type. This fuel was charged into Dukovany NPP, Unit 3 in 2005. Mean planned discharged burn-up of this fuel is the same as for G-1 type and it is about 50 MWd/kgU. First Gd-1 fuel will be discharged in 2008, first Gd-2 fuel will be discharged in 2010.
There was prepared project with the name “Utilisation of Design Reserves” (see [1]). One of tasks included in this project is to increase nominal reactor power from 1375 MWth to 1444MWth(+5%). There was developed new fuel type, known as Gd-2M, which allowed us to preserve 5-years fuel cycle. This fuel has the same enrichment as Gd-1 fuel. It will be used from the year 2009. It is last contracted fuel.
- DNPP Fuel cycle future
Now it is important to find answer of question: Is the 5-year fuel cycle on power level 1444MWththe last one? Or there are other demands which we have to fulfil? We have to analyse planned Dukovany NPP operation to be able to answer these questions. As it was mentioned above, the Dukovany NPP has been operated since 1985. Projected lifetime was 30years but in middle of 90-ties there was stated that there is an interest to operate DNPP 10years more. There was established so called “Dukovany NPP Long Term Operation Team”, which has the task to specify conditions for extension of DNPP lifetime. Team leader of this Team participated in IAEA programme, which was dedicated to safety assessment of long term operation (SALTO, [2]). There was collected information about other NPP’s lifetime extension and there was written the report about DNPP LTO Project. In this technical economic report, thereis specified modernisation of main equipment and other connected problems according to recommendation of IAEA and other institutions. Last version will be negotiated in ČEZ board of directors in 10/2007. There are discussed three variants (+10, +20, +30 years) and board of directors will decide which one will be implemented. There will be modified fuel cycle strategy according to this decision.
Nevertheless, there was found that the main problem is the problem of reactor pressure vessel dose which is connected with possible RPV annealing. Implementation of L3P fuel cycle strategy enabled decrease the RPV dose substantially - see Fig.1, 2, 3. It is possible to conclude, on the base of analyses results, that it is very probable that there will not be annealed RPV in DNPP in all examined variants. But it is very important to follow RPV behaviour during next operation.
Variants +20 and +30 are connected with extensive equipment modernisation, which are very time-consuming. Next question then is, if it is possible to find that fuel cycle strategy which allows minimize power production losses. It could be possible by the way of junction more outages. This way is possible only by the realisation of other fuel cycle strategy. It was decided to analyze possibility of implementation 6-year and 4 x 18-month fuel cycles on VVER-440. There are analyzed some possible changes (fuel enrichment, changes in fuel assemblies construction, operational limits definitions etc.). These analyses allow us to define frontier of possible VVER-440 fuel cycle more precisely.
- CONCLUSIONS
There was shortly described the history DNPP fuel cycle strategy and DNPP LTO Project influence to the next development of DNPP fuel cycle strategy. Reactor pressure vessel dose of L3P loading pattern was shown and the problem of RPV annealing was discussed.
- References
[1] S.Štech, J.Bajgl: Outlook on to fuel cycle perspectives at VVER-440, Proceedings of the fifteenth Symposium of AER, p.215, Znojmo, 3-7 October, 2005
[2] Final Report of SALTO of water moderated reactors, IAEA, 2007
Fig.1 : Relative RPV dose
Fig.2: Gd-2 – 1-st transient loading pattern
Fig.3: Gd-2 – 2-nd transient loading pattern