VGB PowerTech 5 (2011)

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Load cycling capabilities

38 VGB PowerTech 5/2011

Load cycling capabilities of German Nuclear Power Plants (NPP)Holger Ludwig, Tatiana Salnikova, Andrew Stockman and Ulrich Waas

Kurzfassung

Lastwechselfähigkeiten deutscher KKW

In der Diskussion, ob eine Laufzeitverlängerung der vorhandenen Kernkraftwerke die erweiterte Nutzung der regenerativen Energieträger in der Stromerzeugung unterstützt oder behindert, wird teilweise unterstellt, dass Kernkraftwerke als typische Grundlastkraftwerke nur mit mehr oder minder konstanter Leistung betrieben werden könnten.

Tatsächlich sind die Kernkraftwerke in Deutsch-land bereits in den 1970er-Jahren dafür aus-gelegt worden, größere Lastwechsel ausglei-chen zu können. Zu der entsprechenden Aus-legung gehörten folgende spezifische Merk-male: ein Teillastdiagramm mit konstanter Kühlmitteltemperatur (DWR) bzw. eine Um-wälzregelung (SWR) im oberen Teillastbereich, ein besonderes Steuerelement-Fahrkonzept (DWR), aufwändige Messeinrichtungen, be-sondere leittechnische Einrichtungen zum Re-geln und Begrenzen der Leistung und Leis-tungsdichte sowie die Berücksichtigung einer großen Zahl von Lastwechseln für die Ausle-gung gegen Ermüdung der Komponenten.

Die inzwischen vorliegenden Betriebserfahrun-gen bestätigen eindeutig, dass die so ausge-legten, vorhandenen KKW gut geeignet sind, Lastwechsel auszugleichen, wie sie sich z.B. bei weiterem erheblichen Ausbau der Wind-kraftwerke infolge Windschwankungen er- geben werden. Weiterhin zeigt die Auswertung der Betriebserfahrungen auch unter Berück-sichtigung der heute vorliegenden Kenntnisse, dass keine sicherheitstechnischen Gesichts-punkte einem routinemäßigen Lastwechsel-betrieb der vorhandenen KKW entgegenste-hen. Insofern kann, wenn gewünscht, der Wei-terbetrieb der KKW den Ausbau der regenera-tiven Energiequellen in der Stromerzeugung absichern.

Requirements for the load cycling operation of German NPPs

The debate on whether lifetime extension of existing nuclear power plants (NPP) facilitates or impedes the increased use of renewable en-ergy sources (especially wind power and pho-tovoltaics) in power generation has been going on for some time. It is often claimed that NPPs can only be operated to meet base-load demand at more or less constant power output and can-not readily react to changes in load demand (see for instance the policy question raised by the SPD opposition faction in the German Bundestag on 25 February 2010 [1]).

This impression may have been propagated in the general public because German NPPs ran mainly in base-load operation in the past. In actual fact, however, detailed information on load cycling capability was published as early as the seventies and eighties (e.g. fundamental principles published in [2] in 1985). The rea-son behind this design was primarily the fore-cast of larger proportions of NPPs in the in-stalled capacity of small-sized grids, and not the present substantial increase in the share of renewable energy in total power generation. Nevertheless, the question of whether an un-expected spell of calm weather devoid of wind or the half-time break of the World Cup final is the cause of the change in demand for pow-er from the NPP fleet is irrelevant; only the power gradient (rate of change in power) and the power increment (the amount of the power change) are important.

A recent study by the Institute of Energy Eco-nomics and Rational Use of Energy (IER) [3] for the years 2020 and 2030 has examined, on the basis of an estimated 30 to 40 % of elec-tricity being generated from renewable ener-gies, to what extent the power plant fleet prob-ably existing at that time would be suitable for timely compensation of the fluctuating power supply from renewable energies. The study concluded that the fleet would meet this task in the scenario with lifetime extension of ex-isting NPPs better than in the phase-out sce-nario. The “lifetime extension” scenario proved to be more cost-effective and resulted in greater CO2 emission cuts. The IER used a power gradient of up to 65 MW/min for each NPP and a power increment of 50 % for pres-surised-water reactors (PWR) and 40 % for boiling-water reactors (BWR) of rated power. With these parameters, the German NPPs pro-vide a total regulating power of up to 9,600

MW. (In fact, the design allows a little more, namely up to about 10,000 MW regulating power from the NPPs.)

