Mit Pebble Bed Reactor Project

MIT has been developing a conceptual design for pebble bed reactors since approximately 1998, when a student design project concluded that in order to resurrect the nuclear power industry a new, innovative approach was needed, not only in reactor design but also in construction and operation. In their quest to identify the appropriate technology several key conclusions were reached. First was that the reactors did not have to be big to compete, particularly in developing nations, where 1600 MWe reactors are not suitable for most developing nations’ electric grids. Second, new reactors should also be capable of meeting large power demands in a modular, build-out array. Third, they concluded that to meet the competition, new reactors had to have long operational cycles such as combined cycle natural gas plants which rarely shut down for maintenance, certainly not for routine refueling negatively affecting capacity factors. This led to a deep evaluation of current technologies in terms of existing light water reactors and plans for evolutionary plants that were being considered at the time, including the AP-600, the advanced boiling water reactor (ABWR) and the standard PWR designs offered by Combustion Engineering which were currently being developed by Korea. The group also looked at high temperature gas reactors for completeness. The two variants of high temperature gas technology were the prismatic reactor, developed largely by General Atomics, and the pebble bed reactor, which was originally developed in Germany by the Julich Research Institute [1] and promoted by Professor Larry Lidsky at MIT in the late 1980’s. What became quite clear was that if the student design objectives were to be met mainly high efficiency, continuous operating units with greatly improved safety features the down selection rested largely with high temperature gas reactors. These reactors had the benefit of higher thermal efficiency, upwards of 45-50% and, with the pebble bed, online refueling, matching the general performance characteristics of combined cycle natural gas plants. After a careful review of the existing challenges for nuclear power and the expectations of the public relative to new plants, the students chose the pebble bed reactor as their technology of choice for the following reasons: 1) It was naturally safe, namely, it is not physically possible to cause a meltdown and no credible accidents would result in significant fuel damage. 2) It was small. The students judged that 100 to 200 megawatts electric would be the size necessary for international deployment of this technology. While the students recognized the potential advantage of economies of scale, they concluded that economies of production, namely, smaller units with less investment and shorter construction time, would be preferable. These units would be built out in modules to meet demand which should be more economically attractive to many nations and utility companies. 3) On-line refueling was judged to be a major advantage, The conceptual design of the MIT modular pebble bed reactor is described. This reactor plant is a 250 Mwth, 120 Mwe indirect cycle plant that is designed to be deployed in the near term using demonstrated helium system components. The primary system is a conventional pebble bed reactor with a dynamic central column with an outlet temperature of 900 C providing helium to an intermediate helium to helium heat exchanger (IHX). The outlet of the IHX is input to a three shaft horizontal Brayton Cycle power conversion system. The design constraint used in sizing the plant is based on a factory modularity principle which allows the plant to be assembled “Lego” style instead of constructed piece by piece. This principle employs space frames which contain the power conversion system that permits the Lego-like modules to be shipped by truck or train to sites. This paper also describes the research that has been conducted at MIT since 1998 on fuel modeling, silver leakage from coated fuel particles, dynamic simulation, MCNP reactor physics modeling and air ingress analysis.