Hirokazu Ohta

Pyroprocessing of Light Water Reactor Spent Fuels Based on an Electrochemical Reduction Technology

A concept of pyroprocessing light water reactor (LWR) spent fuels based on an electrochemical reduction technology is proposed, and the material balance of the processing of mixed oxide (MOX) or high-burnup uranium oxide (UO2) spent fuel is evaluated. Furthermore, a burnup analysis for metal fuel fast breeder reactors (FBRs) is conducted on low-decontamination materials recovered by pyroprocessing. In the case of processing MOX spent fuel (40 GWd/t), UO2 is separately collected for ~60 wt% of the spent fuel in advance of the electrochemical reduction step, and the product recovered through the rare earth (RE) removal step, which has the composition uranium:plutonium:minor actinides:fission products (FPs) = 76.4:18.4:1.7:3.5, can be applied as an ingredient of FBR metal fuel without a further decontamination process. On the other hand, the electroreduced alloy of high-burnup UO2 spent fuel (48 GWd/t) requires further decontamination of residual FPs by an additional process such as electrorefining even if RE FPs are removed from the alloy because the recovered plutonium (Pu) is accompanied by almost the same amount of FPs in addition to RE. However, the amount of treated materials in the electrorefining step is reduced to ~10 wt% of the total spent fuel owing to the prior UO2 recovery step. These results reveal that the application of electrochemical reduction technology to LWR spent oxide fuel is a promising concept for providing FBR metal fuel by a rationalized process.

Low-Burnup Irradiation Behavior of Fast Reactor Metal Fuels Containing Minor Actinides

Abstract Fast reactor metal fuels containing minor actinides (MAs) Np, Am, and Cm and/or rare earths (REs) have been irradiated in the fast reactor PHÉNIX to examine the effects of adding those elements on metal fuel irradiation behavior. In this experiment, two MA-containing metal fuel pins, in which the test alloys U-19Pu-10Zr-2MA-2RE and U-19Pu-10Zr-5MA/U-19Pu-10Zr-5MA-5RE (wt%) were loaded into part of a standard U-19Pu-10Zr alloy fuel stack, and a reference fuel pin of U-19Pu-10Zr alloy without MAs or REs was set in an irradiation capsule. Two other capsules with this same configuration are also irradiated. Postirradiation examinations are conducted at ~2.5, ~7, and ~11 at.% burnup. For the low-burnup fuel pins, nondestructive tests after irradiation have been performed, and the integrity of the pins was confirmed. The irradiation behavior of MA-containing metal fuels up to 2.5 at.% burnup was analyzed using the ALFUS code. The calculation results, such as the axial swelling distribution of a fuel slug or the extrusion behavior of bond sodium to the gas plenum, are consistent with the measurement data regardless of the addition of MAs and REs to the U-Pu-Zr alloy fuels. This observation result indicates that the macroscopic irradiation behavior of U-Pu-Zr fuels containing MAs and REs of 5 wt% or less is similar to that of U-Pu-Zr fuels up to ~2.5 at.% burnup.

Development of Fast Reactor Metal Fuels Containing Minor Actinides

Fast reactor metal fuels containing minor actinides (MAs) Np, Am, and Cm and rare earths (REs) Y, Nd, Ce, and Gd are being developed by the Central Research Institute of Electric Power Industry (CRIEPI) in collaboration with the Institute for Transuranium Elements (ITU) in the METAPHIX project. The basic properties of U-Pu-Zr alloys containing MA (and RE) were characterized by performing ex-reactor experiments. On the basis of the results, test fuel pins including U-Pu-Zr-MA(-RE) alloy ingots in parts of the fuel stack were fabricated and irradiated up to a maximum burnup of ∼10 at% in the Phénix fast reactor (France). Nondestructive postirradiation tests confirmed that no significant damage to the fuel pins occurred. At present, detailed destructive postirradiation examinations are being carried out at ITU.

