SSN HEU

Date: Feb 8, 2005

SSN HEU

Date: Feb 8, 2005

Will this have much of an affect on reactor burnup and time between refuelings?

Also, any mention of plutonium in the mix?

Date: Feb 9, 2005

I have never seen any figures on the total burnup of naval reactor fuel.

I do know however, that usually the reactors are not refuelled -- they are simply replaced after some dozen years of service.

Their maximum power is something like 80MW thermal, but I don't think that the subs run at top speed very often or for very long. I also think that the wear & tear on the fuel material is more of a limiting factor than the fissile loading itself. Thus advanced fuel materials should "allow, among other things, an increase in [high-power] operating hours per year." Presumably, with higher power density, the same reactor could be run at well over 80MW peak power.

There is certainly no plutonium in the fresh fuel, but due to the high U-235 content, there is a gradual buildup of Pu-238 (RTG fuel) in the core. If you recall, this was mentioned in the AIAA Roundtable web cast linked from the lead page of the NS web site.

From the transcript of the AIAA Roundtable:

GREY I’d like to raise the overall question of materials. And first on the list are nuclearradioisotopes.

NEWHOUSE The most important one is plutonium-238, the nonfissionable isotope used to generate the power for the radioisotope power supplies we’ve been using. We buy it from Russia in 5-kg lots, at a rate of about 5 kg a year. Right now, we have enough for any conceivable NASA mission. However, if we expand our vision into more missions, we will need more. The Russian plutonium can be used only for civilian purposes, but the DOE has other customers besides NASA. The DOE is working to establish a capability to make plutonium-238 for these other customers. It’s in their budget planning. It hasn’t happened yet because it hasn’t been funded, but they have asked for funding, and the plutonium should be available if we need it. Another approach is to generate more power with the plutonium that is available. Our current RTGs are only about 5% efficient. We’re developing new power conversion devices, notably Stirling-cycle generators and others down the road that are on the order of 25% and even 35% efficient. The same amount of plutonium that generates 100W with 5% efficient thermoelectric conversion can generate 500 W with a 25% efficient generator, or else we can produce the same 100 W with one-fifth as much plutonium. So we can extend our plutonium-238 supply by applying these new technologies.

GREY It isn’t just the plutonium-238 that you have to consider; it has to be packaged into a safe power pack. Steve, where does the DOE stand on this?

HOWE The packaging is very important. It’s a very precise, meticulous process. A great deal of time and effort has gone into packaging the plutonium. We put it through a lot of rigorous tests, including fire, impact, and accidents in every kind of scenario. The lab is serious about meeting its responsibilities to supply the plutonium pellets, but that process has now been interrupted with the current shutdown. I think that will impact the Pluto mission and possibly others in the future. We will try to make up time, but these are very stringent processes that you often can’t speed up, so will likely be an impact.

JIM POWELL The plutonium-238 has been made by reprocessing Navy reactor fuel. And as far as I know, they’re not doing that anymore.

NEWHOUSE That’s true; the decision was made to stop processing plutonium-238 during the Carter administration. But in fact, at the time there was more neptunium [an intermediate product of the process, from which the plutonium is made] than we could ever use. And if we ever want to make more plutonium domestically, we can.

GREY Gary, you did most of the testing on the plutonium-238 pellets. Do you have any comments?

BENNETT I agree with Steve Howe: There is rigorous testing, and all the fabrication, assembly, and testing processes are done to detailed standards. The fuel has to be pressed and sintered in a certain way; it has to have a certain grain structure, a certain density. The iridium which encapsulates the fuel pellet has to be manufactured in a certain way. Early in these production campaigns we often had rejection rates of 50% or more. Whenever you start up a highly controlled fabrication process, even with experienced people, there will be high rejection rates initially as people relearn the necessary skills. We need to recognize that we will have a lot of difficulties getting everything to work just right, getting the equipment in place, and so forth. As Steve said, it is a detailed, step-by-step process to manufacture these plutoniumbased heat sources, and it really can’t be accelerated.

Date: Feb 9, 2005

Thanks Jaro. Yeah, I'll bet the US Navy pretty hush, hush about burnups. In fact, they're pretty much mum about anything involved with the submarine powerplants other than they use HEU. and generate steam to drive a turbine which is geared to the propellar. I don't recall ever seeing a photograph or tv of a reactor or engine room (other than the bulk head hatch indicating the passage to the engine room.) I suppose they do this to protect themselves from smart people estimating actual dimensions and 'reverse' engineering to closely estimate actual system performance--which of course would be the goal of any enemy!

As for the build up of Pu-238 in naval reactors--this would add rather significantly to decay heat. Fortunately, these craft operate in a rather larger reservoir of coolant, so as long as coolant pumps continue to operate this shouldn't be much of a problem.

I would love to see a Pu-238 powered Stirling generator developed--just the thing for the MSL. A larger unit of 200 kWt ought to be powerful enough to operate a large, long range manned Mars Rover--when we actually make it to Mars that is.

Date: Mar 19, 2005

I think that once you start going into the hundreds of kilowatts power range, then radioisotope-based generators -- even Stirling type -- are probably not a good idea. You've got a big heat source that can't be turned off when you don't need the power, but cooling must be maintained to avoid overheating. At these power levels (and higher) reactors are the way to go, IMO. They can be turned off when not needed, and the decay heat is pretty insignificant until you hit a power range of dozens of megawatts and long operating runs. Of course with a manned Mars Rover you would have to worry about neutron shielding. I would hope that if the landing/camp site is within reach of ample supplies of water ice, then it might be a simple matter of filling up a shield tank surrounding the reactor, for shielding..... another application of ISRU

PS. I've always wondered whether the Jawas' Sandcrawlers on Tatooine were nuke-powered -- any idea ?

....see pictures at http://members.aol.com/Hggmw3/images/Crawler2.jpg and

http://www.blackbook.org/2003/06/swg_jawa_crawler.jpg and

http://www.technoism.com/technoism/images/JawaSandcrawler.jpg

http://mobz.org/starwars/bin.boa/1368.jpg

http://www.darth-sonic.de/images/TeaserB/3.jpg

http://xpi.org.uk/~swma/archives/other/surface/04/o-sndcrl.zip

.....and the "blueprint" at http://r2d2.rof-aue.de/Baupl%E4ne/jawa_sandcrawler_blueprint.jpg just says "powerplant."