Conceptual design of a passively safe thorium breeder Pebble Bed Reactor

Abstract

More sustainable nuclear power generation might be achieved by combining the passive safety and high temperature applications of the Pebble Bed Reactor (PBR) design with the resource availability and favourable waste characteristics of the thorium fuel cycle. It has already been known that breeding can be achieved with the thorium fuel cycle inside a Pebble Bed Reactor if reprocessing is performed. This is also demonstrated in this work for a cylindrical core with a central driver zone, with 3 g heavy metal pebbles for enhanced fission, surrounded by a breeder zone containing 30 g thorium pebbles, for enhanced conversion. The main question of the present work is whether it is also possible to combine passive safety and breeding, within a practical operating regime, inside a thorium Pebble Bed Reactor. Therefore, the influence of several fuel design, core design and operational parameters upon the conversion ratio and passive safety is evaluated. A Depressurized Loss of Forced Cooling (DLOFC) is considered the worst safety scenario that can occur within a PBR. So, the response to a DLOFC with and without scram is evaluated for several breeder PBR designs using a coupled DALTON/THERMIX code scheme. With scram it is purely a heat transfer problem (THERMIX) demonstrating the decay heat removal capability of the design. In case control rods cannot be inserted, the temperature feedback of the core should also be able to counterbalance the reactivity insertion by the decaying xenon without fuel temperatures exceeding 1600°C. Results show that high conversion ratios (CR > 0.96) and passive safety can be combined in a thorium PBR within a practical operating regime, which means a thermal power of 100 MW or higher, 1000 days total residence time of the breeder pebbles and fuel pebble handling times longer than 14.5 s, like in the HTR-PM. With an increased U-233 content of the fresh driver pebbles (18 w%), breeding (CR = 1.0135) can already be achieved for a 220 cm core and 80 cm driver zone radius. While the decay heat removal is sufficient in this design, the temperature feedback of the undermoderated driver pebbles is too weak to compensate the reactivity insertion due to the xenon decay during a DLOFC without scram. With a lower U-233 content per driver pebble (10 w%) it was found possible to combine breeding (CR = 1.0036) and passive safety for a 300 cm core and 100 cm driver zone radius, but this does require more than a doubling of the pebble handling speed and a high reprocessing rate of the fuel pebbles. The maximum fuel temperature during a DLOFC without scram was simulated to be 1481°C for this design, still quite a bit below the TRISO failure temperature. The maximum reactivity insertion due to an ingress of water vapour is also limited with a value of +1497 pcm.

Analysis of the running-in phase of a Passively Safe Thorium Breeder Pebble Bed Reactor

“…The system's mass balance shows a higher extraction rate of U-233 (and Pa-233) than the insertion rate for the equilibrium core. A more detailed description of the equilibrium core calculation scheme is given by Wols et al (2014aWols et al ( , 2015. Furthermore, the system was also shown to combine breeding with passive safety, as fuel temperatures were shown to remain below 1600°C during a DLOFC without scram and water ingress only causes a relatively small reactivity increase (+1497 pcm), which can be compensated by the temperature feedback only (Wols et al, 2015).…”

“…The running-in phase of a 100 MW th Passively Safe Thorium Breeder Pebble Bed Reactor (PBR) is investigated in the present work. The conceptual design of such a reactor was introduced in a previous work by the authors (Wols et al, 2015). The design combines inherent safety, a high outlet temperature, reduced lifetime of the radiotoxic waste and an enlarged resource availability.…”

“…However, a high fuel pebble handling speed and fuel reprocessing rate is required. During previous design studies by the authors (Wols et al, 2015) the equilibrium core composition was determined.…”

“…The calculation scheme previously applied by Wols et al (2015) for the design of the reactor only provides the option to calculate the equilibrium core composition directly. This code scheme could be extended to a full time dependent version, but the calculations would become very time consuming.…”

Preliminary Neutronic Design of High Burnup OTTO Cycle Pebble Bed Reactor

“…Research activities have been conducted on improving the utilization of fissile materials in pebble-bed reactors by adding burnable poisons [6] or combining the thorium-based fuel [7]. Comparison between the OTTO cycle and the reference multi-passes cycle shows that their core performances are comparable but the fuel burnup of the OTTO cycle is about 21-22% lower than that of the multi-passes cycle [8].…”