Case Forsmark Kraftgrupp AB: Forsmark Boiling Water Reactor simulation applications with Apros® - from power uprate compatibility analysis to safety analysis

Vattenfall Forsmark power plant sea view

Beginning of Apros® use in Forsmark Kraftgrupp AB

In 2004, Forsmark Kraftgrupp AB (FKA) decided to start a project to upgrade F2 power level from 900 MWe to 1118 MWe. The power uprate project included replacing components in the reactor vessels as well as exchange and redesign of parts in other process systems and modifications of control and automation system. Due to the planned extensive changes and modifications in the plant process and control systems, there was a need to perform plant compatibility analyses to make sure that the whole power plant system would function as planned.

FKA assessed the options for performing the complex plant compatibility analyses and chose Apros as the simulation tool since it provided the possibility to simultaneously simulate the dynamics of the complete water/steam cycle including reactor, turbines, condenser, feed water system and control and automation systems using detailed physical models with high accuracy.

Forsmark Kraftgrupp AB

Forsmark Kraftgrupp AB (FKA) operates three boiling water reactors in Sweden. Forsmark 1 and 2 are similar in design, whereas Forsmark 3 operates on higher power level. The total capacity of the three units is 3271 MWe. All plants are BWR reactors designed by the former Swedish company ASEA-ATOM (F1 and F2 are of type BWR69 and F3 is of type BWR75). The three units have been commissioned in 1980, 1981 and 1985. The Forsmark power plant is situated on the Swedish east coast about 150 km north of Stockholm in Östhammar Municipality.

Forsmark 2 model development and use in power uprate project

As a first step towards the power uprate compatibility analyses, an "as-built" Apros model of Forsmark 2 (base model) was developed by Fortum for FKA in 2008. The model was validated against plant transient measurements. During 2009, the model was updated into an uprated model and compatibility analysis were carried out to ensure that the new design performed as planned. Apros was used to investigate and analyze margins and different acceptance criteria to different plant transients. These analyses gave FKA preliminary insight to the future plant behavior and also some insight to how the commission test with the real uprated plant should be carried out.

Following analysis cases were calculated with both the base model and uprated model:

  • Analysis of adapted plant for power uprate without and with increased thermal power
    • partial scram, turbine shutdown, accidental closing of high pressure control valve etc.
  • Analysis of steam reheater
    • Calculation of pressure in steam reheater in case of accidental closure of low pressure shut-off valves
    • Adaption of trip levels to avoid overpressure
  • Fault in feed water controller
    • Maximum feed water flow
    • Increase of reactor water level
    • Investigate the risk of overpressure in reactor tank
  • Accidental closure of main steam valve
    • Calculation of steam flow through remaining valve
    • Risk for self closure of parallel valve due to large flow

The power uprate project for Forsmark unit 2 was implemented during 2009–2011 and this unit has been operating on the uprated power level ever since.

Other use cases besides F2 power uprate project

Besides the power uprate project, FKA has utilized Apros for outage planning and modeling the spent fuel pool at Forsmark 3, for pressure protection analysis of the MSR for Forsmark 3 and containment safety analysis in Forsmark 2.

In outage planning, it is important to ensure there is adequate residual heat cooling available at all times. In some FKA outages, there is a big demand for cooling power to transfer the residual heat away from the nuclear fuel, and planning and handling of the complex system configurations needed for this residual heat removal is a challenging task. To analyze different residual heat transfer rates and system configurations, FKA used Apros to find the balance between the generated residual heat and cooling power, studied the dynamic behavior in case of system failures and also examined the possibilities of new system configurations to handle the residual heat transfer during the outages.

"Apros® has been a great tool to perform detailed process analysis at Forsmark. The detailed models of both the process and automation systems and their interaction have helped us analyze and understand different complex plant events as well as performing safety analysis. Apros is a user-friendly simulation tool and the graphical interface helps to better understand plant behavior as well as showcasing the plant model and its capabilities in the organization.
One of the strengths with our Apros model is that we relatively fast can answer complex questions from projects or in regards to the daily operation and give preliminary but still accurate results in an early phase. This supports the organization in decision making. The Apros model has also been valuable in different projects to compare and verify different simulations or calculations performed by suppliers."
- Magnus Adolfsson, Nuclear Safety Engineer, FKA, Vattenfall

