Post-irradiation description

After final discharge from the reactor, additional data are required to provide a complete description of the fuel assembly.

3.3.3.1. Final discharge burnup

The average discharge burnup of each fuel assembly should be recorded either as a part of irradiation or post-irradiation data category. In certain cases, the burnup distribution within the assembly may also be pertinent. For example, such data would be needed if dismantling of the assembly is planned for purposes of consolidation with fuel pins from other assemblies or where the assemblies may be refabricated into new assemblies for further irradiation.

The variation of burnup within an assembly could also impact criticality analyses and shielding requirements. Although discharge burnup is a derived quantity, because of its extreme importance in calculating the radiological properties of spent fuel and because discharge burnup is often directly available from the utilities, it is included here as a base, or non-derived data element.

The following data, in conjunction with the pre-irradiation data on initial weight of heavy metal and initial isotopic composition, should be sufficient to determine the average discharge burnup of the fuel:

• the date the assembly was installed in the reactor,

• the date the assembly was discharged from the reactor,

• the reactor power history during the residence time of the fuel assembly (i.e. power level as a function of time),

• the fuel channel(s) and/or reactor position(s) in which the assembly was irradiated and times or dates of residence in each location, and

• the positions of reactivity-adjustment-devices during the in-reactor residence time of the fuel assembly.¹⁶

3.3.3.2. Inventory of radionuclides in spent fuel

The inventory of radionuclides in spent fuel may be calculated from the fuel assembly’s pre irradiation physical description, the initial weight of heavy metal, the initial isotopic composition and the in-core fuel management data, including the final discharge burnup. Retaining an assembly’s detailed power history for more than five years after discharge is not essential, because after five years the radiological characteristics are directly related to the final discharge burnup, but are relatively independent of the details of the power history.

Computer codes are available to calculate the isotopic content of the fuel assembly at discharge and, if necessary, radioactive isotopes produced by activation in non-fuel components of the assembly. These codes typically focus on the calculation of the isotopic abundances of three sets of data: actinides and their daughters, activation products, and fission products. The actinides and their daughters will consist of the following:

• isotopes of uranium,

• isotopes of higher actinides produced by neutron activation (Np, Pu, Am, Cm, Bk, and Cf), and

• isotopes of actinide daughters (Pa, Th, Ac, Ra, Rn, Fr, Po, At, Bi, Pb, Tl, and He4, from alpha decay).

Both fission-product and activation-product isotopic inventories can be similarly calculated.

3.3.3.3. Radiological characteristics

From the isotopic inventories detail radiological characteristics can be obtained, including activities, photon spectra, and neutron spectra. Additionally, the variation of these characteristics with time can be calculated.

3.3.3.4. Decay heat

The decay heat can be also calculated on the basis of the isotopic inventories.

3.3.3.5. Non-fuel components

If discharged fuel assembly contains non-fuel components, those components must be included in the description of that assembly. Non- fuel components are not associated with a particular fuel type. These include, but are not limited to, control spiders, burnable poison rod assemblies, control rod elements, thimble plugs, fission chambers, primary and secondary neutron sources and boiling water reactor channels.

16 For reactor types in which power adjustment or trimming within the reactor core may lead to significant power and burnup variations in some fuel assemblies. For example, such records are kept for some CANDU reactors.

3.3.3.6. Results and consequences of spent fuel tests

After final discharge from the reactor, the ‘leak tightness test’, the ‘visual inspection’ and the

‘geometry inspection’ can be performed. As a result of these tests some fuel assemblies can be identified as defective or failed assemblies: such identified assemblies must be handled according to special procedures or require special consideration. Such assemblies may not be able to fit into a spent fuel rack, may not be able to be lifted normally, or may have cladding defects (leakage) greater than established limits.

3.3.3.7. Special considerations

Several special conditions have been identified which, when present in an individual assembly, will require special considerations. These include fuel assemblies that

• are known to contain defective fuel pins,

• have been enclosed in a special containers (“canned”),

• have had fuel pins removed,

• that have been consolidated, or

• that have been altered in some way.

3.3.3.8. Data collection form: US example RW-859

Assemblies meeting any of these conditions should be identified, and they should have details regarding the nature and extent of the condition recorded with other assembly data. For example, if an assembly is known to contain defective fuel pins, the nature and extent of the defect or damage and the location of the defective pins should be recorded, if known. For

“canned” assemblies, a cross-reference to a unique container identifier should be provided.

For assemblies from which fuel pins have been removed, the initial weight of heavy metal should be modified to reflect the removed pins, and the location, mass, and burnup of the removed pins should be specified (preferably by the new unique identifier for the container where they are stored). Containers into which fuel pins from consolidated fuel assemblies are placed should be identified, along with the mass and burnup of the fuel contained.

A survey form in use by USDOE/EIA for collection of data on every fuel assembly irradiated in commercial nuclear reactors operating in the US is the Nuclear Fuel Data Survey Form RW-859 which was formulated with a view to application to the Commercial Radioactive Waste Management Programme in the US.

The data files of the RW-859 Form include the following categories of information:

• Facility data,

• Reactor and operation cycle data,

• Data on permanently discharged fuel,

• Pool storage,

• Reinserted fuel and shipment of fuel,

• Canisters and on-fuel components,

• Dry storage,

• Projected assembly discharge,

• Comments.

Further details on the RW-859 Form is attached in the country report of the USA, attached in the Annex, together with a paper which provides background information [25].