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Conference Article: Fracture mechanics applied to a reactor graphite

Roedig, M.; Kleist, G.; Nickel, H. (Kernforschungsanlage Juelich G.m.b.H. (Germany, F.R.))

Abstract

The reactor graphite investigated belongs to a family of graphites which were considered for the non-exchangeable reflector of a high temperature reactor for nuclear process heat generation (NPH/PNP) or electrical power generation with a helium turbine in a direct cycle (HHT). The reflector vessel consists of the bottom, top and side reflectors. The later concepts differ from the AVR-Reactor (Juelich/FRG) and THTR-Reaktor (Schmehausen/FRG) in the way in which the fuel is loaded. The spherical fuel elements pass only once through the core and then go out (OTTO-loading) and they are not reloaded. This concept leads to a very high neutron irradiation dose of the graphite reflector blocks in the upper part of the side reflector and the top reflector. Maximum fluences in excess of 3x1022 cm-2 (DNE) at temperatures between 300 and 700oC are to be expected. The fast neutron flux and consequently, the accumulated fluence in the block decrease rapidly with core distance whereas the temperature first increases up to a maximum somewhere in the block half near to the core centre and then decreases to a lower value. Due to the irradiation induced decrease of the thermal conductivity of the graphite, the temperature gradients in the block develop with increasing core residence time. The temperature and fluence gradients result in differential dimensional changes due to thermal expansion and Wigner shrinkage. This leads to mechanical stress build up. The magnitude of the stresses are limited by irradiation induced creep. However, even where the initial thermal stresses have been relaxed by creep, they will reappear at shut down with opposite sign, modified by the changes in thermal expansion coefficient and elastic moduli during irradiation

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key words: cracks; damaging neutron fluence; fracture properties; graphite; htgr type reactors; pebble bed reactors; physical radiation effects; stress analysis; carbon; gas cooled reactors; graphite moderated reactors; homogeneous reactors;mechanical properties; neutron fluence;radiation effects; reactors; solid homogeneous reactors
Reference:
Specialists meeting on mechanical behaviour of graphite for HTRs, Gif-sur-Yvette, France, 11-13 June 1979.
International Atomic Energy Agency, Vienna (Austria). International Working Group on High-Temperature Reactors.
IWGHTR--3, pp:76-88