Uranium & Diamonds: Heat Transfer in Nuclear Reactors

[nikkkom]

Oxygen seems to be very resistant to C-14 generation.
O-16 needs to lose two nucleons to generate C-14 via spallation. This is very unlikely.
As to generating a somewhat long-lived (more than a day) radioisotope from oxygen by neutron capture, the first one seems to be Na-22 (!), 2.6 years half-life, it can be reached by a long chain of neutron captures and fast beta decays to Na-23, after which you need one spallation event. Not going to happen.

 

[Astronuc]

We doubtless make a minute amount of C14 in the air surrounding a LWR via leakage neutrons and gamma.

aha somebody has studied that.
Measurements at Ginna report ~a half curie per year from containment air

Assessment of Carbon-14 Control Technology and Costs for the LWR Fuel Cycle, EPA 520/4-77-013, September 7, 1977
That's a good find Jim.

Page 2-4 of the EPA reports indicates that the primary source of ¹⁴C are the ¹⁴N(n,p)¹⁴C and ¹⁷O(n,α)¹⁴C reactions, and the production from ¹²C would be orders of magnitude less. So, replacing O with C would actually reduce ¹⁴C production.

O-16 needs to lose two nucleons to generate C-14 via spallation. This is very unlikely.

The scenario would be more like ¹⁶O(n,γ)¹⁷O followed by the ¹⁷O(n,α)¹⁴C reaction.

As for UN, it reacts with water, so in the event of cladding breach, it would be problematic. Basically, using UN would require a corrosion-resistant coating to prevent UN from reacting with water.

 

[mheslep]

U metal in an LWR/PHWR is pretty though sell in terms of safety. From a reactor physics U metal is great (density, thermal conductivity) but it is far too chemically reactive during accidents and won't contain fission products. It would significantly improve reactor performance in a number of ways. It would be great if you could rely on no cladding failures to eliminate the need for fuel/coolant chemical compatibility, but historical evidence doesn't support that. There are too many failure modes for fuel cladding (core damage, debris fretting, corrosion, over pressure).
...

Yet U metal alloy fuels are apparently used in US Naval reactors (I see multiple web references indicating this but nothing reliable, and I have no first hand knowledge). Perhaps it is possible to make the metal sufficiently nonreactive if combined with other materials. As for the consequences of accidents with existing reactors, visibly the Zr facilitated release of H2 is an issue at high temperatures, and the use of metal fuels might help to reduce maximum temperature in the event of an accident.

As for swelling, the Lightbridge approach is to bump Zr from the 10% alloy that's been studied in the past to 50%, which they claim results in a "significant reduction in irradiation-induced swelling."

The release of fission products would still be a concern, but I think it might be possible to use a non-fuel cladding around metal fuel as is done now with oxide fuels, especially if the fuel swelling can be reduced. Thus the high conductivity feature is retained with metal-fuel and metal cladding.

http://www.ltbridge.com/fueltechnology/generaloverviewoflightbridgesmetallicfueltechnology

http://www.ltbridge.com/fueltechnology/safetybenefitsoflightbridgesmetalfueltechnology

 

[Hologram0110]

Yet U metal alloy fuels are apparently used in US Naval reactors (I see multiple web references indicating this but nothing reliable, and I have no first hand knowledge). Perhaps it is possible to make the metal sufficiently nonreactive if combined with other materials.

As for swelling, the Lightbridge approach is to bump Zr from the 10% that's been studied in the past to 50%, which they claim results in a "significant reduction in irradiation-induced swelling."

I don't know much about naval reactors as they are obviously highly classified. I was under the impression that PWRs were derivatives of naval reactors. I've heard that many of the PWR-SMR designs are actually similar in many ways as well. Its quite possible that there is more than one naval reactor design for different applications. I've heard the Soviets used metal fuel in a few of their designs, but gave up on the design.

It is certainly possible to use metal fuel in a water reactor provided you are sufficiently confident in your ability to maintain cladding integrity. Given how difficult that has proven with UO2 based fuels and the commercial nuclear industry's aversion to risk metal fuels became popular with water coolant.

