1. Field of the Invention
This invention relates to the removal of a layer of film which becomes adhered to the internal surfaces of the reactor coolant system and connected systems during operation of a pressurized water reactor (PWR). More particularly, it relates to a method and apparatus for decontaminating and cleaning the PWR reactor coolant system at a pressure which does not require high pressure components and connections.
2. Background Information
Pressurized water reactors have a reactor coolant system which circulates reactor coolant in the form of light water containing suitable levels of moderating chemicals through at least two loops which include hot leg piping between the reactor vessel and a steam generator and a cold leg returning reactor coolant from the steam generator to the reactor vessel. The reactor coolant system also includes a residual heat removal system containing heat exchangers which remove heat generated by the reactor core when the plant is shut down.
Over years of use, a two layer corrosion film forms on the typically stainless steel surfaces of the reactor coolant system. The inner layer which is a grown in place passivation film bonds to the stainless steel surface and is composed of iron, chromium, and nickel oxides. This inner layer which is several microns thick forms part of the metal surface. The outer layer of the corrosion film is a deposit of iron, cobalt nickel, and other corrosion and wear products. This film which becomes tightly adherent is activated from the passage of its constituents through the neutron flux in the reactor vessel and core. The isotopes formed in this film have long half lives such as five years or more which expose workers servicing the reactor coolant system to gamma radiation.
Most prior attempts at decontamination of PWRs have been limited to portions of the reactor coolant system, such as the steam generator channel heads and the residual heat removal system. These sections of the reactor coolant system could be chemically decontaminated with moderate flow rates at low pressure. Various processes have been developed for such chemical decontamination. Two processes are in general use. These include the LOMI process developed by the Electric Power Research Institute and the Central Electric Government Board of the United Kingdom. This process must be carried out at temperatures of about 180.degree. F. up to about 200.degree. F. The second system is CANDEREM developed by Atomic Energy Canada, Ltd. for heavy water carbon steel reactors. This process operates at temperatures in certain steps of up to 240.degree. F.
In decontaminating an entire reactor coolant system of a PWR, consideration must given to the size of the system. The hot and cold leg piping can be up to for instance 31 inches in diameter. The flow rates for such a decontamination system must be great enough that the process is effective (about one foot per second minimum), and can be completed and the system flushed of the decontaminating and cleaning agents in a reasonably short time to ensure that they do not attack (corrode) the walls of the reactor coolant system. The high flow rates required can best be achieved by circulating the reactor coolant with the decontamination and cleaning agents using the installed main reactor coolant pumps and/or the residual heat removal pumps to provide flow through the reactor coolant system and temperature control for the decontamination process. However, the minimum pressure required for operation of these installed pumps is usually fairly high, 400 psig or higher, and therefore, the decontamination system and its components must be designed to withstand such pressures. In fact, the pressure at the discharge side of the residual heat removal system pumps can be 570 psig or higher. Therefore, a decontamination system utilizing these pumps must be designed to withstand a pressure of 600 psig or higher. As it is noted above, the temperatures required for these chemical decontamination and cleaning processes can also be up to 240.degree. F. Therefore, the components of the decontamination system must be designed to withstand both high pressures and high temperatures. Vessels and conduits designed for these pressures and temperatures can have wall thicknesses of several inches. Thus, the components must be specially adapted, and welded connections and piping rather than flexible conduits are required.
There is a need for an improved method and apparatus for chemically decontaminating and cleaning the full reactor coolant system of a PWR which operates at pressures which permit the use of standard system components including couplings in place of welded connections.
There is also a need for such an improved method and apparatus which provides the required flow rate for the process while using standard components.
There is a further need for such a method and apparatus which can operate at the elevated temperatures required by the chemical decontamination and cleaning processes.