Increasing Operating Level of Underground Nuclear Waste Tanks

[Enginator]

We are trying to determine a path-foreward for increasing the operating level of our underground nuclear waste storage tanks.

Background
These are double-shell tanks, nominally 1 million gallons (75 foot dia primary, ~82 foot dia secondary). They were built to ASME BPVC Section VIII Div 2. Welds were 100% visually inspected, but radiographed only to the cylindrical-haunch interface (at the 420 inch level). The tanks were hydrostatically tested in 1985 up to 480 inch level, which is above the haunch into the dome. The proposed operating level increase is to 460 inches, which is just above the midpoint of the haunch. The tests were performed with water, but the specific gravity of the waste is up to 1.4.

The tanks were not stamped or licensed (government legacy job).

Discussion
ASME BPVC doesn't seem to address this situation. API 653 requires a hydrostatic test, which is not practical. They contain radioactive waste and cannot be cleaned to the point where a leak during a test would be a non-event.

The geometry and hazard conditions are such that we cannot radiograph the welds that were only visually inspected. We can UT some portion of the welds, but not 100%.

It is possible to perform additional visual inspections with remote stereo camera equipment from the inside of the tank. This equipment can get to within inches of the weld. I'm not sure if this is worth it since the welds have already been visually inspected, and the tank was orignally hydrostatically tested above the proposed operating level.

I am going to have to apply my stamp certifying that the tanks are fit for use.

The only other option is to build more tanks to increase plant capacity, but the customer wants to avoid the expense to tax payers.

All suggetions welcome.

Thanks.

 

[zeven]

I would to do a risk assessment
Calculate the probability of failure (High medium or Low)
Assess the concequencies of failure, Fatalities through to no consequence

High risk with fatalities - keep away
Low risk with no connsequenies - do it

To assess the risk of failure ---

A hydrotest is just one method of assessing the component, NDE, calculations and service history of similar designs are the others
Hydrotest only proves that there is no large defect in the design. In practical terms you have a tank which has a defect through 95% of wall and it will still pass.

Much better to reanalyse the tank with FE for the proposed maximum operating loads and calculate the minimum weld thickness required to meet code allowable.
Compare calculated min weld thickness with manufactured min weld thickness
If the calculated value is much less than manufactured one then you have a case for low risk justification.

Concequencies of a leak - I leave to you.

 

[Enginator]

Yes, I forgot to meantion we are also doing an ANSYS FEA on the tank to verify loading and stress on the shell.

The risk assessment is a great idea. Thanks.

 

[saxon]

JOHNHH, As waste tanks, the design and operations are controlled by the EPA through RCRA. There are also reg's established by the AEC and the NRC for the storage of radioactive materials. Although it may be mechanically feasable to increase storge capacity, I doubt that the regulations would allow for it. Check with the EPA, AEC and the NRC. I'm sure they have plenty to say about it.

Hope this helps.
saxon

 

[pablo02]

I personally think "risk assessments" are being used in the wrong manner (what's the probability of the consequences nailing me)-- and I'd be careful how one was here: in this case, I'd use it only to provide you with a comfort level in your assessment analysis, not to make the decision itself -- I think zeven is on the right approach, just be careful how you would apply the process...