Nuclear Material

candice_84

What is the most corrosion resistant alloy other than Zirconium and SS that could be used in reactors but it is not economical?

Astronuc

Is one referring to a cladding/structural alloy for use in an aqueous environment, such as an LWR? It's hard to be the various Zr-alloys, e.g., Zircaloys, Zr-Sn-Nb-Fe, Zr-Nb-O-Fe, and various derivatives.

There are specialty stainless stainless (austenitic and martensitic) and duplex steels (ferritic-martensitic) that have been developed for liquid metal fast reactors and supercritical water reactors.

Gas reactors use SiC and pyrolytic C.

And in fact, there is a program to look at SiC for LWR applications.

candice_84

Thanks for your reply, Do SiC or pyroltic C have low neutron absorption? and also do they crack after 10 or 20 years?

Astronuc

It's possible that they will crack eventually.

Is one asking about fuel structural material or reactor vessel structural material. There is a difference in residence time and fluence levels.

In LWRs, fuel is typically resident for 2 or 3 18-month or 24-month cycles. Design lifetime is however up to 8 years. Reactor vessel structural materials must be resident for the life of the plant which was originally 40 years, but now has been extended to 60 years. However, the fluence/dose rate is about an order of magnitude less than that of the fuel.

Naval nuclear fuel is specially designed for lifetimes greater than those achieved in commercial LWRs.

QuantumPion

Stronger and better alloys exist, as far as structural integrity in a high-temperature/pressure environment is concerned (e.g. inconel). However I don't think there are any other known that also have good neutronic properties.

candice_84

My question is about structural material. For example in Molten Salt Reactor the the temperature of fuel or coolant is very high and I assume salt is corrosive, what can be used that could stay there for 60 years and not let the coolant leak into the moderator? I think it would be a disaster if coolant or fuel leak into moderator since it is carrying fission product. Also what kind of fuel do they use in Naval reactors?

QuantumPion

In a commercial power reactor, the pressure vessel itself is carbon steel with a stainless steel liner. The fuel rods are zirconium alloy (which is not very strong but is transparent to neutrons). The structural supports for the fuel assemblies are made of Inconel, which is probably what you are looking for. It is very resilient to corrosion, radiation, and high temperature environments.

candice_84

According to this website the melting point of Inconel, http://www.engineeringtoolbox.com/me...als-d_860.html the melting point of Inconel is around 1400 C. While the fuel center line temperature of UO2, which is used in PWR is 1400 C. I think Inconel cannot work for Molten Salt Reactor. I am in my 2nd year university, so don't take my opinion as granted, i might be wrong. :)

QuantumPion

First of all, the centerline temperature for UO2 is much higher than the surface temperature due to its low thermal conductivity.

Secondly, the fuel cladding is not a structural support, it only has to withstand the internal pressure of the fuel or external pressure of the coolant. If your original question is thus: "is there a better material than zirconium alloy that could be used for fuel rod cladding" the answer is, for a LWR, not really. Zirconium has decent thermal properties, is transparent to neutrons, and has pretty good resistance to the harsh environment of the reactor core.

Now if you are designing a fast reactor, there may be a better alloy to use since you aren't worried as much with the cladding absorbing neutrons. I believe stainless steel has been used in earlier LFR's. I'm not an expert on metallurgy or fast reactor design though so I don't know what benefit there would be to using newer superalloys for this purpose.

Astronuc

FYI - http://www.gen-4.org/Technology/systems/msr.htm

It's challenging at 650°C, but it has low pressure which is beneficial from the stress on the PV.

Corrosion will be challenge, primarily from fission products in addition to the actinides.

Ferritic steels with 12% Cr are being considered for the SFR, since they possess better strength at high temperatures than austenitic steels. Ferritic steels would have corrosion/IASSC/IGSSC problems in LWRs.


Generation-IV nuclear power: A review of the state of the science <save target as>
http://www.foresight.gov.uk/Energy/G...clearpower.pdf


With respect to LWRs, Zr-alloys are commonly used for most of the fuel structure, e.g., fuel rod cladding, guide tube and spacer grids in the flux regions. Top and bottom grids are usually Inconel-718, but could be Zr-alloy or bimetallic (Zr-alloy strip with Inconel springs).

I can't comment on the materials and conditions in the naval reactors.


I have some additional resources on GenIV materials buried in my library. I'll see if I can dig them up.

candice_84

Thanks for the paper, in the case of MSR, Why don't they mix uranium in water, so it would be less corrosive.

Astronuc

Water is rather corrosive, especially when heated to operating temperatures of nuclear reactor. Uranium eventually hydrolyzed to UO3 which is soluble in water. In addition, fission products would also form compounds with water and each other.

In a reactor environment, radiolysis of water is a factor. That changes the chemical nature and more aggressive species are produced.

High temperature water is generally deleterious to many metals, and more so in a radiation environment.

Hologram0110

A simple example, although not of structural materials, would be the use of heavy-water instead of light-water as a coolant and/or moderator. This offers better neutron economy but significantly higher costs(and physically larger reactors).

Astronuc

Reactor grade zirconium alloys have less than 0.010% by weight (100 ppm) of Hf, which is down from the natural value of ~2% Hf in ziron ores (ores with higher Hf concentrations have been found). Production values can be less than 100 ppm.

The total EBC (equivalent boron content) of cladding and structural material is also limited. This is more of a concern for LWR materials, rather than epithermal or fast reactor materials.

Hf is also commerically valuable.

Hologram0110

You're absolutely right Astronuc. I mixed-up the content of commercial grade and reactor grades. For the commercial grade they do not remove nearly as much hafnium to save on cost.

candice_84

If I understand you correctly, This means if uranium is dissolved in water, at high temperature it breaks from water? (making to different compounds?

Astronuc

Yes - it's one of the problems with degraded fuel rods which have failed. U-metal would oxidize to an oxide, which is one of the reasons that LWR fuel is UO₂. But under reactor operating conditions, UO₂ reacts with water/steam to oxidize first to U₃O₈, then to U₄O₉, and then finally to UO₃, which in water/steam and a radiation environment forms a soluble hydroxide, e.g, UO₂(OH)₂. Irradiated fuel is more complicated because of the presence of volatile and soluble fission products.

In reactor coolant, tramp uranium, and transuranics such as Np-239 and Pu-isotopes are a problem.