TerraPower's Natrium Reactor

"You have power over your mind - not outside events.  Realize this, and you will find strength." - Marcus Aurelius

A commenter, Johnnywoods, recently requested information on a new type of nuclear power plant to be built in Wyoming, by a company named TerraPower.  The reactor is named Natrium, which is the latin word for Sodium.  

You can never be sure if these highly demanding and expensive nuclear projects will make it all the way to completion.  There have been a number of nuclear plants that made it most of the way through construction before being abandoned.  Below are some Google Earth pictures of a few of these half-built, and yet abandoned ruins.  Click any image to improve resolution.

Below: Columbia Generating Units 2 & 3

 

Below:  Satsop Units 1 & 2

 Below:  Hartsville Units 1 and 2

Below: Bellefonte Nuclear station Units 1 and 2

Below: VC Summer Units 3 & 4 - abandoned in 2017 after 9 Billion dollars had been spent on the project, and all the major equipment had been delivered to the site.

A while back, I read a press release about the Natrium reactor.  There didn't seem to be a need to put up a post about it, because there weren't yet many details in the article.  To be honest, I wasn't sure the whole thing wasn't vaporware - and it may still be vaporware.  A look at the above images should remind readers that things can still go sideways even when it looks like the project is nearly complete.  Today we have a bit more information available, so let's take a look to see what they have in mind.  And thanks to Johnnywoods for the timely reminder about this cool subject! 

First off, let's check out the location and current conditions at the site where this new nuclear power plant is intended to be built.  Below: A view of Wyoming, with the town of Kemmerer near the southwest corner of the state.

 Next, a little closer look at the town of Kemmerer.  The Naughton coal-fired power plant is a three-unit plant to the southwest of Kemmerer, which is the proposed site of the Natrium power station. 

Closer in: The coal mine (left), the power plant, and the ash ponds.  Yes, coal is a filthy and destructive means of generating electricity - which is the reason why I posted about global warming and the anti-nuclear lobby recently.  This environmental blight (and the nuclear plant ruins) are their true legacy.

Below:  A final look at the Naughton power plant up close.  Underneath the white steam plumes at the top and to the right, are stacks that release the boiler combustion gas after it has passed through a de-sulfurization process.  Some sulfur still makes it through the process, however.

Let's briefly discuss the old coal-fired units.  The Naughton power plant consists of three coal-fired boilers and three steam turbines, with a combined power output of 820 Megawatts.  The combined Carbon Dioxide released by the three boilers is about 6 million tons per year, Sulfur (which produces acid rain) is about 14 tons/year, and Mercury is about 30 lbs/year.  Mercury of course bats way above its weight in neurological damage, and in its ability to move up the food chain into humans.

The plant is owned by PacifiCorp, which is in turn owned by Mid-American Energy, which is in turn owned by Berkshire Hathaway - whose CEO is Warren Buffett.  I'm pointing this out so that you know who might very well end up pocketing profits from cutting Carbon Dioxide, after having profited from producing Carbon Dioxide for decades.  Unit 3 was shut down in February of 2019, and the remaining units were due for closure in 2025.  Pacificorp had already decided to close the remaining two units, as they were no longer deemed profitable.

A brief aside about trying to site a new-construction nuclear plant...

This place is a good fit for a nuclear power plant because:

• Location - Kemmerer, Wyoming is in a remote location, with no major cities nearby.  Nuclear plants involve making evacuation plans, in the event of a radiological emergency.  The more people that live nearby, the more difficult evacuation planning becomes.  For example: The Shoreham Nuclear power plant on Long Island, NY after many delays, was completely built and conducting low-power testing.  When construction began, the area was sparsely populated, but by the time the plant was completed, there were hundreds of thousands of residences within the evacuation zones - which had been enlarged following the accident at Three Mile Island.  Shoreham never made it to full power.  It was permanently shut down and decommissioned due to the impossibility of quickly evacuating so many nearby residents off Long Island in an emergency.
  
•  Infrastructure - There is already a transmission line in place to support the existing power plant, which is capable of carrying at least 820 Megawatts of power.  This T-line is a cost that can be avoided by siting the new plant near the location of the old one.  Water rights are already in place for the current coal-fired facility, so it may be possible to transfer those to the new facility.  The new facility should use just a fraction of the water the old one does, so from a water-use standpoint, this is a good thing.
  
• Environmental - This is a "brownfield" project, meaning that heavy industry has already made a big footprint on the site.  There will be very little environmental impact to consider at the construction site due to previous development.  Almost any development would be less damaging to the environment than digging up coal, burning it, and burying the ash.
• Political - Rural Wyoming is likely to be much more embracing of a nuclear power plant than any city would be, so there will probably be very little resistance to building such a facility.

According to Wikipedia, TerraPower is a private nuclear power plant design and development company.  TerraPower's founder is Bill Gates, so at least in this respect, he ain't all bad.

Let's finally look at the technology part of Natrium, which is the part that I find the most interesting. 

The power plant design is, in my opinion, a bit edgy.  I don't mean that in a bad way - it shows they are thinking outside the box, and I sincerely hope that it pays off for them.

