Keeping it contained

Since the Fukushima accident there has been a worldwide effort to develop and apply measures that will prevent the consequences of severe accidents. The most effective strategy is in-vessel melt retention (IVMR), which will ensure there are no radioactive releases outside the containment.

The first analytical and experimental work on this IVMR strategy started on the new- design AP1000 (developed by Westinghouse Electric Company). Then it was used for VVER-440 reactors at Loviisa, in Finland, and at other similar units. For higher power reactors – above 1000MW – the strategy has been applied on new reactors in Korea and China. This work was supported using large-scale test facilities.

In Europe a significant research project is in progress as part of Horizon 2020, the European Union’s wide-ranging research and innovation programme, which has total funding of €80 billion over the period 2014 to 2020. The IVMR research programme is being led by France’s Institut de Radioprotection et de Sûreté Nucléaire (IRSN), with participation from key research organisations in Europe. There is also growing interest from outside Europe.

One objective of this project is to perform small and large-scale experiments, supported by extensive analytical work, on the critical heat flux for external pressure-vessel surface cooling for the VVER-1000. This must provide clear confirmation of the safety margin. It is important to realise that this IVMR strategy is to be used for VVER-1000 reactors in operation. This will be the first application on higher power units.

In order to achieve a high safety margin in critical heat flux (CHF), special technology is under development. Work is underway to develop composite powder/coatings systems to be used on the vessel surface.

Small scale experiments results

Our approach was to build and perform a set of small-scale experiments.

Steel surface samples were placed in the cooling channel. The sample inclination can be varied between 0 and 90 degrees. It was also very important to verify the explosive welding of the thin pressure vessel material, using a copper heater block, which generated heat flux above 2.1MW/m2.

Tests at different inclinations provided a first estimation of the safety margin in the CHF curves, which had been ascertained after extensive analytical calculations, using different codes, and recently also with different corium pool configurations. As mentioned, the surface effect is also very important.

The small-scale facility could be used to test several types of surface treatment; the most effective treatment will be used on the large-scale facility.

The image shows the small-scale facility, and the chart below compares measured heat flux curves with analytical results.

Large-scale facility THS-15

More than 120 experiments were performed using the small-scale facility. They had very positive results in respect to the safety margin, to the CHF values defined by different qualified analytical codes and also with the most recent corium pool configurations.

However, large-scale experiments are definitely needed.

The facility must be designed to fully simulate a semielliptical VVER-1000 pressure-vessel lower head, the exact dimensions of the vessel cavity, cooling water input to the cavity and the exact available space for the steam-release generated. We have not described here the technical details of available passive and active cooling media sources and other very important technical issues, all of which have been the subject of technical studies.

The large-scale tests and experiments are due to begin at THS-15 in 2017 and the project will run until 2019.

Conclusion

It is very important to have a verified severe accident mitigation strategy for nuclear plants in operation. The IVMR strategy, if approved after large-scale experiments using the exact design of the pressure vessel and cavity of a VVER-1000/320, will significantly increase safety of these plants.

Results from the small-scale experiments are very positive. There is a strong chance of increasing the safety margin in new Generation III designs as a result of this research work. 

About the author: Dr. Jirí Žd’árek is vice-president for Business Development Division of Integrity and Technical Engineering at ÚJV R? ež.