Ocean-Based Passive Decay Heat Removal in the Offshore Floating Nuclear Plant

Abstract

The Offshore Floating Nuclear Plant (OFNP) concept offers the potential for superior economics and safety. The 300 MWe version of the OFNP features an integral primary system PWR, and adopts an ocean-based direct reactor auxiliary cooling system (DRACS) which provides passive and indefinite decay-heat removal from the reactor pressure vessel during abnormal occurrences without primary system depressurization, e.g. a loss of flow accident. In this paper we present analyses aimed at sizing and evaluating the performance of the DRACS loops and heat exchangers, to ensure adequate core cooling and a compact layout. We assume that all the residual heat is removed by the DRACS for the condition that the reactor coolant pumps have stopped and thus the system operates in natural circulation mode. The DRACS effectively consists of three coupled flow loops: first, natural circulation in the primary system from the core to the core makeup tank (CMT); second, natural circulation from the CMT to the ultimate heat exchanger; third, natural convection of seawater in the shell of the ultimate heat exchanger. The asymptotic (quasi-steady) behavior of these loops was first modeled using hand calculations, which allowed estimation of the size of the heat exchangers. Then a transient analysis of this preliminary design was performed with the code RELAP5, to confirm that the principal safety margins (margin to boiling, MDNBR, and maximum allowable reactor coolant pressure) are not challenged.