Citation: NUCLEAR TECHNOLOGY vol. 181 no. 2 p. 317-330
Publication Year: 2013
JRC N°: JRC68089
ISSN: 0029-5450
URI: http://www.new.ans.org/pubs/journals/nt/a_15786
Type: Articles in periodicals and books
Abstract: As early as in the 1970s, attempts were made to reduce the peak fuel temperature in pebble bed type 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 averaged 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 burn-up. To assess such improvements, calculations were performed using the PANTHERMIX code. Among others, 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 fractions (with the CRYSTAL code) and fissile material cost can be determined. It is demonstrated that this fuel type impacts positively on the fuel cycle, reduces the production of minor actinides (MA) and improves the safety-relevant parameters of the reactor. A comparison of these characteristics with pebble fuel of PBMR characteristics is presented: By comparison with PBMR fuel, the “wallpaper” design results in an increase of the effective neutron multiplication coefficient (by about 925 pcm). This reactivity increase can lead to a burn-up extension (from 96.4 to 101.3 MWd.kg-1), therefore improving burn-up of HTRs, or to an enrichment reduction (from 9.6% wt. to 9.277% wt.). Both options decrease MA production (between 5.9% and 34.5%), making fuel reprocessing easier, and reducing fuel cost (by 4.6% for the high burn-up option and 3.7% for the low enrichment option). Safety is also improved with particle temperature being reduced during steady-state operations (by more than 55K for the most exposed ones and by almost 10K 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 (FP) release. While an increase of the graphite density in the Central Fuel Free Zone increases thermal inertia, initiates a faster reactor shut down and delays re-criticality, 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 DLOCA from 1872 K for the PBMR case to 1876 K, 1917 K and 1895 K for the three wallpaper designs proposed, respectively. The fuel changes suggested in this paper offer more versatility to the HTR concept. The conversion ratio can be decreased, leading to lower MA build-up and fuel reprocessing cost, or raised, leading to lower fuel consumption and fuel cost.
JRC Directorate:Energy, Transport and Climate

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