Patent Number: 047864656
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to the flow of bypass coolant through nuclear reactor internals and more particularly to the conversion of a downward flow to an upward flow to reduce the pressure difference between the bypass and core regions. 2. General Background Many existing nuclear reactor internals were originally designed to be cooled by coolant bypass flow entering through the side of the core barrel adjacent the top and then flowing downwardly through holes in the former plates between the core barrel and the baffle plates. The coolant then joined the main coolant flow which flowed upward through the core region. However, the two flow directions resulted in a large pressure difference between the bypass region and the core region. This pressure differential of as much as 20 p.s.i. caused flow jetting at the gaps between adjacent baffle plates. This flow jetting resulted in damage to fuel pins which led to fuel pin failures. Attempts to reduce the pressure differential to a level which does not result in damage to fuel pins have traditionally plugged the horizontal core barrel flow holes and added vertical flow holes to the upper former plates. This form of upflow conversion results in the pressure differential being reduced from approximately 20 p.s.i. to approximately 3-4 p.s.i. However, even this reduced pressure differential still has sufficient driving pressure to cause jetting which leads to fuel pin failures for certain gap sizes between adjacent baffle plates. It can be seen that an upflow conversion method is needed which will reduce the pressure differential between the bypass and core regions to a level which does not result in jetting of coolant with subsequent fuel pin failures. SUMMARY OF THE INVENTION What is provided is a method of converting a vertically downward flow of bypass coolant through a core barrel and former plates in a nuclear reactor to a vertically upward flow. This method reslults in a pressure differential of approximately one-half (1/2) p.s.i. between the bypass and core regions. The method involves providing holes in the top former plates. Coolant flow holes in the core barrel adjacent the top former plates are plugged. Plugs are inserted through the top and intermediate former plates and used to plug selected holes in the lower former plates. Also, some holes in the lower former plates may be partially plugged. This causes a reduction in the quantity and velocity of coolant flow through the bypass region which results in a reduction in the pressure differential between the core and bypass regions to near zero. An expander tool designed to allow for a certain offset between flow holes in the former plates is used for ease of plug installation. In view of the above, it is an object of the invention to provide an upflow conversion method which may be accomplished through the use of remotely operated tooling. It is another object of the invention to provide an upflow conversion method which reduces the pressure differential between the core and bypass regions to near zero. In view of the above objects, it is a feature of the invention to plug selected holes in the lower former plates. It is another feature of the invention to partially plug selected holes in the lower former plates.