Risk Assessment of Abandoned Radioactive Logging Sources in Oil Wells in Nigeria

An integrated model for the risk assessment is developed for abandoned radioactive logging sources (Am-241 and Cs-137) in oil wells. The model is composed of four components: Source term, Barrier failure model, a geosphere model, a biosphere model, and finally a dose and health effect model to analyse the radionuclide dispersion phenomenon from the abandoned source term to the risk of serious health effects on members of the public (cancer death). In addition to time-dependent annual release rate and dose rate for each radionuclide, the ultimate risk in terms of cancer death rate is estimated.  The results show that the highest value of the annual dose for the abandoned radioactive logging sources is less than the individual dose limit to the human body, and the calculated highest cancer death rate is much lower than that of background radiation but higher than that of low-level waste repository. Subsequently, an abandonment procedure was developed in an effort to manage the risk and reduce it to the barest minimum.  It is concluded that the use of simplified mathematical modelling for the risk assessment of abandoned radioactive sources in oil wells could be an effective tool for development of efficient and acceptable abandonment procedure. Keywords: risk assessment, geosphere model, dose, cancer death, logging sources, low-level repository, radionuclide, abandonment procedur

Science and Technology of Supercritical Water Cooled Reactors: Review and Status

An overview of the global development of Supercritical Water-Cooled Reactors (SCWRs) has been given. The SCWR concept is the natural design path and ultimate evolution of conventional reactors today as the vast majority of modern power nuclear reactors are water cooled units. The move from subcritical to supercritical pressures by the thermal power industry over the past 50 years has been successful. The SCWR concept follows two main types of large reactor vessel as found in conventional Pressurized Water Reactors (PWR) and Boiling Water Reactors (BWR); and distributed pressure tubes as found in Canadian and Russian nuclear reactor designs. The concept has been designed to operate with a thermal spectrum core, fast spectrum core or mixed spectrum core. SCWRs promise to increase the efficiency of modern nuclear power plants from 30 - 35% to about 40 - 45% and reduce capital and operational costs. Most of the SCWR designs are still at the conceptual stage and are not expected to be implemented till 2030 because of the challenges faced in thermal-hydraulics and construction materials chemistry that would withstand supercritical conditions. Keywords: SCWR, PWR, BWR, thermal spectrum, fast spectrum, mixed spectrum, capital and operational costs

Sensitivity Analysis of Ex-Vessel Corium Coolability Models in MAAP5 Code for the Prediction of Molten Corium–Concrete Interaction after a Severe Accident Scenario

A postulated progressing severe accident scenario has been simulated using MAAP5 code with the focus on ex-vessel cooling of molten corium in the reactor cavity. Various parameters associated with the prediction of molten corium–concrete interaction (MCCI) are identified. Accordingly, a sensitivity analysis is performed to assess the impact of these parameters on the predicted cavity floor erosion depth during this MCCI postulated accident. The sensitivity index of each variable parameter is determined using the Cotter indices method and Sobol′ indices method. At the early stage of the accident, the predicted cavity floor erosion depth is found to be highly sensitive to the downward heat transfer coefficient parameter with Cotter and Sobol′ indices of 94% and 50%, respectively. At the late phase of the accident, however, the cavity floor erosion depth becomes sensitive to melt eruption (Cotter index of 40%), water ingression (Cotter index of 13%), and particulate bed (Cotter index of 15%) parameters alongside the downward heat transfer coefficient (Cotter index of 16%) with the melt eruption parameter becoming dominant. Thus, the sensitivity of the code′s predictions can be minimized by improving the physical models associated with these parameters. Moreover, the sensitivity indices of these parameters can be used by model developers to identify unimportant parameters in a bid to reduce the dimension of the problem with the aim of improving the current predictive capabilities to conduct MCCI-related safety analyses