Radiological parameter control

Waste acceptance criteria (WAC) for disposal, a principal requirement for qualification of a produced waste package, are established predominantly on the radiological parameters of waste packages. Radiological parameter control is, therefore, considered to be the main component of a RAW control system. The WAC are country-specific; however, the IAEA recom­mendations for the establishment of WAC are accepted as the basis world­wide. Besides surface dose rates, maximum permitted activity concentrations (or total activity per entire waste package) of several radionuclides is usually defined in a WAC. The list of considered radionuclides is different for each country’s WAC for disposal. Besides common and simple measur­able radionuclides (such as Cs-134, Cs-137, Sr-90, etc.), a declaration of the activity concentration of 10-40 so-called critical radionuclides for disposal (alpha emitters, biologically important radionuclides, long-lived radio­nuclides usually with half-life over 30 years, etc.), is also required in a WAC. These radionuclides are often difficult to measure. To facilitate declaration of waste package compliance with a WAC, non-destructive (mainly gamma spectrometry) as well as destructive radiochemical procedures (with radio­chemical processing of the samples) are routinely applied.

Radiological control is applied in the entire life cycle of RAW. However, analogous to chemical parameters, the main effort is put on the radiological control of ‘as generated’ (raw) waste and then on the declaration of RAW package compliance with a WAC. For radiological control of ‘as generated’ waste, carefully selected combinations of non-destructive instrumental methods and radiochemical analysis with separation and subsequent deter­mination of difficult to measure radionuclides (some fission products, transuranium elements, etc.) are applied. The information obtained is widely used in waste processing planning for each waste stream and in the estima­tion (prognosis) of final waste package parameters. Results of radiochemi­cal analysis of input waste are also used for determination of radionuclide vectors, necessary for application of scaling factor methods (see below).

Most often a non-destructive check of the entire waste package is used for a declaration of final waste package compliance with a WAC. Gamma scanning, gamma tomography and in some cases also neutron tomography, all in combination with advanced data processing, are commonly used by both waste package producer as well as by disposal facility operator. The above-mentioned techniques allow determination of the major gamma — emitting radionuclides and along with using neutron tomography deter­mines the major actinides and fissile material. In general, non-destructive determination of minor radionuclides, critical for disposal, is very compli­cated, expensive, and in some cases even impossible. Destructive determina­tion with sampling of the waste form and waste package material and subsequent laboratory radiochemical analysis is not only technically com­plicated but can cause unacceptable damage to one or more of the waste isolation barriers in the waste package. The situation is more substantial for processed liquid waste, where critical disposal radionuclides can be expected with higher probability. The way around this situation is the application of scaling factors and a nuclide vector methodology [8]. The substance of this method is simple; however, implementation is more complicated and requires special software tools. Careful and precise radiochemical analysis of homogenized waste before the start of its processing is used to establish the nuclide vectors — a mathematical relationship between the activity con­centration of major or easy-to-determine radionuclides (usually strong gamma emitters) and the activity concentrations of minor (usually difficult — to-determine) radionuclides is developed. Using nuclide vectors and thor­ough knowledge of the waste processing procedure and waste package materials, it is possible to calculate and declare activity concentrations of minor radionuclides in a waste package using measured data on the activity of the major radionuclides, obtained by non-destructive gamma scanning of the entire waste package. Such a procedure should be, of course, qualified and approved by the regulator and disposal facility operator.

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