The Volgodonsk Branch of JSC AEM-technology Atommash (part of the Rosatom engineering division Atomenergomash) completed the local heat treatment of the Steam Generator bottom weld for the “Rooppur” NPP.
The bottoms were welded to the Steam Generator Vessel previously. Local heat treatment is a mandatory operation to relieve stress in the welded joint. The volume heat treatment in the furnace is not possible due to the thin-walled stainless SG tubes installed inside the vessel.
When carrying out local heat treatment, the weld zones are covered with ceramic elements – thermocouples. Then the heating elements are gradually brought to 620-660 degrees Celsius. Experts control the temperature in 12 zones of the weld. The temperature is recorded every half hour. The difference in thermocouples shall not exceed 40 degrees.
Local heat treatment includes not only gradual heating of the weld but also gradual cooling of the joint. The entire process takes one and a half to two days.
Further, the Steam Generator shall be subjected to test operations: evacuation and helium tests, as well as eddy current test of SG tubes.
The Steam Generator is a heat exchanger, is part of the reactor plant equipment, and refers to the items of the first safety class.