EVOLUTION OF CORE DESIGN AND OPERATIONAL REQUIREMENTS IN GERMAN PWRS FROM A TECHNICAL EXPERT ORGANIZATION´S POINT OF VIEW

Sebastian Schoop,Kai-Martin Haendel

doi:10.1051/epjconf/202124711005

Abstract

In the last three decades, core design and operational requirements of German PWRs have evolved significantly in order to optimize plant operation and to react to changed market conditions. In the first phase, steady state operation was optimized to limit plant operation costs or increase availability and power output. The second phase of the evolution was triggered by the necessity of flexible plant operation in order to compensate fluctuating power generation by wind and solar power and by optimization of phase out cores. The changes of core design and operation parameters had among the direct, expected impacts on safety analyses and reactor physics also several indirect and unexpected ones. Examples for expected impacts are changes of reactivity coefficients or boron worth compared to safety analysis assumptions or changes of possible initial conditions of incidents. Unexpected impacts showed for example on rod bow, cladding corrosions or neutron noise. The tasks of TÜV NORD EnSys as technical expert organization is to identify all these impacts of core design and operational parameter evolution, evaluate whether these impacts compromise plant safety, evaluate models and correlations regarding unexpected effects, assess if intended countermeasures are effective and acceptable and evaluate whether the set and ranges of plant parameters used for core limitation and protection are still valid. The paper gives an overview over selected experiences and tasks performed by the Reactor Physics and Criticality Safety Group of TÜV NORD EnSys for the evolution of core designs and operational requirements in German PWRs.

Highlights

The evolution of the core design and the operational requirements of German PWRs in the last three decades can roughly be divided in two phases
The first phase, starting in the late 1980s was marked by the economical optimization of steady-state operation, which was the normal mode of operation in German NPPs
The task for TÜV NORD EnSys as technical expert organization with respect to this type of corrosion is to assess models of the plant operator and the manufacturer, to identify possible key parameters linked to the corrosion and to evaluate possible operational restrictions in order to assure that the total maximum oxide layer thickness remains below the acceptable limit

Summary

INTRODUCTION

The evolution of the core design and the operational requirements of German PWRs in the last three decades can roughly be divided in two phases. Safety analyses can be affected by changes of key parameters such as boron worth and reactivity coefficients or the set of possible transient or accident initial conditions In addition to these rather direct effects, the changed operational conditions can have indirect and unexpected effects like rod bow, increasing neutron noise or cladding corrosion. REASONS AND MEASURES FOR THE EVOLUTION OF CORE DESIGN AND PLANT OPERATION IN GERMAN PWRs. The economical optimization of the reactor operation was mainly achieved by the uprate of the thermal reactor power the improvement of the fuel usage by x Increase of the fuel enrichment up to 4.95 wt-% U235 and target fuel assembly burnups up to 65 MWd/kgHM, x Implementation of U-Gd-, ERU- (enriched reprocessed uranium) and MOX-fuel, x Implementation of new fuel assembly designs (e.g. spacer grids) and materials x Modification of cycle lengths from 6 to 18 month cycles x Reduction of neutron leakage / systematic change of the core design strategy x Design of cores with little or no fresh fuel assemblies due to preparation of phase-out cores In the last two decades, the operational requirements of Nuclear Power Plants and PWRs in Germany have changed significantly due to changing political requirements and market conditions.

Boron Worth

Cladding Corrosion

Findings

CONCLUSIONS