Nucengable said:
Hi,
I was reading about PWR control system and I saw that control rods are stainless steel tubes encapsulating a Hafnium absorber material , it has Excellent mechanical properties and exceptional corrosion-resistance properties allow its use in the harsh environment of a pressurized water reactors and The German research reactor FRM II uses hafnium as a neutron absorber.
so the question is if I'm designing a nuclear reactor based on what I should choose the control rods' material ?
and as a Hafnium absorber why its better than Gadolinium [Gd] as control rods while Gd has much higher absorption cross section..?
is Hf used as pure or in some composition ?
and what's the Hf interactions with neutron?
Hf has 5 stable isotopes (Hf-176, 177, 178, 179, 180) and one quasi-stable isotope, i.e., an isotope with a long half-life, Hf-174.
Hf-174 has a thermal σ(n,γ) cross-section of 561.7 b, where σ(n,γ) = thermal neutron capture cross section at 300 K, in barns.
Hf-176 13.76 b
Hf-177 373.6 b
Hf-178 84.05 b
Hf-179 43.59 b
Hf-180 13.01 b
http://www.nndc.bnl.gov/chart/reCenter.jsp?z=72&n=106 select σ(n,γ) and Zoom 1. http://www.nndc.bnl.gov/chart/reCenter.jsp?z=64&n=91
A subsequent daughter from transmutation, Ta-182, has a cross-section of 8289 b.
Gd by comparison has a much higher cross-section for thermal neutrons
Gd-155 60740 b
Gd-157 253700 b
However, one must look at the isotopic vector for natural Gd.
One also has to consider the temperature or 1/v effects, because PWR control rods are sitting above the reactor, they start at about average core exit coolant temperature, usually in excess to 600K, because the tips get some shine from the core. One also has to look at resonance absorption. So one has to look at the PWR neutron spectrum and use that with the σ
n,γ(E)
Gd density 7900 kg/m
3 and a melting point of 1312 °C
Hf density 13310 kg/m
3 and a melting point of 2233 °C
Also, one has to look at the cost. Gadolinium is a relatively expensive rare-earth element. Hf is a by-product of Zr production. Hf is present at about 3-4% of zircon sand, and to get nuclear grade Zr, one has to remove the Hf.
The biggest problem with Hf is its tendency to absorb hydrogen. The formation of hafnium hydrides HfHx, x ~ 1.6 to 2, causes Hf to swell and this can be a problem for RCCA insertion. In some cases, the stainless steel cracks.
Traditionally, PWR absorber has been 80%Ag-15%In-5%Cd (AIC), and in some cases, B
4C has been used, often with B enriched in B-10. Hybrid control rods may use B
4C with AIC tips. And traditionally, the neutron absorbing material is sealed in a tube of SS-304 or 316, and stainless steel is somewhat permeable with respect to hydrogen.
If Hf has a nice adherent oxide, the oxide can prevent uptake of hydrogen, so ideally, Hf is either oxidized initially, or is allowed to have contact with the coolant, such that it always maintains an oxide coating. Then one has to determine the degree of oxidation over the lifetime of the control rod, which is usually ideally 15 years. If it can go 20 years, that's better.
Alternatively, the form of Hf could be a rolled foil that allows for some hydrogen uptake without significant swelling. Another alternative could be a Hf tube containing B
4C.
The Russians have looked at dysprosium titanate as an absorber material.
Most BWR control rods use B
4C with some Hf slugs or rods in high exposure locations, such as the tips. Hf swelling due to H-update is also a concern.
The stainless steel fingers (rodlets) are attached via endplugs with long shanks to spiders, the spiders are attached to the hub, and the hub forms a housing which interacts/couples with the control rod drive mechanism. The spider is often cast, but could be manufactured from forged plate which is then welded to the hub.
