Study On Hydraulic Characteristics Of A SCWR Nuclear Energy System Conceptual Design

Under supercritical pressure, thermo-physical properties of waterexperience abrupt variation around the pseudo-critical point, whichcouples nonlinearly with heat transfer and flow and makes heat transferand flow features rather complex. At present, there is still little study onstatic flow characteristics of supercritical water circulation loops (forboth natural and forced circulation), therefore a systematicunderstanding of hydraulic characteristics of the loop has not yetreached. By simplifying typical natural and forced circulation loop undersupercritical pressure, a unified one-dimensional steady-statethermal-hydraulic model is established, with equations of hydrauliccharacteristics of the loop system derived. Considering the stronglynonlinear characteristics of equations under supercritical loop system,appropriate nonlinear DERPAR numerical algorithm based oncontinuation concept is applied to develop a hydraulic analysis code forsupercritical water loop. With the code programmed, several existing supercritical water loop are taken advantage to verify the code. It isdemonstrated through the validation that the code works well in analysiswith acceptable accuracy.Furthermore, considering a typical conceptual design of a UnitedStates Super Critical Water Reactor (US-SCWR), simplified andequivalent loops of its Reactor Coolant System (RCS) and PassiveResidual Heat Removal System (PRHRS) are identified. Complicatedhydraulic features and heat transmission/removal capabilities of boththe RCS and PRHRS of the US-SCWR are analyzed using the codedeveloped. Detailed calculation and discussion reveal thenon-monotonic relationship between the RCS/PRHRS flow and the coreoperation/decay power, as well as its mechanism. Furtherly, influenceof such factors as core inlet temperature, power shape, pumpcharacteristics, system height and hydraulic resistances, etc. on systemstatic flow characteristics are demonstrated and discussed.It is demonstrated through analysis that system flowrate of the mainloop experiences a rather flat change (increases slowly) with core powershifting higher, then reaches a maximum followed by a substantial drop.The nominal operation point of US-SCWR locates within a slightlyadverse region in view of hydraulic characteristics. Thus it is suggestedthat some operation margin should be considered in the conceptual design. It is also observed that an over-decreasing of system flowratemight be encountered with the increasing of core inlet temperature, andthus over-high inlet temperature should be avoided through startup ofprotection system. Influences of main pump features on hydraulics andheat transmission of RCS of US-SCWR are also observed: in situation ofrelatively high flowrate, it is beneficial to RCS system heat transmissionwith increasing of main pump’s effective power and decreasing of itssteepness of the flowrate-lift curve.Compared with that of the main pump driven forced circulation inRCS, hydraulic curve of the natural circulating PRHRS seems evenmore “uneven”. When the residual heat power rises so that coolant at theexit of the core is almost of pseudo-critical, PRHRS flowrate reaches itsmaximum. However, the actual operation points is far away from theadverse heat transmission region, which implies that the SCWR systemis safe and within favorable region for residual heat removal. Moreover,it is found that there is certain degree of self-adjusting and adaptivecapacity for the system when the core inlet temperature varies. Furtherly,significantly elevating PRHR heat exchanger seems to have ratherlimited effect on the enhancement of PRHRS residual heat removalcapacity.