A Design Study on the FBR Metal Fuel and Core for Commercial Applications

A design methodology for the FBR metal fuel and core is developed. It consists of the fuel integrity prediction by a mechanistic analysis code, and the core neutronic/thermal-hydraulic design in which the effect of the characteristic fuel behavior and fuel cycle technologies are properly considered. Based upon this methodology, a fuel and core design study for reactors of various output scale (150–1500 MWe) is conducted, and the feasibility of metal fuel FBRs for the future commercial applications is reviewed. The results show that the averaged burnup of as high as 90–150 MWd/t is achievable at the refueling interval of 1–2 years with 3–4 batches, regardless of the output scale. The maximum allowable cladding temperature is assumed to be 650°C to avoid the liquid phase formation, which leads to the achievable core outlet temperature of ∼510°C based on the current hotspot factors estimation. It is found that high breeding ratio of ≥1.2 can be enabled with relatively small blanket amount, owing to the very high internal conversion capability. Another advantage is that it is possible to significantly reduce the burnup reactivity loss at the slight expense of the burnup, in which case the core excess reactivity becomes as small as a few dollars.

Irradiation of Minor Actinide–Bearing Uranium-Plutonium-Zirconium Alloys up to ˜2.5 at. %, ˜7 at. %, and ˜10 at. % Burnups

An irradiation experiment on minor actinide (MA)-bearing uranium-plutonium-zirconium (U-Pu-Zr) alloys, in which contamination by rare earth (RE) elements was considered, was performed up to ˜2.5 at. %, ˜7 at. %, and ˜10 at. % burnups in the Phenix fast reactor. All the irradiated metal fuel pins were subjected to nondestructive tests such as cladding profilometry and gamma spectroscopy. Then, cross-sectional metallography of the low-burnup and medium-burnup fuel alloys was performed, and the redistribution of the fuel matrix constituents”U, Pu, and Zr”in the low-burnup fuels was analyzed by energy dispersive X-ray spectroscopy. As a result, the irradiation growth of MA-rich and RE-rich precipitates was observed by comparing the low-burnup and medium-burnup fuels. From the postirradiation examinations carried out so far, it was confirmed that the irradiation swelling, the cross-sectional structures, and the migration of matrix constituent in metal fuels containing 5 wt% or less MAs and REs are almost the same as those in conventional U-Pu-Zr fuels.

Concept of capture credit based on neutron-induced gamma ray spectroscopy

Capture credit (CapC) based on neutron-induced gamma ray spectroscopy (NIGS) is proposed to confirm the subcriticality of fuel debris in which nuclear fuel and structural materials are co-melted or mixed. By NIGS, rates of some capture reactions can be measured in relation to fission reactions. By the ratio, we can credit the negative reactivity inserted by the capture reactions. The theory of CapC is described. In order to demonstrate the benefit to take CapC for storage of the fuel debris, numerical simulations are performed for a hypothetical array of canisters in which the fuel debris is stored. A procedure of CapC based on NIGS is also proposed, which consists of several technologies: (1) NIGS, (2) simulations of a response and an efficiency of the γ ray detection, and (3) unfolding of the γ ray pulse height spectrum to obtain reaction rates. Experimental studies of NIGS have been launched in Kyoto university critical assembly facility. NIGS is firstly studied for simulated fuel debris of a few kinds of mixture of stainless steel and uranium in subcritical systems. The measured γ ray pulse height spectra and preliminary analyses indicate that CapC based on NIGS is worth to be investigated further for the efficient storage of fuel debris.

Design Study on a Fast Reactor Metal Fuel Attaining Very High Burnup

Abstract Innovative fuel design measures to attain a much higher burnup than that obtained using the conventional concept were investigated for a fast reactor (FR) metal fuel. Considering the typical mechanism of metal fuel degradation, three innovative design measures were proposed: (a) a decrease in plenum pressure by adopting the fission gas vent design, (b) prevention of fuel-cladding chemical interaction by lining the cladding inner wall, and (c) mitigation of fuel-cladding mechanical interaction by reducing the fuel smear density. The effects of these design measures on increasing the burnup were analyzed with ALFUS, an irradiation behavior analysis code for FR metal fuels. The ALFUS analysis revealed that a very high burnup of >40 at. % can be attained under the conventional design criteria for securing fuel integrity by applying these innovative measures. Neutronic analysis of a metal fuel core employing these design measures indicated that a high burnup of >40 at. % at the assembly peak can be attained while suppressing the burnup reactivity swing to almost the same level as that of conventional cores with normal burnup through the use of a minor actinide–containing fuel.