 

[John d Marano]

It is certainly possible to use metal fuel in a water reactor provided you are sufficiently confident in your ability to maintain cladding integrity.

I'm glad i got this conversation going (I'm not following it entirely) but if your getting at cladding metal fuel with diamond consider use diamonds insulation abilities. Once covered sufficiently a diamond coating shouldn't allow electrons to flow into the metal fuel; submerge the pellet in an electrolyte and test the solutions resistance to see if the cladding's effective.

 

[mheslep]

...
As for swelling, the Lightbridge approach is to bump Zr from the 10% alloy that's been studied in the past to 50%, which they claim results in a "significant reduction in irradiation-induced swelling."
...

Ah, I'm reminded now of Astronuc's earlier comment on enrichment limits. If the Zr fraction is increased, this displaces U atoms, so to maintain power density enrichment percentage would have to be increased, which is prohibited. One thread may pull the idea apart: no high enrichment means no high Zr fuel ratio, without which significant swelling causes the fuel to expand out of its cladding.

Edit: Yes

Lightbridge fuel with enrichments near 20 wt % may requires special containers ...

http://www.ltbridge.com/assets/27.pdf

I suppose advoidance of the enrichment issue is one reason why Lightbridge originally worked on Thorium fuel designs.

 

[John d Marano]

This is slightly unrelated but still on the topic of reactor efficiency;

NASA is considering using superconductors to protect astronauts http://www.nasa.gov/pdf/637131main_radiation shielding_symposium_r1.pdf so I was wondering instead of just having "a mass of absorbing material placed around a reactor" could a superconducting field be used to concentrate the radiation within the core to improve the thermal yield?Regards,
JDM

 

[nikkkom]

No, magnets can't do that. Much of heat transfer is by neutrons, which are uncharged.
Even if they could do that, cost and engineering difficulties of placing -200 C cold magnets near +300 C water make it a non-starter.

 

[Astronuc]

NASA is considering using superconductors to protect astronauts http://www.nasa.gov/pdf/637131main_radiation shielding_symposium_r1.pdf so I was wondering instead of just having "a mass of absorbing material placed around a reactor" could a superconducting field be used to concentrate the radiation within the core to improve the thermal yield?

As nikkkom indicated, besides the matter of placing a cryogenic system inside a high temperature system, a magnetic field will not confine neutrons or photons (gamma rays). In fact, neutrons are moderated quickly in water (hydrogen) and otherwise, interact with matter via absorption into nuclei. Similarly, gamma rays mostly scatter off electrons in various materials, and in the process, provide some heating (2-3%) of the thermal energy from fission. Gamma-rays from the decay of radionuclides provide a similar amount of energy in a reactor (which is part of the decay heat one has to address with a shutdown reactor and then discharged spent fuel). There is also alpha and beta decay, and most of the energy is deposited in the fuel, typically UO₂ and related fission products.

 

[mheslep]

Just as an aside, I'm curious as to why a NASA proposal with no gamma protection is a contender for long term space missions. Gamma bursts, etc?

 

[Astronuc]

Just as an aside, I'm curious as to why a NASA proposal with no gamma protection is a contender for long term space missions. Gamma bursts, etc?

Radiation (cosmic rays and gammas for GBs) shielding for spacecraft is a topic for another thread. Is doesn't appear that the final configuration for a manned spacecraft has been determined yet - assuming that there will ever be one.

About 30 years ago, when we looked at missions to Mars, we looked at ways of using the spacecraft structure to provide shielding. For example, one needs radiators and propellant storage, so the radiators, propellant and storage tanks could be used for shielding, in addition to whatever shielding was necessary immediately around crew quarters. There could always be a small shielded volume, which would serve in the event of a large solar magnetic event or extra-galactic event.

The shielding and other systems also faced the constraint of minimal mass in order to minimize the energy required to transfer the mass to the final destination. The power system on the other hand was designed to maximize specific energy, which then challenges materials with respect to technical limits based on creep and material degradation, which are also the same challenges with maximizing plant efficiency by pushing temperatures as high as possible.