Elsevier

Annals of Nuclear Energy

Volume 61, November 2013, Pages 96-101
Annals of Nuclear Energy

Fission product release from nuclear fuel II. Validation of ASTEC/ELSA on analytical and large scale experiments

https://doi.org/10.1016/j.anucene.2013.03.045Get rights and content

Highlights

  • A wide range of experiments is presented for the ASTEC/ELSA code validation.

  • Analytical tests such as AECL, ORNL and VERCORS are considered.

  • A large-scale experiment, PHEBUS FPT1, is considered.

  • The good agreement with measurements shows the efficiency of the ASTEC modelling.

  • Improvements concern the FP release modelling from MOX and high burn-up UO2 fuels.

Abstract

This article is the second of two articles dedicated to the mechanisms of fission product release from a degraded core. The models of fission product release from nuclear fuel in the ASTEC code have been described in detail in the first part of this work (Brillant et al., this issue). In this contribution, the validation of ELSA, the module of ASTEC that deals with fission product and structural material release from a degraded core, is presented. A large range of experimental tests, with various temperature and conditions for the fuel surrounding atmosphere (oxidising and reducing), is thus simulated with the ASTEC code. The validation database includes several analytical experiments with both bare fuel (e.g. MCE1 experiments) and cladded fuel (e.g. HCE3, VERCORS). Furthermore, the PHEBUS large-scale experiments are used for the validation of ASTEC. The rather satisfactory comparison between ELSA calculations and experimental measurements demonstrates the efficiency of the analytical models to describe fission product release in severe accident conditions.

Introduction

In the first contribution of this work (Brillant et al., this issue), the models implemented in the ASTEC/ELSA code have been presented in detail. The ELSA module aims at estimating the release of FP and SM from a degraded core during a severe accident. The ELSA modelling is based on a semi-empirical approach. Therefore, only the governing mechanism of the release, identified as the limiting phenomenon, is modelled in each of these categories. The release governing mechanism for the highly volatile FP is diffusion in fuel pellets. The semi-volatile/low FP behaviour is governed by evaporation and mass transfer processes. Lastly, the very low-volatile FP release is linked to the uranium volatilisation. A model for the release of FP from in-vessel molten pools has also been developed, completing the release in a severely degraded-core configuration of a PWR.

In this article, we present the validation of the ASTEC/ELSA code. This validation is performed on both analytical and large scale experiments. Firstly, FP release from fuel fragments is estimated using the MCE1 test series. Then, cladded fuel samples are considered, in a second part, using the results of VERCORS experimental program and HCE3 test series. All these analytical tests have extended experimental conditions with different values of temperature, gas atmosphere composition or fuel burn-up. Lastly, FP release during the PHEBUS FPT1 large scale experiment is presented.

Section snippets

MCE1 tests

The MCE1 experiments were part of the Canadian out-reactor experimental program on severe accident FP source term research (Cox et al., 1991). The objective of these experiments was to measure FP releases from bare UO2 fragments in inert and oxidising atmospheres at high temperatures. The MCE1 experiments were performed in a zirconia-resistance furnace with samples placed in a vertical zirconia reaction tube with gas-tight connections on both ends. During the tests, the FP release was measured

VERCORS tests

Due to the potentially severe radiological consequences of a nuclear accident, the VERCORS program (Pontillon et al., 2010, Pontillon and Ducros, 2010a, Pontillon and Ducros, 2010b), which simulates severe PWR accidents, was initiated in order to quantify the emission rates and release kinetics of FP from irradiated nuclear ceramics (enriched UO2 and MOX fuels). Both the experiment and the test matrix were defined to devote to the understanding of the mechanisms that lead to the release of FP.

Release from a whole core

The international PHEBUS Fission Product (FP) programme was initiated in 1988 by IRSN and the European Commission within a framework of worldwide cooperation (Schwarz et al., 1999). The aim of PHEBUS FP was to study the key phenomena involved in the core degradation and the behaviour of the released fission products in the reactor coolant system and the containment building during severe accidents. In this objective, PHEBUS FP consisted of five in-pile large scale experiments, named FPT0, FPT1,

Conclusions

A wide range of experiments has been considered for the validation of the ELSA module of the ASTEC code: analytical tests such as AECL, ORNL and VERCORS; large-scale PHEBUS FPT1 test. With respect to solid fuel, calculation of volatile FP release is noticed to be satisfactory. Regarding semi-volatile and low-volatile FP release, a semi-empirical approach, with analytical correlations derived from thermodynamic equilibrium calculations, has been chosen. The rather good agreement with

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