EUROTRANS - Helium cooled EFIT Probabilistic assessment of different DHR designs Karlsruhe, November Sophie EHSTER, Laurent VINCON

> Eurotrans – He cooled EFIT DHR study – November Study description - 1  Study general objectives:  To compare several schemes of Decay Heat Removal (DHR) architecture of helium cooled EFIT with the help of reliability studies  Decay Heat Removal safety function is performed by the Shutdown Cooling System  To provide a first assessment of the global reliability of each scheme  To provide safety input (e.g. number of SCS trains, redundancy, diversity, reparability) for helium cooled EFIT DHR design

> Eurotrans – He cooled EFIT DHR study – November Study description - 2  Safety requirements :  General safety objectives for He-cooled EFIT: A high level of prevention of severe core damage has to be provided. With respect to Decay Heat Removal (DHR) safety function, its complete failure has to be made extremely rare.  In accordance with available background (e.g., EFR), a quantitative probabilistic objective is defined: the failure of the DHR function (i.e., temperature higher than Cat. 4 limits), considering all initiators, should have a probability lower than ~ per reactor year.  This will lead to implement a very reliable SCS.  To improve this reliability (“practical elimination” is aimed at), SCS capability to operate in natural convection during accidental conditions is requested.

> Eurotrans – He cooled EFIT DHR study – November Study description - 3  This leads to request: Capability to operate the primary circuit in natural convection at nominal pressure (70 bar) in accidental conditions (Cat. 4) (Case 1 in SCS design specification table), Capability to operate the primary circuit in natural convection in case of intermediate pressure for fuel handling (10 bar) in accidental conditions (Cat. 4) (Case 3 in SCS design specification table).  In case of lower frequent event occurring in depressurised conditions (1 bar), capability to operate the primary circuit in natural convection is not requested. This corresponds to the case of an helium leak that cannot be compensated (Cat. 3 initiating event) (Case 2 in SCS design specification table).

> Eurotrans – He cooled EFIT DHR study – November Study objective - 4  In intermediate pressure conditions, it is not reasonably feasible to remove decay heat by natural convection with the current reactor design (this leads to implement primary SCS heat exchangers at a too high level – about 75 m)  The objective of the study is to assess the reliability provided by equipment needed for forced convection cooling and to check the consistency with respect to the safety requirements:  The current design is provided with 3 SCS systems (each SCS capability is 100 % with regard of Cat. 4 limits).  SCS diversity, redundancy and power supply by diesel- generators and batteries are assessed with different assumptions.  The impact of period needed for repairing the SCS is assessed (non reparable, 3-weeks, 50-hours).

> Eurotrans – He cooled EFIT DHR study – November  Architecture 1: « 3 x 100 % » concept = 3 diverse systems, each one comprising one line with 100 % capability  3 configurations for power supply are studied:  2 then 3 redundant electrical trains, diverse or not  Electrical trains are supplied by external power, diesel-generators and batteries (<6h) Assessment of different schemes - 1 SCS1SCS2SCS3 100% SCS1SCS2SCS3 A B SCS1SCS2SCS3 ABC SCS1SCS2SCS3 A B

> Eurotrans – He cooled EFIT DHR study – November Assessment of different schemes - 2  Architecture 2: « 2 x 2 x 100% » concept = 2 diverse systems, each one comprising 2 redundant lines with 100% capability SCS1-1 SCS % SCS1-2 SCS %  3 configurations for power supply are studied:

> Eurotrans – He cooled EFIT DHR study – November Assessment of different schemes - 3  Architecture 3: « 2 x 3 x 50% » concept = 2 diverse systems, each one comprising 3 redundant lines with 50 % capability SCS150% SCS250%  3 configurations for power supply are studied: A B C A B A B

> Eurotrans – He cooled EFIT DHR study – November Assessment of annual occurrence frequency of complete loss of DHR function  « 3 x 100% » concept:  « 2 x 2 x 100% » concept:  « 2 x 3 x 50% » concept:

> Eurotrans – He cooled EFIT DHR study – November Conclusions - 1  “2 (diverse) x 2 x 100%” and “2 (diverse) x 3 x 50%” SCS concepts are more feasible than 3 (diverse) x 100 % concept. The study has shown that the reliability is improved.  Results show the importance of natural convection.  The best configuration in terms of reliability is the “2 x 3 x 50%” concept. This design is also the less sensitive to the time to repair SCS systems.  “2 x 2 x 100%” concept could be acceptable if it can be demonstrated that the time to repair SCS remains short (lower than 50 hours).

> Eurotrans – He cooled EFIT DHR study – November Conclusions - 2  The study has been performed considering that core by- pass is acceptable  Possible by-pass:  Through PCS if flow shutter closure has failed and the primary blower is shut down,  Through failed SCS if check valve closure has failed and the SCS blower has failed  The feasibility of these assumptions has to be assessed:  SCS circuit head loss (shutdown of SCS blower) must be sufficiently important with respect to core head loss but this could be penalizing for natural convection  PCS circuit head loss (shutdown of PCS blower) must be sufficiently important with respect to core head loss  Confirmation by thermal–hydraulic calculations has to be performed

> Eurotrans – He cooled EFIT DHR study – November Conclusions - 3  During shutdown conditions, natural convection could be not required in case of fuel handling operation at low pressure conditions:  If “2 x 3 x 50 %” concept is selected,  If depressurized conditions are initiated after sufficient delay (about 3 days) allowing 6 x 100 % redundancy.

> Eurotrans – He cooled EFIT DHR study – November Issue of reports  Specific report about the present study : issue in early December 2008  AREVA contribution about He-cooled EFIT safety approach to be included in D1.44 : issue in February 2009

> Eurotrans – He cooled EFIT DHR study – November Appendix 1  Normal operation:

> Eurotrans – He cooled EFIT DHR study – November Appendix 2  SCS operation:

> Eurotrans – He cooled EFIT DHR study – November Appendix 3  SCS scheme:

> Eurotrans – He cooled EFIT DHR study – November Appendix 4  Decay heat curve:

> Eurotrans – He cooled EFIT DHR study – November Appendix 5 Case/ Condition 1 Case/ Condition 2 Case/ Condition 3 Case/ Condition 4 Helium pressure (bar)70110 Active / Passive modePAPA Decay heat to remove (MW th ) Helium temperature at the HX inlet (°C) Helium temperature at the HX outlet (°C) Blower compression ratio1.2 Event /CategoryLoss of normal + emergency supply Cat.4 Helium leak (not compensated) Cat.3 Fuel handling shutdown + Loss of normal + emergency supply Cat.4 Fuel handling shutdown Cat.1  Specifications for the SCS design: