R2.01/03 - Transfer of international best knowledge for development of hydrogen safety systems (ref AP 7.1.2.1)

One of the adverse consequences of serious reactor accidents involving reactor coolant leaks and core meltdown can be generation of substantial amount of hydrogen due to the water-zircon reaction at high temperatures. Due to disrupted integrity of the reactor coolant system, the hydrogen may leak into the containment. Considering the high explosiveness of the hydrogen-air mixture, this process can seriously endanger containment integrity incurring risk of radioactivity release into the environment. Mitigation of consequences of such an event has been therefore identified as a significant safety issue.

To help to adequately address the issue also at Russian VVER-based nuclear power plants, the EC-funded design safety project R2.01/03 was established in cooperation with the Concern Rosenergoatom and other Russian institutes and organizations.

The project was implemented within two separate contracts: (1) a fee-based contract 76775 with an EU Consultant, and (2) a global price contract 127699 with a Russia-based Director of Experiments (DoE). The overall project management was under the direction of the Consultant.

The general objective of the project was to address the risk of hydrogen explosion in reactor containment following severe (Beyond-Design-Basis) accidents involving core damage.

The specific objective was to transfer the know-how on the hydrogen risk mitigation techniques from the western nuclear power plants to the Beneficiary. The main focus was on design development and testing of the monitoring part of a hydrogen control system which would be capable of working in VVER-1000 reactor containment in the Beyond-Design-Basis Accident (BDBA) conditions including high pressure, temperature, moisture and increased radiation.

The project was implemented in five tasks: (1) project management, (2) transfer of know-how on hydrogen control at NPP to the Beneficiary, (3) design development of a hydrogen monitoring system (HMS), (4) qualification testing of the HMS and (5) conceptual design of a comprehensive hydrogen control system, including hydrogen removal system (passive catalytic hydrogen recombiners). No specific computational analyses have been envisaged in the project.

The fee-based contract No. 76775 was concluded with the EU Consultant TES s.r.o. (Czech Republic). The contract implementation started on 11 February 2007 and was finished on 10 February 2009.

The Consultant focused his activities on transfer of western know-how related to normative documentation, technical requirements and operational practice in hydrogen control at western NPPs. He worked on improvement of design of the HMS single module existing at the DoE, and on elaboration of conceptual design of the comprehensive hydrogen control system using his knowledge of the overall VVER-1000 plant design. The Consultant also assisted the DoE in preparation of the qualification test programs, the test performance at the DoE experimental facility in IPPE Obninsk, and in evaluation of the test results. In order to properly identify initial and boundary conditions of the qualification tests and verify the experimental results, the Consultant performed additional benchmark pre-test and post-test CFD calculations of the conditions in the DoE experimental facility.
In close cooperation with the DoE, the Consultant prepared and delivered to the Beneficiary a complete design documentation of the improved hydrogen-oxygen gas analyzer assembly. Beyond the scope of the project, to increase the added value of it, the Consultant also manufactured and delivered to the Beneficiary a prototype of the hydrogen-oxygen gas analyser assembly. No design development has been performed at the hydrogen and oxygen analysers themselves upon explicit requirement of the DoE, the know-how owner.

Conclusions

The experiments combined with the additional CFD calculations verified proper performance of the hydrogen gas analyzers in BDBA conditions. The oxygen gas analyzer readings, however, showed significant deviations from the assumed conditions in the experimental facility and from the results of the benchmark calculations. The correct oxygen concentration data are necessary for full evaluation of actual hydrogen risk during a BDBA, as required by Russian regulatory documents. The Consultant has therefore recommended that further developments be made at the oxygen gas analyzers to improve their performance under BDBA conditions.

In general, the project can be considered successful, fully satisfying ToR requirements and contributing to improvement of the VVER-1000 reactor safety.
For additional information, see also the related project summary on the Contract No. 127699.