The High Temperature Gas Reactor (HTGR) is designed to use an all ceramic fuel form which supports the ‘inherent safety’ of HTGR. This fuel form is call TRISO, and production of this fuel in the US and in Germany in the 1980s resulted in excellent fuel, but not as good as HTGRs demand. The degree of manufacturing flaws in the TRISO fuel have to be exceptionally low and the fuel has to perform under all normal and abnormal operations without significant damage to the fuel.
The current work at Idaho National Laboratory (INL) provides high confidence that the new production process for TRISO fuel, first demonstrated at Oak Ridge National Laboratory (ORNL), and turned into a reliable production process at Babcock and Wilcox, will perform at the high levels expected by the NGNP Industrial Alliance.
INL, in conjunction with ORNL, recently announced some remarkable news about TRISO fuel. The NGNP Alliance has been tracking this very specialized and sophisticated work at these National Laboratories closely for the past several years . And we’re impressed!
First, what is TRISO fuel and why do we care?
TRISO is a shortened version of TRIstructural-ISOtropic. Say that mouthful a few times in a row and you’ll understand why they nicknamed it TRISO. TRISO fuel is tiny balls of uranium coated with carbon, then silicon carbide, then carbon.
The ball (or spherical) shape means that the fuel maintains its strength in every direction. LWR fuel is small cylinder shaped pellets; that shape means that they behave a bit differently along the axis of the cylinder than they do across the diameter. The spherical shape of TRISO fuel is important in helping to ensure its integrity under normal or any possible accident condition. Isotropic simply means that something is the same in every direction.
How tiny are those balls? Each one is about 1mm in diameter – about the same size as the tip of a ballpoint pen. About 96,000 of them could fit in a chap-stick tube.
This type of fuel has been known for decades. The Germans first developed it in the 1980s and several countries have considered it for various next generation reactors. It works particularly well in HTGRs by allowing much higher temperatures and much more effective use of the uranium within the tiny sphere. Technicians at INL and ORNL have been working with a U.S. version of the fuel to see if they can make it even better.
So what’s the big news?
The US has been testing how the fuel will behave in high temperature next generation reactors. This is where INL and ORNL come in. They put approximately 300,000 TRISO fuel particles into one of their test reactors and irradiated them for 3 years.
This TRISO fuel was subjected to neutron radiation much like it would experience in a real HTGR. After three years, they took it out and baked it at extremely high temperatures to simulate conditions beyond even a worst case scenario accident situation.
They baked it to about 1800 degrees Celsius. That’s more than 3700 degrees Fahrenheit, more than 1500 degrees (Fahrenheit) hotter than any current generation nuclear fuel is expected to withstand. It is nearly 200 degrees (Celsius) hotter than any accident scenario for the HTGR. Only a few of these tiny balls of fuel leaked ANY fission products at that high level. This level of leakage is so small, that even at 1800 C, no on-site or off-site consequence would result.
Compared to the German TRISO designs and experience, about 10 times fewer particles failed at 1600C compared to assumptions used by designers and about 100 times fewer failed at 1800 C than historic German data (which is well above temperatures expected in a worst case accident where all coolant is lost and the operators take no action.). If this level of performance continues in the next series of tests (qualification series), it will provide powerful evidence that the HTGR will be so safe that no emergency plan outside of the plant would be required on the basis of contamination or radiation.
Such high temperatures and so few failures demonstrate the robustness of TRISO fuel and the inherent safety of the prismatic HTGR concept, another step forward in achieving the deployment of HTGRs that we, at NGNP Industry Alliance, are excited about.