R2.07/93VVER 1000 Core Analysis

Some Reference Documents indicated the tendency to unstable behaviour of VVER-1000 reactor cores (e.g. Xe oscillations).
Knowing that the safety margins of these cores are smaller than for the VVER-440, V-230 and 213 reactors and that the core control system may present some weak points (control parameters, control logic, Instrumentation and Control), a joint review of the problems with the counterpart was considered to be of importance.
In addition, an engineering feasibility study should be performed to evaluate the possibility to increase the core burn up and so to expand the period between reloadings.
The work was divided into 7 tasks:

Task 1: Start of the project-reference data-calculation methods:

• Establishment of the working group;
• Definition of the calculation methods to be used in the project;
• Choice of the available experimental data to compare the calculation methods;
• Cross evaluation of the methods;
• Establishment of detailed listing of the core characteristics to perform the foreseen studies;
• Definition of the nominal operating conditions for the input of the calculation.

Task 2: Calculation of the nuclear key parameter for safety:

• Determination of the equilibrium core characteristics/3 years fuel cycle;
• Calculation of Beginning of Cycle (BOC) and End of Cycle (EOC) main parameters for the safety analysis;
• Issue of the report on the key parameters for safety analysis.

Task 3: Calculation of the nuclear key parameters for safety.

Task 4: Study of the reactivity control:

• Analysis of the current situation in the PWRs contra VVERs;
• Parametric studies for the VVER standard core with burnable poison;
• Issue of the recommendations on the reactivity control management.

Task 5: Control of the power and Xe distribution in case of load change:

• Analysis the present situation in the VVERs compared to PWR;
• Definition of the adopted control philosophy;
• Parameter study;
• Deduction of conclusions and recommendations.

Task 6: Synthesis of the recommendations deducted from Task 4 and Task 5.

Task 7: Analysis of low leakage fuel management strategy:

• Choice of the VVER core type to be studied, based on PWR experience;
• Parametric studies of low leakage patterns and effects on vessel fluency, etc.;
• Recommendations inside the working group;
• Issue of the report.

The project has been performed to a large extend according to the tasks. However the project was delayed due to the procurement process for equipment. Because of the very slow progress, caused by problems in local computing capacity due to the pending equipment delivery, the project could not be completed in its full scope by the official end date, which was postponed already by six months.
The reports provided reveal, however, that although they have been issued with a minor delay to the official end date, the original TOR objectives were achieved.

The working method for calculating the safety related data with the help of independently reviewed methods used by independent Russian and Western expert teams seems to have been reasonable for this kind of project. Regular working group meetings, cross-evaluating the results and discussing the progress, essentially contributed to the successful transfer of the Western methodology and expertise.
The conclusions validate in principle the use of Western methods for application to VVER 1000 core analysis and confirm the safe performance of the three year cycle Standard Core, which is already employed by several operating VVER 1000 reactors.

The decision of the Joint Working Group to make an amendment to the original TOR requirements by considering two Advanced Core designs rather than one, as well as carrying out benchmark Monte Carlo calculations, offered a larger basis for comparisons and recommendations on further work.
Recommendations for short-term implementation were given, e.g. the use of a 5% control rod working group insertion, rather than the current 20% insertion, which gives an improvement in both safety margins and profitability. However, implementation of this recommendation required the examination of the core behaviour under the full range of transient conditions as applied to the western PWRs, which was beyond the scope of this project.

Further improvements of the neutronic safety parameters for the standard core were possible, especially through the use of gadolinium integral poisons as illustrated in the Advanced Core case.
The deliverables formed in general a basis for further considerations in improving the VVER 1000 core performance and safety.

The recommendations are useful also as reference for operating procedures in respect to clear and effective control strategies.