Community Research and Development Information Service - CORDIS


IVMR Report Summary

Project ID: 662157

Periodic Reporting for period 1 - IVMR (In-Vessel Melt Retention Severe Accident Management Strategy for Existing and Future NPPs)

Reporting period: 2015-06-01 to 2016-11-30

Summary of the context and overall objectives of the project

The In Vessel Melt Retention (IVMR) Severe Accident Management strategy consists in cooling the reactor vessel by flooding it externally, in order to extract the residual heat and prevent the failure of the vessel. Currently, the IVMR strategy is only applied in reactors of relatively low power (VVER-440). New designs like AP-1000 (USA) or HPR-1000 (China) involve IVMR as a means for termination of a severe accident. However, those reactors are not in operation yet. The main objective of this project is to evaluate if the In Vessel Melt Retention strategy could be applicable to Light Water Reactors (PWR, VVER, BWR) of total power around 1000 MWe (which represent a significant part of the EU Nuclear Power Plants fleet).
The ambition of the project is to provide knowledge and numerical tools that will allow a less conservative evaluation of safety margins and provide recommendations for a more efficient and safer implementation of IVMR in reactors of medium and high power. It will elaborate an updated and harmonized methodology for the analysis of IVMR that will be used for various types of reactors and implemented in various codes used in Europe.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

This first 18 months of the IVMR project were dedicated to set-up the general organization, start some of the work packages, consolidate the directions of research selected in the project proposal and initiate international contacts as well as communication about the IVMR project. The different work packages within the project have been launched in coordination with the associated work package leaders.
WP2.1: work on the methodology has started by a state-of-the-art about the implementation of IVR in VVER-440 plants in Europe.
WP2.2: work started with a synthesis of developments planned for each code. A PIRT for the phenomena related to in-vessel retention is being built.
WP2.3: So far, activities have been devoted to validation and code development. Relevant experiments, all performed at the CEA Grenoble in the 90s, have been identified regarding thin layers, homogeneous pools and external vessel cooling.
WP2.4: The main goal of WP2.4 is to assess the capabilities of mechanical models to predict vessel resistance or failure near the location where the vessel wall is significantly thinner because of high local heat flux and to use the results of mechanical models to assess simpler models that could be used in SA codes.
WP2.5: A set of reactor calculations was performed. All participants were able to calculate severe accident (SA) scenarios with External Reactor Vessel Cooling (ERVC), for the reactor and with the code of their choice. Very high values of heat flux are always associated with transient conditions leading to temporary presence of a thin metallic layer on top or inversion of stratification. In that last case, the peak heat flux depends on the superheat of the metal phase and not on the residual power.
WP3.1: small scale experiments with prototypic corium have been made at NITI (Russia). The first two experiments were made to study the influence of crusts between oxide pool and top metal layer on the heat and mass transfer. The results confirm that, as predicted in some of the reactor calculations presented in section 2, inversion of stratification is likely to occur, driving superheated metal to the top part of the pool, with possible high heat flux.
WP3.2: The objective is to perform measurements on some key thermophysical properties of real corium having importance in the In-Vessel corium configuration. This first period has allowed to start technical and scientific exchanges between CEA, ITU and UJV for the future measurements.
WP3.3: In order to complement the very small-scale corium experiments, larger scale experiments with simulants are being designed for the study of heat transfers in various configurations of stratified pools. The two main facilities are SIMECO-2 (Under design) and LIVE (2D and 3D, Under renovation). New simulants have been selected allowing tests at higher temperature.
WP3.5: During the first period of the project, most of the activity was focused on the design of the heated wall that would simulate the molten pool boundary. Preliminary tests have shown that it is possible to reach heat fluxes up to 600 kW/m2.
WP3.6: During this period, CVR has designed and installed two new induction furnaces dedicated to melt either metal or oxide (or both) in cold crucibles.
WP4.1.1: Among the most important experiments performed by UJV were the tests made with vessel samples modified by surface treatment (“cold spray” coating). It was shown that this coating could enhance the local heat flux up to 30-50% locally.
WP4.1.2: Large scale facility THS-15 is to be built by UJV as a full-scale representation of a VVER-1000 geometry. The target is to finish the facility early March 2017 to be ready for first tests starting in April 2017.
WP4.2: Study of spray cooling of RPV lower head. The KTH group has designed SPAYCOR (SPrAY COoling of Reactor vessel), a surface heat flux of up to 3.8 MW/m2 experimental facility operated with upward-facing multi-nozzles.

WP5: Review of innovation and technical engineering applicable to IVMR –
One way to avoid the IVMR failure is to increase the CHF by modifying the surface state of the vessel wall. In this framework UJV tested cold spray porous coating in its small scale experiment BESTH. A positive effect of the cold spray coating was noticed whatever the inclination of the wall. In the same framework, EDF studied the effect of the oxidation at the vessel wall. Samples from French reactor steel were prepared after 8 hours of heat treatment. The porous superhydrophilic layer of 40µm obtained enhances the CHF of at least 30%. Based on those results experiments and a review will be carried on.
Another way to enhance the CHF is to increase the mass flux the use of baffles. Studies will be carried on at IVS by studying the design of ERVC loop for existing and newly designed NPPs.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

So far, the project has attracted the attention of several organizations outside Europe (mainly from Korea and China, but also Russia and the USA), which confirms its potential impact. Up to now, the results obtained are not sufficient to claim that progress have been made “beyond” the state of the art. However, it is already possible to claim that the issue of IVR for high power reactors has been investigated from various apsects, some of them being more detailed than before, such as the consideration of transient evolution of corium layers, the use of CFD to obtain data on thin metallic layer thermal behaviour and the use of detailed mechnaical codes to study the behaviour of an ablated vessel.

The project will contribute to reinforce research cooperation on reactor safety at EU level by bringing together research organizations, TSOs, utilities and designers who all have an interest at investigating the benefits of IVMR, either for backfitting of existing reactors or for safety studies on future reactor designs. The case of VVER-1000 is of key importance nowadays and it is likely that the project results will contribute to the décisions taken about implementing IVR strategy in VVER-1000 plants in Europe.

The project is already likely to have impacts out of the group of participants considering that Korean organisations will join the Consortium and that exchanges with Chinese organisations have already started.

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