So, from the rumors I’ve seen Twitter, One of the nuclear reactors in Texas ( @stpnoc - unit 1) may have gone offline at some point today on a trip of a feed-water pump due to the historically-cold temperatures.

South Texas Project is one of our nation’s newest nuclear power plants (1988/89) , producing a net power of 2560 MWe. Given their young age, these might be the coldest temperatures the two units have ever seen! In the photo, I’ve highlighted a turbine (blue).

The two STP units work by heating water within a rector vessel, which is housed in one of the two large, domed structures. This vessel contains fuel rods that with a uranium oxide that gets mined from the ground and processed (through enrichment of an isotope, uranium-235).

In the specific arrangement with water, physics (neutron moderation and absorption) causes some of the U-235 in rods to fission (split) and produce heat, which is removed by the water. The heated water travels to a heat exchanger in the concrete building called a steam generator.

Another set of pipes carries steam from the steam generator out to the blue turbine, which is spun by the high pressure/temperature/enthalpy steam, which in turn cranks a generator at a frequency that can later load electricity onto the grid.

This steam then passes to a condenser before going to a feed-water pump to repeat its cycle in the steam generator. This is where the rumored problem occurred. An unusual feature about STP is (as shown from my Google Maps satellite photo) a lot of these components are outdoors.

So, this may help explain why this happened at STP when the temps got cold, but not at other nuclear plants in the US that have been through similar temperatures. It’s an extremely unusual condition for STP. Similar weather + supply effects may be harming other TX generators, too

One benefit of nuclear is that the fuel in the reactor is designed to last 18/24 months, and even then, they only swap out one-third or one-fourth of the core. Coal plants usually have coal supplies that can last days, while gas plants require online/functioning infrastructure.

All-in-all, this weather has created havoc on the TX grid (supply & demand), and I’m sure we’ll get lots of lessons learned in the weeks ahead. But, for now, please stay safe & try to stay warm TX. I’m hoping the people supporting your power are able to meet your needs soon!

One final thought—be cautious about alarmists on social media. The power grid situation is dire, but the nuclear plants remain very safe, based on the rumors I’ve seen. This “trip” operated as designed: to keep the system in its defined operating space and maintain safety margin.

@Atomicrod has a statement from @stpnoc in one of his blog: https://atomicinsights.com/south-texas-project-unit-1-tripped-at-0537-on-feb-15-2021/ . The tl;dr version is that a sensor provided a false pressure signal that caused the reactor trip from a perceived loss of feed-water.

To illustrate the defense-in-depth (redundancy) of the design, IF the feed-water was actually lost to ALL of the legs, and the reactor didn’t initially trip, a trip of another system would occur OR the reactor would naturally power down as the water heated and the physics changed

With the handful of other trips, fail-safe safety rods, and nature, the main concern becomes removing residual heat from the reactor. This residual heat is from nuclear radiation (decay power) plus the thermal mass or enthalpy.

There are a number of systems that can provide water to remove heat from the reactor at high to low pressure. With offsite power, this is pretty simple, but without offsite power, passive systems (for example water thats been previously accumulated at a high pressure) may be used

Or one of three (a somewhat-unique feature to STP, I believe) redundant “trains” of power provided by diesel generators could be used to power residual heat removal pumps or even building water sprays to condense steam and lower containment pressure. The containment building...

(the domed, concrete building) is actually designed to recirculate this water for days in catastrophic scenarios. Unfortunately the containment building is airtight, otherwise maybe they could just open a window in this scenario on a day like this. Stay safe everyone!

Oh, and for more info on the basic safety and design of U.S. nuclear power plants (including schematics of how a feed-water pump ties into a PWR system like the one at STP), check this out (or ask away)! https://www.nrc.gov/reading-rm/basic-ref/students/for-educators/04.pdf

Update: STP-1 listed as offline in the morning reactor status report at https://www.nrc.gov/reading-rm/doc-collections/event-status/reactor-status/ps.html