R2.01/02A Study of radiation phenomena in steel 08X18H10T and its welded joints under irradiation to improve safe operation of VVER internals (PA ref. 7.1.2.1)

Implementation of the project

The project was implemented by means of two contracts, the first with an EC Consultant (Contract 61820, also called project R2.01/02A) and the second with a so-called Director of Experiments [DOE] (Contract 96732, also called project R2.01/02B). The DOE was a Russian design organisation which was both a contributor to and beneficiary of the project results and generated knowledge, in addition to the official beneficiaries Rosenergoatom and Rosatom.

The EC Consultant for this project was Iberdrola in consortium with Ciemat and SEA Ingeneria. A sub-contract was also placed with the Russian CRISM Prometey Institute in Russia. The DOE of the project was EDO Gidropress in co-operation with NIIAR Dimitrovgrad and CRISM Prometey in ST Petersburg/Gatshina. The beneficiaries of the project were Rosenergoatom and Rosatom.

The EC Consultant Iberdrola was carrying out the stress calculations of the internals of the selected VVER 1000 and 440 NPPs. SEA Engineria was responsible for the neutron fluence calculations and Ciemat was advising and following the testing of the radioactive materials in CRISM Prometey in Gatshina. The Russian DOE EDO Gidropress was responsible for the stress analyses and the neutron fluence calculations of the internals on the Russian side. The irradiation of the internals materials took place in NIIAR Dimitrovgrad. This task was the most time consuming and obviously also the most expensive part of this project. CRISM Prometey provided the material for irradiation and testing. CRISM was also responsible for the testing of the irradiated materials in their Hot Cells in Gatshina. Some basic materials testing took also place in NIIAR.

The starting date of the project was the 1st of March 2006 and the project took a total of 49 months. One extension was needed due to the long time needed for irradiation in Dimitrovgrad in BOR 60 FBR. The total project budget was 1,4 M€.
Short Tasks review

In Task 3 the neutron fluence was determined for Kola NPP unit 2 and Balakovo NPP unit 1 RPVI (Reactor Pressure Vessel Internals). For Kola 2 a full core loading pattern and a reduced core pattern with 36 dummy elements were analysed. For Balakovo 1 full core and low leakage core pattern were analysed. The neutron fluence was calculated separately by the Consultant (SEA) and the DoE (EDO Gidropress) and the results were compared for each of the selected core loadings. SEA used a Monte Carlo 3-D code while DoE used an old 2-D method based on discrete ordinates transport codes DOT-III and ANISN (1-D). The results showed that SEA calculations gave systematically somewhat lower (20-40%) neutron dose levels than DoE calculations. This means that the old DoE calculations can be considered conservative as far as neutron dose evaluation is concerned.

In Task 4 the structural integrity of the RPVI was evaluated by both the EC Consultant (Iberdrola) and the DoE (EDO Gidropress). The structural integrity calculations on core internals were calculated by Iberdrola Engineeria and the main plant designer Gidropress. Balakovo NPP unit 1 and Kola NPP unit 3 were used as pilot plants for the study. The aim of the integrity calculations were to:

• Determine the stress-strain state of the internals at, start-up, normal operation conditions, shut down and LBLOCA (Large Break Loss Of Coolant Accident)
• Determine the stresses at selected spots with high neutron fluence
• Determination of temperature and stress distribution in the internals in above mentioned conditions
• Determine critical crack size for selected highly stressed locations
• Compare the results obtained by Iberdrola and Gidropress

The results of the calculations showed that the residual plastic strains in the different parts of the RPV internals are rather small and do not impose any problems as regards reactor trip in case of a scram in EOL (End of Life) conditions. Furthermore dismantling of parts or supply of ECCS water is not restricted after a LBLOCA incident.

Based on the calculated temperature and stress distribution the behaviour of postulated surface crack on selected positions of the internals where evaluated. It was shown that there is no risk of initiation of a crack in EOL condition. The main reason to this finding is the beneficial stress distribution with tensile stresses on the surface which turn to compressive stresses just below the surface.

The aim of Task 5 of the TACIS project was to evaluate the experimental strategy, the material and specimen preparation, the testing scope, the irradiation conditions as well as the testing procedures of the test specimens. As a result a detailed description of the strategy was reported by the DoE, supported by the Consultant.

