Editorial - VGB PowerTech Journal 5/2017

Flexible operation of nuclear power plants – first steps for paradigm change worldwide?

The motivation for going flexible for a Nuclear Power Plant (NPP) can vary significantly according to the regional context. For some NPPs, several relevant measures for flexible operation are implemented and well-proven. For others, flexible operation is still not preferable and some actions are even not authorized, e. g. remote secondary control by the grid dispatcher.

The German power plant operators have been demonstrating advanced, safe and reliable flexible operation for many years. NPPs are among the most flexible plants in the grid. Load following operation is typically performed with a power gradient of 2 percent per minute in the range from 40 to 100 percent generator power. The design margin could even allow greater values.

NPPs of the predecessor company of today’s AREVA NP, German KWU, had been already designed for enhanced load-following, primary, secondary and tertiary power regulation capability. Over a long time these capabilities were however only occasionally used to cope with some grid-related events, whilst NPP Unterweser already systematically experienced dynamic power adjustments during the warm months due to the cooling temperature limitations of the river Weser.

The need for frequent flexible operation appeared recently in order to compensate fluctuating, annually increasing power generated by renewable energy sources and lead to first modernization projects related to the improvement of the turbine instrumentation & control (I&C) systems.

Since 2008, the German electricity market allows negative electricity prices. On the one hand, the annual volume of electricity sold at negative prices is worth millions of Euros. On the other hand, the reserve and balancing markets together with redispatching and intraday trading allows new price models.

Hence various German operators of pressurized water reactors decided on full automation of plant operation to achieve maximal flexibility, also with respect to the core loading. The introduction of the digital technology in the field of I&C was very beneficial for the needed upgrade of reactor control, including introduction of the World Association of Nuclear Operators (WANO)-favored reactivity management. This allowed the power level ranges for the control reserve to be increased incrementally, including the necessary prequalification procedure, which created more commercial opportunities on the spot market.

The first plant to begin with this modernization of reactor control was Philippsburg-2 in 2008, achieving the world record in primary power regulation. Further NPPs followed in 2014/2015, introducing remote controlled secondary power regulation in the worldwide leading range from 60 and 100 percent of generator power.

Various feasibility studies were carried out to evaluate the impact of flexible operation on I&C, NSSS and Fuel performance. Thereby, items like power plant chemistry, lifetime of components and power maneuvering/pellet cladding interaction (PCI) – guidelines were stated to be of particular importance.

Developed tools and the existing operational experience related to fatigue, flow-accelerated corrosion (FAC), vibration phenomena etc. are also beneficial for non-nuclear plants in flexible operation (e.?g. fatigue monitoring systems). Related new evaluation methods based on local temperature measurements and corresponding realistic loads are being implemented. In this way, fatigue margins can recover with respect to flexible operation as well as other new requirements such as Environmentally Assisted Fatigue (EAF). For some components even the need for replacement could be waived. Proactive action is of great importance for safe and economic long term operation of any NPP taking into account flexible operation.

Overall, today almost 25 percent of the NPPs worldwide are flexible, providing hundreds of reactor years of operational experience. The largest wealth of experience comes from the French and German fleets. France, with 58 NPPs, has a strong need for flexibility due to the high nuclear share of its energy portfolio. Also one of the Swiss NPPs is now in the final stage of the flexible operation modernization project for reactor control considering additionally heat and steam extraction for external purposes. In the USA, EPRI initiated a technical advisory group to support utilities moving from base load to flexible power operation. Growing renewable energy and strong competition from low-priced carbon fuel required in some American states to promote flexible operation of NPPs. In Spain, with Europe’s second largest wind power capacity and with currently decreasing overall demand, NPPs have begun last year to perform a limited number of slow power ramps. Also for countries using VVER-type NPPs, the grid requirements are changing, e. g. in Russia with respect to primary regulation. Ukraine is considering replacing the control reserve service in future, provided until now by aged fossil fired plants, with existing NPPs. Chinese NPP operators are analysing various flexible operation scenarios and some of them are facing seasonal part load operation due to the grid requirement, e. g. during the rainfall period. For all new builds worldwide, advanced flexibility is an important design requirement anyway. IAEA is close to release a detailed guideline for non-base load operation of NPPs, considering both new builds and existing plants.

What about the current market situation for reserve and balancing energy? The European markets are growing together through cross-border trading. However, the prices are decreasing due to growing competition. In the medium term, the control reserve price is still considered attractive due to the further planned growth of renewables. Offer and demand will determine the price level. Hereby, further development of various new technologies such as the large scale battery storages have to be borne in mind.

In summary, flexible operation is needed to meet the current and future grid requirements and at the same time to ensure profitability for NPPs. In the future, every power plant is expected to become capable of flexible operation, and NPPs will be no exception!