There are four type WWER-440 units of Paks NPP installed between 1982 and 1987. Total safety evaluation of the units was accomplished in 1994. As a result of measures taken between 1996 and 2002, in the framework of a programme for increasing safety, the safety level reached that of Western NPPs of similar age. An efficiency enhancement due to reconstruction of the secondary loop and replacements of the turbines increased the original 440 MW of electric power to 470 MW. Further enhancement of efficiency can be realized with increasing the heat power of the reactor. International experiences showed that capacity upgrading could be accomplished this way. Prudent preparations and multilevel reconstructions will have led to reaching 500 MW nominal power output of the four units for 2009.
2. Principals of capacity upgrading
The advance of nuclear technology in recent years provides more accurate knowledge about:
In the United States the power capacity of nuclear power plants was increased by 6010 MW on 125 units in 2007. Enhancements were realized between 0.4 and 17.8 per cent. The new energy programme aims for a further 1380 MW increase on another 26 units.
Two WWER-440/213 units (similar to ones operating in Paks NPP) of Loviisai NPP in Finland have 510 MW power capacity.
A provisional programme in Russia aims for a power capacity increase of between 4 and 7 per cent.
Without constructing new units the nuclear capacity of Spain increased by 578 MW (6.1%), which is equal to a small NPP.
A 7 per cent power increase of the 3rd and 4th unit of the Bohunice NPP is planned in Slovakia. Reconstruction works are in execution and introduction of a new fuel has started. These two units will be operating on 490 MW from 2010.
3. Earlier achievements in Paks
In the framework of Project AGNES, complete re-evaluation of safety of the units was finished in 1994. Between 1996 and 2002 the costs of the Programme of Safety Measurements (PSM) amounted to 60 billion forints. PSM resulted in a safety level of the Paks units equalling the safety level of Pressurized Water Reactors (PWR) in Western countries with a similar age and this was certified in the 2001 annual report of the Western European Nuclear Regulators‘ Association (WENRA).
Before the power capacity increasing programme started in 2002 our efforts concentrated on enhancement of the efficiency of the secondary loop. Comleting the modernizing programme involving eight turbines of the NPP resulted in raising the nominal electric power from 440 MW to 470 MW. This enhancement means a more than 4 % increase of electric power without increasing the heat power of the reactor-moreover, with reduced environmental heat burden.
4. Capacity upgrading programme
A feasibility study of power enhancement was completed in November 2001. The study considered possible solutions and their impacts on the systems of the NPP. Its main conclusions were:
The development conception of PE was completed in October 2002. The Management and the General Meeting accepted the project plan noted that the aim of the project is increasing the heat power of the units from 1375 MW to 1485 MW, namely reaching 500 MW of electric power output. The project plan recorded the necessary reconstruction and the schedule of realization. Realization of the PE has to be completed with in line with the following principals:
Increasing heat power by 8% means 5 °C increase of primary loop heat exchanger medium. This can be realized with the following resources:
5. Security analyses
As a consequence of increasing nominal power to 108% all the analyses included in the Final Safety Reports (FSR) had to be repeated for revalidation, that is the values regarding the new power satisfy safety levels in proposed failure states of the NPP.
Analyses of transients were accomplished with a conservative 108+4% increase of power considering the necessary technology modifications.
During the analysis of proposed failures checking the signals and set values of both the Reactor Defence System (RDS) and the Failure Core Cooling System (FCCS) were accomplished. According to the results, modification of CFCS signals was not necessary. However, some of the RDS values have to be altered according to the new power level.
The results of analyses confirmed unambiguously that neither exceeded limits nor a considerable decrease of reserves could be expected, that is capacity upgrading could not result in violation of approval criteria.
In addition to the deterministic calculation, the impact the of the planned power enhancement on safety was numerically evaluated with the Probabilistic Safety Assessment (PSA) method. In the case of 108% operation, Core Damage Frequency (CDF) in relation to the nominal operation showed a negligible (1.4%) increase above the reference (100%) PSA results.
Realization of the project and execution of modifications require authorizations strictly controlled by Nuclear Safety Regulations (NSR). There are no examples of such complex reconstructions in Hungary. As competent authorities even the National Public Health and Medical Officer’s Service and Duna-Völgyi Környezetvédelmi, Természetvédelmi és Vízügyi Felügyelőség are the participants of the authorization process. Via consultations with the authorities, Paks NPP has established the conception of a multi-step authorization process:
Several experienced Hungarian and foreign scientific and design institutions have participated in the preparation of the authorization process. The Hungarian institutions are: KFKI Atomic Energy Research Institute, Institute for Electric Power Research, ETV-ERŐTERV Power Engineering and Contracting Co. Russian institutions are: Kurchatov Institute as the main constructor of the Paks units, TVEL Corporation and its scientific institutions as the provider of the fuel, and the Experimental Design Bureau Hydropress. In addition, Ukraine's Turboatom Joint Stock Co. is also a participant.
