Power production in LWRs is economic and very efficient. A by-product of the fission process, which generates the thermal power, is radioactivity associated with fission and activation nuclides. This article describes the ways in which radionuclides are produced and then dissipated
in the systems of a LWR. Radionuclides in and on the surface of components create radiation fields which can cause harm to workers. The article describes methods which help to minimize the buildup of these radiation fields. The difficulties in determining the radionuclide inventories for structural
materials in LWRs are discussed and appropriate code packages are discussed. The basic principles adopted in the context of treatment of radioactive waste are summarized, including an overview of typical conditioning strategies. Radiation protection issues are briefly described and methods
used to protect workers are discussed.
The Swiss National Cooperative for the Disposal of Radioactive Waste, partly in collaboration with ETH Zurich and the Swiss NPP utilities, has developed MCNP NPP models for neutron activation analysis and NPP decommissioning studies. These models exhibit mainly rotational symmetry and are bound to the reactor pit/drywell area. In this paper, the PWR Goesgen model is chosen as a reference model for further investigation and extended to incorporate detailed reactor pressure vessel support structures, cooling pipes/penetrations and farther ex-reactor pressure vessel areas such as the adjacent steam generator or cooling pump compartment as well as the layout of the basin area above the reactor pressure vessel. The impact of these azimuthal asymmetries on the ex-reactor pressure vessel neutron distribution is quantified and the variance reduction methodology used is outlined. Finally, a description of the in-situ foil irradiation campaign initiated in collaboration with the Swiss NPP Goesgen for the purpose of future model validation is presented.
Disposal of low- and intermediate-level activated waste generated at nuclear power plants is being planned or carried out in many countries. The radioactivity concentrations and/or total quantities of long-lived, difficult-to-measure nuclides (DTM nuclides), such as C-14, Ni-63, Nb-94, α emitting nuclides etc., are often restricted by the safety case for a final repository as determined by each country’s safety regulations, and these concentrations or amounts are required to be known and declared. With respect to waste contaminated by contact with process water, the Scaling Factor method (SF method), which is empirically based on sampling and analysis data, has been applied as an important method for determining concentrations of DTM nuclides. This method was standardized by the International Organization for Standardization (ISO) and published in 2007 as ISO21238 “Scaling factor method to determine the radioactivity of low- and intermediate-level radioactive waste packages generated at nuclear power plants”.
However, for activated metal waste with comparatively high concentrations of radioactivity, such as may be found in reactor control rods and internal structures, direct sampling and radiochemical analysis methods to evaluate the DTM nuclides are limited by access to the material and potentially high personnel radiation exposure.
In this case, theoretical calculation methods in combination with empirical methods based on remote radiation surveys need to be used to best advantage for determining the disposal inventory of DTM nuclides while minimizing exposure to radiation workers.
Pursuant to this objective a standard for the theoretical evaluation of the radioactivity concentration of DTM nuclides in activated waste, is in process through ISO TC85/SC5 (ISO Technical Committee 85: Nuclear energy, nuclear technologies, and radiological protection; Subcommittee 5: Nuclear fuel cycle). The project team for this ISO standard was formed in 2011 and is composed of experts from 11 countries. The project team has been conducting technical discussions on theoretical methods for determining concentrations of radioactivity, and has developed the draft International Standard of ISO16966 “Theoretical activation calculation method to evaluate the radioactivity of activated waste generated at nuclear reactors”.
This paper describes the international standardization process developed by the ISO project team, and outlines the following two theoretical activity evaluation methods:
— Point method
— Range method
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.