Research in Progress

Although nuclear power accounts for about 20% of electric power generation in the United States (second only to coal), no new nuclear plants have been built or ordered for a considerable time. Several aging or troubled plants have been shut down because the economic case for continued operation could not be made. Adding to the uncertain future for nuclear power is the controversial but accelerating movement toward deregulation of the electric utility industry. In this climate, it is imperative that nuclear utilities maximize the availability and efficiency of their plants while continuing to satisfy regulatory requirements. To achieve these goals a variety of problems must be addressed, a number of which are related to corrosion and chemistry in the primary coolant system.

My research over the past 10 years has addressed such problems through the design, construction, and operation of a set of unique facilities that operate in the 5MW MITR-II research reactor at MIT’s Nuclear Reactor Laboratory (NRL). The technical challenge of constructing accurate analogues of power reactor coolant circuits arises from the low temperature and pressure at which the MITR-II operates, and from the limited physical space available in core. The incentive to perform such experiments at a research reactor, rather than in a commercial plant, arises from the accessibility of the facilities for operation and measurement, the broad parameter range that can be investigated, and very large cost savings over typical in-plant pilot programs. Despite the large absolute power disparity between the MITR-II and commercial nuclear power plants, the power density and irradiation environments are remarkably similar. It is therefore possible to study aspects of the primary coolant system where radiation effects on the cooling water or the materials of construction are important.

One such aspect is the problem of radioactive corrosion product transport. In the light water cooled reactors (LWRs) used by American nuclear utilities, there is a small but significant content of radioactive material carried in the coolant. This inventory arises from release of fission products from fuel elements, from activation of in-core materials such as the fuel cladding and structural components, and principally from activation of material corroded or eroded from surfaces outside the core and deposited in-core by the flowing coolant. Deposition of radioactive materials outside the core is a significant source of worker radiation exposure during plant refueling and maintenance.

The first LWR simulation facility installed at the MITR-II was used to demonstrate that careful control of coolant pH is a useful strategy for reducing the out-of-core activity levels. The facility is a one-third-scale reproduction of a single unit flow cell (one steam generator tube plus one inter-fuel-pin channel) in a commercial pressurized water reactor (PWR). It is unique in that it closely simulates most of the parameters thought to be important in corrosion product transport, including coolant velocities, heat fluxes in in-core and out-of-core components, and the surface area ratios of the principal primary circuit materials. The small size and low cost of the wetted portions of the experiment make possible the unusual strategy of complete replacement of these surfaces for each run at a specific coolant condition, in contrast to the more usual technique of installing and replacing small sample coupons. The MIT approach allows complete post-irradiation radioactive and chemical product inventory and ensures that there is no "cross-talk" between the runs at different conditions.

More recently, facilities using passively and actively loaded mechanical test specimens to study environmentally assisted cracking (EAC) have been operated in the MITR-II. Various types of EAC are of concern to nuclear power plant operators. Our studies have focused on irradiation assisted stress corrosion cracking, in which cumulative irradiation effects on in-core components as well as instantaneous radiation-induced water chemistry effects are known to be important. The actively loaded system, in particular, exploits the unusual accessibility of the MITR-II with the installation of a standard servo-mechanical test machine on the reactor tank lid. The system permits one or more specimens to be tested at a controlled load in the core of the reactor under coolant conditions similar to those found in LWRs. One of the goals of this research is to separate the effects of cumulative and instantaneous irradiation to better understand the significance of data generated by testing irradiated materials in an out-of-core environment.

These examples (of two of the five major loops) illustrate the important role of research reactor experiments in areas where irradiation effects are integral to the problem being studied. Well-designed experiments permit relatively low cost investigation of pressing reactor problems and the freedom to increase our understanding of important underlying mechanisms. Results can be applied to the continued efficient and safe operation of the installed base of nuclear power plants and thereby contribute to a stable and responsible system of electric power generation.