NPP reliance on the grid a meltdown risk : US NRC

REGULATORY GUIDE 1.155 Tables 1, (Task SI 5014) 5, and 6.

“The term “station blackout” refers to the complete loss of alternating current electric power to the essential and nonessential switchgear buses in a nuclear power plant.

Station blackout therefore involves the loss of offsite power concurrent with turbine trip and failure of the onsite emergency ac power system, but not the loss of available ac power to buses fed by station batteries through inverters or the loss of power from “alternate ac sources.”

Station blackout and alternate ac source are defined in § 50.2. Because many safety systems required for reactor core decay heat removal and containment heat removal are dependent on ac power, the consequences of a station blackout could be severe.

In the event of a station blackout, the capability to cool the reactor core would be dependent on the availability of systems that do not require ac power from the essential and nonessential switchgear buses and on the ability to restore ac power in a timely manner.

The concern about station blackout arose because of the accumulated experience regarding the reliability of ac power supplies.

Many operating plants have experienced a total loss of offsite electric power, and more occurrences are expected in the future. In almost every one of these loss-of-offsite-power events, the onsite emergency ac power supplies have been available immediately to supply the power needed by vital safety equipment.

However, in some instances, one of the redundant emergency ac power supplies -has been unavailable. In a few cases there has been a complete loss of ac power, but during these events ac power was restored in a short time without any serious consequences. In addition, there have been numerous instances when
emergency diesel generators have failed to start and run in response to tests conducted at operating plants.

The results of the Reactor Safety Study (Ref. 1) showed that, for one of the two plants evaluated, a station blackout event could be an important contributor to the total risk from nuclear power plant accidents. Although this total risk was found to be small, the relative importance of station blackout events was established. This finding and the accumulated diesel generator failure experience increased the concern about station blackout.

In a Commission proceeding addressing station black out, it was determined that the issue should be analyzed to identify preventive or mitigative measures that can or should be taken. (See Florida Power & Light Company (St. Lucie Nuclear Power Plant, Unit No. 2) ALAB-603, 12 NRC 30 (1980); modified CLI-81-12, 13 NRC 838 (1981).)

The issue of station blackout involves the likelihood and duration of the loss of offsite power, the redundancy and reliability of onsite emergency ac power systems, and the potential for severe accident sequences after a loss of all ac power.

References 2 through 7 provide detailed analyses of these topics. Based on risk studies performed to date, the results indicate that estimated core melt frequencies from station blackout vary considerably for different plants and could be a significant risk contributor for some plants.

In order to reduce this risk, action should be taken to resolve the safety concern stemming from station blackout. The issue is of concern for both PWRs and BWRs. end quote. Source: as given above at

source: as given above.

The following post uses a Tepco document which provides a chronology of events covering the time taken to restore grid power to the Fukushima Diiachi reactors 1,2,3 and 4 and the consequences of that grid failure. Coupled with multiple failures of the FD Emergency Core Cooling Systems each reactor there is fitted with, the emergency proceeded. The consequences and costs continue to mount as of April 2018