Nuclear power plant operators (VGB) have requested the manufacturer (now Areva) to review these assumptions under consideration of present experience and knowledge from a technical perspective, so as to make a techni-cally sound statement on the interplay of nu-clear power with other CO2-free energy pro-duction systems.These are outlined below:

load cycling for PWRs and BWRs, −the most important existing design charac- −teristics,the operating experience with load cycling −gained to date, andthe effects of more frequent load cycling on −the systems and components.

Description of load cycling

PWR

The schematic part-load diagram of a PWR ( F i g u r e 1 ) shows the temperature of the coolant (CT) at the reactor pressure vessel (RPV) inlet, the RPV outlet, and the average CT across the steam generator as a function of the reactor power (100 % = RTP – rated thermal power). The range of the constant coolant tem-perature between (in example) 40 and 100 % can be clearly seen. In the event of a power re-duction, the so-called D bank (4 groups, each comprising 4 control assemblies distributed across the reactor core) is inserted into the reac-tor core until the desired power change is achieved. The remaining control assemblies (typically 45) are assigned to the so-called L bank, which always remains at a high position during power operation and thus preserves the shutdown margin, which is a variable that is important to safety. Slower changes of power distribution in the core occurring after reaching the desired output, and the concentration of xe-non isotopes in the nuclear fuel are compen-sated by minor movements of the L bank and changes in boron concentration in the coolant. On a power increase, the D bank is first to be withdrawn from the core. If necessary, the pow-er increase can be boosted by withdrawing the L bank simultaneously; however, this depends on the position of the L bank (for overview of the above components, see F i g u r e 2 ).

The power gradient for a power increase is limited, for example, by the permissible power

Authors

Dipl.-Ing. Holger Ludwig Dr.-Ing. Tatiana SalnikovaAndrew Stockman Dipl.-Phys. Ulrich WaasAREVA NP GmbHErlangen/Germany

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VGB PowerTech 5/2011 39

density. Power reduction is possible at virtu-ally any desired rate.

BWR

The output of BWRs can be regulated either by manoeuvring control rods or by changing the speed of the forced circulation pumps and thus the coolant flow rate (recirculation control). Recirculation control is perfectly suitable for load cycling in the upper power range (about 60 to 100 % of rated electrical output – REO). The amount of steam in the reactor core increases with a reduction of coolant flow rate, thus re-ducing both moderator density and reactivity. By contrast, increasing the coolant flow rate leads to an increase in the moderator density and reactivity and thus to an increase in power. F i g u r e 3 shows the characteristic curve for recirculation control; this plots the reactor pow-er as a function of the core flow for a constant control rod position. A major advantage of re-circulation-only control is that the relative pow-er distribution in the core is not significantly affected upon load changes, because no ma-noeuvring of control rods is required for this purpose. This minimises stressing of the fuel

rods caused by load cycling and consequent changes in temperature in the fuel rod. The maximum feasible load change rates are about 10 % REO/min in the recirculation control power range. Power changes beyond the recir-culation control range are done by manoeuvring control rods. With optimised control rod ma-noeuvring sequences power increments total-ling between 20 and 100 % can be achieved at sufficiently high power gradients (for overview of the above components, see F i g u r e 4 ).

S t a r tup and shu tdown

The 9,600 MW regulating power assumed in the IER study [3] can be provided by the Ger-man NPPs particularly well in the upper power range in which they are readily maneuverable. In addition, it would be possible to disconnect units temporarily from the grid, and then re-start later. In this case, an NPP does not re-quire the often mentioned 1 to 2 days to reach full load, but only 1 to 2 hours. The difference lies in the different plant states at shutdown conditions. While 1 to 2 days are required for starting up a NPP after a refuelling, starting from the hot standby state takes just about 2 hours to reach nearly full load. It is even faster if the unit is kept running at house load, name-ly the power demand of the power plant is pro-vided by its own generator. The generator re-mains synchronised with the grid. Run-up to full load is then possible in less than an hour. This mode of operation is designed for special situations and meets the requirements for NPPs formulated by the grid operators (re-quired maximum startup time is 3 h) [4].

Design characteristics

Stringent requirements with regard to control-lability were placed on the German NPPs even

Figure 1. Schematic part-load diagram of a PWR.

