VVER/01-D : Licensing Related Assessment

This project addresses the licensing related assessments of the primary circuit integrity of nuclear power plants (NPP) and the application of the Leak-Before-Break (LBB) concept to the main piping in VVER-440N-230 type reactors.

The whole work has been divided into seven tasks, which are specified in the Terms of Reference and in the internal work programme. For performing these tasks, the results of the corresponding industrial project as well as the present knowledge and progress of science and nuclear engineering were taken into account.

For precluding breaks in safety related piping such as main coolant lines and surge lines the following topics have to be reviewed in particular:

Task 1:
The design basis safety, such as material properties, design basis loads, piping static evaluation, inspectability and inspection programmes were to be re-assessed.

Task 2:
The loading history, such as deviations from design basis including the influence of unspecified loads and potential for environmental assisted attack were to be re-evaluated.

Task 3:
Upgrading measures, especially seismic ones were to be re-evaluated.

Task 4:
The scope and methods of In-Service Inspection (ISI) including the assessment of limitations of inspectability were to be re-evaluated.

Task 5:
The implementation of operation monitoring systems, such as leak detection, vibration and fatigue monitoring were to be re-assessed.

Task 6:
The operational behaviour to be expected of through-wall cracks compared with the sensitivity of leak detection systems implemented was to be re-evaluated.

Task 7:
Based on the results obtained by reviews of Tasks 1-6 a recommendation for licensing was to be proposed to GAN RF.

The reference VVER-440 plants were the units 3 and 4 of Novovoronezh NPP with version V-179 reactors and the units 1 and 2 of Kola NPP with real version V-230 reactors, which were designed in the middle of the 1960's and commissioned in the beginning of the 1970's.

That is the reason why some systems, components and pipelines were considered not satisfying the presently applied safety concept mainly due to:

Absence of a containment designed for maximum possible state of pressure and temperature waived by assuming only the failure of a pipe with DN (nominal diameter) = 32 mm;
Impossibility to supply a sufficient emergency heat removal from the reactor core for all possible primary leak areas due to loss-of-coolant accidents (LOCA).
As a consequence of previous basic discrepancies, the effect of a guillotine break of the main primary piping may have here higher impact than in Western European plants. Taking this into account, the aim of this project was to give support to Gosatomnadzor of Russian Federation to install a licensing methodology for the safeguarding of the primary circuit integrity and the application of the LBB concept to its main piping.

Task 1: Reassessment of the design basis safety.
Sufficient generic information is available to assess the given design basis safety; however, plant specific data have to be addressed for a qualified leak-before-break evaluation.

Task 2: Re-evaluation of the loading history.
Some loading conditions, important for the piping integrity and not considered in the design phase, have to be added to the load spectrum; the primary water chemistry is no matter of concern.

Task 3: Re-evaluation of upgrading measures.
Some upgrading measures are required to cope with some design relevant loading such as breaks inside the reactor building and earthquake, this especially for Novovoronezh NPP.

Task 4: Re-evaluation of in-service inspection concept.
This concept is in full compliance with regulatory requirements and very close to Western European control practice; however, preference should be given to automatic control methods.

Task 5: Reassessment of operational monitoring systems.
The installed monitoring systems correspond to the present industrial practice; however, the leak detection systems have to be improved with respect to sensitivity and seismic resistance.

Task 6: Re-evaluation of operational behaviour of cracks.
The LBB concept based on the U.S. procedure is here not applicable. LBB behaviour can be proven only for the German procedure at Kola NPP; Novovoronezh NPP requires upgrading.

Task 7: Technical recommendations for main piping licensing.
Based on the results and the recommendations of the first six tasks, technical recommendations for main piping licensing with respect to LBB application have been formulated.
If some upgrading measures will be performed, LBB behaviour can be proven for the main primary piping of NPPs with VVER-440N-230 type reactors. However, this demonstration is not sufficient to justify the deficits in the design of the containment and the emergency heat removal system.

In addition, it has to be demonstrated and monitored for these reactors that there is no larger risk with leakage resulting from failure of bolted joints, e.g. man holes, and malfunction of isolation and safety valves.
Only with respect to the restricted lifetime of these reactors, the deficits in the design of containment and emergency heat removal system can be accepted. To compensate these general design deficits and to improve the plant safety, it is strongly recommended to implement appropriate accident management measures in the plant operation concept and to train the operators in the use of these measures.