EMERGENCY COOLING TOWER In Nuclear Power Plants

Transcription

1 EMERGENCY COOLING TOWER In Nuclear Power Plants Jean-Christian Martin Hamon Thermal Europe 1 Emergency Cooling Towers Foreword Environmental Constraints Choice of the Cooling System Cooling Tower Specification Design Characteristics References 2 1

2 Foreword In most Nuclear Power Plants, the Essential Service Water System (ESWS) uses a oncethrough cooling system In some specific cases due to particular environmental conditions, this system cannot be used. 3 Emergency Cooling Towers Foreword Environmental Constraints Choice of the Cooling System Cooling Tower Specification Design Characteristics References 4 2

3 Environmental Constraints Need of a permanent cold source. Possible threats: obstruction of the water intake in the pumping station (e.g.: seism leading to the collapse of a dam located upside the NPP. This would drain muddy water in front of the NPP) Low flow rate of the river can leads to insufficient water intake 5 Emergency Cooling Towers Foreword Environmental Constraints Choice of the Cooling System Cooling Tower Specification Design Characteristics References 6 3

4 NUCLEAR POWER PLANT: Various cooling circuits (main + auxiliaries/emergency) The Cooling System LOSS OF COOLANT ACCIDENT : Need for a safe emergency cooling circuit (redundancy, ) Possible need for Ultimate Heat Sink Cooling Tower (UHSCT). 7 For a 1450 MW NPP : How to ensure a safe cooling? Thermal power to exhaust during normal operation (80% of the time) = 10 to 25 MW Thermal power to exhaust during shut down period = 50 MW In case of accident and loss of primary cooling circuit, ESWS will cool the safety injection system and reactor vessel spraying system 8 4

5 Closed loop options Dry Cooling Tower with dry bundles : Very large cells with very high fan power consumption High risk of freezing during no operation period No evaporation : no plume & no make-up water 9 Closed loop options Huge water storage: a cooling lake with a surface of 30 ha per reactor unit. Spraying basin that would requires about 3 ha per unit. This solution would be very noisy and very depending upon wind conditions. Wet cooling tower 10 5

6 Wet Cooling Tower Cooling tower could be : Natural Draft : for a 1450 MW NPP it would require towers with 31 m base diameter and 41 m high. Almost impossible to achieve resistance against earthquake and explosion Freezing issues under very cold temperature due to important variation of thermal power to exhaust 11 Mechanical Draft Cooling Tower: Wet Cooling Tower Safety System Schematic Diagram Source : EDF 12 6

7 Emergency Cooling Towers Foreword Environmental Constraints Choice of the Cooling System Cooling Tower Specification Design Characteristics References 13 UHSCT Specification The efficiency and the reliability of emergency cooling tower are based on : Correct cooling tower duty (process side) (definition & choice of internal components to insure thermal performance) Design based on extreme operating conditions (environment, internal and external risks) Special construction 14 7

8 Redundancy : 2 redundant system per unit 1 system = 1 tower with 10 cells UHSCT Specification No structure, systems or components share with other NPP units 15 Operating modes: Normal Cooling of some power plant auxiliaries during normal, full load operation and normal shutdown sequence Normal ambient conditions Normal water quality 16 8

9 Operating mode: Accidental (main purpose) Thermal duty for Accidental conditions (defined by the nuclear reactor manufacturer) in worse ambient conditions Accidental ambient conditions External accidental/risk Internal accident 17 Why to use the same Cooling equipment for Normal and accidental conditions? To insure the permanent availability of UHSCT the safest way is: to operate it continuously to foresee redundant cell(s) that allow maintenance without interruption of the service. With this safety philosophy an additional normal operating mode can be defined. 18 9

10 Input: Environmental and External Risks Environmental risks: Acc. to return periods (10, 100 & 1000 years) Seism Tornado, wind Extreme temperatures Snow Hail 19 Input: Environment and External Risks External Risks: Explosion (shockwave) Seism Impact (Aircraft ) Loss of make-up water Power supply variation (Hz, V) Fire (for structural resistance) Internal Flooding 20 10

11 Emergency Cooling Towers Foreword Environmental Constraints Choice of the Cooling System Cooling Tower Specification Design Characteristics References 21 Reinforced concrete structure Design Characteristics Redundancy: n+1 identical, independent cells Make-up water reserve basin underneath the cooling tower Forced draft configuration (direct fan drive, no transmission shaft, special fan hub) Reinforced concrete fan stack, being integral part cooling tower structure 22 11

12 Design Characteristics Holding in position of heat exchange fill hold in position, drift eliminators and lifting equipment (if any) Mechanical protection of cooling tower equipment (fan, drift eliminators) Sideway outlet openings Buried water outlet canal 23 Design Characteristics 24 12

13 Sample Data Sheet Design Characteristics 25 Qualifications Earthquake ITEM QUALIFICATION Reinforced concrete structure Validation of civil design by finite elements and 3-D model at full seism design spectrum Fan blades Vibration test Mechanical test Fan-Motor assembly Vibration tests of the whole assembly Heat exchange fill Calculation note of the holding-down system Drift eliminators Calculation note of the holding-down system 26 13

14 WIND, TORNADO ITEM QUALIFICATION Reinforced concrete structure Validation of civil design by finite elements and 3-D model under tornado conditions Protection of equipment Internal equipment is protected from flying debris by mechanical protection. ITEM Heat exchange fill Fan blades EXTREME AMBIENT TEMPERATURES, HAIL, SNOW QUALIFICATION Qualifications Mechanical, physical and chemical qualification Ageing test Ageing test (40 years) Mechanical test (flexion, traction, hardness, material) 27 EXTERNAL EXPLOSION ITEM QUALIFICATION Reinforced concrete structure Validation of civil design by finite elements and 3-D model under explosion generated shockwave Fan-Motor assembly Vibration tests of the whole assembly Heat exchange fill Calculation note of the holding-down system Drift eliminators ITEM Calculation note of the holding-down system IMPACT QUALIFICATION Qualifications Reinforced concrete structure Validation of civil design by finite elements and 3-D model 28 14

15 Emergency Cooling Towers Foreword Environmental Constraints Choice of the Cooling System Cooling Tower Specification Design Characteristics References 29 Civaux Nuclear PP 30 15

16 Hamon References for safety classified CT Nuclear power plant Civaux EDF France Doel Electrabel Belgium Neckar Westheim EnBW Germany Grundremmigen RWE Germany Vandellos AIE Spain 31 16