Method of refueling reactor

The method of refueling, in a relatively short time interval, a nuclear reactor having a pressure vessel, a core therein having fuel assemblies to be replaced;

• fuel in said fuel assemblies having an enrichment of about 2.7% to 3.2% in fissionable material, an upper package including the head of the pressure vessel, control rods and control rod drive mechanisms and housings, electrical cables connected at one end to said control rod drive mechanisms and adapted for connection at their other ends to a power source, a lifting device and said head being held to said vessel by studs having stud tensioners and detensioning means for detensioning said studs, the said method comprising the steps of:
• increasing the concentration of neutron absorbing material to its refueling magnitude in coolant circulated through the core, said refueling magnitude being that amount and concentration of neutron absorbing material which corresponds to the fissionable material enrichment in the fuel assemblies and which is needed to compensate for neutron activity upon withdrawal of control rods from the core;
• actuating said detensioning means to unlock and remove said studs thereby unlocking said head from said vessel,
• lifting said upper package in a single lifting operation to a prepared position away from said pressure vessel thus affording access to said core, flooding borated water equal to said refueling magnitude into a pit in which the reactor is located before said upper package is removed from the pressure vessel;
• raising the level of said water in the pit to a point just below the stud detensioners and cables, and then maintaining said level by uniformly increasing the level of said water in the pit during the time of lifting said upper package from the pressure vessel;
• and
• by virtue of such access replacing said fuel assemblies in said core at relatively short intervals by replacing about ten percent of the spent fuel assemblies when the reactor is refueled at three month intervals;
• and replacing about twenty percent of the spent fuel assemblies when the reactor is refueled at six month intervals.