The SAR project evaluated the capability of the Ignalina NPP reactor cavity structures to withstand coolant discharges that might be encountered during accidents which involve fuel channels ruptures. The range of coolant discharge conditions from the ruptured pressure tubes into graphite moderator stack has been quantified. The consequences of various coolant discharges into the hot and rather confined reactor stack in terms of peak pressures within the reactor cavity have been evaluated. The venting capacity of the reactor cavity over-pressure protection system is expressed in terms of the number of fuel channels that can rupture simultaneously or sequentially without damaging the reactor by exceeding the 214 kPa peak pressure load on the upper lid of the reactor cavity. The capacity of the existing reactor cavity over-pressure protection system introduced at the end of 1996 is 9 ± 4 simultaneous or closely-spaced-in-time channel ruptures at full system pressure. If the above channel ruptures occur at reduced system pressures, the discharge of the coolant will be smaller, and hence capacity of the reactor cavity over-pressure protection system to relieve this discharged coolant will be higher. This capacity rises to 25 ± 12 at 4 MPa.
The range of uncertainty associated with the analysis is quite large, i.e. about 50 %. The primary reasons for this are the scenario-specific variability of the break flow, and the uncertain characteristics of the flow path through the reactor cavity. In addition, there are uncertainties concerning the deformation of the graphite stack after channel rupture, uncertainties regarding the graphite surface area that will be in contact with the discharged water and the stored heat that will evaporate this discharged water. If the stack deformation remains small, the discharge water-steam mixture flow rate will be rather “isotropic” and evaporation could be almost complete, i.e. 100 %. If the stack deformation creates vertical free “channels”, the discharged water will be quickly forced out of the graphite stack and additional evaporation will be small. Therefore the range of the amount of steam generated from the discharge into reactor cavity is quite wide - from about 30 % to almost 100 %. Improved analytical methods which might decrease the noted uncertainties are not available in the short term. Since the consequences of the multiple pressure tube rupture can be catastrophic, it is necessary to continue investigations related to this issue in order to better understand physical phenomena which can lead to multiple pressure tube ruptures and to develop more accurate prediction methods for the reactor cavity over-pressure issue.