Transmutation of long-lived fission products with use of available excess neutrons in BWR core

Feasibility of transmutation of long-lived Fps in BWR is studied from the view point of neuron balance. Pointing out the equilibrium mass amount of LLFP such as ¹²⁹I, ⁹⁹Tc, and ¹²⁶Sn in BWR core, the neutron balance analysis shows that the available neutrons produced by the current BWR core modification is satisfactory to the required amount of neutrons for LLFP transmutation. Core performance of the LLFP-recycled BWR is then analyzed and the result shows satisfactory results for the present core design criteria.

The results confirm the potential of the transmutation of LLFP in BWR and that the eventual radioactive hazard of the radwaste coming form the LLFP recycled BWR can be reduced well below the hazard level of the burned uranium.     

Evaluation of recycle P&T treatment by using BWR to perform geologic disposal

Geologic disposal scenario combined with multiple-recycle P&T (partitioning and transmutation) treatment of MA (minor actinide) was thought to have an important potential merit to carry out the geologic disposal. P&T treatment was thought to have a role to avoid an uncertainty caused by geologic behavior, such as underground water migration rate, reductive and inorganic environment in a super-long time. Partitioning and grouping of LLRN (long-lived radionuclide), i.e. MA and LLFP (long-lived fission product), has an essential role to the transmutation treatment. B/T (burning and/or transmutation) treatment of MA, with R&P (reprocessing and partitioning) process, could be recycled and SLFP (short-lived fission product) was removed to immobilize in GSC (glass solidified canister). If R&P has a high performance, the multi-recycle system which combines B T-BWR (B/T boiling water reactor) and R&P can transmute of U&Pu, MA and LLFP etc. with small inventory. B/T fraction of MA is high in case of multi-recycle system, and then the remain of mass of MA or LLFP is recycled many times with low inventory. The B/T fraction of MA or LLFP could be high, if the flux is high, i.e. the time needed for high B/T fraction is relatively short in case of high flux BWR, or long in case of normal BWR. The optimum period for discharge of B/T fuel could be determined by the net difference between the reduced mass of MA burned and/or transmuted in the B/T fuel and the accumulated mass of MA in the normal fuel, and with the additional mass of MA accumulated by U&Pu, which was unrecovered in the reprocessing existed in B/T fuel.     

Evaluation on transmutation performance of minor actinides with high-flux BWR

The performance of high-flux BWR (HFBWR) for burning and/or transmutation (B/T) treatment of minor actinides (MA) and long-lived fission products (LLFP) was discussed herein for estimating an advanced waste disposal with partitioning and transmutation (P&T). The concept of high-flux B/T reactor was based on a current 33 GWt-BWR, to transmute the mass of long-lived transuranium (TRU) to short-lived fission products (SLFP). The nuclide selected for B/T treatment was MA (Np-237, Am-241, and Am-243) included in the discharged fuel of LWR. The performance of B/T treatment of MA was evaluated by a new function, i.e. [F/T ratio], defined by the ratio of the fission rate to the transmutation rate in the core, at an arbitrary burn-up, due to all MA nuclides. According to the results, HFBWR could burn and/or transmute MA nuclides with higher fission rate than BWR, but the fission rate did not increase proportionally to the flux increment, due to the higher rate of neutron adsorption. The higher B/T fraction of MA would result in the higher B/T capacity, and will reduce the units of HFBWR needed for the treatment of a constant mass of MA. In addition, HFBWR had a merit of higher mass transmutation compared to the reference BWR, under the same mass loading of MA.     

次锕系核素在铅冷快堆中的嬗变性能

乏燃料中大部分次锕系（minor actinides, MA）核素半衰期较长,对环境具有长期放射性危害。分离 嬗变技术将次锕系核素从高放废液中分离出来,并通过反应堆嬗变为短寿命或稳定核素,从而消除其放射性危害。为研究次锕系核素与燃料均匀混合、制成嬗变棒和做燃料芯块镀层装载方式下在铅冷快堆中的嬗变特性,采用MCNP和SCALE程序进行模拟计算。结果表明,三种方式下²³⁷Np、²⁴¹Am、²⁴³Am和混合次锕系核素使有效增殖因数k_eff降低,而²⁴⁴Cm和²⁴⁵Cm使k_eff升高,且²⁴⁵Cm可使k_eff大幅度增加。不同质量的混合次锕系核素装载后,三种方式下堆芯k_eff都随装载量的增加而降低,降低幅度由小到大分别为嬗变棒、均匀混合和镀层。不同次锕系核素装载量以均匀混合方式在堆芯经过550 d辐照后,²³⁷Np、²⁴¹Am和²⁴³Am嬗变率均为正值,其中²⁴¹Am嬗变率最大,而²⁴⁴Cm和²⁴⁵Cm嬗变率均为负值,²⁴⁵Cm增加明显,总的次锕系核素嬗变率为14%,可为次锕系核素在铅冷快堆中嬗变性能评价提供参考。     