According to the fact sheet, the power plant should have 345 MW net electrical output, with a provision for thermal storage, and the ability for that stored thermal energy to boost plant output to 500 MW for about 5-1/2 hours.  This is quite a modest-size nuclear plant.  900-1300 MW is typical for utility-scale reactors.

Below are the basic design elements of the primary and intermediate loops of this interesting reactor.  

• The primary reactor coolant will be liquid Sodium.  I've posted before about sodium-cooled reactors, and listed their significant nuclear and thermal benefits, as well as noting their deficiencies - mainly regarding safety.  I highly recommend following the above link above for a deeper (but layman friendly) discussion of fast neutron spectrum reactors and liquid metal cooling.
• The choice of liquid metallic sodium as a coolant necessitates using an intermediate coolant loop for heat exchange between the primary coolant and the steam system.  The reason an intermediate heat transfer fluid is required is because the steam system operates at higher pressure than the sodium reactor coolant.  Therefore any potential leak in the steam generator will result in water entering the sodium reactor coolant.  Water and sodium will chemically react to form sodium hydroxide, which is caustic, and will damage the nuclear fuel cladding.  Using a less chemically reactive intermediate fluid at low working pressure to transfer heat avoids this issue.
  
• The intermediate coolant loop of this design will contain molten salt, which should be inexpensive and chemically not too reactive with either sodium (reactor coolant) or water (steam cycle). 
  
• As discussed in the link above, this reactor will run with a fast neutron spectrum, which requires highly enriched fuel, near the 20% U-235 level that could be used to develop a crude nuclear weapon, if enough of it could be diverted.
• Most interestingly, this project will use the intermediate coolant loop for thermal energy storage, in the form of a insulated tanks filled with hot molten salt.  This thermal storage will be used to rapidly augment steam production (and power output) - giving this power plant a peaking capability that no other nuclear power plant has.  This is a very innovative use of the intermediate fluid - which has previously only been used as a closed circulating heat transfer loop in these reactors.
  
• The thermal storage system will allow the plant to have good synergy with renewable energy sources, by being ready to pick up generation when renewable output falls off.  Recharging the energy of the thermal storage system will of course reduce the electrical output of the plant for the duration of the heating process.
• Also, according to the fact sheet, this plant will be air-cooled.  To my knowledge this will be the first-ever air-cooled nuclear power plant.  This is a very environmentally friendly choice from a water conservation standpoint.  It will be interesting to see how well that will work, and also whether the plant will be "passively safe" - meaning that nuclear decay heat can be handled with no operator intervention.

The Wikipedia page for this reactor is here.

I have not been able to learn any precise details regarding the physical design of the power plant - no data on the core design, the containment structure, the coolant loops, the thermal storage system, nor the steam system.  Nearly all of the currently available information is marketing and PR stuff.  Even the NRC pre-application activity files are largely blank, due to proprietary information.  We have no engineering drawings or other hard data, sadly.

Below is a cutaway of the reactor island, with spent fuel storage at the right, and molten salt pump room on the left.

The TerraPower website has a nice isoschematic drawing of the process, which I've copied below.

At the bottom of the image is the reactor - our heat source.  Hot liquid sodium inside the reactor vessel transfers heat into the intermediate loop containing molten salt  - the red and orange lines.  The red lines contain non-radioactive hot molten salt leaving the intermediate heat exchanger inside the reactor.  The heated salt is pumped to a hot storage tank on the left side of the image.  

The hot molten salt is pumped out of the tank, into two small heat exchangers.  One heat exchanger superheats the high pressure steam, and the other one reheats the steam after it has passed through part of the steam turbine.  Superheating and Reheating increase steam cycle efficiency.  The molten salt streams come together again and then pass through a steam generator, which boils the water for the steam turbine.  After leaving the steam generator, the much cooler salt flows through an economizer - which also pre-heats the water prior to boiling it, increasing cycle efficiency.  After this final heat extraction, the much cooler molten salt is returned to the reactor for heating. 

During peaking periods, additional intermediate fluid will be pumped from the hot tank on the left, through the steam generators and into the the warm tank on the right, increasing steam production (and electrical output) until the hot tank runs low on level.

During recharge periods, molten salt from the cooler orange tank on the right will be pumped into the intermediate loop, heated by the reactor, and used to refill the red storage tank on the left, while still also circulating a portion of the fluid through the steam generators to run the steam turbine.  Electrical output of the plant will fall while refilling the hot storage tank, which would make this plant an excellent solar-following power source. 

I would expect that the process of transitioning to and from re-heating the molten sodium would require a great deal of time and extra caution from a reactor controls standpoint, and possibly from a steam turbine/steam plant standpoint as well.  Major changes in heat flows are never casual things in a power plant.

Finishing up with the steam cycle though:  The spent steam from the steam turbine is sent to the air-cooled condenser at the top right.  There the steam is condensed back into water, then is returned to the high temperature section of the cycle by the little blue pump.  For a brief primer on the steam cycle, see this post.

As with all these projects, I'll believe it's a reality when the generator breaker closes and we push some electrons. 

EDIT 23 November 2021:

Linked article from PowerMag, which gives a similar overview to this post, and got statements from some of the principal actors in the project.