The handheld Cerenkov Viewing Device (ICVD), such as those seen being used in this photograph, is an image-intensifier viewing device sensitive to the ultraviolet radiation which can be found in the water in which spent fuel is immersed.

(Source: Dean Calma/IAEA)

A method for qualitative verification of irradiated nuclear fuel in storage pools. Irradiated fuel emits fast electrons that induce a characteristic blue glow in water. Electro-optical image intensifiers have been adapted to observe this glow from above a storage pool. They are optimized for ultraviolet radiation and are capable of operating with facility lights turned on. When aligned vertically above the tops of fuel assemblies, a Cerenkov viewing device can distinguish irradiated fuel items from non-fuel items.

The improved Cerenkov viewing device (ICVD) and the digital Cerenkov viewing device (DCVD) are image intensifier viewing devices that are sensitive to ultraviolet radiation in the water surrounding spent fuel assemblies. The hand-held ICVD (Fig. 16) is the instrument most commonly used by safeguards inspectors to obtain qualitative confirmation (attribute testing) of the presence of spent fuel in storage pools. The viewing device is capable of operating with facility lights turned on in the spent fuel pond area. The ICVD is optimized for ultraviolet radiation by filtering away most of the visible light and by having an image intensifier tube primarily sensitive to the ultraviolet light frequencies. Cerenkov radiation is derived from the intense γ radiation emanating from spent fuel, which, when absorbed in the water, produces high energy recoil electrons. In many cases these electrons exceed the speed of light in water (which is slower than the speed of light in a vacuum) and therefore must lose energy by emitting radiation (Cerenkov radiation). Spent fuel also emits β particles (which are also energetic electrons), adding to the Cerenkov radiation. Spent fuel assemblies are characterized by Cerenkov glow patterns that are bright in the regions immediately adjacent to the fuel rods. The variation in light intensity is apparent when viewed from a position aligned directly above the fuel rods. With careful alignment and appropriate assessment of the object being viewed, an irradiated fuel assembly can be distinguished from a non-fuel item that may look the same to the naked eye. Typically, a row of fuel assemblies is viewed vertically from the bridge while the facility operator slowly runs the bridge down the row. One inspector views the items in the row through the ICVD and verbally declares each item as spent fuel, as a void or as some other object, while a second inspector compares the observed results with the facility declarations.

The DCVD is used to verify assemblies with long cooling times and/or low burnups, which have weak Cerenkov signals that cannot be seen with a standard ICVD. Apart from its higher sensitivity, the DCVD can record and document individual scans for subsequent re-analysis. It has the potential to quantify the Cerenkov glow from spent fuel assemblies as a function of irradiation history and cooling time.

Improved Cerenkov Viewing Device (ICVD)

 (Source: IAEA Safeguards Techniques and Equipment)

Picture taken from the top of a spent fuel assembly with a Digital Cerenkov Viewing Device (Source:  IAEA-CN-184/338).