Corrosion in Nuclear Reactors


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FACSIMILE


FACSIMILE is a very efficient robust and versatile suite of closely related computer programs for the modelling of complex steady state and dynamic chemical reactions with diffusion and/or advection.

In this application FACSIMILE has been used to simulate the reaction chemistry within a specific nuclear reactor type where a large number of operational and physical mechanisms interact.

WATER RADIOLYSIS IN BWR PRIMARY COOLANT

Boiling Water Reactors (BWR) use high purity water as the neutron moderator and primary coolant in the production of steam. As a result of water radiolysis, gas stripping in the core and recirculation, reactor recirculation water contains from 100 to 300ppb of hydrogen peroxide. This concentration of hydrogen peroxide under normal water chemistry operation increases the susceptibility of austenitic stainless steel to intergranular stress corrosion cracking (IGSCC), when other factors such as stress and sensitisation are present.

Reducing the peroxide level to 1 to 10ppb results in a decrease in the electrochemical potential to less than -230mV, which can effectively eliminate IGSCC in austenitic stainless steels. Experimental work has shown that the oxidant concentration can effectively be reduced by adding hydrogen and that IGSCC can be stopped.

A model has been developed for simulating the effect of adding hydrogen to prevent corrosion due to high oxidant concentrations. This model incorporates:

  • profiles of the gamma and neutron dose rates around the reactor circuit
  • a set of G-values (neutron and gamma) that describe the rate of production of the primary species formed during the irradiation of the water
  • a reaction mechanism in which these primary species react together.
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The reactor is modelled in sections. The figure above shows a cross-section of the various modelled regions of the reactor. flow in each part is treated as one-dimensional representing mass flux and dose rates as radial averages. The FACSIMILE program is used to construct a set of simultaneous differential equations in time from the rates of radiolytic input, the reaction scheme and the velocity profile. In the core region the velocity is obtained by thermal balance. The equations are integrated to give concentrations as a function of distance around the reactor circuit.

In a model of this complexity, which is dependant on a large number of determinants and inputs, a sensitivity analysis is essential to gauge the effect and interdependence of the various parameters. FACSIMILE contains sensitivity analysis routines that are ideal for this purpose.