Shielded containments are commonly referred to as hot cells, the word ‘hot’ being used as a synonym for radioactive. Hot cells are used in both the nuclear power and the nuclear medicines industries. They are required to protect individuals from radioactive isotopes by providing a safe contain­ment box in which they can control and manipulate the equipment required.

Hot cells are used to inspect spent nuclear fuel rods and to work with other items, which are high-energy gamma ray emitters.

The methods utilized in a hot cell examination vary with the objec­tive of the program, but usually begin with a visual inspection. Visual inspections are used to characterize the external surfaces of a fuel rod, including the crud and oxide layer, and to identify features of interest for subsequent examinations. High resolution diametral profilometry and EC lift-off measurements are also used to characterize the thickness of the crud and oxide layers relative to axial and azimuthal position, partic­ularly if such measurements were not performed before transporting the rod to the hot cell.

EC measurement with encircling, pancake or pencil coils can detect the axial and tangential location of cracks and other damage that might not be readily discernible in visual inspections; for example, small or incipient cracks due to PCI.

Neutron radiography — If a test reactor is adjacent to a hot cell, neutron radiography can be performed on the fuel rod to non-destructively locate regions with different hydrogen levels in the cladding. Depending on the method used for capturing and displaying the neutrographic image, accu­mulations of hydrogen are indicated by light or dark areas. Neutron radi­ography can also reveal fuel washout in the case of degraded failed fuel. In addition, neutron radiography provides geometrical information of the fuel column for the selection of cutting positions of samples for ceramographic examinations.

Destructive examinations — The fuel and cladding can be examined by means of a number of destructive methods including:

• Fission gas collection and analysis.

• Cladding oxide thickness (ID and OD) measurement.

• Hydride concentration and morphology.

• Second phase particle size and distribution.

• Microstructure characterization.

• Ceramographic characterization of fuel pellets.

• Micro-gamma scanning across the diameter of fuel pellets.

• Electron microprobe analysis of the fuel pellets and the pellet-cladding interface.

The fission gas that was released from the fuel to the free volume within a rod is normally collected and analyzed as the first step in the destructive examinations of sound fuel rods. The collection process involves punctur­ing the cladding and collecting the free gas in one or more containers of known volume, pressure and temperature. This process not only captures the gas from the open volume within a fuel rod, but also provides data for the calculation of the net internal void volume and the total quantity of gas in the open volume. The gas sample is analyzed for composition by tech­niques such as gas chromatography to identify the concentration of Xe, Kr, CO-CO2, CH4 and other constituents. The results are adjusted for decay and combined with calculations or measurements of exposure to establish the fraction of the generated fission gas that was released to the free volume within the fuel rod.

Metallographic mounts of the cladding and fuel may be prepared, pol­ished and photographed. The mounts may be examined for clad oxide layer (external and internal), fuel/clad interactions, fuel restructuring, rim effect and agglomerate behaviour. The mounts may also be etched to reveal grain boundaries for an analysis of the grain size.

Hydrogen/hydride analysis — A hot vacuum extraction process may be used to determine clad hydrogen content such as that employed by the LECO method. (LECO is a corporation which provides instrumentation for elemental determination in organic and inorganic materials.) When such analyses are done, it is important to separate the total hydrogen evolved from the sample into concentrations from the surface layer(s) and from the metal itself (which is of interest). Specifically for samples with thick oxides, water molecules adsorbed at the cracked and porous zirconium oxide may contribute to the recorded hydrogen content and give a metal hydrogen content that is too large. It is therefore crucial that the adsorbed water mol­ecules are removed before the hydrogen content in the sample is measured. This can be done by grinding of the oxide. but in this case care is needed so as not to remove massive hydrides at the outer metal surface of a fuel rod. For components without a surface heat flux there is no concentration of hydrides at the outer metal surface and there is lower risk of removing hydrides from the sample by grinding.

In addition to gas-extraction methods, detailed destructive post-irradia­tion examination (PIE) can be used to characterize the local oxide thick­ness and hydrogen concentration. Backscattered electron imaging (BEI) of polished cladding in the SEM may be used together with image analysis to determine the local hydrogen concentration and radial hydrogen profile through the cladding wall. This is a relatively new application of BEI which allows the relative positions of hydrides and oxides to be located more pre­cisely than the gas-extraction methods.

Alternatively, the hydride distribution and orientation in the cladding material is determined by optical examination after etching the sample to reveal the hydride locations.

Crud analysis — The morphology of the crud may be examined by electron probe microanalysis (EPMA).

Mechanical tests — Mechanical tests of differently shaped samples from

the fuel cladding can be carried out in the hot cell. Some of the tests that can

be done with irradiated cladding are:

• Room and elevated temperature axial tensile test yield strength (YS), ultimate tensile strength (UTS), uniform elongation (UE) and total elongation (TE).

• Cladding room and elevated temperature ring tensile test (YS, UTS, UE and TE).

• Burst testing.

• Cladding thermal creep rate test.

• Hardness testing.