Figure 2. Illustration of typical RPV internals of a PWR.

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Figure 3. Schematic characteristic curve for recirculation control (BWR).

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during the design phase. Therefore, they have special design features that stand out from the international standard (see e.g. [2, 5, 6]).

Con t ro l rod manoeuvr ing p rog ram (PWR)

The manoeuvring of control rods for the pur-pose of power control always has an effect on the axial power distribution in the core which ideally is cosinusoidal in shape. A power in-crease will generally result in increased local power density. In this case, the installed power and power density limitation functions ensure that permitted maximum values of power den-sity are not exceeded despite such changes. However, as activation of the limitation system slows down the power increase, it is important for effective and especially fast reactor control that power and power distribution can be con-trolled independently. The control rod manoeu-vring program of the German PWRs relies on standard control assemblies, all of which can be used for both regulation and shutdown of the reactor. From a functional perspective, the control assemblies are grouped in two banks: the D bank made up of 4 sequentially moving groups each consisting of only 4 control as-semblies; and the L bank with the majority of the control assemblies. The D bank is used for regulating the integral reactor power. As a re-sult, the axial power distribution is only slight-ly distorted by the four moving control assem-blies. The L bank can be used to back up reac-tor power control. Its main task is to control the axial power distribution, for which a change in position of only a few centimetres can yield a sufficient effect. (The L and D banks have to be manoeuvered in a mutually compensating sequence for changing the power distribution at a constant reactor power). This control rod manoeuvring concept enables rapid power changes but requires prompt and accurate measurement of power distribution by appro-priate in-core instrumentation.

In -co re in s t r umen ta t ion (PWR and BWR)

The power density, together with burnup, is significant for fuel management. Therefore, there are limits for the power density that may not be exceeded. In case of load cycling (by control rod manoeuvring), the pattern of the power distribution changes and, with it, the maximum local power density. The difference between the relevant maximum local value of power density and its limit defines the margin available for such redistributions. The more reliably and more precisely the local power density is measured and controlled during op-eration, the greater the operating margins for load cycling. The core monitoring system used in German NPPs, especially in interna-tional comparison, is fast and accurate, and thus provides larger margins for load cycling. This is made possible by an in-core instru-mentation system with both in-core power distribution detectors (PDD) which continu-ously measure the power distribution directly

in the core (therefore fast), and an Aeroball flux measuring system (PWR) or traversing in-core probe system (BWR), with which the PDDs are regularly calibrated during opera-tion (therefore precise).

Pa r t - load d i ag ram (PWR)

There are basically two types of part-load dia-grams for PWRs, one with constant average CT and the other with varying CT but constant main steam pressure. In German PWRs, a mix-ture of both types is applied (see Figure 1). Load cycling is done with constant average CT in the power range from 60 to 100 %, in which most load cycling operations are run. This has the advantage that the coolant volume remains relatively constant and the change in reactivity with varying CT is low in case of power changes. Thus, the requirements for the volume control system and reactor power con-trol (control rod manoeuvring) are reduced. Moreover, the mass exchange between the pressuriser and the rest of the reactor coolant system is reduced, leading to lower tempera-ture fluctuations in the surge line.

Rec i r cu la t ion con t ro l (BWR)

Expressed simply, the above-explained recir-culation control for BWRs (see Figure 3) com-bines the benefits of the optimised control rod manoeuvring program and the part-load dia-

gram of the PWR for BWR conditions. Con-sequently, it is intrinsic to the BWR that it can change power in the upper load range faster and with less power distribution deformation than the PWR, because no manoeuvring of control rods is needed. Temperature and pres-sure remain constant in the main steam sys-tem, so that also the loads on the components of the reactor coolant system and adjoining systems remain low.

Limitat ion systems (PWR and BWR)

There are safety-related limits for key operat-ing parameters in NPPs that may not be ex-ceeded. To ensure this, sequential instrumen-tation and control systems provide step-raised automatic actuation of counteractions:

Deviations from the normal state are at first −detected and restored to normal by the op-erational controls.

If the controls are unable to do this, priori- −tised limitation systems intervene and re-turn the plant to the control range of the operating controls.

If this is insufficient in case of serious mal- −functions, a safe state is established by the reactor protection system, in particular by initiating reactor trip (RT).

