Thermal Hydraulic Analysis of Sub-channel Blockage Accident Using RELAP5/MOD3.2 in a Pool Type Research Reactor

Thermal–hydraulic analysis was carried out to assess the consequences of full and partial blockage of narrow rectangular sub-channels in a fuel assembly of a pool type nuclear research reactor using RELAP5/MOD3.2 best estimate code. The highest rated fuel assembly considered in the analyses is based on 40 MW High Flux Research Reactor (HFRR) under development at BARC. The study involves modelling of the fuel assembly (SFA) in RELAP5 code using heat structures and hydrodynamic components along with implementation of axial and local nuclear peaking factors. Blockage of the sub-channel in vicinity of the hottest plate was simulated for two cases separately (i) at the inlet—i.e., at the bottom location of the fuel channel and (ii) at the outlet—i.e., at the top location of the fuel channel. For the assessment of Departure from Nucleate Boiling Ratio (DNBR), correlation proposed by Sudo and Kaminaga was implemented in RELAP5 which could accurately predict DNBR in rectangular sub-channels heated by fuel plates under relatively low pressure and low temperature conditions as prevalent in research reactors. Transient analysis was carried out to estimate the fuel temperature and DNBR for different extent of blockages and also with respect to the location of blockage i.e. at the inlet and outlet of the affected sub-channel. Critical blockage, represented in terms of % of sub-channel flow area, was assessed beyond which the safety criteria could not be met during the blockage transient. Results showed that DNBR acceptance criteria are met when the sub-channel blockage is less than 80% of the flow area at the inlet location. For the sub-channel blockage at the exit location, minimum DNBR criterion was met till the blockage reaches 95% of the flow area. It was also found that for 80% sub-channel blockage at the inlet, the fuel and clad temperatures remained well below the safety limits. Moreover, no voiding in the sub-channel was encountered during the blockage transient up to 80%.