In my ideal world we would use the closure of Yucca Mountain to launch efforts into reprocessing. The back end of many reprocessing streams flows nicely into the start of the SolGel process for making the kernel for TRISO particles. The direct formation at one location from used fuel into a coated particle removes any reasonable concern about proliferation. The long term stability of carbon also makes the particles an ideal waste form. With a little careful engineering there is no reason why the particles couldn’t retain fission products for 10 million years.

Across the front end to the back end of the fuel cycle the HTGR concept has a wealth of advantages. I just wish we could move a lot of the funding from renewables (looking right at you, biofuel) to help further the work along.

I understand NGNP’s decision to use helium, it has a number of attractive properties that resulted in its selection as the de facto coolant of choice for gas cooled reactors sometime in the late 1950s to early 1960s.

Up until that time, the favored gas CO2, which was the design choice for the British Magnox and AGR reactors. Though no new CO2 cooled reactors have been built in 30 years, the UK still produces about 10-15% of its electricity each year with CO2 cooled reactors. By far, it is the coolant gas with the most run time in nuclear reactors.

There is one thing we already know about that alternative; CO2 breaks down at too low of a temperature to make it useful for the high temperature reactors that NGNP is planning. It’s maximum temperature limit is somewhere close to 600 C.

Helium got selected for Peach Bottom, the AVR and for the Dragon HTR. There were promising results in those small reactors. The challenges associated with helium’s physical properties (monatomic gas with very low specific mass) did not really make much impact until attempts were made to produce much larger machines and to consider using helium as the working fluid as well as the reactor coolant by directly coupling a gas turbine to the reactor heat source.

Then, factors like leakage and the challenge of completely redesigning turbines and compressors (and building up an entirely new supply chain) became much more important.

In my opinion, helium-related technical challenges have been a primary reason why gas cooled reactors have not had much commercial success, despite their promise and despite the fact that GA once had an order book of more than 10 large units, none of which were ever really started. Helium gas challenges are no more insurmountable than the challenge of using light water as a coolant at temperatures high enough to produce useful steam; the difference is that the LWR hill was climbed in the 1950s. That means there is a knowledge base and a supply chain.

I avoid climbing big hills. I’m a lazy cheap skate (actually, I am just not terribly wealthy, so I needed to find a cheaper way around the issues). Early in my Adams Engines days, I determined that nitrogen gas has “good enough” nuclear properties and with the advantage of having aerodynamic properties that are essentially identical to air. Using N2 allows atomic engine designers to take advantage of the vast technology and knowledge base associated with air breathing combustion turbines.

http://adamsengines.blogspot.com/2009/09/nitrogen-n2-gas-cooling-for-closed.html

The market application I was interested in serving is different from the one that NGNP is aiming to serve. Their choice of helium makes perfect sense; there are nuclear related challenges with using nitrogen that cannot be solved, they must instead be mitigated. I think the mitigations will be effective; nitrogen’s inherent neutron absorption and activation issues discourage most nuclear engineers from even considering using it as a coolant gas.

In NGNP’s defense, the technical challenge of preventing leaks in a helium cooled reactor can be solved, my hope is that we have gained enough engineering and manufacturing experience in the 50 years since the ill-fated water cooled reactor coolant pump bearings so that NGNP has more success than Ft. St. Vrain.

I have selfish reasons for wanting NGNP to succeed. Not only do I want more nuclear fission power to replace fossil fuel combustion, but I also want regulatory acceptance and a supply chain to be established for TRISO coated fuel. Those tiny particles are a key building block that just might make it possible for another attempt at building Adams Engines sometime in the distant future.

I would like to have an Adams Engine-powered yacht ready for my retirement someday around the 2040 to 2050 time-frame (except I will likely need some partners to afford such a luxury).

As you have pointed out, there is plenty of room out there in the energy markets for divergent ideas. I am rooting for your success in getting the TRISO fuel form licensed; just be aware that I have never lost hope in putting that same fuel to use in a different physical form that can be used for reactor heat sources that are as small as 10 MWth and can be cooled by a more useful gas – from a heat engine point of view – than helium.

May all fission power sources succeed! The world will be a cleaner, safer and more prosperous place than it would be without that success.

One of the reasons I like pebbles is that the fuel form is more flexible, with exactly the same elements used in both medium and small reactor cores. In my technical and economic analysis opinion, the HTR-PM is moving in the right direction, essentially borrowing the scaling concept used by large diesel engines. Need more power and bumping against technical limits for cylinder size but still have technical headroom in crankshafts, reduction gears and generators? Simple answer; add more cylinders.

When the people building HTR-PM type machines want more power they will add more cylinders (cores) that are each identical in manufacture and construction. They clearly understand how to build 250 MWth passive cores and have good experience with steam turbines that are as large as 1200 MWe per unit.

They can match the core thermal production with the turbine capacity by producing the heat and steam in a dozen or so cores and piping it to a single steam header.

Building many units of identical design is often cheaper than trying to produce the same output by using larger individual components that each stretch the bounds of technology.

Google uses this philosophy in their server farms; which replace a lot of big iron mainframes by ganging thousands of individual microprocessor based server units.

Even if your goal is industrial process heat, I am sure that you realize that many of your potential customers cannot put 600 MWth to good use. If that is the smallest unit you offer, they will not buy.