Thread by @wang_seaver

Ketan mocks inertia chatter re: Spain outage while misunderstanding inertia. Inertia doesn’t scale with generation output but rather with spinning generator mass. Spain had 4 reactors online, 2 at 70% power. Inertia equivalent to 4 reactors at full power!

More thoughts below.🧵

In his blog post + thread, Ketan shares ~10 graphs of Spanish grid data to show how in April 2025 Spain’s nuclear fleet was generating at historic record lows. Except it’s largely misleading as nuclear generation is a poor proxy for the % of grid-forming resources on the grid.

Inertia scales w kinetic energy of spinning generators, which are synced to grid at fixed rpm even if output to the grid is reduced.

Spain: 2/4 nuclear units at 70% power but inertia equiv to 4 units at full power. This is in line with other past spring/fall shoulder seasons.

Ketan later casts shade at what he claims are the high costs of nuclear “reliable” services… except on the day of Spain’s outage from a pure inertia perspective 4 units were providing the same amount of inertia to the grid... for less compensation than usual.

Re: drawing attention to low nuclear generation… In the past others have criticized nuclear for *not* load following + adjusting output to match RE generation. Now load following + output adjustment to match RE is evidence of nuclear’s unreliability. Pick one!

Very crucially, nuclear-powered generators are hardly the only generators providing inertia on the grid. The actual relevant metric is the total inertia from all synchronous generators operating at a given moment: hydro, gas, nuclear, coal, cogen, even solar thermal.

Ketan’s own graphs show how relative to last April, Spain’s nuclear output was lower… but gas generation this April was higher than last April by enough to compensate for much of lower nuclear output. Then we must consider other technologies’ inertia.

April is also historically a shoulder season with lower power demand given people are running heat/air con less frequently. If Ember data is correct, April 2025 may have been Spain’s third-lowest electricity monthly demand in the past decade. Important context!

Overall Ketan vigorously argues nuclear wasn’t providing “reliable” services yet hasn’t gotten scrutiny renewables have re: Spain. But the reason why there’s little criticism of nuclear is b/c low availability of nuclear is a non-story.

Let's consider power markets.

Grid operators are required to keep a stable grid, and procure power in advance to meet likely demand, picking lower-priced generators first. If more assets are needed to stabilize grid, operators can buy supplemental power + bring more gens online.

eldiario.es/economia/nucle…

Imagine a factory where employees compete each morning on the wage they’re willing to get. Spain’s grid opted not to work some nuclear employees in April, and didn’t call them on an emergency basis.

If the production line fails, does one blame the employees on unpaid leave?

The weeks prior to the outage don’t particularly matter. What matters is the day of the outage, and the combination of operational decisions + market factors that meant the resources actually online that day couldn’t handle the contingency they encountered.

Does the failure of some nuclear to clear Spain’s power auction in mid-late April mean nuclear is expensive? Spanish industry says they need 60 EUR/MWh to break even. But ~1/3rd of this is a spent fuel management tax, else they could do 40 EUR/MWh.

decouple.media/p/the-iberian-…

Yet frankly I imagine some pro-100% renewable academics would fall on their knees crying uncontrollably in gratitude if they could get a green hydrogen-powered combined cycle power plant running today for 60 EUR/MWh.

https://x.com/Dr_A_Stein/status/1919920366287233107

Likely many gas peakers need 40-60+ EUR/MWh to run. These will need analogues in a future fully clean grid which won’t be cheap either!

Likely new Portuguese offshore wind would need 40-60 EUR/MWh or more, subsidies inclusive, to justify projects. So what is “expensive”?

Infantry are cheaper than tanks, artillery, and surface-to-air missile systems, but one doesn’t build a modern army with nothing but men holding rifles. It turns out a system operates more effectively overall with a mix of cheap and expensive capabilities!

Furthermore, charging Spanish nuclear plants an expensive spent fuel management tax is a political choice. One could reduce this tax. One could levy this tax on petrol sales or gas generation. One could cooperate with European partners on a joint spent fuel solution.

@guillemtweets explains for @DecoupleMedia the spent fuel tax is costly b/c while Spain once approved a centralized fuel repository, opposition incl from enviro groups killed the project. Now plants are duplicating effort, building onsite storage parks.

decouple.media/p/the-iberian-…

This raises a final important point which is that while Ketan perceives attacks against renewables from pro-nuclear advocates, nuclear power has been under political attack for a long time too that aims to even deny it fair parity alongside clean renewables.

Sure, Ketan + many others often get triggered by criticism of RE from anon pro-nuclear “reply guys” with generic profile pictures + names... as if that really should color their thinking about a valuable clean energy technology that has much to offer energy transition efforts.

Yet criticism of nuclear is coming from even the Spanish Prime Minister who during the outage blamed nuclear for coming back online slowly (despite this being prescribed by regulations + best practices) + is dismissing calls to rethink nuclear phaseout

bloomberg.com/news/articles/…

From 1983-1997 Spain implemented a moratorium on new nuclear construction, causing the cancelation of up to 5 planned reactor projects—history Ketan completely omits to mention.

Then of course there’s the policy of nuclear phaseout planned for 2027-2035.

I’d also point out that there are indeed at least some articles that provide level commentary on whether nuclear has been slow to restart, and on the fact that some units were offline pre-outage.

eldiario.es/economia/nucle…

Ultimately yes, there are some nuclear advocates who want nuclear-only and will put down renewables to get it (unreasonable imo). But many nuclear advocates just want fair treatment for nuclear alongside other clean energy, which Spanish policy currently opposes.

My perspective is simple + I’ve been very consistent: - Smart inverters, synchronous condensers + storage can enable high wind + solar % on the grid, but you have to **actually** build those things. - Spain should keep using nuclear energy.

https://x.com/wang_seaver/status/1918370447785071088

In response, Ketan has singled me out in this piece + on social media as a “evidence-free bro” trying to create a "Fukushima moment" for RE. I likely partly motivated him to write this piece calling attention to the lack of discussion of Spanish nuclear’s shortcomings.

But Ketan’s effort to build a counternarrative re: low Spanish nuclear generation in April 2025 is flawed, despite all his graphs.
- Available inertia from Spanish nuclear was consistent with past yrs
- What matters is total system inertia, not just inertia from nuclear

- It is the responsibility of grid operators to procure resources needed to keep the grid stable. They had multiple mechanisms to procure more nuclear and chose not to.
- Spanish nuclear is not that expensive for what it represents: clean firm generation.

Ultimately Ketan presents a paradox.

Low nuclear pre-outage only deserves discussion if nuclear *is* useful for complementing high RE.

If nuclear doesn’t usefully support RE reliability-wise, then it’s *correct* for nobody to be talking about low Spanish nuclear.

So which is it? You have to pick!

As I’ve argued in this thread and elsewhere, nuclear provides value. The reliability value provided by nuclear on the day of the outage was a relatively normal level chosen by the grid operators. Hence this is a non-story.

END

P.S. re: fairness, quote-tweeting + tagging. Ketan Joshi left Twitter but I tagged him in a similar thread over in the land of bluer skies + we are currently duking it out there in the replies. So he's aware of this exchange.

(He generally was not so generous towards me)