This document describes a proposed passive safety system for CANDU nuclear reactors using isolated condensers to remove decay heat over 72 hours following a station blackout accident. The system would use noncondensable gases in the isolated condensers to self-regulate heat transfer and natural circulation flow without secondary system depressurization. Modeling with RELAP5/MOD3.2 validated that the isolated condensers could maintain natural circulation cooling for over 72 hours to safely handle decay heat loads. The passive system would significantly increase the time window before core damage in an accident scenario compared to the current approximately 23.5 hour capability with active safety systems alone.

1. 1
A COMPLETE PASIVE SAFETY SYSTEM FOR CANDU
6- A NOT TOO FAR BRIDGE
Iulian Pavel NIȚĂ Luminița NIȚULESCU
RATEN CITON RATEN ICN

2. PASSIVE SAFETY SYSTEM FOR CANDU
2
CANDU6
Starting point
- ALFRED LFR DEMONSTRATOR REACTOR
- ISOLATING CONDENSER for Normal and Abnormal operation
- ANSALDO - PATENT
- Using noncondensable gases as GOOD environment
- Low heat exchange – Low Pressure – High nonconsable concentration – Reduce heat transfer –
increase pressure – good for natural circulation – SELF REGULATING System - Novelty

3. PASSIVE SAFETY SYSTEM FOR CANDU
3
INPUT DATA
Starting point
- System requirements: 3 days Station Black out Accident
- Thermal load: reactor decay heat for 72 hours
- Cooling option considered: Isolated Condenser
- Cooling regulating option: using noncondensable gases to reduce
- Low heat exchange – Low Pressure – High nonconsable concentration – Reduce heat transfer –
increase pressure – good for natural circulation – SELF REGULATING System – Novelty – ANSALDO
PATENT

4. PASSIVE SAFETY SYSTEM FOR CANDU
4
Decay Heat
0
20
40
60
80
100
120
140
0 50000 100000 150000 200000 250000 300000
CANDU 6 PHWR Reactor Decay power [MW]
0
500000
1000000
1500000
2000000
2500000
3000000
3500000
4000000
0 50000 100000 150000 200000 250000 300000
Total energy to be taken by IC pool [MJ]
ENERGY (Power*Time)
Time Time Time
Power
percent
Thermal
Power Duration
[seconds] [hours] [days] [%] [MW] [seconds]
0 0 0 100.000% 2180 0.1
0.1 0.00003 0.00000 6.530% 142.354 0.9
1 0.00028 0.00001 6.180% 134.724 1
2 0.00056 0.00002 5.790% 126.222 3
5 0.00139 0.00006 5.290% 115.322 5
10 0.00278 0.00012 4.900% 106.82 10
20 0.006 0.000 4.450% 97.01 20
40 0.011 0.000 4.010% 87.418 40
80 0.022 0.001 3.560% 77.608 70
150 0.042 0.002 3.190% 69.542 150
300 0.083 0.003 2.810% 61.258 200
500 0.139 0.006 2.520% 54.936 250
750 0.208 0.009 2.300% 50.14 250
1000 0.278 0.012 2.140% 46.652 250
1250 0.347 0.014 2.010% 43.818 250
1500 0.417 0.017 1.900% 41.42 500
2000 0.556 0.023 1.740% 37.932 3000
5000 1.389 0.058 1.320% 28.776 5000
10000 2.778 0.116 1.050% 22.89 5000
15000 4.167 0.174 0.948% 20.6664 5000
20000 5.556 0.231 0.882% 19.2276 10000
30000 8.333 0.347 0.790% 17.222 20000
50000 13.889 0.579 0.693% 15.1074 20000
70000 19.444 0.810 0.629% 13.7122 20000
90000 25.000 1.042 0.581% 12.6658 20000
110000 30.556 1.273 0.546% 11.9028 30000
140000 38.889 1.620 0.507% 11.0526 30000
170000 47.222 1.968 0.475% 10.355 30000
200000 55.556 2.315 0.448% 9.7664 59200
259200 72 3 0.381% 8.3058 259200
Total Decay heat power for CANDU 6 NPP [1], [2]

5. Solution 1
5
POOL INSIDE REACTOR BUILDING
Advantage
- Closed to heat sources
- Low pressure loss
- Great for natural circulation
Disadvantages
- Heat is not transported away from reactor building
- Vaporization of water inside pool leads to overpressurization of RB and very bad Temperature and
humidity conditions for equipment inside RB – could lead to over 100 bar inside R/B if pool
evaporates
- Big pool volume (15000 m3)

6. Solution 2
6
POOL OUTSIDE REACTOR BUILDING
Advantage
- No problem with steam produced by evaporation
- No influence on RB working condition
- No issue with space for even big pool
- Less volume (2000 m3) required due to vaporization of pool inventory
Disadvantages
- Heat is not transported away from reactor building
- Vaporization of water inside pool leads to overpressurization of RB and very bad Temperature and
humidity conditions for equipment inside RB

7. Solution adopted 2 – in Unit
2 proximity to containment
7
Section through the water tower
IC HEAT ECHANGER Concept
130 m2 4*66%
⁓ 15 MW
General arrangement - lateral view
of the DHR system.

8. System process diagram
8

9. Relap5/MOD3.2 Modeling –
Validate of natural circulation
9
Main scheme of CANDU 6 NPP used in RELAP5/MOD3.2
General modeling for CANDU6
The scheme adopted in the modelling of the
secondary side of the steam generator The scheme adopted in modelling the
isolation condenser coupled to Steam
Generator 1 and 2
-1.00E+06
0.00E+00
1.00E+06
2.00E+06
3.00E+06
4.00E+06
5.00E+06
6.00E+06
7.00E+06
8.00E+06
0 2000 4000 6000 8000 10000 12000
pressure
[Pa]
time[s]
p 655050000-IC1 p 615050000-IC1 p 630010000-IC1
p 645050000-IC2 p 617050000-IC2 p 620010000-IC2
p 139010000
Evolution of pressure in the steam
generators and in the passive isolation
condense during SBO event

10. Conclude and Thanks
10
Current situation
Current situation in case of SBO
- ⁓23.5 hours window without affecting reactor core
- Active safety system can be operated as passive safety
- Any accidents lead to steam generators depressurization
With new IC passive system
- More then 72 hours window without affecting reactor core
- BUT Great modification to safety procedures
- Steam generators stay with pressure inside and no secondary flow thrown to ambient