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In-Vessel Melt Retention Severe Accident Management Strategy for Existing and Future NPPs

Periodic Reporting for period 2 - IVMR (In-Vessel Melt Retention Severe Accident Management Strategy for Existing and Future NPPs)

Reporting period: 2016-12-01 to 2018-05-31

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.
This first 36 months of the IVMR project were dedicated to set-up the general organization, start and complete 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. Among the main results already achieved, we may mention:
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. A generalization of the methodology is now proposed for high power reactors in order to take into account the very thin remaining part of the vessel wall. It also allows to take into account the possible peaks of heat transfer which can lead to more ablation than the standard steady-state approach would predict.
WP2.2: work started with a synthesis of developments planned for each code. A PIRT for the phenomena related to in-vessel retention was built. New models were developed by some partners and implemented in some of the codes.
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. Satisfactory turbulence models for oxide pool were identified. The study of thin metal layer is going on in order to derive new correlations for integral codes.
WP2.4: A benchmark was defined, to predict vessel resistance or failure near the location where the vessel wall is significantly thinner because of high local heat flux. It is planned to make use of results to assess simpler models for SA codes.
WP2.5: 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.
WP3.1: small scale experiments with prototypic corium have been made at NITI (Russia). The results confirm that inversion of stratification is likely to occur, driving metal to the top part of the pool, with possible high heat flux.
WP3.2: Measurements of oxide conductivity, metal density and metal surface tension were performed.
WP3.3: Larger scale experiments with simulants were 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. New simulants have been selected allowing tests at higher temperature.
WP3.5: A heated wall simulates the molten pool boundary, surrounded by a debris bed. Preliminary tests have reached heat fluxes up to 600 kW/m2. CHF was reached for small (2mm) particles.
WP3.6: CVR has designed and installed two new induction furnaces dedicated to melt either metal or oxide (or both) in cold crucibles. Preliminary tests with stratified oxide/metal were performed.
WP4.1.1: Tests with vessel samples modified by surface treatment (“cold spray” coating) were performed. 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 was built by UJV as a full-scale representation of a VVER-1000 geometry. Preliminary tests, with representative heat flux profiles were performed.

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. 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. The porous super hydrophilic layer of 40µm obtained enhances the CHF of at least 30%. Possibilities of injecting water in the vessel after molten pool formation were also proposed.
So far, several organizations from countries outside Europe (Korea, China, Japan, Russia, Ukraine) have officially joined the project, which confirms its potential impact. Up to now, the results obtained provide new tools and an updated methodology to analyze IVR for high power reactors. In particular, the consideration of transient evolution of corium layers according to thermochemical kinetics, the use of CFD to obtain data on thin metallic layer thermal behaviour and the use of detailed mechanical codes to study the behaviour of an ablated vessel appear quite important. New data on the CHF along the vessel wall show less dispersion than previous data and new corrélations are available.

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 decisions 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 there are already proposals for international collaborative actions (IAEA and OECD/NEA) following some of the tasks initiated in the IVMR project.