A comprehensive Thermo-hydraulic neutronic and safety analysis of a 100MWth pebble bed reactor core

Abstract

The HTMR100 is a 100MWth High Temperature Gas-cooled Reactor (HTGR), which is helium-cooled and graphite moderated. This reactor is of the Pebble Bed type featuring a Once-Through-Then-Out (OTTO) fuelling scheme, this cycle was chosen for the simplicity of the fuel handling system.The HTMR100 Pebble Bed Reactor (PBR) has a core volume of 27.73 m3 (core diameter 2.6 m, core height ∼ 5.2 m) which results in an average core power density of 3.6 MW/m3. This reactor utilizes uranium dioxide fuel (UO2) at a U-235 enrichment of 10 wt% and 10 g of heavy metal (total uranium) per Fuel Sphere (FS). Although there are various types of fuel options that can be utilized in the HTMR100 reactor, ranging from LEU to mixtures of Th/LEU, Th/HEU and Th/Pu, this study will only focus on LEU in the form of UO2 fuel.This study investigates the neutronic and thermal–hydraulic modelling of the 100MWth HTMR100 at equilibrium (100% power with the control rods situated at the nominal position for a core having an equilibrium fuel burnup profile) and at transient/accident conditions. The purpose of the investigation is to determine if the maximum fuel temperature remains below the set limit of 1600 °C during a design-base accident event that is defined as a Loss of Coolant Accident (LOCA).The equilibrium core was simulated using the Very Superior Old Programs (VSOP99) suite of codes to obtain burnup results. The transient behaviour of the core was modelled by a code called Multi-group Time Dependent Neutronics and Temperatures (MGT). MGT assesses the dynamic transient results of the accident scenarios such as the Depressurized Loss of Forced Cooling (DLOFC), Pressurized Loss of Forced Cooling (PLOFC), Control Rod Withdrawal (CRW) and a Control Rod Ejection (CRE) as well as normal operating transients of which a Load Following (LF) operation is a typical example.The HTMR100 reactor utilizing the enrichment and heavy metal loading as specified does indeed produce the targeted 80 000 MWD/THM burnup for the OTTO fuel cycle. The study also proves that the VSOP99 and the MGT codes do in fact yield similar results for the HTMR100 reactor with regards to fuel centreline temperatures, outer sphere surface temperatures as well as moderator temperatures for the postulated accident scenarios that were analysed. The results also indicate that the design-base transient safety analysis simulations prove that the fuel temperatures remain below the set value of 1600 °C for the oxide-based fuel. A temperature-volume analysis of a DLOFC design-base event shows that only ∼ 2 % of the fuel reaches the higher temperatures of 1500 °C to 1600 °C in a DLOFC event.The beyond-design base events with a very low probability of occurrence do however exceed the set value of 1600 °C but the fuel only exceeds this set limit for short periods of time while the transient occurs. The probability that this will lead to fuel damage should be very small since the time at these high temperatures is short and the volume fraction of the core exposed to these high temperatures is small.