What does it mean when the Nuclear generating needle is in the red?

It means that all, or almost all of the available generating capacity is in use.

Of limited relevance in fact for nuclear generation, since this is almost always run on an "all available" basis, unlike fuel burning plant that is run on an "as needed" basis.

It means that the nuclear stations are running really hard above nominal capacity because the price is really high. It is a very windy day but still, wind is below nameplate power. Gas is 15GW or so because there is no alternative, and of course no large-scale storage. Interestingly diesel generation is significantly cheaper than gas at the moment.

You could think that it says that the folk who manage the website have not set up the 100% of the display 'meters' quite right, or at least that the available capacity has changed since they did.They do periodically re-organise the scales on the dashboard - the full scale deflection for coal has shrunk over time and renewables have expanded. Nuclear has shrunk during maintenance shutdowns - perhaps something has come back on a bit early . As  others have said, once fueled up, a nuclear plant just runs at more or less full chat - you could take unfinished fuel rods out to shut down, and waste the heat they produce, but you'd not normally do so.

The level indicated on the meter is not really a concern,

Compare the scales with a few older versions

ten years ago....

or

5 years ago

look different to each other and also

A few months ago...

Mike

Yes, the small daily and weekly graphs are good but maybe they could use filled in colours to total up the generation types one above the other; say bottom green for hydro, followed by wind above it in blue, then CCGT for hydro-carbons in red, above which is coal in black at the top as a very last resort. 

Note that wood burning should not be considered green as it is a carbo-hydrate is mostly imported causing deforestation and also emits CO2.  To fill the gap when the wind drops off the old nuclear stations should be rehabbed ASAP while we wait for new ones to be commissioned..

Interesting to note that today (6th Jan) Nuclear is back in the black (just).  Presumably a generator has been taken off line.

David