Session: 01-05: Nuclear Plant Operation, Modification, Life Extension, Maintenance and Life Cycle - V

Paper Number: 134916

134916 - Ways to Solve the Safe Operation of Vver-1000 Reactors in Case of Loss of Original Rod Cluster Control Assemblies Supply 

Abstract: 

For NPPs with soviet-designed VVER-1000 reactors, the only supplier of the rod cluster control assemblies (RCCAs) was the russian company jsc tvel. The Ukrainian nuclear operator Energoatom has completely refused russian nuclear fuel and announced the transition to fuel produced by Westinghouse after the russian invasion of Ukraine. Therefore, Ukraine faces an urgent issue of supplying with RCCAs, as they are an integral part of the core, but Westinghouse does not manufacture RCCAs for VVER-1000 reactors.

Currently, Ukrainian experts have developed a technology for manufacturing control rods and RCCAs that are comparable in design and materials to the standard russian RCCAs. However, the implementation of new RCCAs requires some time to substantiate their safe operation, carry out laboratory tests and trial operation. Taking into account that and a limited reserve of "fresh" RCCAs, work is being carried out to extend the service life of RCCAs that are already in operation. Two approaches were considered: substantiation of new criteria related to neutron absorption efficiency and maintain the integrity of RCCA structural elements; estimation of residual service life based on the approach that takes into account actual position of RCCAs in the core. However, this is a temporary solution that will help to gain time for the implementation of nationally manufactured RCCAs with same characteristics as the original ones.

But in the future, there would arise the issue of creating a design of the RCCAs based on national recent scientific and technical developments that exceed the characteristics of the operating RCCAs in terms of service life and efficiency. To find ways of improving the efficiency of RCCAs, it is necessary to identify the characteristics of neutron-absorbing materials operated at different periods in VVERs and PWRs. The main way, which is acceptable for all considered materials, is increasing the density and diameter of the neutron-absorbing core of the RCCAs. The density increase is possible with the tablet version of the RCCA filling instead of the vibro-compacted one (original RCCA).

Among the proposed absorbers, the most acceptable neutron absorption efficiency is provided by metallic hafnium with an absorbing part diameter of 8.2 mm (without a cladding) and Dy₂O₃·HfO₂ in the tablet version. The use of these materials in the lower part of the control rod will increase the efficiency even if the lower part is increased from 300 mm to 500 mm.

The composition Eu₂O₃·HfO₂ can be used instead of B₄C in the upper part of control rod (from 300 mm to 3500 mm). According to available state diagrams, the structure of this material is radiation resistant. Eu₂O₃·HfO₂ with a density of 8.0 g/cm³ and control rod tablet filling (6.85 mm) will exhibit an initial neutron absorption efficiency of 95.6% from that of B₄C. The high neutron absorption efficiency will allow the use of less efficient absorbers with an extended service life (Dy₂O₃·HfO₂, Hf) with a length of more than 300 mm in the lower part without violating the neutronic criteria for the performance of control rods.

The article briefly summarizes the results and experience of extending the service life of existing RCCAs and implementation of nationally developed RCCAs with characteristics similar to Russian RCCAs. However, the main part of the article is devoted to the ways of increasing the efficiency and service life of RCCAs and development of control rods with improved performance. It is important that VVER-1000 reactors are operated not only in Ukraine but also in the Czech Republic (Temelín NPP) and Bulgaria (Kozloduy NPP), where they also stopped purchasing Russian RCCAs, so Ukraine's experience is important and can be implemented in these countries.

Presenting Author: Oleksandr Mazurok ES Group LLC

Presenting Author Biography: I am the technical director of the company, but despite my position I continue to engage in research to ensure the NPPs. At the moment, the issue of diversifying everything russian for Ukrainian nuclear power plants is urgent. My report will be devoted precisely to this, in terms of reactor power control. My scientific degree is Ph. D. in Nuclear Energy. I graduated from Kyiv and Odessa Polytechnic Institutes. Also I participated in workshops, seminars and scientific conferences in Ukraine and abroad. Member of Ukrainian Nuclear Society, American Nuclear Society, American Society of Mechanical Engineers etc

Authors:

Oleksandr Mazurok ES Group LLC
Valeriy Zuyok National Science Centre “Kharkov Institute of Physics and Technology”
Vadym Ivanov ES Group LLC
Mykhaylo Tretyakov National Science Center “Kharkov Institute of Physics & Technology”