Studying RPV ageing

29 June 2017

As most existing light-water reactors are considering extending their operational life, more data about the thermal
ageing and irradiation induced degradation of pressure vessel steels becomes essential. Research conducted by the Dutch company NRG, supported by unique surveillance specimens from the Armenia’s Metsamor reactor, helps assure safe long-term operation of these plants.

Light-water reactors (LWRs) are the most common type of nuclear reactor. There are 350 reactors of this type in the world. Most of them were designed to operate safely for 30 to 40 years, a period that in many cases is elapsing. For most plants, a lifetime extension to 60 or even 80 years is being considered. To determine whether this can be done safely, an assessment of the reactor pressure vessel integrity is essential. The pressure vessel is considered to be irreplaceable or prohibitively expensive to replace. But during operation neutron irradiation and thermal ageing cause hardening and embrittlement of the pressure vessel steel. Degradation means it could be the life-limiting component, and to predict when it will reach this limit the rate of mechanical degradation of steel during plant operation has to be established.

Limited data

Independent supervisors at nuclear plants execute regular ageing monitoring programmes, examining specimens of steel to determine whether the nuclear plant can operate safely. Such surveillance specimens are typically placed inside the reactor vessel. They are closer to the core than the vessel and receive a larger neutron dose, so surveillance specimens age faster than the pressure vessel and can be used to predict the behaviour of the material.

However, these monitoring programmes were also designed to last for 30 to 40 years of reactor operation. Even though appropriate measures are taken to monitor the material behaviour beyond 40 years on an individual reactor basis, the available data and knowledge on how the steel ages after that period is limited. To extend the operational life of existing nuclear plants to 60 or 80 years, a deeper insight is required into these ageing processes and any adverse changes in the properties of the metal.

To obtain these data, in 2013, Dutch nuclear research institute NRG launched the international project Structural Materials for Light Water Reactor Pressure Vessels (Strumat), after it gained access to surveillance specimens from Armenia’s Metsamor plant. As a part of collaboration agreement between NRG and Metsamor, the last (sixth) specimen of the Armenian surveillance programme, which had been present for 27 years in the reactor core, was withdrawn in 2012 and transported to NRG for investigation.

This surveillance chain comprises both thermally aged and neutron irradiated surveillance specimens. “These materials are unique,” say Murthy Kolluri and Lida Magielsen of NRG. “Those samples received a dose of neutron radiation five times larger than the maximum amount of neutron radiation that steel in a nuclear reactor must withstand for 60 years. A study of these samples will allow us to learn how the materials behave and how the internal structure of reactor vessel steel changes after long-term operation of a nuclear reactor. This clearly illustrates whether there will be a break in the trend with the data collected on these materials in the monitoring programmes.”

Thermal ageing alone is not the problem

The Strumat programme is meeting the worldwide demand for more data and tests.

In the programme, NRG is working with the European Commission’s Joint Research Centre as well as nuclear research centres in Armenia. The first results of a study of thermal ageing effects in these surveillance specimens, which have been thermally aged (but not irradiated), for 27 years (~200,000 hrs) at 290°C, were published in the Journal of Nuclear Materials in January 2017. The results show that thermal ageing is not a problem for lifetime extension of VVER-440 type pressure vessel steels.

The results – measured after tensile and impact tests – indicate there is no of significant long-term thermal ageing effect on the Armenian steel. There is no difference between the impact properties of as-received and thermal-aged weld metals. The phase composition of the microstructure did not change significantly. Minor changes observed in the microstructure did not influence mechanical properties. And fracture morphology remained essentially the same after long-term thermal ageing.
Most of the previous investigations of long-term thermal ageing-induced effects had had similar conclusions about VVER-440-type steel. However, the effect of thermal ageing could be important for steels used in other reactor types.

Irradiation study

After this first study, NRG’s researchers are studying irradiation-induced degradation, which has become more significant. “Neutron radiation has clearly the most effect on the hardening and embrittlement mechanisms,” says Magielsen. “For instance, the first results from tensile tests to measure tensile and elongation properties show significant embrittlement of these materials.”

NRG also uses transmission electron microscopy (TEM) to conduct extensive microstructural research, trying to assess new mechanisms of steel degradation, like so-called ‘late blooming phases’. “No one knows whether these phases occur. That could be an important parameter to establish how this material will behave at high neutron fluencies,” says Magielsen.

Lyra helps predict

One of the most important objectives of Strumat is to acquire more data on the mechanical properties and hardening and embrittlement mechanisms after long-term ageing and irradiation of LWR structural steels.

Apart from the surveillance specimens from the Metsamor reactor, the Lyra-project (part of the Strumat programme), provides valuable data and knowledge that can be used to validate existing predictive models, which often apply no further than 40 years.

At the High Flux Reactor in Petten samples of steel used in reactor vessels in Western and Russian nuclear plants are being irradiated with a total radiation dose which is comparable to the total dose in a reactor core that has been operational for 60 to 80 years. Various compositions of steel are being studied to determine which chemical elements have the most impact on the quality of the steel after lengthy exposure to neutrons.

The resulting data and knowledge are important for validating currently available correlations, and prediction models, and developing procedures to aid safe long-term operation of nuclear plants. Results from the Lyra project can provide a better understanding of these factors.

NRG is also using a method called reconstitution to re-use the research materials so that more data about the properties of the metal can be collected. Research is being carried out to identify annealing methods to restore the properties of the irradiated materials. “In Russia, this method has been applied previously for lifetime extension of VVER-440 reactors. We are going to investigate such methodologies for other varieties of pressure vessel steels. The success of such research can contribute to a safe long-term operation and lifetime extension of other types of nuclear power plants,” says Kolluri. 

About NRG

NRG is an internationally operating nuclear service provider. The company produces isotopes, conducts nuclear technological research, is a consultant on the safety and reliability of nuclear installations and provides services related to radiation protection.

Hot Cell laboratories at NRG
Comparison of TEM microstructures of as received and aged RPV steel Reference: M. Kolluri et al., J. Nucl. Mater. 486 (2017) 138-147.
View inside the RPV at Ringhals (Credit: Vattenfall)

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