Hiroshi Akie

A new fuel material for once-through weapons plutonium burning

AbstractFor the burning of plutonium derived from nuclear warheads, once-through type oxide fuels have been studied by considering their proliferation resistance and environmental safety as well as their technological backgrounds of fuel fabrication and reactors.From phase relations of ceramic materials and their chemical properties, it seems that a two-phase mixture of a fluorite-type phase and alumina has favorable characteristics as a once-through-type fuel of plutonium burning. It also seems that the fluorite-type phases such as thoria and fully stabilized zirconia are acceptable as host phases of plutonium because of high solid solubility of the actinide elements and fission products, irradiation stability, and chemical stability. The spent fuels finally obtained will become mineral-like waste forms, which could be buried under deep geological formations without further processing.From reactor burnup calculations with the use of the fuels, light water reactors (LWRs) with the larger volume ratio of m...

Rock-Like Oxide Fuels and Their Burning in LWRs

Research on the plutonium rock-like oxide (ROX) fuels and their once-through burning in light water reactors has been performed to establish an option for utilizing and disposing effectively the excess plutonium. The ROX fuel is a sort of the inert matrix fuels and consists of mineral-like compounds such as yttria stabilized zirconia, spinel and corundum. A particle-dispersed fuel was devised to reduce damage by heavy fission fragments. Some preliminary results on swelling, fractional gas release and microstructure change for five ROX fuels were obtained from the irradiation test and successive post-irradiation examinations. Inherent disadvantages of the Pu-ROX fuel cores could be improved by adding 238U or 232Th as resonant materials, and all improved cores showed a nearly the same characteristics as the conventional UO2 core during transient conditions. The threshold enthalpy of the ROX fuel rod failure was found to be comparable to the fresh UO2 rod by pulse-irradiation tests simulating reactivity initiated accident conditions.

Core design study on rock-like oxide fuel light water reactor and improvements of core characteristics

A rock-like oxide (ROX) fuel - LWR burning system has been studied for efficient plutonium transmutation. A zirconia based ROX (Zr-ROX) core has problems such as a small negative Doppler coefficient and a large power peaking factor, which causes severe transients in accidents and high fuel temperature even under nominal condition. For the improvement of these characteristics, two approaches were considered: the additives UO 2 , ThO 2 and Er 2 O 3 , or a heterogeneous core with Zr-ROX and UO 2 assemblies. As a result, the combination of the additives UO 2 and Er 2 O 3 is found to sufficiently improve the accident behavior, while a further power peaking reduction may be necessary for the Zr-ROX + UO 2 heterogeneous core. The plutonium transmutation rate is extremely high in Zr-ROX assemblies in the heterogeneous core, to be more than 85% and 70%, respectively for weapons- and reactor-grade plutonium. The plutonium transmutation rate becomes smaller in the full-ROX core with the UO 2 or ThO 2 additive, but the annual transmutation amount of plutonium is large, in comparison with the full-MOX fuel core.

Irradiation behavior of rock-like oxide fuels

Two irradiation tests were performed on the rock-like oxide (ROX) fuels in order to clarify in-pile irradiation stabilities. In the first test small disk-shape fuel targets were irradiated in the JRR-3 in JAERI. In the second test pellet-type fuels were employed. Irradiation behaviors such as swelling, fractional fission gas release (FGR) and phase change were examined by puncture test, profilometry and ceramography. Swelling and FGR behavior of the pellet-type fuels improved considerably compared with the disk-type fuels. Yttria stabilized zirconia (YSZ) single-phase fuel showed an excellent irradiation behavior, i.e. low FGR (<3%), negligible swelling and no appreciable restructuring. The particle dispersed fuels showed lower swelling and higher FGR than those of mechanically blended fuels. Spinel decomposition and subsequence restructuring in the spinel matrix fuels was observed for the first time in the present investigation. It would be possible to reduce the FGR of the spinel matrix fuels to that of the corundum ones, if the maximum fuels temperature is limited below 1700 K where neither spinel decomposition nor restructuring was observed. Damaged area of spinel matrix due to fission fragment irradiation seemed to be confined to thin layers around the surface of YSZ particles as expected.

A new idea of excess plutonium once-through burning in light water reactor

Abstract Once-through burning process has been proposed for the disposition of excess plutonium. A new stable fuel material of multi-phases is fabricated based on conventional MOX fuel technologies. After irradiation in LWR, the spent fuels would be geologically stable, and become high level radioactive waste (HLW) without further processing. From the chemical properties and crystal structures, two oxide systems have been proposed: PuO 2 -ThO 2 - Al 2 O 3 -MgO and PuO 2 -ZrO 2 (Y,GD)-Al 2 O 3 -MgO systems. The experimental study has been made to examine the phase relations of the fuel materials and the distribution of fission products. From the burnup calculation study, it was estimated more than 80% of plutonium is transmuted after irradiation and the quality of plutonium becomes very poor. For the zirconia type fuel (PuO 2 ZrO 2 (Y,Gd)-Al 2 O 3 ), the void and Doppler reactivities were calculated to be very small. The two modified fuel systems are therefore proposed to avoid these small reactivities, i.e. the zirconia-thoria fuel system (PuO 2 -(Zr,Th)O 2 -Al 2 O 3 -MgO) and zirconia fuel system with W or Yb additive (PuO 2 -ZrO 2 (Yb)-Al 2 O 3 -MgO-(W).

