ERMSAR 2012, Cologne March 21 – 23, 2012 Authors: PANTYUSHIN S.I., FRIZEN Е.А., SEMISHKIN V.P., BUKIN N.V, BYKOV M.А., MOKHOV V.А. (OKB «GIDROPRESS», Podolsk (RUS) Consideration of a possibility for corium retention (reactor internals and core melt) in the vessel of WWER reactor with power from 600 to 1300 MW

ERMSAR 2012, Cologne March 21 – 23, 2012 INTRODUCTION Since the middle of the 80s after accident at the «Three Mile Island» (USA) the in-vessel corium retention is a key point of strategy of severe accident management both for the pressurized water reactors being in operation and for the reactors of this type designed newly. The concept of external cooling of the vessel was justified for WWER-440 reactor at «Loviisa» NPP (Finland) and «Paks» NPP (Hungary). For reactors of new generation such as АР-600, АР-1000 of «Westinghouse» (USA), APR-1400 reactor (Korea) and design of the Russian WWER-600, WWER-640 reactor of new generation an engineering decision on arrangement of external cooling of the vessel is taken to keep integrity of the reactor vessel under severe accident.

ERMSAR 2012, Cologne March 21 – 23, 2012 INTRODUCTION Calculational-analytical investigations of the processes inherent to in- vessel corium retention are carried out for base medium-power Unit with WWER-600 at the OKB «GIDROPRESS» and the NRC «Kurchatov Institute» in Within the framework of the assumptions accepted when computing simulating it was shown that for WWER-600 RP none of the criteria, namely, integrity of the vessel and critical heat flux for provision of in-vessel corium retention is not exceeded. It was demonstrated, that there is a DNB ratio when forming the corium pool on the vessel lower head. Further, calculation analyses of possible application of the developed concept for in-vessel corium retention to high-power reactors, particularly, to reactors of WWER-1200 type and WWER-TOI (WWER-1300) under design were performed.

ERMSAR 2012, Cologne March 21 – 23, 2012 BASES OF IN-VESSEL CORIUM RETENTION CONCEPT [1/4] For implementation of the in-vessel corium retention concept in WWER RP the «System for corium retention and reactor vessel cooling» is designed at the OKB «GIDROPRESS». The system is purposed functionally for prevention from yield of core corium outside the reactor vessel under severe beyond design basis accidents. - retention of internals and core corium inside the reactor vessel should be ensured due to external cooling of the reactor with water; - in-vessel pressure of the reactor should be low (ensured due to accident management with tight primary circuit, and with small breaks in case of partial rupture of pipelines or implemented with large breaks of MCP); - reactor vessel flooding with water from outside to the required level shall be ensured due to a special geometry and structure of the containment, building structures and reactor pit equipment; - water supply for reactor vessel flooding from outside shall be ensured from maximum number of various available sources (depending on accident scenario) – from the reactor coolant system, ECCS and SCPF hydroaccumulators, from internals inspection wells, from sources outside the containment;

ERMSAR 2012, Cologne March 21 – 23, 2012 BASES OF IN-VESSEL CORIUM RETENTION CONCEPT [2/4] - water shall be supplied to the reactor vessel from steam generator sumps via the channels leading to the reactor concrete pit sump; during reactor cooling the upward flow of water shall be under the conditions of natural circulation; - steam shall be removed via channels in the reactor pit equipment into SG boxes and then into a space under the containment dome, where it shall be condensed on heat exchanging surfaces (containment PHRS or SG PHRS), and flows by gravity into the sumps of steam generator boxes; - heat removal from the outside surface of reactor vessel to cooling water shall be calculated in such a way that heat flux from corium to the reactor vessel wall was removed due to water cooling on the outside wall of the reactor vessel under the conditions of water nucleate boiling. With this, heat flux from corium through the reactor vessel wall shall be lower than critical heat flux determined for real geometry of WWER reactor vessel; - with available thermal loads onto the reactor vessel it is necessary to provide the absence of meltthrough of the reactor vessel due to external heat removal;

ERMSAR 2012, Cologne March 21 – 23, 2012 BASES OF IN-VESSEL CORIUM RETENTION CONCEPT [3/4] - design solutions shall restrain boric acid sedimentation on the surfaces forming a channel for reactor vessel cooling; - heat removal from the containment to atmospheric air shall be provided within long time (within the period required for in-vessel corium retention); - the components of the « System for corium retention and reactor vessel cooling », as well as adjacent systems shall be equipped with instrumentation required for control and management of core melting BDBA; - if a need is verified, it is allowed to apply passive features (that do not require the off-site source of energy) for intensification of heat transfer between the vessel and cooling water; - introduction of the corium retention system into a set of the design of the Unit and RP shall not result in degradation of operating characteristics (capacity factor, availability factor, timetable and dose rates during PM), increase in thermal losses from equipment; - the system shall not prevent from operation of ventilation channels in the concrete pit and passing of cooling air between the vessel thermal insulation and metalwork of dry shield; - a provision shall be made for water drain from the reactor pit.

