Visual and Profilometric Fuel Rod Inspection

Problem Definition
Current inspection of nuclear fuel cells requires the use of cumbersome hot cells. INL is investigating an alternative process for inspecting these fuel cells that can be located closer to reactor sites and does not require a dedicated building for operation. Our task is to specify segments of this process that will perform non-destructive profilometric and visual inspection of irradiated test specimens. The processes must be fast and easily serviceable while maintaining a high level of fidelity and flexibility.

Background
Inspection of nuclear fuel rods via current methods is quite cumbersome. Irradiated materials must be transported to an examination facility such as INL's HFEF. There, the fuel is inspected inside hot cells, large led lined rooms with argon atmospheres where technicians must use robot arms to interact with the material. It is difficult to implement new inspection solutions within this testing environment due to the long testing time. It is expensive to test and to improve testing methods. INL is devising a new environment for inspecting materials known as the Modular Examination Instrument for Transportable Nuclear Energy Research (MEITNER). This device is a modular stack of examination equipment that can simultaneously inspect materials via non-destructive, non-contact measurements.

Deliverables
Our mission is to design and specify the equipment of the profilometric and visual test cell. A mock-up cell featuring analog equipment is to be designed as a proof of concept of the INL cell design. An extensive design document is to be included detailing equipment specifications, testing procedure, limitations, and avenues for future development.

Profilometry
Within the HFEF at INL profilometric inspection is currently performed using a pair of LVDT sensors within their hot cell. These sensors are placed against the material and removed for every measurement at a resolution of 1,2,5, and 10 measurements per millimeter. This causes some examinations to last 8 hours for a single element. LVDT are contact tools for dimensional inspection though they do not present a significant risk of damaging test elements. Non-contact inspection methods are preferred, such as those using lasers. Laser systems may additionally save time by eliminating the time required to reset the sensor for every measurement. Promising laser imaging techniques include laser interferometry and laser triangulation, laser micrometry, and line scan imaging.

Visual Inspection
Within HFEF standard digital cameras take pictures of test elements through 4 feet of oil filled glass. The discoloration of the glass causes images to lack true color. It is also difficult to image the entire specimen due to the hot cells fixed viewpoint and cumbersome manipulation of specimens. Some inspection is done by using binoculars located at each window. Due to MEITNERs closed off environment, the current techniques for visual inspection are not possible, but the ability to significantly reduce the radiation impact on test equipment presents an opportunity. Standard digital cameras, who live for a very short time within the hot cell, will be able to withstand the duration of a parts testing cycle allowing true color photos of the element in 360 degrees. This is due to the lower rad/hr dose that is present within MEITNER as well as the potential to position the camera where there is a significant amount of shielding between it and the fuel specimen. Rad shielded cameras may be used to extend the time between replacement and may allow them to be placed much closer to the test element. Snake cameras present an opportunity for testers to look around the far side of a specimen within the cell through a tortuous through hole, though the small length of articulating armature reduces versatility and may necessitate multiple insertion points.

Design
Each lead shielded ring of MEITNER will be capable of housing multiple inspection tools. Stepped wedge-shaped plugs will be through cut from the walls of MEITNER. Testing methods may be devised using one of these wedges as their access point to the nuclear material within. In order to minimize radiation escaping from the vessel, any through cut hole in the shielding must follow a tortuous path, a path that does not have a line of site connection between each end. This is critical to the safety of the facility and allows equipment to operate in a lower rad environment which reduces degradation. Due to the meandering of the through hole light must be bounced to meet testing equipment or equipment must possess enough radiation tolerance to last within the inner cell.

In our investigation, the use of mirrors to reflect light through the walls of MEITNER to reach a camera on the exterior would prove more problematic than needed. Mirrors would be difficult to replace and the cameras field of view would be restricted to a window the size of the through hole without the use of fish-eye lenses that would distort its image. For this reason, radiation tolerant cameras were preferred. Few non-contact inspection methods were found that possessed the radiation tolerance and flexibility to function to specifications in the MEITNER system, though through our investigation we concluded that the best course of action would be to internalize this part of the module as well. The method that stood out was laser interferometry, specifically NovaCams Microcam. To accurately position the laser within the cell a linear motion stage is needed with at least a stroke length equal to the largest plate with to be inspected within MEITNER, 11.5 inches. Though radiation tolerant equipment has a higher investment it will need to be replaced less frequently, which will outway the costs of decontamination and service.

Experimentation
Two different testing procedures were investigated to demonstrate how the profilometric setup could be used to capture information about the dimensions of a subject. The first was a radial inspection for viewing cylindrical specimen. The second is a sweeping method for more diverse geometry such as curved plates.

Document Archive
[[Media: ClientInterview1.pdf|First Client Interview]] [[Media: .pdf|All Meeting Minutes]]