SNF

This paper provides on overview of the SNF computer code1 of the Studsvik code package. SNF has capabilities for back-end calculations consistent with, and utilizing in- core fuel management (ICFM), multi-dimensional analyses performed with e.g. the CASMO/SIMULATE system2. SNF generates isotopic concentrations, decay heat and radiation source terms of fuel assemblies and/or individual fuel rods for cooling times up to 100,000 years. The basic methods of SNF, including some recent development is reviewed as well as examples of validation and application of the program.

The SNF program calculates the isotopic inventories and radiation source terms of spent LWR fuel assemblies based on the multi-dimensional methods employed in in-core fuel management (ICFM), thus utilizing models with precise neutron spectra, detailed power histories and a nodal representation of the fuel assemblies. It is applicable to all fuel types that may be represented by the ICFM system.

Acceptability for loading of spent fuel assemblies into dry casks depends on both gamma- and neutron source rates, as well as on burnup, decay heat power, etc. Conservatism in the applied calculation model may lead to overestimation of the cooling times required to reduce the radiation to acceptable levels and thus there is considerable incentive to apply high accuracy methods for the source terms calculations. The aim of this paper is to review the SNF method with emphasis on prediction of the neutron emission rates from spent LWR fuel assemblies and to present the underlying, experimental validation.

Efficient analysis of spent fuel source terms is a task of growing importance for many utilities. As the on-site fuel pool storage capacity becomes exhausted, it is necessary to utilize dry storage casks for additional, on-site storage.

Since the first release in 2004, the SNF program has been extended with e.g. more detailed isotopic decay chains and new user features, such as a general restart capability. The most important new validation was the completion of a benchmark using measured decay heat data of spent fuel assemblies.

SNF has been successfully applied for calculation of gamma- and neutron radiation parameters relevant to cask loading criteria. Other applications, besides traditional back-end calculations include ‘on-line’ core and fuel pool decay heat calculations using a special decay heat module (DHM) that may be executed by the reactor engineer.

A comparison of calculated and measured decay heat for 16 BWR fuel assemblies from the CLAB interim fuel storage facility in Sweden is presented. The calculations were carried out by Studsvik Scandpower in cooperation with the Swedish nuclear utilities. The calculations are based on the isotopic summation method using nodal, isotopic concentrations from standard 2-D/3-D CASMO/SIMULATE core-tracking calculations and the new spent fuel module, SNF, of the CMS system. The purpose of the work is to evaluate the capability of this analytical method to predict the decay heat of heterogeneous, BWR spent fuel assemblies. A corresponding evaluation of PWR fuel assemblies will be added later as experimental data becomes available.

The experimental data utilized in this project is provided by Svensk Kärnbränslehantering AB (SKB).

A method for performing spent nuclear fuel (SNF) analyses using in-core fuel management (ICFM) calculations has been developed and validated as described in this paper. Parameters such as decay heat, neutron source rates, and photon spectra of discharged LWR fuel assemblies are calculated on the basis of ICFM core tracking data. This approach offers highly automated and accurate SNF analyses and eliminates the need for special fuel irradiation calculations to obtain the radiation source terms. The new SNF method has been validated against LWR spent fuel isotopic measurements, by comparisons with ORIGEN-ARP calculations and comparisons with a decay heat standard. A further validations effort against experimental decay heat data from a Swedish storage facility is underway.