4.5.1 Plant States and Core States
The Plant States in two columns recognize the accident initiators and the availability of support and safety functions. The Core States comprise four columns. The fuel states progress in the order S = Safe stable cooling, V = Violation, D = Damage, A = severe Accident from lesser to larger emission fractions (e) of volatile noble gases, iodine, cesium from the fuel. The largest emission fractions in class seven can be caused by reactivity induced accidents (RIA), like the one that occurred at Chernobyl in 1986. In that accident s = e L was larger than 0.5 for iodine and 0.3 for cesium, according to the recent review [10]. The pressure tube states progress from small local damage via single tube rupture to multiple pressure tube rupture at low as well as high pressure. The significance of this column is of course that it recognizes the potential for upper core plate upraise and lift off into zone 4 as occurred at Chernobyl. Some large LOCA accident initiators by themselves eliminate lid lift by relieving primary system pressure. The hydrogen states are important because a burn or explosion of large amounts of evolved hydrogen can damage or fail containment structures. The graphite stack states can progress from locally heated spots, via overall stack heat-up to air ingress and large scale fire, which will damage the fuel and emit its radioactive inventory.
4.5.2 Containment States
Zone 3 states comprise the core cavity compartment. As long as this compartment remains intact, source terms can be expected to remain small. Fuel melting in the channel can be expected to flow down and melt out into the ACS compartments. Pressurization of zone 3 by multiple tube failure at high pressure causes upper plate rise up into zone 4. Simultaneous upper and lower end failures of fuel channels open a path for natural circulation through the core of hot gases including hydrogen evolved from zirconium oxidation caused by steam and air ingress. Such a scenario transports a substantial part of the part of radioactive inventory emitted in fuel-air oxidation into zone 4.
Zone 4 states are mainly that the structures may remain sound throughout the sequence, and thereby mitigate releases by directing them via filters and the stack to the atmosphere. Alternatively, the zone 4 structures can be failed by steam and/or hydrogen-burn pressurization beyond relief capabilities, in which case most of the zone 4 gasborne radioactive content is released to the atmosphere.
The ACS sturdy structures can be expected to remain sound and confine radioactivity in most scenarios. Concern is with the above state of natural circulation of gases via the core to the atmosphere. There is also a concern for basement meltthrough as was imminent at Chernobyl.
4.5.3 Source Term Data and Formalism
Source Term escalation on the INES scale in classes 1 through 7 is exemplified in the last column according to the formalism s = e× L, at present, for illustrative purpose only. Reliable quantitative source term data must be calculated on specific Ignalina NPP accident sequences using the available source term technology.
4.5.4 Three Mile Island and Chernobyl
At Three Mile Island severe damage, melting, to large fraction of the core caused emission from the fuel into the containment of large amounts of radioactivity,
e = 0.5 ®1.
However the containment structure remained essentially leaktight throughout the accident and release to the atmosphere remained small,
L = 10 -3.
Then according to the rule of thumb:
s = 1.0x1 = 1 for noble gases and
s = 0.5x10-3 = 0.0005 for iodine and cesium.
Real releases at the Three Mile Island were:
s = 0.05 for noble gases,
s = 0.5x10-7 for iodine, and
s = 0 for cesium.
This illustrates the point made below (see 4.6), that thumbrule estimates represent an upper bound.
At Chernobyl the violent Reactivity Induced Accident damaged at the same time the fuel and the confinement structures and caused release of a large fraction of the core radioactivity directly to the atmosphere. Real releases at the Chernobyl were:
s = 1.0 for noble gases,
s = 0.5 for iodine, and
s = 0.3 for cesium.