If a nuclear power plant is suddenly disconnected from the grid carrying the electricity away from the plant, what is the risk to the plant itself?

Mudassir Ali
Feb 06, 2020 04:03 PM 0 Answers
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Mudassir Ali
- Feb 06, 2020 04:03 PM

Years ago I worked in a nuclear power plant for a few months and experienced the plant suddenly and unexpectedly coming off line. Nuclear power plants (or at least the typical US commercial plant) NEED to have electricity at all times and they invest a lot in making sure that it is provided. This imperative is not, as implied in the question, to take the power away, it is to make sure the control systems and key electrical pumps kept working.

Following is the basic scenario for how a sudden disconnect might work.

Something happens that disconnects the generator from the grid (this can be a safety response to a grid problem, or a physical issue).
The very first thing that happens is the computers detect the lack of load on the generator and reduce the flow of steam to the turbine to prevent the turbine and generator from overspeeding, this steam is diverted to be expanded and then feed into the normal cooling loops.
Then the computer will detect that there is no need for generating heat in the reactor and shutdown the reactor (drive the control rods fully in, slowing then stopping the reaction and the generation of heat).
Now the systems will look to say, can I still get power from the grid? Yes no worries, No – Start up standby diesel generators to restore power.
Make sure the reactor cooling pumps keep circulating water (it will take sometime for the decay products to further decay and substantially reduce the heat going into the core, so the cooling pumps need to run to move that heat to the turbine steam loop. This then continues (in detail steam is still flowing to the turbines and is throttled to reduce the temperature smoothly and the turbines at some point are kept spinning on jacking gear until they are fully cooled down).
In the case I watched (which was slightly different as the turbine trip happened first, rather than the generator trip), the whole sequence up to halfway through 5, took the amount of time necessary to jump out of a chair, drop a cigarette and kick off a coffee cup while lunging for a control panel (in a vain attempt to override the instrument fault that triggered the trip).

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