There have been a number of comments on this blog, in other places in the media, and on the web registering concern about the amount of waste that is produced by a pebble bed type reactor. I answered some of those comments in a recent response in a comment thread on this blog, but there appeared to be an issue with Haloscan when I posted the comment. Besides, I am not sure if people take the time to dig through comment threads and this is worth its own blog post. (Heck, blog posts of something I have already written are almost free, so that may not be saying too much.)
There is a certain amount of truth to comments about the increased volume of pebble bed fuel compared to light water reactor fuel, but IMHO the interpretations that some have applied to the facts almost amounts to spreading FUD.
Pebbles as proposed for the PBMR, the Chinese HTR, and the Adams Engine include both fuel and moderator in a single unit. Each pebble contains about 8-9 grams of heavy metal, about 200 grams of graphite and a small amount of SiC.
Unlike a water or liquid metal cooled reactor, the moderator mass becomes and integral part of the fuel left overs once its time in the reactor is complete. In traditional reactors, the left over fuel is easily removed from the coolant/moderator and handled separately.
At a first order level of analysis, that seems to be a disadvantage, but more details are needed to really understand the trade offs.
The first is the burnup achieved. A reasonable value for modern light water reactors is about 50,000 MW days per ton, or about 5% of the initial heavy metal. There may be ways to keep improving on that, but the improvements seem to have been quite gradual over the years.
The AVR pebble bed, on the other hand, was able to demonstrate a burnup of about 160,000 MW-days/ton. For a variety of reasons, there might be an initial hesitation on the part of pebble bed operators to go much above 80,000 MW-days per ton, but there is also some evidence pointing to an ultimate goal of 500,000 to 750,000 MW-days per ton (50-75% consumption of the initial heavy metal load). I do not want to get into the math here, but you can certainly see that getting significantly higher burnup can reduce the ultimate volume of fuel needing disposition.
Another factor to consider is the fact that water and liquid metal cooled reactors have other waste products that do not require the level of concern of spent fuel, but they cannot be casually discharged either.
One more factor that enters into consideration is the fact that the pebbles can be put into dry storage immediately and do not need to decay in a cooling pool first. This eases some handling issues and might lower overall costs.
The choices are complex and worth careful analysis. Overall, I like the projected ‘S’ curve of gas cooled pebble bed technology better than the ones that I see for other coolant types. I could be wrong. Feel free to bet against me and to share your thoughts and concerns about my choices for Adams Engines.
However, if you are an advocate of a different type of technology, I hope that you do not ask the government to put too many hurdles in the path of pebble bed reactor development in hopes of encouraging your own favorite technology. I love competition, but let’s put up some swim lanes and compete without trying to interfere in each other’s progress.