ERMSAR 2012, Cologne March 21 – 23, 2012 BASES OF IN-VESSEL CORIUM RETENTION CONCEPT [4/4]

ERMSAR 2012, Cologne March 21 – 23, 2012 EVALUATION OF IN-VESSEL CORIUM RETENTION [1/6] Evaluation of thermal loads onto the reactor vessel for high-power WWER Initial data: A governing sequence of events and failures resulting in the quickest meltdown of the core is under consideration: a) guillotine break of main coolant pipeline D nom 850 b) failure of active safety systems c) RP cooldown and decay heat removal using the passive safety systems d) core melting and forming of corium pool in the lower part of the vessel e) long-term heat removal from the outside surface of the vessel and corium retention inside the vessel. Using code – severe accidents code – SOCRAT/B1 & SOCRAT/B3 Thus, calculation of this phase begins 24 and 72 h after the onset of severe accident. A conservative assumption is accepted that at this moment a pool of corium of the core and the internals is formed on the vessel lower head. Corium moving on the reactor lower head is assumed to be instantaneous; this creates the harder conditions related to corium retention as compared with long-term period of moving.

ERMSAR 2012, Cologne March 21 – 23, 2012 EVALUATION OF IN-VESSEL CORIUM RETENTION [2/6] Initial composition and temperature of corium components for base calculations Thermal power of the reactor under nominal conditions is 3300 MW. Rate of decay heat in corium without regard for a standard deviation is equal to 20 MW for 24 h, 14,5 MW – for 72 h. In calculations power decrease due to heat entrainment with violable fission products in case of fuel rod damage and fuel melting is not taken into account. MaterialMass, kgInitial temperature, K UO 2 (100% mass in core) Zr22000 (var. from 100% to 0% oxidation) 2850 ZrO 2 var. from 0% to 100% oxidation Zr2850 Steel65000 (var. from to )2400 Total mass corium~

ERMSAR 2012, Cologne March 21 – 23, 2012 EVALUATION OF IN-VESSEL CORIUM RETENTION [3/6] maximum heat flux onto the reactor vessel wall when corium comes onto the reactor vessel lower head in 24 h maximum heat flux onto the reactor vessel wall when corium comes onto the reactor vessel lower head in 72 h

ERMSAR 2012, Cologne March 21 – 23, 2012 EVALUATION OF IN-VESSEL CORIUM RETENTION [4/6] Calculation of the reactor vessel damage and deformation An axially symmetric problem of temperature and mechanical loading of the reactor vessel under severe accident for WWER-1300 is considered as a rough approximation. Arrangement of corium pool and its phase composition leads to maximum heat fluxes in the area of transition of the lower head to the cylindrical part of the vessel. In this area due to high temperature the wall is melted particularly and its thickness becomes variable along the generatrix. Temperature distribution, as a heat conductivity problem, and deformation process, as a creep problem, were considered using FEM (finite element method) by the code MSC.MARC. As a result of calculation of stressed-strained state of the reactor vessel in progression of the accident to the severe stage in 24 h the distributions of temperatures, stresses, accumulated creep deformation, as well reactor vessel damage following meltdown are obtained. It follows from the results of analysis of stressed-strained state of the reactor vessel that melt through of the vessel does not occur, vessel damage does not also take place because of temperature-induced creep.

ERMSAR 2012, Cologne March 21 – 23, 2012 EVALUATION OF IN-VESSEL CORIUM RETENTION [5/6] Temperature distribution for the reactor vessel (t=29,27 h) Distribution of equivalent creep deformations for the reactor vessel (t=51,30 h)

ERMSAR 2012, Cologne March 21 – 23, 2012 EVALUATION OF IN-VESSEL CORIUM RETENTION [6/6] Distribution of reactor vessel damage as a result of melting of base metal (t=38,00 h) Displacement of the reactor vessel lower head pole during severe accident

ERMSAR 2012, Cologne March 21 – 23, 2012 CONCLUSION A problem on implementation of in-vessel corium retention for design of RP with medium- and high power WWER was under study since 2009 at the OKB “ GIDROPRESS ” with participation of a series of scientific and design organizations. The key trends of activities are as follows: analytical-calculational, design, process, circuit-mode designs and experimental activities. Preliminary evaluations of heat fluxes on the outside surface of the reactor vessel and residual thickness of the vessel wall make possible to conclude concerning a possibility in principle for implementation of the in-vessel corium retention in the medium-power WWER-type reactors on the assumptions of modernization and improvement of passive safety systems for WWER-type high-power reactors. For WWER-type high-power reactors the in-vessel corium retention with operation of passive safety systems during 24 h is possible only in case of application of heat transfer intensifiers in the design. With operation of passive SS during 72 h corium can be retained inside the vessel without additional heat transfer intensification. Application of subreactor metalwork in the design to arrange an annulus and to accelerate coolant flow the DNBR and residual thickness of wall shall be higher. This fact is reflected in foreign experiments of ULPU type.

ERMSAR 2012, Cologne March 21 – 23, 2012 CONCLUSION In implementation of the in-vessel corium retention concept, it is planned to perform experiments in the following trends: - study of innovation methods for heat transfer intensification on the outside surface of the vessel; - determination of high-temperature physical and mechanical properties of vessel steel; - study of the effects occurring in restrained channels during heat removal from the outside surface of the vessel; - study of DNB in a cooling channel formed by the outside surface of the reactor vessel and its thermal insulation, as well as search for ways for increasing in the value of critical density of heat flux, for example, by means of channel configuration profiling; - a series of bench rig experiments for verification of calculation procedures and computer codes for the areas of change in the parameters inherent to WWER design.