Following the widely publicized nuclear reactor accidents since the 1980's, scientists have worked towards reactor designs with emphasis on safety, in order to improve public perception. The pebble bed reactor was among the first of the Generation IV reactors to implement the latest safety features.
Originating from the German HTR (High temperature reactors), the pebble bed reactor continues with the same concept of a gas cooled reactor with graphite moderation. The gas turbine creates ease of power production and provides high output efficiency. After several proof of concepts and feasibility studies, countries such as South Africa and China have started the process of implementation the PBR.
One primary interest of reactor designers was to create a system where accidents would be prevented without usage of engineered control systems. The PBR achieves a high level of inherent safety. The keys to this is the materials that it employs, and also the fuel form. The fuel sphere of the PBR is a 60mm sphere enclosing uranium dioxide fuel coated with carbon (TRISO) and a 5mm graphite layer.  All fission products will remain within the fuel itself. This design results in a very low particle failure so long as the temperature is under 1600 degrees Celsius.
Other safety capabilities of PMR include the large negative temperature coefficient, and automatic heat removal of the reactor. This will stop the fission process with any deviations in core temperature even without the need for engineered controls or other manual shutdown systems.
One of the earliest prototypes of the PBR was tested in South Africa in 1993, as a highly anticipated facility to meet electrical needs. Target specifications for this plant included a Rated output per module of > 165 Mwe, and a continuous stable power range of 40-100%, and a Load rejection with reactor trip of 100%.  Funded by the South African government, the South African plant a successful proof of concept providing 400 Mwt of energy per module. However in 2010, the South African government cut funding to the project due to pressure from special interest group.
Since the PBMR in South Africa, there have been many more attempts to utilize the safety and efficiency of PBR including the HTR-10 in Tsinghua University at China, and also in the Netherlands. Despite successful demonstrations however, the PBR also comes with a unique set of challenges and risks. The risks of reactor meltdown are largely alleviated by the passive safety of the PBR, but one issue of interest is the use of the PBR to develop weapons grade plutonium.
Consider a country that invests in a PBR for stated purposes of energy generation. Typically, the safeguard against clandestine usage includes careful counting and safeguard of spent fuel, and no new fuel until all previous amounts have been accounted for.
One possibility is that they will manufacture a small amount pebbles that made to look like normal fuel pebbles. These pebbles would have identical composition compared to legitimate fuel pebbles with the exception of replacing enriched uranium with normal uranium, allowing the reactor to receive new supplies of fuel. Meanwhile, a small amount of fuel is utilized to produce weaponizable material while causing minimal effects on the energy production. 
The passive safety of the PBR is certainly an attractive feature, but risks such as these show that implementations of the PBR should be just as carefully scrutinized as any other reactor should it be widely adopted in the future.
© Jerry Zhou. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 A. Weinberg and I. Spiewak, "Inherently Safe Reactors and a Second Nuclear Era," Science 224, 1398 (1984).
 S. Ion et al., "Pebble Bed Modular Reactor - the First Generation IV Reactor to Be Constructed," Nucl. Energy 43, 1 (2004).
 A. M. Ougouag, W. K. Terry and H. D. Gougar, "Direct Deterministic Method for Neutronics Analysis and Computation of Asymptotic Burnup Distribution in a Recirculating Pebble-Bed Reactor," Ann. Nucl. Energy 29, 1345 (2002).