Patent Number: 050892169
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of decontamination of nuclear reactor primary systems. More specifically, it relates to a unique method of integrating a chemical injection and clean-up sub-system into a nuclear reactor primary system for chemical decontamination of the entire primary system. 2. Description of the Prior Art The problem of excessive personnel exposures caused by high background radiation levels in a nuclear reactor primary system, such as in pressurized water reactor (PWR) systems, and the resultant economic cost of requiring personnel rotation to minimize individual exposure is significant at many nuclear plants. These background levels are principally due to the buildup of corrosion products in certain areas of the plant. The buildup of corrosion products exposes workers to high radiation levels during routine maintenance and refueling outages. The long term prognosis is that personnel exposure levels will continue to increase. As a nuclear power plant operates, the surfaces in the core and primary system corrode. Corrosion products, referred to as crud, are activated by transport of the corroded material to the core region by the reactor coolant system (RCS). Subsequent release of the activated crud and redeposition elsewhere in the system produces radiation fields in piping and components throughout the primary system, thus increasing radiation levels throughout the plant. The activity of the corrosion product deposits is predominately due to Cobalt 58 and Cobalt 60. It is estimated that 80-90% of personnel radiation exposure can be attributed to these elements. One way of controlling worker exposure, and of dealing with this problematic situation, is to periodically decontaminate the nuclear steam supply system using chemicals, thereby removing a significant fraction of the corrosion product oxide films. Prior techniques had done very little to decontaminate the primary system as a whole, typically focusing only on the heat exchanger (steam generator) channel heads. Two different chemical processes, referred to as LOMI (developed in England under a joint program by EPRI and the Central Electricity Generating Board) and CAN-DEREM (developed by Atomic Energy of Canada, Ltd.), have been used for small scale decontamination in the past. These processes are multi-step operations, in which various chemicals are injected, recirculated, and then removed by ion-exchange. Although the chemicals are designed to dissolve the corrosion products, some particulates are also generated. One method of chemical decontamination, focusing on the chemistry of decontamination, is disclosed in U.K. Patent Application No. GB 2 085 215 A (Bradbury et al.). There is little disclosure, however, of the methodology to be used in applying that chemistry to system decontamination. While these chemical processes had typically been used on only a localized basis, use of these chemical processes has now been considered by the inventors herein for possible application on a large scale, full system chemical decontamination. Consequently, there is now a need for a subsystem design that efficiently integrates the chemical decontamination process into the current types of nuclear reactor primary systems. Such an integration will necessarily have to meet the requirements of pressure and temperature used in the chemical decontamination, requirements that had not previously been necessary to have been met since full system decontamination had not been undertaken. While some work has been done in the boiling water reactor (BWR) programs, the BWR scenarios examined by those in the field involved decontaminating fuel assemblies in sipping cans employing commercial processes at off-normal decontamination process conditions with little regard for the effects of temperature, pressure, and flow that would be mandated by an actual application of the process to the full RCS. The estimated collective radiation dose savings over a 10-year period following decontamination is on the order of 3500-4500 man rem, depending upon whether or not the fuel is removed during decontamination. At any reasonable assigning of cost per man-rem, the savings resulting from reduced dose levels will be in the tens of millions of dollars. As a result of the examination of potential full system decontamination, a need now exists for an effective and economic method for integrating a chemical decontamination system into current primary systems. SUMMARY OF THE INVENTION The present invention is directed to a chemical decontamination system to be used in conjunction with a nuclear reactor primary system to achieve full primary system decontamination. To this end, a chemical decontamination system is integrated with a nuclear reactor primary system by utilizing the residual heat removal sub-system. Processed fluids, after passing through one of the residual heat removal heat exchangers, are diverted to the chemical decontamination system. After passing through the decontamination system, wherein the fluid is decontaminated and/or decontamination chemicals are injected into the fluid stream, the fluid stream is returned to the residual heat removal system downstream of the location of initial diversion. In this manner, the residual heat removal pumps and heat exchangers can be utilized to assist the chemical decontamination process, both in terms of fluid flow and temperature control. Typically, one or more reactor coolant pumps will be needed to augment the residual heat removal pumps during chemical decontamination. Therefore, the pressure within the system must be maintained above the minimum required for operation of the reactor coolant pump. This is accomplished by use of a nitrogen blanket in the primary system pressurizer. Additionally, various fluid streams collected from seal leakages and equipment drains can be optimally fed to the chemical decontamination system and, thereafter, reinjected into the primary system along with the other process fluids or removed with the decontamination wastes. Accordingly, it is an object of the present invention to provide a method for integrating a chemical decontamination system into a nuclear reactor primary system to economically and chemically decontaminate substantially the entirety of the nuclear reactor primary system. These and further objects and advantages will be apparent to those skilled in the art in connection with the detailed description of the invention that follows.