Stainless Steel Contamination 2

[agmotes]

I am new here , and i'm an engineering intern, not a real engineer...but i'm trying to find out if there is any way that an oil-filled capacitor with a stainless steel case can become contaminated with cobalt-58 or 60. I was told that when heated the metal becomes pourous and absorbs small particles of cobalt-60, which then becomes trapped in the metal upon cooling. The capacitor has been thoroughly cleaned on the outside and is still contaminated. I beleive the dose-rate on the capacitor was approx. 50 rad/hour, and the capacitor has been in service for 3 years. Any help would be awesome.

 

[agmotes]

there may also be cobalt-58 and a chromium isotope, though i can see where the chromium isotope is coming from.

 

[SMF1964]

I'm not sure if the temeperatures are high enough to allow diffusion of the cobalt into the stainless steel - typically diffusion temperatures are above (roughly) half the melting point (in absolute, °K or °R) and at temperatures like this, you are dealing with a pressure vessel filled with oil and a weaker steel (the tensile strength drops off dramatically at these temperatures).

If the alloy is an austenitic stainless steel (304, 316 are examples), then the composition sometimes lists the Nickel content as "nickel + cobalt" and has a small amount of cobalt present as a tramp element. If this capacitor is in a high rad area, then I would expect neutron bombardment to affect the cobalt directly.

 

[EdStainless]

I also doubt that it is particles of Co, but rather the trace levels of Co in the SS. People ask for 304 with 0.05% max Co, the only problem is that you can't make it on a large scale. Given the similarities between Ni and Co they are very difficult to separate. If you work from virgin feed you can make 0.15% Co in large commercial heats.

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Plymouth Tube

 

[UconnMaterials]

agmotes,

How are you characterizing the contamination? Are you doing a AES (Auger Electron Spectroscopy) Analysis, EDS (Energy Dispersive Spectroscopy), or SIMS (Secondary Ion Mass Spectrometry)? Low abundance isotopes such as cobalt 58 can only be detected using SIMS. Cobalt-59 is the only stable and therefore the highest abundant cobalt isotope. In fact, naturally occurring cobalt is made up of 99 At% Cobalt-59. What is the source of radiation? The reason being that knock-on damage of a atoms core can occur as a result of high energy bombardment.

 

[agmotes]

Again, I'm an intern...I don’t have access to toys like that (though i wish i did). The source of the radiation is a Nuclear Reactor...so I’m Guessing mainly U-235 and a small amount of plutonium-239 (<3%) If I'm not mistaken, I believe that Cobalt-60 can be formed when Cobalt-59 is bombarded with a Slow Neutron Source, and being that this reactor is a water/boron moderated reactor I would suspect that the area is swarming with slow neutrons.

agmotes

 

[plasgears]

Remelt of SS scrap is common. That's where apparent contam. could be originating.

 

[agmotes]

all parts are checked for any count when it comes to us so we know what comes in and what goes out...just a procedure to keep radioactive material from getting out. The part originally checked out with no count.

agmotes

 

[SMF1964]

Agmotes: Yes, your nuclear reactor is where the neutrons are coming from. The use of Cobalt alloys (such as stellite) in nuclear power plants is an issue because the cobalt becomes radioactive over time as the non-radioactive Cobalt becomes radioactive Cobalt-60. Corrosion of the stellite discharges the Cobalt-60 into the liquid stream, transporting it around the system. I see know reason why the same bombardment could not transform the tramp levels Cobalt in the stainless steel into Cobalt-60 as well.

The use of SS scrap is common, but I don't think radiologically contaminated scrap used in new stainless steel is the source of your contamination issue.

 

[UconnMaterials]

agmotes

I'm still wondering why you believe you are detecting Cobalt-58 or Cobalt-60 in your oil capacitor instead of the stable cobalt-59 isotope. If in fact you are detecting Co-58 or Co-60 you will have to change the ways you are disposing of the cooling oil.

 

[SMF1964]

At this point, it becomes a question of whether the oil itself is radiologically contaminated. If, upon removal of the oil, a RadCon (or Health Physics, HP, or whatever your department is called) survey of the oil determines that it is not radiologically contaminated, then it can be disposed of as 'normal' waste capacitor oil.

UconnMaterials: I'm not sure as to the survey instrumentation Agmotes is using, but I do know our nuclear plant's HP department claims they can identify the actual isotope that is decaying based upon what comes off the isotope.

 

[agmotes]

UconnMaterials,
I had been told before (obviously this was wrong) that cobalt-60 was mainly formed from cobalt-58, upon further research and your guidance I realize that the majority of the contamination must have come from either contaminated scrap SS, or tramp cobalt-59 to cobalt-60 from the neutron source. Sorry for the confusion, and SMF1964 is right about RP telling us that it was cobalt-60 on or in the capacitor. Again, we do a test to check for contams before installing components, are you suggesting that there may be nonradioactive materials in the scrap that are becoming radioactive over time?

agmotes

 

[SMF1964]

My only experience in the nuclear industry is in the U.S. and the NRC (or Naval Reactors, if we consider the nuclear Navy) does not let any component arrive on site without a check for radiological contamination and the consequences of accepting receipt of such a component for the power plant or it's owners/corporate partners are SEVERE. I cannot imagine someone accepting some unpedigreed item that shows up as contaminated in teh receipt inspection.

But stranger things have happened.

 

[UconnMaterials]

SMF1964,

Examining the decay mode certainly would be a way to narrow down the isotope. The Wikopedia lists Co-59 as decaying by electron capture and in this case a neutrino gets emitted. I'm not a nuclear physicist (but rather a materials scientist) and therefore cannot comment on how to detect a neutrino. The best way to detect an isotope is to use SIMS analysis. I've ion implanted Cr-52 into high speed steel and been able to do depth profiling chemical analysis with a SIMS unit that detected just the Cr-52 isotope alone. SIMS allows one to sort atoms based on atomic mass.

agmotes,

Certainly, if one bombards an atomic core with high energy neutrons the atomic core will become damaged and/or the neutron will become implanted into the core. Besides nickel, tramp cobalt is probably the heaviest element in stainless steel and therefore is more likely to be effected by neutron bombardment than say iron or chromium. This begs the question of why just radioactive Co-60 was found and not any nickel isotopes.

 

[agmotes]

We don’t allow radioactive equipment to pass into the plant unless its fuel assemblies. And all I've been told is that cobalt-60 was the main contaminant; others are probably there, but not in large enough amounts to really make a difference.

agmotes

 

[SMF1964]

Uconn - there may be nickel isotopes, but it's a question of shielding - if the decay of the nickel involves a weak evolution of alpha or beta only, it can be more easily shielded. (If the capacitor has a lacquer coating on the outer surface, would that be sufficient to block any nickel related radiation?) A Cobalt-60 source, however, yields two gamma rays with each decay, which requires greater shielding. We use Co-60 for our weld radiography, so the zoomies that come from it are pretty energetic.

With regard to contamination of 'new' steel from scrap - I just wiki'd Cobalt-60 and found reference to a March 1 2008 incident. Wow.