Comparative analysis of high conversion achievable in thorium-fueled slightly modified CANDU and PWR reactors

We study here the conversion performance of thorium-fueled standard or only slightly modified CANDU and PWR reactors with unchanged core envelope and equipments, to be eventually used as the third and last tier of symbiotic scenarios. For instance, plutonium extracted from the spent fuel of UOX PWRs could be converted in Th/Pu CANDUs to uranium (mainly 233U), finally used to feed a thorium-fueled water-cooled high converting third component. This could be a convenient way to replace likely delayed Generation IV in the case of an important increase of uranium-based energy demand. In order to assess the competitiveness of such symbiotic scenarios, detailed burnup and conversion data are obtained by means of a core-equivalent simulation methodology developed for CANDU-6 and adapted to N4-type PWR.Once-through cycles in CANDU are firstly evaluated for various Th/Pu and Th/233U fuels as regards detailed conversion and basic safety performance. Breeding in Th/233U CANDU is achieved for a 1.30 wt% homogeneous fissile enrichment and a relatively short burnup of 7 GWd/t. Small increase of enrichment (to 1.35 wt%) considerably extends cycle length (to 14 GWd/t) at the cost of slight sub-breeding. Heterogeneity of fissile load can bring another 70% gain on burnup with no significant impact on conversion. Multirecycling gives even shorter burnup (about 5 GWd/t) for the breeding case, while performance close to the once-through 1.35 wt% case is obtained for a slightly sub-breeding regime sustained by a small add of uranium from Th/Pu CANDU. Th/U cycle neutronic analysis explains the convenient feature of almost constant burnup as 233U load is unchanged at each recycle. Two symbiotic scenarios based on UOX PWRs, Th/Pu CANDUs and Th/233U CANDUs in a first open version or optimized Th/U CANDUs in a second closed version are compared.At standard power and moderation levels, Th/233U PWR conversion performance is much lower than CANDU with only a bit more than half of initial fissile load remaining after 50 GWd/t. Contrary to CANDU, fuel heterogeneity does not increase burnup. Conversion is mainly improved by enhanced sub-moderation down to minimal acceptable water over fuel volume ratio of 0.8 at standard power. In this limit case, a 3.00 wt% enrichment ensures a burnup of 33 GWd/t with 80% of initial fissile load remaining. By comparing a few Th/233U CANDU and PWR high converting cases, we understand that main part of the CANDU-PWR conversion gap results from neutron-economical CANDU operation conditions based on frequent online refueling and therefore why sub-moderation improves PWR conversion. From this better understanding, we deduce and preliminarily evaluate two possible ways to really higher conversion with thorium fuel in PWR envelope based on faster spectra either with light water and power derating or with heavy water and Spectral Shift Control.

► CANDU and PWR as thorium-fueled high converting water reactors.
► Core-equivalent simulation methodology based on deterministic and Monte Carlo codes.
► CANDU enables (near-)breeding with thorium at (acceptable) low burnup levels.
► Comparison with CANDU shows that PWR needs faster spectra for real high conversion.