Inspection of Hanford Double Shell Tank Bottom Using Electromagnetic Acoustic Transducer (EMAT) Techniques

摘要

The Hanford Nuclear Reservation in Washington State contains twenty eight, million-gallon waste-storage tanks that store radioactive and hazardous waste resulting from the US weapons program. The typical double shell tank (DST) design includes a primary tank wall to hold the slurry waste and an outer secondary liner. The primary tank is resting on an insulating concrete pad (aka refractory pad) which is traversed by small channels radiating from the center. The first leak in a double-shell tank at Hanford was discovered on Tank AY-102, and the failure was observed to be from the primary tank bottom. The exact failure location and damage mechanism are still undetermined. Inspection of the tank bottom is extremely challenging. Access to the bottom requires fitting the equipment through a small opening on the top, travel through the annulus between the primary and secondary tank, and slide the sensors through the refractory pad channels. Moreover, the technique used for inspection needs to be capable of coupling through a rough, rusted surface and address several feet of material between channels. This paper presents the different MRUT (Medium Range Ultrasonic Testing) techniques developed by Innerspec to inspect the bottom of the tanks, and the results on two mockups prepared by Pacific Northwest National Laboratory (PNNL) on behalf of the company managing the site. All the MRUT techniques are based on Electro Magnetic Acoustic Transducer (EMAT), an ultrasonic inspection technique that uses electromagnetic induction to generate the ultrasound in the material. EMAT does not require the use of liquid couplant, is not affected by surface conditions, and is easier to deploy than conventional ultrasonic transducers, making it a potentially ideal technology to solve this complex inspection project.