One problem of pebbles in a reactor is that they are not welded into place. Long-lived pebbles are hard to make stay in one place with pneumatic forces from He flow and thermal transitions, expansion from pebble internal pressures and other loadings in a reactor. They do not hold radioactive stuff as well as welded tubes of Zr. However, they are cheap and easy to make.
I would not want one near me. The X-energy web page notes that the design concept has been around since 1944. I think I know why it did not catch on. The only good thing about the suggested reactor design is that it is relatively small and probably can be contained within a good pressure vessel. Maintenance, refueling and decomissioning of a reactor plant could be a significant headache. Disposal of pebbles, particularly those that are broken, does not have a good conceptual design in hand. I would like to know where Axil got his information about the German population deciding to ban nuclear power—I do think it focused on pebble bed reactors. I think the safety of their large existing reactors, some with the same ill-design for fuel storage that caused the Fukushima reactor disaster to get way out of hand, was the primary cause of the decision. Bob Cook From: Axil Axil Sent: Friday, January 22, 2016 11:11 AM To: vortex-l Subject: Re: [Vo]:DoE Funds Two Advanced Nuclear Programs Pebble bed reactors have many unresolved problems. A pebble bed reactor was the cause that persuaded the German population to ban nuclear power from that country. On Fri, Jan 22, 2016 at 12:30 PM, Eric Walker <[email protected]> wrote: On Fri, Jan 22, 2016 at 10:03 AM, Bob Cook <[email protected]> wrote: X-Energy is a pebble bed reactor designer. As I read about pebble bed reactors, the approach seems fundamentally more sane than current reactor designs, which appear to have low tolerances for error and catastrophic failure modes. Pebble bed reactors have few moving parts and assume high temperatures. The more complex a design, the more potential points of failure. I do not know how the NIF people thought that they could get inertial confinement fusion to work reliably by sending an enormous number of hohlraums through a high-energy laser. The approach does not sound dangerous, but it sounds like it would be hard to get everything properly tuned. And, apparently, hard to build at a small scale in order to iterate on the design. If transmutations and fission are more than a trivial component of LENR, as I suspect they are in some cases, you could get all kinds of toxic elements that build up as a result, such as arsenic. They may not have long-lived radioactivity, but lots of the byproducts would be poisonous. If this ends up being the case, there might be a way to control what elements are produced. Eric
