Nuclear-Heated Gas Producing Superheated Steam

The first HTR-PM (High Temperature Reactor – Pebble Module), one of the more intriguing nuclear plant designs, is currently under construction on the coast of the Shidao Bay near Weihai, China.

This system uses evolutionary engineering design principles that give it a high probability of success, assuming that the developers and financial supporters maintain their steady progress. Considering the fact that the plant is a logical follow-on to a successful prototype that has been operating since 2000 and that it is being developed by long-term thinking Chinese engineering and constructors there does not appear to be much development risk. Since the project appears to have the solid backing of the Chinese government, there does not appear to be much risk of sudden funding removal.

Here is a link to an April 2014 update presentation given to the IAEA – HTR Progress in China.

It does a good job of describing the technical foundations of the plant design and the reasons why the system is considered to have a high degree of inherent safety.

In basic layout, the power plant will share a number of features with the second stage of a modern combined cycle power plant. In a combined cycle power plant, the exhaust gases from combustion turbines are directed to a heat recovery steam generator (HRSG).

Those combustion product gases enter the HRSG at a temperature somewhere close to 750 C and leave that HRSG at a temperature of about 250 C. On the other side of the HRSG heat transfer tubes, feed water enters and boils, leaving the HRSG as superheated steam with a temperature somewhere close to 565 C and a pressure of 13-15 MPa. In most cases, the steam output of two or more gas turbine/HRSG modules is combined to drive a single steam turbine train, which might include both a high pressure and a low pressure turbine.

Interestingly enough, those are the same conditions produced in the HTR-PM.

For the demonstration plant, two reactor modules, each producing 250 MWth in a large, low power-density pebble bed reactor produce high temperature gas that enters the reactor at 250 C and leaves the reactor at 750 C. That hot gas (helium in the case of the HTR-PM) is moved by a circulator (the gas equivalent of a pump) into a steam generator that has feed water coming in and steam going out. The steam conditions are 565 C and 13.2 MPa. The output of the two steam generators is combined to drive a single 210 MWe steam turbine.

As described in the literature, this demonstration configuration was chosen to gain experience with multiple modules with the full intention of eventually producing larger output power plants by using more reactor/steam generator modules connected to larger steam turbines.

There are conceptual designs for 4, 6, 8 and even 10 reactor modules all connected to a single steam turbine. The designers are sticking with smaller power output reactors. Calculations tell them that if they keep total output power less than 300 MWth they can make a testable claim of inherent safety. No conceivable event can lead to a situation where the temperature in any part of the core exceeds the 1600 C design temperature for the TRISO particle fuel.

If no accident leads to temperatures that can cause fuel damage, there is no need to devise additional safety systems or features to remove heat.

There are several evolutionary paths available based on this design advancement. One path would be to implement a phased replacement of coal fired boilers with HTR-PM reactor/steam generator modules. China has a large and rapidly growing inventory of modern steam plants that currently require burning about 3.5 billion tons of coal per year, resulting in places where the air is almost too foul to breathe.

Moving all of that coal from the source to the power plant is also a major burden on the country’s straining rail and barge transportation network. Replacing coal boilers with nuclear heat sources would eliminate the main drawbacks of the power plants while fully using the rest of the installed infrastructure of cooling water, steam plant, transmission lines, and trained operators/maintenance staffs.

Another direction available is to gradually increase the temperature capability of the pebble bed to the point where the gas is hot enough to drive a direct cycle gas turbine whose exhaust can then be directed to the steam generator for a higher efficiency, higher power output combined cycle system.

The Chinese purchased their initial TRISO fuel manufacturing capability from the Germans and their designs have a great deal in common with the HTR program being pursued in Germany up until the end of the 1980s. In that program, demonstrated gas temperatures reached 950 C with future plans of hitting 1100 or 1200 C as the manufacturing techniques improved.

As demonstrated in the German program, TRISO fuel particles do not have to be UO2, a wide variety of actinide compounds including UC, PuO2, and ThO2 have been tested and are available for future use.

One of the things that I find incredibly invigorating about nuclear technology is the almost endless horizons and options for creatively using energy dense, ultra-low emission fuel sources to create useful heat that does not require wholesale reengineering of our basic infrastructure. We can reuse a large portion of what we have already built and already learned how to effectively operate and maintain.

If you are as interested in high temperature reactors as I am, you might want to learn more about HTR-2014, The 7th International Topical Meeting on High Temperature Reactor Technology. It is being held in China in Weihai, close to the HTR-PM construction site, from October 27-31. Tours of the site will be offered as part of the conference program.

Additional reading

Next Big Future (April 2013) HTR-PM High Temperature Pebble-Bed Modular Status in China March 2013

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