This isn't a separate containment structure surrounding the reactor, but part of the reactor itself. From your own link, page 42, section 5.8 Containment syste

This isn't a separate containment structure surrounding the reactor, but part of the reactor itself. From your own link, page 42, section 5.8 Containment systems: > Thus, there seems to be a barrier less in AGR compared with LWR reactors. In AGR's the pressure vessel and containment is one unit, but the vessel contains the total primary circuit.The main reason for this difference is the single phase CO2 coolant used in AGR compared with the H2O coolant in LWR. CO2 cannot undergo suddenly phase change as a result of an unexpected rise in temperature or pressure, i. c. it cannot flash as water. It means that there can be no sudden discontinuity of cooling under fault conditions, and changes in flows, temperatures and pressures progress rather slow.