In Task 6 of the project the experimental reactor facilities of reactor BOR 60 in Dimitrovgrad and the irradiation conditions of the test specimens were described. Details on the irradiation program including loading scheme of test specimens, temperature and neutron fluence monitoring, detailed irradiation schedule and positioning of the irradiation capsules in the reactor core were also provided. The specimens were irradiated to 3 different neutron fluence levels; 5-7 dpa, 25 dpa and up to 50 dpa, which is close to the EOL fluence of the VVER PVIs. The irradiation of the high dose specimens in BOR 60 took about 3 years time. It was not possible to visit the irradiation facilities in Dimitrovgrad during the project. Nevertheless, proper irradiation conditions were confirmed by measuring the radioactivity of test specimens and calculating the fluence (based on neutron spectra and operating history).

Task 7 includes tensile- and fracture toughness and fatigue testing of the reference and the irradiated specimens. The test results showed that the yield- and tensile strength of the material is increasing remarkably at an early stage with relatively low neutron dose. Saturation could be observed already at a dose level of 5 dpa, which level will be reached after a few years of operation in the hot spot locations of the PVIs.

Fracture toughness was measured for dose levels 25 and 40 dpa. The results showed that the J-integral (JIc) decrease to about 1/10 of toughness values in reference (non-irradiated) condition.
Fatigue testing was carried out in air environment. The test results showed that the crack growth rate decrease slightly due to neutron irradiation to small neutron fluences (5-7 dpa), but increase again in higher doses (40 dpa). The fatigue crack growth rate remain below the trend curve for the material.

In Task 8 the Stress Corrosion Cracking properties (IASCC = Irradiation Assisted Stress Corrosion Cracking) were evaluated by testing the selected test specimens in an autoclave in the Hot-Cell laboratories of CRISM Prometey in Gatshina (near ST Petersburg). The test specimens were irradiated to all 3 above mentioned (Task 6) neutron doses. The test method used was SSRT (Slow Strain Rate Testing), where small tensile specimens are pulled very slowly (1 x 10 -7 1/s) to rupture. The autoclave provided the testing environment simulating the primary water chemistry of the VVER NPPs. Based on the stress elongation curves and on the fracture surface characteristics it is possible to evaluate whether the material in sensitive to SCC in selected environment. The test results showed that the PVI material is not susceptible to IASCC in any of the irradiated conditions.

The final technical issue in this project was Task 9; investigation of swelling effects. The swelling of the material was determined by using the Archimedes method with hydrostatic weighing of reference and irradiated samples. The results of the measurements showed that with small neutron doses some shrinkage could be observed, but with higher doses slight swelling occurred (0,164% in 40 dpa).

As a final Task (Task 10) a general assessment and a scientific/technical value of the results of the project and recommendations were given.

The main objective of the project was to evaluate neutron irradiation embrittlement, swelling and hydrogen embrittlement kinetics, as well as IASCC resistance of the austenitic stainless steel due to neutron irradiation in the core. Specific objectives were to determine neutron flux and dose as well as stress distribution in different regions. Furthermore, the objective was to assess integrity of ageing VVER reactor internals in order to ensure safe operation of the NPP in all plant operating and accident conditions. The aim was at preventing failure of the core internals and to avoid stuck-up and blocking of control rods as well as disturbance in the water flow through the reactor core. Blocking of control rods could have serious effects on plant operability in accident conditions and make it impossible bring the plant to a safe mode. Disturbance in water flow through the reactor core would cause overheating and consequently bursting of fuel element cladding and subsequent release of radioactivity in the primary circuit. The most important objective of this project is to avoid the risk of severe accidents in the VVER NPPs.

The results of the project showed the following:

• The neutron fluence calculations for selected NPP RPVIs the Russian calculations have been conservative.
• The temperature and stress distribution calculations showed that the integrity of the RPVI meet the requirements even based on conservative assumptions.
• The mechanical tests showed that the hardening and embrittlement of the irradiated RPVI material was as expected and with rather early saturation.
• The mechanical fatigue of the irradiated RPVI material was below the trend curves for austenitic stainless steel.
• SSRT testing of irradiated RPVI material showed that the material has good resistance against IASCC
• Only a small tendency to void swelling could be observed in the highest neutron dose levels.

Conclusion

This TACIS project was one of the 1st direct contract projects, were EC made separate contracts with the EC Consultant and the Russian DoE (Director of Experiments). The project was technically very successful providing all the results, reports and deliverables expected on a high scientific level for the Russian nuclear industry. The results showed that the integrity of the RPVI can be assured to the EOL (40 years). The results also showed that the VVER RPVI have a good resistance against IASCC during the life time. Furthermore the RPVI material showed only a small tendency to swelling due to neutron irradiation.