The intervention of the reactor protection sys-tem (e.g. RT) is averted for most malfunctions

Figure 4. Illustration of typical RPV internals of a BWR.

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VGB PowerTech 5/2011

by the limitation systems, which thus mini-mise life-limiting loadings on plant systems. As a result, the limitation systems not only fulfil a safety function but also increase the availability of the plant. The German NPPs are international leaders in the automation of such limitation functions. A wealth of thought and experience has gone into the limitation sys-tems concept of German PWRs and BWRs with respect to load cycling. In particular, the power and power density limitation systems with their various constituent functions, and the control rod manoeuvring limitation sys-tems contribute significantly to maintaining sufficient margins to safety limits, taking load cycling especially into account.

The advantages of the automated limitation functions with regard to load cycling capabili-ties of German NPPs are:

The controls can be primarily designed −from the point of view of optimum function without regard for safety issues which are covered by the limitation systems.

The operating staffs are relieved of monitor- −ing responsibilities and can therefore devote special attention to load cycling opera-tions.

Plant availability and thus, grid reliability −can be improved.

The high accuracy and reliability of the −limitation systems (in conjunction with the in-core instrumentation) allow greater scope for load cycling.

Des ign measu res to mi t iga t e and con t ro l cyc l i c l oad ings (PWR and BWR)

Load cycling within the scope outlined above is predominantly associated with only minor changes in global plant parameters such as pressure and temperature in the reactor cool-ant system. The resulting low thermal stresses are not relevant to the fatigue of the affected components. Large temperature gradients with correspondingly higher loads can occur when different hot fluids meet in individual compo-nents. In addition to a low-stress mode of op-eration (see part-load diagram), such loadings are well-controlled by a suitable design, name-ly the choice of suitable materials and appro-priate dimensioning or mechanical design to reduce temperature changes, for example in the area of injection nozzles. German NPPs are designed for the stresses associated with load cycling. This design is based on defined numbers of service conditions (in this case load cycles), which bound the frequencies ex-pected during the lifetime of the plant.

Fat igue monitor ing (PWR and BWR)

Continuous fatigue monitoring (measuring, recording and analysis of wall temperatures), which facilitates both graduated quick evalua-tion and detailed fatigue analysis is available for components susceptible to fatigue. Moreo-ver, by the comparing of the history of service conditions, such as load cycles or operational

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transients, with the design assumptions docu-mented in thermal load specifications, it can be ensured that the component design imposes no restrictions on the operation of the plants. In addition, periodic, non-destructive testing at specified intervals is performed especially for safety-related components. Despite all of this, if unexpected effects should occur, these would be detected in time.

Operating experience

Extensive experience with load cycling exists both in Germany and internationally. In par-ticular, so-called primary control, in which reactor power control is linked to the grid frequency directly and without intervention of the operator, is established practice. The power increments are limited to a maximum of 5 % REO (equivalent to 65 MW per unit) in which the power change can take place quick-ly. This mode of operation stresses the system only minimally due to the limited power incre-ments, and experience with it is positive (Isar 2 (KKI 2) [5] and Isar 1 (KKI 1) [6]).

Larger load ramps are usually actuated manu-ally by the operator. Preparations for the use of an automated secondary control, in which the plant output is controlled by an outside signal, are already complete or are currently being performed for various plants. All plants have run corresponding load ramps in the course of their operating lifetimes. In 2009, some units such as Phillipsburg 1 (KKP 1) and Neckar-westheim I (GKN I) operated almost entirely in the load follow mode (see F i g u r e 5 [7]).

In this case the load gradients were up to 2 %/min, the power increments were generally in the range of constant CT (PWR) and the recirculation control (BWR). Existing opera-tional experience with this mode of load cy-cling is also positive. The mentioned 2 % REO/min still means approximately 400 MW of power change in a quarter of an hour for only one of the larger NPPs, so that the re-quirements for compensating wind fluctua-tions are practically met.

With regard to experience with load cycling, it is also worthwhile to take a glance across the Rhine, to France where, in 2008, approximate-ly 58 % of power generation capacity was nu-clear, and its share in electricity production was as high as 76 %. Naturally, the French NPPSs contribute to both frequency stabiliza-

tion and balancing out medium-term load fluc-tuations. Load gradients of up to 5 %/min in a load range from 30 to 100 % are specified.

