What happens step by step during station blackout before core damage occurs?

[matt222]

I have Q regarding station blackout, the causes of it complete loss of Onsite and offsite power. However if there is no make up what is happening exactly before core damage by steps

 

[Astronuc]

I have Q regarding station blackout, the causes of it complete loss of Onsite and offsite power. However if there is no make up what is happening exactly before core damage by steps

That would largely depend on what event precipitated the SBO.

Station blackout means the complete loss of alternating current (ac) electric power to the essential and nonessential switchgear buses in a nuclear power plant (i.e., loss of offsite electric power system concurrent with turbine trip and unavailability of the onsite emergency ac power system). Station blackout does not include the loss of available ac power to buses fed by station batteries through inverters or by alternate ac sources as defined in this section, nor does it assume a concurrent single failure or design basis accident. At single unit sites, any emergency ac power source(s) in excess of the number required to meet minimum redundancy requirements (i.e., single failure) for safe shutdown (non-DBA) is assumed to be available and may be designated as an alternate power source(s) provided the applicable requirements are met. At multi-unit sites, where the combination of emergency ac power sources exceeds the minimum redundancy requirements for safe shutdown (non-DBA) of all units, the remaining emergency ac power sources may be used as alternate ac power sources provided they meet the applicable requirements. If these criteria are not met, station blackout must be assumed on all the units.

Ref: 10 CFR 50.2 - http://www.nrc.gov/reading-rm/doc-collections/cfr/part050/part050-0002.html

See also - http://www.nrc.gov/reading-rm/doc-collections/cfr/part050/part050-0063.html

Reevaluation of Station Blackout Risk at Nuclear Power Plants (NUREG/CR-6890)
http://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr6890/

The concern with a SBO or loss of critical AC power would the effect on core coolability, basically the loss of decay heat removal. The progression to core damage would depend on the how decay heat removal is impaired and for how long.

If the RCS is intact, there is a chance that some cooling will occur. In PWRs, it's possible to get some natural convection, but the steam generators would also need some circulation on the secondary side.

Basically, as cladding temperature increases the oxidation rate increases. Under normal operation, cladding oxidation is very slow - less than 0.1 microns per day, and preferably less than half that or < 0.05 microns per day. Once that cladding temperature increases by more than 100°C above normal operating temperature, the oxidation rate increases. The concern then is that the cladding would breach and release fission gases Xe, Kr and volatiles, I, Cs. Ingress of steam would allow oxidation of the ceramic fuel and more fission products. The oxidation of Zr-alloys produces hydrogen gas, which is combustible. Ignition of hydrogen could lead to containment overpressure and failure.

In the worse case, e.g., Fukushima, the core could experience near adiabatic conditions, in which case, the fuel would reach melting temperatures, or at least severe chemically reactions. If fuel melts, then there is an increase risk of damage to the reactor pressure vessel or loss of integrity.