Modelling of a passive reactor cavity coolingsystem (RCCS) for a nuclear reactor core subject toenvironmental changes and the optimisation of theRCCS radiation heat shieldheat shield

[摘要] ENGLISH ABSTRACT: A reactor cavity cooling system (RCCS) is used in the PBMR to protect the concretecitadel surrounding the reactor from direct nuclear radiation impingement and heat. Thespeci ed maximum operating temperature of the concrete structure is 65 ±C for normaloperating conditions and 125 ±C for emergency shut-down conditions. A conceptual designof an entirely passive RCCS suitable for the PBMR was done by using closed loopthermosyphon heat pipes (CLTHPs) to remove heat from a radiation heat shield over ahorizontal distance to an annular cooling dam placed around the PBMR. The radiationshield is placed in the air space between the Reactor Pressure Vessel (RPV) and the concretecitadel, 180 mm from the concrete citadel.A theoretical heat transfer model of the RCCS was created. The theoretical modelwas used to develop a computer program to simulate the transient RCCS response duringnormal reactor operation, when the RCCS must remove the excess generated heat fromthe reactor cavity and during emergency shut-down conditions, when the RCCS must removethe decay heat from the reactor cavity. The main purpose of the theoretical modelis to predict the surface temperature of the concrete citadel for di erent heat generationmodes in the reactor core and ambient conditions.The theoretical model assumes a 1D geometry of the RCCS. Heat transfer by bothradiation and convection from the RPV to the radiation heat shield (HS) is calculated.The heat shield is modelled as an. Then e ciency was determined with the experimentalwork. Conduction through then is considered in the horizontal direction only.The concrete structure surface is heated by radiation from the outer surface of the heatshield as well as by convection heat transfer from the air between the heat shield andthe concrete structure surface. The modelling of the natural convection closed loop thermosyphonheat pipes in the RCCS is done by using the Boussinesq approximation andthe homogeneousow model. An experiment was built to verify the theoretical model. The experiment is a fullscale model of the PBMR in the horizontal, or main heat transfer, direction, but is onlya 2 m high section. The experiments showed that the convection heat transfer betweenthe RPV and the HS cannot be modelled with simple natural convection theory. A Nusseltnumber correlation developed especially for natural convection in enclosed rectanglesfound in literature was used to model the convection heat transfer. The Nusselt numberwas approximately 3 times higher than that which classic convection theory suggested.An optimisation procedure was developed where 121 di erent combinations ofn sizesand heat pipe sizes could be used to construct a RCCS once a cooling dam size was chosen.The purpose of the optimisation was tond the RCCS with the lowest total mass.A cooling dam with a diameter of 50 m was chosen. The optimal RCCS radiation heatshield that operates with the workinguid only in single phase has 243 closed loop thermosyphonheat pipes constructed from 62.72 mm ID pipes and 25 mm wideatbarns.The total mass of the single phase RCCS is 225 tons. The maximum concrete structuretemperature is 62.5 ±C under normal operating conditions, 65.8 ±C during a PLOFC emergencyshut-down condition and 80.9 ±C during a DLOFC emergency shut-down condition.In the case where one CLTHP fails and the adjacent two must compensate for the loss ofcooling capacity, the maximum concrete structure temperature for a DLOFC emergencyshut-down will be 87.4 ±C. This is 37.6 ±C below the speci ed maximum temperature of125 ±C. The RCCS design is further improved when boiling of the workinguid is inducedin the CLTHP. The optimal RCCS radiation heat shield that operates with the working uid in a liquid-vapour mixture, or two phaseow, has 338 closed loop thermosyphonheat pipes constructed from 38.1 mm ID pipes and 20 mm wideatbarns. The totalmass of the two phase RCCS is 198 tons, 27 tons less than the single phase RCCS. Themaximum concrete structure temperature is 60 ±C under normal operating conditions,2.5 ±C below that of the single phase RCCS. During a PLOFC emergency shut-downcondition, the maximum concrete structure temperature is 62.3 ±C, 3.5 ±C below that ofthe single phase RCCS and still below the normal operating temperature of the singlephase RCCS.By inducing two phaseow in the CLTHP, the maximum temperature of the working uid isxed equal to the saturation temperature of the workinguid at the vacuum pressure.This property of water is used to limit the concrete structure temperature. Thise ect is seen in the transient response of the RCCS where the concrete structure temperatureincreases until boiling of the workinguid starts and then the concrete structuretemperature becomes constant irrespective of the heat load on the RCCS. An increasedheat load increases the quality of the workinguid liquid-vapour mixture. Workinguidqualities approaching unity causes numerical instabilities in the theoretical model. Thetheoretical model cannot capture the heat transfer to a control volume with a densitylower than approximately 20 kg/m3. This limits the extent to which the two phase RCCScan be optimised.Recommendations are made relating to future work on how to improve the theoreticalmodel in particular the convection modelling in the reactor cavities as well as the twophaseow of the workinguid. Further recommendations are made on how to improvethe basic design of the heat shield as well as the cooling section of the CLTHPs.

[发布机构] Stellenbosch University