7. Technical conditions, modifications
The real extent of the capacity upgrading is determined by the properties of the core and fuel. Keeping the limitations of reactor physics and providing safe operation of the units requires the following significant modifications.
Introducing new fuel in two steps
– Phase one: rode grid frequency of the normal, average 3.82% U-235 enriched fuel cassette changed from 12.2 mm to 12.3 mm. Control and Safety (CS) cassettes provide more uniform power distribution in the upper part of the fuel cassettes.
– Phase two: optimization of fuel consumption of cassettes with average 4.20% enrichment and with three burnable poison (Gd2O3) containing absorption rods. There will be 18 test cassettes installed into the unit 4 in 2009. The application of the modernized cassettes in the standard operation is scheduled for 2010.
Reconstruction of primary loop pressure controller
Total replacement of the control system will be accomplished with application of a continuous controller. The new device provides more accurate pressure keeping, the former 122 to 124 bar primary loop (over)pressure range will be reduced to 123.6±0.4 bar core pressure. For more accurate pressure control, the Core Control System calculates saturation temperature from the actual values releasing reserves for PE.
Reconstruction of the Core Control System
The PDA-Verona system controls the status of the core and the fuel cassettes. After its reconstruction the system provides higher accuracy and data stream, reduced data processing cycles and calculation with the pressure dependent, actual primary loop saturation temperature for the higher reactor power.
Modification of Reactor Defence System
Three of the failure defence signals, the neutron flux conversion coefficient and the power coefficient were modified in the system according to the new 108% power output requirement.
Modification of the parameters of the hydro-accumulators (HA)
Hydro-accumulators serve for the passive failure core cooling. Parameters of HAs are modified from 58 bar pressure and 40 m3 stored water capacity to 35 bar and 50 m3, respectively, thus during failure the hydro-accumulators can quench the reactor with more water and more reliable timing. As a consequence of this modification, replacement of the scaffold of the drain safety closing and the level measuring equipment is necessary.
Modification of the main circulator pumps (MCP) of Unit 2
Primary loop coolant streams are different in Paks NPP. The primer loop stream of Unit 2 with the smallest coolant delivery capacity needed enhancement via replacement of the impeller of its MCP. The modernized rotors were produced with new forging and welding technology, thus they were tuned to the required characteristics on manufacture. Installation of the new impellers was finished in 2008.
Modification of turbines
Rising the mass flow rate of the primary steam requires increasing the throughput of the turbine’s nozzles, that is application of a new nozzle ring with a larger cross section in the high pressure housing. Requirements of safe economic and reliable steam distribution compel modification of the control system, too.
Increasing the concentration of boric acid in the failure systems
The new core with the same campaign time should have larger reserve reactivity. In the beginning of the campaign this higher reactivity can be absorbed with higher boric acid concentration. The maximum critical boric acid concentration will increase to -12 g/kg and both the minimum boric acid concentration in the failure systems and the stopping boric acid concentration will increase from 12 g/kg to 13.5 g/kg.
The proposed budget of the PE investment programme involves 4.777 billion forints according to the project plan. Economic analyses were prepared by comparison of the project plan with the establishment of an advanced combined cycle gas turbine power plant and by investment return calculations.
According to the calculations, the investment will turn profitable after 3 years and result in 0.50 Ft/kWh estimated cost reduction. The average sale price of the Paks NPP was 9.43 Ft/kWh in 2007, and it was 10.16 Ft/kWh in 2008. This price was 10.67 Ft/kWh in 2009.
After completion of the project the nominal power of each of the four units of Paks NPP reached 500 MW, thus altogether 120 MW of power capacity was established. Analyzing the specific costs of generation of the extra-power, the investment cost of the extra-capacity enhancement was shown to be the lowest as compared to the cost of building new different type power plants.
Specific investment costs [bFt/MW]
New gas turbine
Capacity upgrading of Paks NPP
9. Execution, recent situation
After completion of individual modifications and general maintenance, powering up the units was carried out in three steps according to the authorized operation programme: 100%, 104% and 108% output levels. Comprehesive reactor physics, technology and chemistry investigations, moreover resonation measurements on the designated secondary loop section should be accomplished at each power output level.
Unit 4 and Unit 1 reached 1485 MW heat power (108%) for the first time on 28 September 2006 and 19 June 2007. The Unit 2 reached 1485 MW heat power on 5th December of 2008. Unit 3 had been operated at 104% since 31 October 2008 and the Unit 3 reached the final capacity on 13 November 2009. Since then the powered up units have operated stably on 108%. The parameters are in the permitted ranges and the limiting parameters of the core have adequate reserves. Radiochemical and chemical parameters are in accordance with the criteria. Absolute values of the vibrations measured in the secondary loops are in the permitted range according to the calculations based on ASME standards.
Between 2005 and 2009 the power enhancement programme provided to reach 500 MW electric power with an 8% reactor heat power increase for all the four units. With observance and continuous priority given to safety, the power enhancement and elongation of the operation time of the power plant can satify the majority of Hungary's electric power consumption and ensure the future of national nuclear energy.