The fuel and the containment are the major entities in source term predictions. Their states are represented by s (release), e (emission) and L (leakage), respectively, in the prediction formalism:
s = e×L ,
that is suggested for application in this Emergency Preparedness Handbook. This formalism will be explained in discussing below how accident scenarios affect source terms and environmental consequences. Table 4.1 provides the scenario map and organizing structure for this discussion. Confinement zones of Ignalina NPP are shown on Fig. 4.1.
Fig. 4.1 Ignalina NPP confinement zones [9]:
1 - steam separator; 2 - suction header; 3 - pressure header; 4 - main circulation pump; 5 - check valve
Table 4.1 Scenario map. Overview of plant, core and confinement states with their corresponding source terms, adopted from [7]
|
I N E S |
PLANT STATES |
CORE STATES |
CONTAINMENT STATES |
SOURCE TERMS |
REMARKS |
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|
Accident Initiators |
Failure of Support & Safety Functions |
Fuel States |
Pressure Tube States |
Hydrogen States |
Graphite Stack States |
Zone 3 States |
Zone 4 States |
ACS States |
As Now Operated |
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|
0 |
S = Safe stable cooling |
Operating temperature |
Inerted, unburnable |
Inerted, unburnable |
Inerted, unburnable |
s = e× L |
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|
1 2 3 |
LOCAs Transients |
Available apart from single failures |
V = Violation of heat remove from fuel at preserved geometry |
Small local damage |
Small amounts |
Heated locally |
e = 10-5à 10-3 L = 0.01 s = 10-7® 10-5 |
Inside Design Basis |
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|
4 |
Local flow blockages Station blackout |
Temporary |
D = Damage to fuel by local melting |
Multiple pressure tube |
Medium amounts |
Graphite stack cooled via control rod coolant flow |
Fuel channel top end heat up to failure into Zone 4 |
Structure of Zone 4 remain sound |
Molten fuel opens vent path via Zone 3 |
e = 0.002 L = 0.1 s = 0.0002 |
Somewhat beyond Design Basis |
||
|
5 |
Station blackout |
Longer |
A = Severe Accident |
rupture at low pressure |
Large H2 evolution from fuel |
Control rod channel boil dry and graphite heat up |
Low end fuel meltout into ACS |
to Zone 2 |
e = 0.1® 0.5 L = 0.1® 0.2 s = 0.01 |
Clearly beyond Design Basis |
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|
6 |
Station blackout |
Prolonged |
Global melting |
Multiple pressure tube rupture at high pressure |
and pressure tubes |
Air ingress to hot graphite causes fire |
Upper core plate uprise into Zone 4 Low end fuel meltout into ACS |
Zone 4 structure failed by press beyond relief Transport of FP+H2+ steam via Zone 4 to atmosphere |
Hot gases from Molten fuel open vent path from Zone 3 to Zone 4 |
e = 0.5® 0.8 L = 0.2® 1 s = 0.1 |
Severe Accident far beyond Design Basis |
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|
7 |
RIA induced power transient, e.g. Chernobyl |
Damaged |
Large amounts |
H2 and graphite fires |
Open upwards to Zone 4 |
Open to the atmosphere |
e = 0.8® 1.0 L = 0.5® 1 s = 0.5® 1 |
Extreme event |
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