Effect of load cycling on the plant

The effect of load cycling on the plant was taken into account by the manufacturer in de-sign and is now being reviewed from a modern perspective. In the following, the effects on the reactor core are described and then the effects of load cycling on other components and sys-tems and on transient processes are assessed from mechanical, chemical, radiological, proc-ess control and electrical standpoints.

Reac to r co re

The feasibility of the specified load ramps of up to 10 % REO/min was proven for both BWRs and PWRs in the course of commis-sioning. In the subsequent operating time, the plants were modified to some extent (e.g., en-richment increases, power uprates) and fuel management strategies were optimized for base-load operation. This generally resulted in the margins available for load cycling (inter-vals until power and power density limits are reached) becoming tighter. The margins have been partly increased again through improve-ments in fuel technology (e.g. transition to larger number of fuel rods per fuel assembly in BWRs). The power gradient of 2 % REO/min (corresponds to about 25 MW/min in most plants) desired for an decrease of wind energy can be achieved in any case. Larger power gra-dients of up to 100 MW/min can be achieved for BWRs in the range between 60 and 100 % REO without violating design limits relatively easily by changing the speed of the forced cir-culation pumps. In PWRs, it is now possible in certain circumstances for power increases with large gradients and increments to cause limits on maximum power density to be reached at which the limitation functions would automat-ically reduce the rate of power increase. (Fast power reductions are always possible.) The ac-tuation of the limitation systems is permissible for safety reasons but undesirable from the op-erational point of view. Therefore – where de-sirable for grid stabilization – operational im-provements can also be undertaken to allow greater power gradients for large power incre-ments. These include:

Optimised fuel management strategy (PWR/ −BWR)

Optimised control rod manoeuvring con- −trols (PWR/BWR)

Optimised measurement selection for the −PDD signals (I&C (instrumentation & con-trol) consideration of masking effects of the D Bank) (PWR)

Fuel rods and fuel assemblies are generally de-signed for the loadings of load cycling. The Pellet Clad Interaction (PCI) damage mecha-nism is especially relevant with regard to such loadings. PCI is the combination of Pellet Clad Mechanical Interaction (PCMI) and mecha-nisms of stress-corrosion cracking in the inte-rior of the fuel rod. PCMI is caused by contact between fuel pellet and cladding due to differ-ential thermal expansion. Fuel expansion oc-curs as a function of power, and accordingly, the “expansion differences” between pellet and cladding depend on the power increment. The PCI threshold defines the power level below which no PCI damage can be expected. Stress relief through relaxation of the cladding at higher power levels can raise the PCI thresh-old. This process is known as conditioning.

A specific limitation function takes into ac-count the power history of the reactor and thus the conditioning of the fuel in the limits for avoiding PCI. Both analyses and the opera-tional experience demonstrate the effective-ness of this measure. However, as a precau-tion, further operational experience, including with higher power gradients, should be col-lected and evaluated.

On the whole, implementation of the above measures ensures that fuel rod failures are not to be expected even in load cycling operation. If isolated fuel rod failures do occur (leaks in individual fuel rods of the more than 40,000 present in the reactor core – although rare – have occurred for other reasons), the coolant purification system is designed to remove ra-dioactive materials from the coolant and thus limit impact on personnel and the environment to acceptable levels. Furthermore, general practice is not to carry out load cycling on a large scale if fuel rod failures have occurred.

NPPs can carry out load changes more easily (faster, larger increment), if the operator is in-formed of the impending load change a couple of hours earlier. With better prediction of the power yield from renewable energies, the po-tential for optimised load sequence of NPP also grows. For example, if the required load change is known an hour in advance, the reac-tor operator can prepare the plant (setting of optimum parameters with regard to available control rod reactivity, power distribution, PCI margin) so that the transient processes in-volved in the power increase are simplified.

Mechanical phenomena

Mate r i a l f a t igue

Components made from metallic materials have the property of losing mechanical

Figure 5. Load follow operation of Neckarwestheim I (GKN I) NPP in 2009. [7]

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Load cycling capabilities

Table 1. Number of load cycles considered for design (example KONVOI PWR).

