The invention relates to a method for decontaminating nuclear reactors employed to generate electric power and more particularly to a method for performing full system decontaminations on boiling water reactors.
During on-line power generating operations of commercial boiling water nuclear reactors, thin layers of metal oxides tend to build up on the internal surfaces of vessels and other components and piping in contact with circulating primary coolant (essentially high temperature water). Activated metal ions in the central core regions in reactor pressure vessels are entrained in the primary coolant and then are absorbed in the metal oxides, which results in relatively high radiation levels on these surfaces. It is desirable to reduce the radiation levels to xe2x80x9cAs Low As Reasonably Achievablexe2x80x9d levels in order to reduce the exposure of personnel working near the reactors during periodic plant outages and/or plant decommissioning operations. Thus, the industry may employ one or a combination of various known chemical decontamination treatments, e.g., acid permanganate, alkaline permanganate, Citrox, CAN-DEREM, LOMI and/or other processes, in order to dissolve or break up the oxide films. Conventionally, these decontamination processes involve the addition of permanganate, oxalate, citrate, EDTA and/or other ions to the primary coolant to form decontamination solutions and then the circulation of the solutions through the components to be decontaminated. In addition to removing the oxide layers, it may be desirable to remove several microns of base metal in order to better protect personnel during decommissioning processes. Dilute chemical decontamination solutions generally contain less than about 3-5% by weight of such decontamination agents. Chemical decontaminations may be performed upon full primary coolant systems or upon selected subsystems. Full system decontaminations are the preferred approach when the goal is to reduce dose rates on multiple subsystems throughout the plants. In addition, full system decontamination processes are generally performed with nuclear fuel assemblies out of the central core regions of the reactor pressure vessels, but the fuel assemblies may be retained in the central core regions in some cases.
The activated metal ions that are removed from the internal surfaces of the primary coolant systems in the course of the decontamination operations are collected on cation exchange resins. The activated resins must then be removed to remote disposal sites.
The majority of the activated oxide deposits in boiling water reactor primary coolant systems are located in the central core regions of reactor pressure vessels. These deposits do not substantially contribute to personnel exposure. Thus, it would be very desirable to decontaminate only those systems that substantially contribute to personnel exposure and bypass the central core regions. This would substantially reduce the total exposure of personnel while reducing resin and disposal costs.
It is an object of the present invention to decontaminate a portion of a reactor pressure vessel in a boiling water reactor and its appurtentant recirculation system while bypassing its central core region. It is a further object to substantially decontaminate a boiling water reactor with lower overall personnel exposures to radiation and lower resin costs.
With these objects in view, the present invention resides in a method of decontaminating a boiling water reactor having a plurality of reactor recirculation loops hydraulically connected in parallel with a reactor pressure vessel. Such a reactor pressure vessel has: a central core region; an annulus region surrounding the central core region and in hydraulic communication with the recirculation loops; and a lower internals region in hydraulic communication with the central core region. In the practice of the present invention, a decontamination solution is circulated through at least one of the reactor recirculation loops and the annulus region of the pressure vessel without circulating through the central core region. In a preferred practice of the present invention, the decontamination solution also circulates between the annulus region and the lower internals region without circulating through the central core region. Thus, a boiling water reactor can be substantially decontaminated while reducing overall personnel exposure and generating less resin wastes.