ULOF and UTOP Analyses of a Large Metal Fuel FBR Core Using a Detailed Calculation System

ULOF and UTOP analyses of a large metal fuel FBR core (1,500 MWe, averaged discharge burnup: 150 GWd/t) are conducted. The effect of core radial expansion is considered as the major negative feedback during the transient. A detailed analysis system is used, in which a transient core thermal-hydraulic code is coupled with three dimensional core radial deformation and reactivity feedback calculation codes, in order to calculate the radial expansion feedback. In ULOF analysis, the pump flow halving time is assumed to be 10 s, which is reasonably long and effective in avoiding too large power to flow ratio. The reactivity insertion during UTOP is set to be 34¢, based on the control rod reactivity design. As the analysis results, it is found that the core shows benign responses to both events, owing largely to the radial expansion feedback. No significant coolant boiling or fuel failure is predicted. The response during ULOF is compared to that of an oxide fuel core of similar design, and it is confirmed that the negative Doppler effect associated with the fuel temperature rise plays the major role in the quick power decrease.

Minor actinide transmutation in fast reactor metal fuels irradiated for 120 and 360 equivalent full-power days

An irradiation experiment on uranium–plutonium–zirconium (U–Pu–Zr) alloys containing 5 wt% or less minor actinides (MAs) and rare earths was carried out in the Phénix fast reactor. The isotope compositions of the fuel alloys irradiated for 120 and 360 equivalent full-power days (EFPDs) were chemically analyzed by inductively coupled plasma–mass spectrometry after 3.3–5.3 years of cooling. The results of chemical analysis indicated that the discharged burnups of the fuel alloys irradiated for 120 and 360 EFPDs were 2.1–2.5 and 5.3–6.4 at%, respectively. The changes in the isotopic abundances of plutonium, americium, and curium during the irradiation experiment were assessed to discuss the transmutation performance of MA nuclides added to U–Pu–Zr alloy fuel. Multigroup three-dimensional diffusion and burnup calculations accurately predicted the changes in these isotopic abundances after fuel fabrication. An evaluation of the MA transmutation ratio based on the results of chemical analysis revealed that the quantity of MA elements in the U–19Pu–10Zr–5MA (wt%) alloy decreased by about 20% during the irradiation experiment for 360 EFPDs.

Analytical Evaluation of Core Bowing Reactivity in the 4S Reactor

The 4S (super-safe, small and simple) reactor is a sodium-cooled small fast reactor. The core reactivity is controlled by moving the reflectors installed around the core, and the reactor has a fixed absorber at the core center to accomplish a long core lifetime. To evaluate core bowing behavior and the resulting reactivity feedback in the 4S reactor, an analytical evaluation was conducted under various core power to flow ratios (P/F). The core bowing reactivity under the BOC (beginning of core life) condition becomes increasingly negative with increasing P/F up to 2.0, then becomes less negative with increasing P/F from 2.0 to 3.0, and finally becomes positive at P/F = 3.0. The bowing reactivity under the EOC (end of core life) condition becomes increasingly negative with increasing P/F up to 1.5, then becomes less negative then positive with increasing P/F from 1.5 to 3.0; the core bowing reactivity is positive when P/F ≥ 2.0. These results are mainly caused by the following two mechanisms originating from the structural characteristics of the 4S reactor: - a decrease in neutron absorption by the fixed absorber due to the radial displacement of the inner core subassemblies (under the BOC condition); - a decrease in neutron streaming caused by the small gaps between the outer core subassemblies and the reflectors due to core radial expansion (under the EOC condition).Copyright