In the pressure protection analysis of the moisture separator reheater (MSR) for Forsmark 3, FKA had ordered the analysis work from external supplier to calculate transient scenario where all Low Pressure Turbine Control Valves (LPTCV) would close simultaneously. FKA also compared the supplier analysis to the Apros analysis results done by FKA, and differences in the expected pressure behavior were found between these two analysis results. Thus, it was important to identify and explain these differences between the analysis results before proceeding further in the project. After making detailed analysis and sorting out different possible causes, it was found out that the difference was due to omission of relevant pipeline between High Pressure Turbine (HPT) exhaust to LPTCV in the supplier model. This pipeline was included in the extensive Apros analysis model. The detailed piping model included in the Apros model brought up the fact that there was a certain delay caused by this piping, which then caused LPTCV to open later in Apros analysis, and thus the pressure in the piping would increase to the acceptance criteria level.

Forsmark simultaneous closure of all LPTCV

In the containment safety analysis in Forsmark 2, FKA wanted to investigate if Apros containment modelling tools would be suitable for their needs, as it was in FKA interest to find a modern software for containment system analysis purposes. A master thesis work was carried out to evaluate Apros in containment analysis area and a similar containment model was developed by FKA personnel with Apros as was used in COPTA simulation code, which was in use at FKA for containment analysis purposes. FKA calculated two containment incidents with both codes: a valve-failure in the reactor pressure relief system and large steam pipe break (LOCA) case. Overall, a good agreement between Apros and COPTA code was found out as the result of this master thesis work. 

Post-Fukushima requirements and Independent Core Cooling system analysis

After the Fukushima accident in Japan in 2011, the European Council stated that the safety of all EU nuclear power plants should be reviewed. On May 25th, 2011, the Swedish Radiation Safety Authority SSM required all Swedish Nuclear Power Plants (NPP) to perform reassessments in accordance with the joint specifications for the stress tests as agreed between European Nuclear Safety Regulatory authorities and the European Commission, within the framework of ENSREG. In 2014, SSM presented its Independent Core Cooling (ICC) requirements to FKA. The requirement for an ICC function is to maintain the ability to ensure reactor core cooling for at least 72 hours in the case of a long-term loss of external power supply. This capability has to be sustained even in combination with extreme conditions such as exceptional weather conditions, earthquakes, etc.

Vattenfall Forsmark nuclear power plant in sunset

In order to support the ICC project, FKA decided to use Apros to calculate the needed analysis for Safety Assessment Report (SAR) update, since over the years FKA had developed their own in-house competence in using Apros. This analysis work consisted of creating the model of the new ICC system and including that as part of their existing plant analysis models, but also to document the developed models in very detailed way. Also, the Apros code capability to calculate official safety analysis needed to be proven to SSM, since FKA had not used Apros to calculate their official safety analysis in Sweden before. So extensive validation and verification work needed to be done. Fortum and VTT had earlier prepared a comprehensive certification package of Apros, which included the general code description and validation procedures of Apros code. In addition to this certification package, FKA calculated other validation cases specific for their own BWR plant and ICC design to SSM.

As part of this work, FKA calculated following analysis cases:

  • Calculation of real Forsmark plant transients and comparison to plant data
    • Load rejection with transition to house load operation
    • Main steam valve isolation
    • Loss of feedwater pumps
    • Etc.
  • Comparison calculations to GOBLIN and COPTA codes, which have been used for FKA SAR analysis earlier
    • LBLOCA
    • Loss of feedwater pumps with delayed scram
    • Etc.
  • BWR integral test program FIX-II (Studsvik, SSM and Asea Atom):
    • Experiment 3051: 10% (area) split break in the external recirculation loop with ADS activated 40 s after the initiating event
    • Experiment 3061: 100% (area) split break in the external recirculation loop with activation of ADS around 1 s after the initiating event
    • Experiment 4011: Double ended guillotine break in one of the four external recirculation loops
    • Experiment 5052: Double ended guillotine break in one of the four external recirculation loops

FKA submitted their SAR including the Apros calculated deterministic safety analysis results to SSM in spring 2020 and in December 2020 FKA received the response from SSM stating that the FKA planned ICC system was accepted to be taken into use, as well as acceptance of the SAR report and the included Apros analysis results.

"In the future the use of simulations with Apros at FKA will most notably be focused towards general process optimization, interoperability analysis as a part of new regulatory requirements and as a simulation tool in different modernization projects."

Peter Höök, Nuclear Safety Engineer, FKA, Vattenfall