As early as the 1970s, attempts have been made to reduce the peak fuel temperature in pebble bed-type high-temperature reactors (HTRs) by means of so-called "wallpaper fuel," in which the fuel is arranged in a spherical shell within a pebble. By raising the particle packing fraction, fuel kernels are condensed to the outer diameter of the fuel zone, leaving a central part of the pebble free of fuel. This modification prevents power generation in this central fuel-free zone and decreases the temperature gradient across the pebble.
Besides the reduction of maximum and average particle temperature, the wallpaper concept also enhances neutronic performance through improved neutron economy, resulting in reduced fissile material and/or enrichment needs or providing the potential to achieve higher burnup. To assess such improvements, calculations were performed using the PANTHERMIX code. Among other tests, investigations of fuel cycle under steady-state conditions and loss-of-coolant-accident calculations were conducted. Based on PANTHERMIX steady-state conditions, both particle failure fraction [with the CRYSTAL code (Code foR analYsis of STress in coAted particLes)] and fissile material cost can be determined.
It is demonstrated that the wallpaper fuel type positively impacts the fuel cycle, reduces the production of minor actinides (MAs), and improves the safety-relevant parameters of the reactor. A comparison of these characteristics with those of Pebble Bed Modular Reactor (Pty) Limited (PBMR) type of fuel is presented: In comparison with PBMR fuel, the wallpaper design results in an increase of the effective neutron multiplication coefficient (by [approximately]925 pcm). This reactivity increase can lead to a burnup extension (from 96.4 to 101.3 MWd/kg), therefore improving the burnup of HTRs, or to an enrichment reduction (from 9.6 to 9.277 wt%). Both options decrease MA production [as defined in g/TW(thermal)h, between 5.9% and 34.5%], making fuel reprocessing easier and reducing fuel cost (by 4.6% for the high-burnup option and by 3.7% for the low-enrichment option).
Safety is also improved, with particle temperature being reduced during steady-state operations (by >55 K for the most exposed particles and by almost 10 K on average). This positively impacts particle failure fraction as calculated by the fuel performance code CRYSTAL, leading to a reduction of up to 85% of the particle failure fraction over its in-core lifetime. This reduces the in-core fission product release.
While an increase of the graphite density in the central fuel-free zone increases thermal inertia, initiates a faster reactor shutdown, and delays recriticality, it also disturbs the thermal flux that raises pebble powers in the inner part of the core. This increases the highest kernel temperature during a depressurized loss-of-coolant accident from 1872 K for the PBMR case to 1876, 1917, and 1895 K, respectively, for the three wallpaper designs proposed.
The fuel changes suggested in this paper offer more versatility to the HTR concept. The conversion ratio can be decreased, leading to lower MA buildup and fuel reprocessing cost, or raised, leading to lower fuel consumption and fuel cost.