Load cycle (% RTP) Number

10 (step change) 100,000

100 – 80 – 100 100,000

100 – 60 – 100 15,000

100 – 40 – 100 12,000

strength under cyclic loadings which exceed certain limits; this is known as fatigue. Such cyclic loadings can also be generated by load cycling, especially when large temperature transients occur in the material. However, the design of German NPPs, as already men-tioned, was based on load cycling spanning the entire lifetime. Accordingly, the number of load cycles has been set relatively high (see Ta b l e 1 for load changes relevant in this context). The loadings associated with the re-spective load cycles were determined for com-ponents susceptible to fatigue and included in their sizing. Furthermore, ongoing fatigue evaluations are conducted based on the data obtained by fatigue monitoring.

Since the German NPPs have mainly been op-erated at constant power in their operating life-times to date, there are still significant reserves with respect to material fatigue. A simple ex-ample for illustration: a power plant which has run a typical number of 2,000 load cycles run between 60 and 100 % would be capable of a further 13,000 such load cycles before using up the remainder. Under the (unrealistic) as-sumption that such load cycles would be need-ed daily to compensate for changing wind power generation, the design number of load cycles would be reached only after 35 years. It is to be borne in mind that the design in any case still includes a significant safety margin until damage (e.g. leakage) might occur.

Moreover, continuous fatigue monitoring and assessment ensure that any new findings and

requirements (e.g. mechanical strength prop-erties) can be responded to quickly and effec-tively.

The plants can also take preventive measures to further mitigate the impact of load cycling on material fatigue. Optimization of the oper-ational controls is to be mentioned here first. Further developments in fatigue analysis to-wards more realistic methods will also be helpful in the assessment of load cycling. De-spite all of this, if a single component or length of piping should reach its design limits, re-placement of the affected component or pipe would also be possible in principle.

Eros ion co r ros ion

In the fluid systems of a NPP, throttling of flow rates may be required in load cycling opera-tion, e.g. at valves of the main feedwater sys-tem (PWR), which could result in increased erosion corrosion due to local flow rate in-creases. This applies in principle to operational systems only, but not to safety systems as these are usually on standby during the normal op-eration of the plant. For operational reasons, plant-specific assessment and monitoring of these effects may be useful.

Wear and t ea r

Active components (e.g. control valves, pumps) can be exposed to increased wear and tear due to load cycling. In general, these are operational systems with no importance to safety. The effects of load cycling can be con-sidered in maintenance and repair plans.

Where components of safety systems (e.g. control rod drive mechanisms) are activated often as a result of load cycling, this has been considered as a specified number of load cycles (see above) in the design. If necessary, the effects of load cycling would be detected at an early stage through periodic inspections.

Chemis t r y / r ad io logy

The fluctuations of the physical parameters (pressure, temperature, flow rate) of the reac-tor coolant system caused by load cycling tend to be of secondary importance for corrosion mechanisms in the reactor coolant system. Variations of chemical parameters are detected by continuous monitoring of the coolant. If necessary, corrections are initiated by the staff. Operational experience shows no or only a mi-nor effect of load cycling on corrosion and dose rates as a result of corrosion product par-ticles in the coolant [8].

P rocess eng inee r ing

The process engineering sequences which take place during load ramps were already roughly

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described in Section 2. A NPP consists of nu-merous interacting controls and systems. There are power states in which the system can be operated more smoothly than in others (mini-mal challenging of operational controls). From an operational perspective, such “smooth-run-ning” power states are to be preferred, and it is desirable to extend this power range as much as possible. An example of such an improvement of operational processes would be the use of variable speed pumps instead of throttling valves. This would also reduce the electrical house load demand of the plant.

E lec t r i ca l equ ipmen t

No significant effect of load cycling on the generator is to be expected, as the generator has to provide less active power in the part-load range, and therefore the reserves are higher. The effect is marginal also for the transformers, as the generator transformer is less loaded in the part-load range. The auxil-iary transformers are virtually unaffected by load cycling because the house load hardly varies with the load condition of the plant.

Trans i en t s / acc iden t s

Safety analyses are performed for abnormal operating transients and accidents. As required by German regulations, the worst conditions for normal operation are assumed as initial conditions for the transients and accidents. In-sofar as part-load conditions are unfavourable, they have been generally taken into account in the analysis in that the potential reactor states due to load cycling were approved in the orig-inal licensing procedure.

