EUROTRANS - Helium cooled EFIT Probabilistic assessment of different DHR designs Karlsruhe, November 27-28 2008 Sophie EHSTER, Laurent VINCON

EUROTRANS - Helium cooled EFIT Probabilistic assessment of different DHR designs

Karlsruhe, November 27-28 2008

Sophie EHSTER, Laurent VINCON

> Eurotrans – He cooled EFIT DHR study – November 27-28 20083 3

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



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 ~ 10-7 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.



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).


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).


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

SCS1 SCS2 SCS3

100% 100% 100%

SCS1 SCS2 SCS3

A

B

SCS1 SCS2 SCS3

A B C

SCS1 SCS2 SCS3

A

B



Architecture 2: « 2 x 2 x 100% » concept = 2 diverse systems, each one comprising 2 redundant lines with 100% capability

SCS1-1 SCS2-1100% 100%

SCS1-2 SCS2-2100% 100%

A

B

SCS1-1 SCS2-1100% 100%

SCS1-2 SCS2-2100% 100%

A

B

SCS1-1 SCS2-1100% 100%

SCS1-2 SCS2-2100% 100%

A

B

SCS1-1 SCS2-1100% 100%

SCS1-2 SCS2-2100% 100%

A

B

SCS1-1 SCS2-1100% 100%

SCS1-2 SCS2-2100% 100% AB

SCS1-1 SCS2-1100% 100%

SCS1-2 SCS2-2100% 100% AB

SCS1-1 SCS2-1100% 100%

SCS1-2 SCS2-2100% 100%




Architecture 3: « 2 x 3 x 50% » concept = 2 diverse systems, each one comprising 3 redundant lines with 50 % capability

SCS1 50% 50%

SCS2 50% 50% 50%

50%


A

B

C

A

B

A

B


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:


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).


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


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.


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


Appendix 1

Normal operation:


Appendix 2

SCS operation:


Appendix 3

SCS scheme:


Appendix 4 Decay heat curve:

Time (second) 50%PuO2/ 50%MAO2

0.000E+00 7.134E-02 3.000E-01 7.009E-02 1.000E+00 6.782E-02 3.000E+00 6.373E-02 1.000E+01 5.763E-02 3.000E+01 5.180E-02 6.000E+01 4.816E-02 1.800E+02 4.272E-02 6.000E+02 3.762E-02 1.800E+03 3.257E-02 3.600E+03 2.954E-02 1.080E+04 2.609E-02 3.600E+04 2.373E-02 8.640E+04 2.227E-02 2.592E+05 2.079E-02 8.640E+05 1.933E-02 2.592E+06 1.743E-02 7.776E+06 1.403E-02 3.156E+07 6.797E-03 9.467E+07 3.609E-03 3.156E+08 3.075E-03


Appendix 5

Case/ Condition 1

Case/ Condition 2

Case/ Condition 3

Case/ Condition 4

Helium pressure (bar) 70 1 10 10

Active / Passive mode P A P A

Decay heat to remove (MWth)

18 18 9 9

Helium temperature at the HX inlet (°C)

1000 1000 1000 250

Helium temperature at the HX outlet (°C)

350 100 100 100

Blower compression ratio 1.2

Event /Category Loss 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:

Documents

EUROTRANS - Helium cooled EFIT Probabilistic assessment of different DHR designs Karlsruhe, November 27-28 2008 Sophie EHSTER, Laurent VINCON