R2.06/96 - surveillance programme VVER 1000

The main objectives of the project were:

• Define adequate and reliable methods for determining of the real irradiation conditions of the surveillance specimens of VVER 1000 RPV (neutron fluence and temperature monitoring)
• Procurement, installation and commissioning of a welding machine in RRCKI for reconstruction of broken surveillance specimens in order to improve the reliability of test results
• Re-assess the current situation of the surveillance programs of VVER 1000 NPPs
• Define corrective actions and alternative surveillance programs

Implementation of the project

The consortium consisted of Belgatom SA and Framatome ANP (former Siemens AG), Belgatom being the main contractor. SCK-CEN was the local sub-contractor. The Russian Beneficiary was Rosenergoatom and the Local Sub-contractor in Russia was Atomstroyexport. In Russia, mainly RRCKI (RRC-Kurchatov Institute) and ZNIIKM Prometey were carrying out the technical work. The irradiation of the temperature monitors and dosimeters to determine irradiation conditions was implemented in Balakovo Unit 1. Reconstitution and testing of surveillance specimens was performed in RRCKI and the work on the advanced technologies in toughness measurements was carried out in ZNIIKM Prometey and Framatome ANP. Part of the dosimetry was carried out in SCK-CEN in Belgium. The project was originally scheduled for a time period of 24 months. There were some problems in the irradiation schedule, which caused some delays of Task 2. The delay was due to the plant operation/outage schedule of Balakovo 1 and was thus a Force Major situation for the consortium. This delay was then seen in other Tasks, which were dependent on the results from Task 2. Due to this delay, AIDCO confirmed an extension of the project by 6 months. The starting date of the project was 09.04.1999 and the deadline after extension was 09.10.2001. The budget of the project was 800 000 Euro.

Short Tasks review

In Task 1, a general “Description of the surveillance program” was given. Furthermore a total of 11 surveillance sets of VVER-1000 RPVs were reviewed and evaluated. So far sets number 1 and 2 of surveillance program have been withdrawn and tested in the VVER-1000 plants in operation in Russia. Only in Novovoronesh 5 also the 3rd set was tested already. According to the normal schedule the 3rd set shall be withdrawn after 17 years of plant operation. According to Deliverable 2 it is not possible to elaborate a reliable Charpy V or KIc transition curve for the tested material since the variation in neutron fluence in one set is too large, >170%. According to standards the variation in neutron fluence must be <15% within the same set of specimens of 12 samples. Despite this situation the results from 11 selected surveillance sets were evaluated. The evaluation of the Charpy V results showed that the Russian embrittlement coefficient given in the Russian Guidelines was exceeded for the weld metal in most cases and to some extent also for the HAZ (Heat Affected Zone). This was later confirmed in Task 3, when some of the broken weld specimens of Kalinin 1 were reconstituted and tested once more in order to improve the quality and reliability of the results.

The main objectives of Task 2, “Real Irradiation Conditions” were to evaluate the real irradiation temperature and neutron flux of the surveillance specimens of the VVER 1000 NPPs. Two specific sets of surveillance assemblies were produced including advanced temperature monitors and neutron dosimeters. The surveillance assemblies were irradiated in Balakovo 1 during 1 year. The results showed, that the irradiation temperature of the surveillance specimens does not exceed 300 oC. This result is extremely important, since it confirms that the temperature of the surveillance specimens is close to the down-comer temperature of the RPV during normal operation. The results from the neutron fluence evaluation showed, that there is a need to improve the current surveillance specimens dosimetry. The most important recommendations were to introduce Nb dosimeters (wire) in the surveillance program and to extract Nb from austenitic stainless parts of the assemblies for activation measurements. Also current surveillance data need to be updated by systematic re-analysis of all surveillance capsules taking into account detailed local, cycle by cycle, power history in the upper part of the adjacent fuel assemblies.

Regarding Task 3, “Advanced Technologies-Toughness Determination”, the most important result was the successful commission of reconstitution technique (Task 3.1) of broken Charpy (Charpy V and pre cracked Charpy V) surveillance specimens at RRCKI. A stud-welding machine was procured and installed in the hot-cell laboratories in RRCKI. The reconstitution technique was at first qualified for use and then applied for reconstruction of tested, radioactive Charpy V specimens of Kalinin NPP Unit 1, as mentioned above. It is most important, that the reconstitution technique is now available in Russia. This technique could now be used in practice in a broader scope in the Russian and especially in Ukrainian VVER-1000 NPPs in order to improve the reliability of the surveillance results, especially the transition curves and temperatures.

In Task 3.2, “Non Destructive Techniques” the state of the art of some methods was summarized. The conclusion was that there is no fully qualified technique to date applicable to a real RPV to monitor and quantify non destructively the irradiation embrittlement.

The objective of Task 3.3, “Master Curve”, was to obtain directly FT (Fracture Toughness) information of the material by means of a limited number of Charpy V surveillance specimens. The fundamental advantage of the Master Curve is, that it gives a direct measure of the FT avoiding the use of indirect correlation through impact Charpy testing as in current Guidelines. Some tests were carried out on thermally embrittled VVER 1000 material, as well as on reconstructed surveillance specimens of Kalinin 1. The results on the Kalinin 1 material were very good and in agreement with the shift in transition temperature in testing the reconstructed Charpy V specimens mentioned above in Task 3.1. According to the deliverable, tests on thermally embrittled material indicated, that the shape of the Master Curve could be non-conservative for highly embrittled material in higher FT regions. According to the reviewer this finding is quite academic and is of no practical concern for normal surveillance testing.

Task 3.4, “Local Approach” was devoted to study the new Russian approach on FT testing based on “local approach”. At present the new Russian method is applicable, but needs a broader validation before it can be used in safety assessment of NPP components. Since the Master curve approach, which is adopted in Western Countries to a broad extent, seem to be very well applicable for Russian RPV steels as well, further support for the development of advanced methods should be considered as code development.

In Task 4, “Corrective actions-alternative surveillance programs” different actions and measures to improve the reliability and transferability of standard surveillance programs were described. The most appropriate corrective action is the use of reconstruction technique in order to improve the quality of the transition curves and temperature (Charpy V and Kjc) of the surveillance specimens.
Another important action is the combination of several surveillance sets (1, 2 and 3) in order to increase the number of specimens with proper fluence in given ranges. This is necessary not only in order to increase the reliability of the individual set, but gives also a good opportunity to systematically investigate the flux effect when the 3rd surveillance set will be withdrawn since the relative fluence is overlapping due to the high neutron gradient of each set.

In this TACIS project there are two results which deserve special attention. The temperature of the surveillance specimens has been measured successfully by using melting alloys in a specific surveillance set in Balakovo 1. In the standard surveillance program the temperature is normally measured using diamond powder only. The irradiation temperature was determined by measuring the change of the lattice parameters of diamond by X-ray diffraction. According to results from such measurements one could conclude, that this method cannot be used for this purpose. The results from the measurements in this project are of utmost importance; since it is the 1st time a reliable method has been used and implemented in the surveillance set of a VVER 1000 Unit. It was confirmed, that the irradiation temperature of the surveillance specimens is rather close to the down comer water temperature in the RPV. Accordingly the results from the surveillance tests of the VVER 1000 NPPs in general can be considered much more representative and reliable for use in integrity analyses and life time assessments of the RPV.

Another important result is the successful commissioning and adoption of the reconstruction technique for broken surveillance specimens in the hot cells at RRCKI. By using reconstruction of broken Charpy specimens the quality and reliability of the transition- and fracture toughness curves can be improved considerably since the deviation in neuron fluence for each specific transition curve can be considerably reduced (standard requirement <15%).

Conclusion

Some of the results received in this project are very good and extremely important for the continuing safe operation of the VVER 1000 NPPs. The determination of the irradiation temperature by melting alloys was a great success, which increases confidence in the surveillance program and the received surveillance results. The dosimetry work carried out under this project also gave some good results and important recommendations to improve the system were presented. Also the re-constitution technology provided to RRCKI has been successful and will be of great help for improving the quality of the surveillance results. Despite the positive results of this project there still remain problems relating to integrity of the RPV of the VVER 1000 plant in PTS, which need to be addressed. According to TACIS projects U1.02/92 A2 and R2.09/94 there is an urgent need to increase the temperature of the emergency core cooling water in order to reduce thermal stresses in the RPV wall in a possible PTS and to ensure safe operation of the plants.