Current status of researches on the plutonium rock-like oxide fuel and its burning in light water reactors

Abstract Intention of the ROX-LWR system research is to provide an option for utilization or disposition of surplus plutonium. Researches on inert matrix materials and irradiation performance shows that the most favorable candidate for the ROX fuel is a particle dispersed fuel where small particles consisted of yttria stabilized zirconia, PuO 2 and some additives are homogeneously dispersed in spinel matrix. Reactor safety analyses show that the ROX fueled PWR core has nearly the same performability as the existing UO 2 fueled PWR under both reactivity initiated accidents and loss of coolant accidents.

Radiotoxicity hazard of inert matrix fuels after burning minor actinides in light water reactors

Abstract Ingestion radiotoxicity hazard index of inert matrix spent fuels are investigated after burning minor actinide (MA) isotopes in LWRs and compared with the hazard index of MOX and MA burning MOX (MOX+MA) spent fuels. As U-free fuels, ROX: (PuO 2 +ZrO 2 ) and TOX: (PuO 2 +ThO 2 ), are considered, in which MAs are added as oxides. The radiotoxicity hazard index of ROX+MA spent fuel is less than that of TOX+MA and MOX+MA spent fuels due to the lower density of actinides in spent fuel. Some of cooling years the toxic yield of ROX+MA spent fuel is even less than that of MOX spent fuel, if the initial loaded MA in ROX is about 0.5 at %.

COMPARISONS OF CELL CALCULATIONS FOR URANIUM-FREE LIGHT WATER REACTOR FUELS

An effective way to reduce the large quantities of Pu currently accumulated worldwide would be to use uranium-free fuel in light water reactors (LWRs) so that no new Pu is produced. Such a possibility could be provided by an LWR fuel consisting of Pu in a neutronically inert matrix. It may be necessary to add a burnable absorber or thorium to reduce the reactivity swing during burnup. The methods and data currently used for LWR analyses have not been tested in conjunction with such exotic fuel materials. An international exercise has accordingly been launched to compare the relative performance of different code systems and the accuracy of the basic data. Comparison of the results of cell calculations done with fixed isotopic densities against reference Monte Carlo results shows fairly small but systematic differences in the multiplication factors. A sensitivity analysis done with different basic cross section libraries and the same code system allows one to distinguish between the effects of the codes and those of the databases. The results of the burnup calculations indicate a fair agreement in k∞ both at beginning of life (BOL) and after 1200 days of irradiation [end of life (EOL)] under conditions representative of a present-day pressurized water reactor. At BOL, the fuel temperature coefficients agree fairly well among the different contributions, but unacceptably large differences are observed at EOL. The void coefficients agree well for low voidage, but for void fractions >90%, there are significant effects mostly due to the databases used. The agreement in the calculated boron worths is good.

Core burnup calculation and accidents analyses of a pressurized water reactor partially loaded with rock-like oxide fuel

Abstract A rock-like oxide (ROX) fuel – light water reactor (LWR) burning system has been studied for efficient plutonium transmutation. For the improvement of small negative reactivity coefficients and severe transient behaviors of ROX fueled LWRs, a partial loading core of ROX fuel assemblies with conventional UO 2 assemblies was considered. As a result, although the reactivity coefficients could be improved, the power peaking tends to be large in this heterogeneous core configuration. The reactivity initiated accident (RIA) and loss of coolant accident (LOCA) behaviors were not sufficiently improved. In order to reduce the power peaking, the fuel composition and the assembly design of the ROX fuel were modified. Firstly, erbium burnable poison was added as Er 2 O 3 in the ROX fuel to reduce the burnup reactivity swing. Then pin-by-pin Pu enrichment and Er content distributions within the ROX fuel assembly were considered. In addition, the Er content distribution was also considered in the axial direction of the ROX fuel pin. With these modifications, a power peaking factor even lower than the one in a conventional UO 2 fueled core can be obtained. The RIA and LOCA analyses of the modified core have also shown the comparable transient behaviors of ROX partial loading core to those of the UO 2 core.

A concept of nitride fuel actinide recycle system based on pyrochemical reprocessing

Abstract A fuel cycle system coupled with nitride fuel fast reactors and a pyrochemical reprocessing has been investigated in order to establish an actinide transmutation recycle system with long-lived radioactive nuclides. Core performance of the nitride fuel fast reactor can provide design flexibility of excellent safety characteristics and a new concept of core composed with Na- and He- bonded fuel assemblies is proposed. The effect of 15 N enrichment on nuclear characteristics and the evaluation of toxicity of 14 C generated from 14 N are appeared, and futhermore, excellent performance for the minor actinide (MA) transmutation is shown. The study of the pyrochemical process shows that the actinides are reasonably separated from fission products in the nitride spent fuels, and that the high level wastes are of nearly actinide-free form.