Improving the cooling conditions of the irradiation grid of a 22 MW multipurpose research reactor

ETRR-2 is a 22 MW Open-Pool Research Reactor designed and constructed by INVAP for the Atomic Energy Authority of Egypt (AEA) at Inshas site (located 35 km away from Cairo city). The Reactor is aimed to produce radioisotopes for medical and industrial applications, Cobalt-60 and Silicon Doping. It has facilities for neutron activation analysis, neutrongraphy as well as neutron beam and other irradiation facilities useful widely used for basic or applied research. During the commissioning tests unexpected non-condensable gas bubbles appeared when power was reaching 15 MW and over. As a consequence a careful investigation was undertaken to find out the root causes of the phenomena and it was concluded that bubbles came from certain positions located at the irradiation grid while the fuel elements and the reactor core cooling system (i.e. nuclear safety) were not involved. The analysis made showed that hot spot appears in structural materials due to the lack of coolant flow resulting in locally favourable conditions for degassing. The problem could not be solved by a simply increase of the available mass flow because of mechanical and geometrical design flaws resulted in uneven flow distribution through the existing channels. Different proposals were studied and their effectiveness was evaluated using modelling tools and experimental observation (laboratory and on site test). Finally a set of specially designed mechanical devices placed at the irradiation grid flattened the flow distribution and assured the clearance of the channels. In addition the total mass flow was also increased. The modifications proved to be successful and the reactor was able to run at its full power without producing bubbles while it maintained its design functionality and capabilities. This paper gives a brief description of the reactor jointly with the mechanical and flow characteristics of the affected zone. The methodology used, the main conclusions reached for the diagnosis, experiences held in mock-ups and at the reactor test results obtained are also presented. (authors)