Load changes have an impact on various pa-rameters that describe the initial state of pos-sible transients, such as reactor power, coolant temperature, core flow, xenon concentration and distribution, critical boron concentrations,

loadings caused by load cycling. Basically, the stresses due to load cycling are already cov-ered by the design of the NPP.

The safety of NPPs is not affected by load cy-cling, since:

all relevant plant states (even different load −conditions) were considered in the applica-ble safety cases,

reliable limitation systems are available, −

continuous monitoring of fatigue of ex- −posed parts is provided and

periodic inspections of critical safety-relat- −ed components are carried out.

Therefore, from a safety perspective, there is no objection to load cycling as would be need-ed in combination with a further expansion of renewable energies.

From the perspective of efficient energy use, further expansion of renewable energy in-creases the demand for power plants that are suitable for large, rapid load changes. Com-pensation for feed-in fluctuations through cor-respondingly large storage capabilities is not foreseeable in the near future. Hence, NPPs are particularly suitable for combination with renewable energies.

References

[1] Deutscher Bundestag, Drucksache 17/832, Große Anfrage der Fraktion der SPD vom 25.02.2010: Verlängerung von Restlaufzeiten von Atomkraftwerken – Auswirkungen auf die Entwicklung des Wettbewerbs auf dem Strom-markt und auf den Ausbau erneuerbarer Ener-gien.

[2] Aleite, Werner: Lastfolgefähigkeiten von Kern-kraftwerken mit Druckwasserreaktoren; VGB-Kongress „Kraftwerke 1985“.

[3] Hundt, Barth, Sun, Wissel, Voß: Verträglichkeit von erneuerbaren Energien und Kernenergie im Erzeugungsportfolio. Institut für Energiewirt-schaft und Rationelle Energieanwendung, Uni-versität Stuttgart, 2010.

[4] Deutsche Verbundgesellschaft e.V.: Das versor-gungsgerechte Verhalten der thermischen Kraftwerke; Heidelberg, Oktober 1991.

[5] Müller: Lastfolgebetrieb und Primärregelung – Erfahrungen mit dem Verhalten des Reaktors KKI 2; KTG Fachtagung Reaktorbetrieb und Kernüberwachung, Rossendorf, February 2003.

[6] Frank: Lastfolgebetrieb und Primärregelung – Erfahrungen mit dem Verhalten des Reaktors KKI 1; KTG Fachtagung Reaktorbetrieb und Kernüberwachung, FZ Rossendorf, February 2003.

[7] atw – Internationale Zeitschrift für Kernener-gie: Kernkraftwerke in Deutschland – Betriebs-ergebnisse 2009.

[8] Rich, Stellwag, Böttcher, Jürgensen, Holz, Markgraf, Schütz, Rübenich: Zink-Dosierung in Siemens-DWR Anlagen; VGB PowerTech 5/2010.

[9] DEWI/E.ON Netz/EWI/RWE Transportnetz Strom/VE Transmission: Energiewirtschaft- liche Planung für die Netzintegration von Windenergie in Deutschland an Land und Off-shore bis zum Jahr 2020; DENA-Netzstudie, Köln, 24. February 2005. □

Figure 6. Regulating power necessary for compensation of fluctuating energy production of wind power plants [9].

power distribution and control rod positions. The above parameters are controlled or limited within a narrow range by various controls, the control rod insertion limits and various power limitations (with margins, e.g. for loss-of-coolant accidents or loss of offsite power). Even if load changes alter various parameters, the parameters are maintained within the con-trol ranges and the setpoints of the limitation functions regardless of the power history.

Conclusions

Extensive operational experience exists for load cycles with power gradients of up to 2 %/min and power increments in the range from about 50 to 100 %, partly also for higher gra-dients. Such load cycles in the existing NPP, with a total of up to 10,000 MW, are certainly sufficient to compensate for fluctuations in the wind-generated electricity supply required according to the present analyses [9] (see F i g u r e 6 ).

Larger power gradients and increments required by the grid can also be achieved. Depending on the plant, the current maximum achievable gra-dients/increments can be limited, for example, by the margins to the setpoints of the limitation functions. But there are plenty of possibilities for optimising systems and achieving higher power gradients. Approaches include:

fuel management strategy optimised for −load cycling

optimised control rod manoeuvring controls −and

look-ahead operating strategies. −

Likewise, there are options for optimising low-stress operation, preferably in the range of operational controls to reduce the additional

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