Transmutation characteristics of MA and LLFP in a fast reactor

Systematic studies were implemented to investigate the flexibility and attractive core concepts of MA and LLFP transmutation in fast reactors. The MA transmutation in the fast reactor core has no serious drawbacks in terms of core performance, provided that the homogeneous loading method can be employed with a small fraction of MA fuel (2˜5wt%). The recycling of MA in the fast reactor is feasible from neutronic and thermal-hydraulic points of view. For FP transmutation, the introduction of target subassemblies using duplex pellets — a moderator annulus surrounding a Tc-99 core — gives the maximum transmutation rate of Tc-99 in the radial shield region of the fast reactor. The fast reactor has an excellent potential for transmuting MA and LLFP effectively. The fast reactor will be able to play an important role for reduction of environmental burden in future energy system.     

A new back-end cycle strategy for enhancing separation, transmutation and utilization of materials (Adv.-ORIENT cycle)

To minimize the ecological burden originating in nuclear fuel recycling, a new R&D strategy, the Adv.-ORIENT (Advanced Optimization by Recycling Instructive Elements) cycle was set forth. In this context, mutual separation of f-elements, such as minor actinide (MA)/lanthanide (Ln) and Am/Cm, are essential to enhance the MA (particularly ²⁴¹Am) burning. Isotope separation before transmutation is also inevitably required in the case of some long-lived fission products (LLFPs) like ¹²⁶Sn, ¹³⁵Cs, etc. The separation and utilization of rare metal fission products (RMFPs: Ru, Rh, Pd, Tc, Se, Te, etc.) are offering a new direction in the partitioning and transmutation (P&T) field. ⁹⁹Tc and ¹⁰⁶Ru, well-known interfering nuclides in reprocessing, should be removed prior to the actinide stream. Separation of exothermic nuclides ⁹⁰Sr, ¹³⁷Cs as well as MA will significantly help to mitigate the repository tasks.

A key separation tool is ion exchange chromatography (IXC) by a tertiary pyridine resin having soft donor nitrogen atoms. This method has provided individual recovery of pure Am and Cm products with a Pu/U/Np fraction from irradiated fuel in just a 3-step separation. A catalytic electrolytic extraction (CEE) method by Pd_adatom has been employed to separate, purify and fabricate RMFP catalysts. Differently functioned ion exchangers, e.g., ammonium molybdophosphate (AMP), have been investigated for the separation of Cs⁺. Theoretical and laboratory studies on the isotope separation of LLFPs were begun for ⁷⁹Se, ¹²⁶Sn and ¹³⁵Cs.     

基于启明星1#的ADS嬗变研究

以ADS次临界试验平台启明星1#为研究对象,计算分析了MA和LLFP共9种核素的嬗变反应率。通过实验测量了LLFP中¹³⁷Cs的实际嬗变反应率,发现该装置对¹³⁷Cs嬗变速度是其本身自然衰变的10倍。实验结果表明启明星1#具有一定的嬗变能力,经分析确认实验测量结果和理论计算结果吻合。     

压水堆中嬗变99Tc和129I的计算研究

选取大亚湾压水堆作为嬗变参考堆,研究在压水堆中嬗变长寿命裂变产物⁹⁹Tc和¹²⁹I的可行性。计算结果表明：在1个换料周期（18个月）内,⁹⁹Tc的最大嬗变率为15.69%,¹²⁹I的最大嬗变率为9.18%。通过对不同堆芯方案进行安全性分析发现：添加⁹⁹Tc和¹²⁹I后,堆芯有效增殖因数k_eff降低且随燃耗变化的幅度变小;堆芯径向中子通量密度分布无明显变化但径向功率峰因子降低;考虑燃料温度系数、慢化剂温度系数、硼微分价值以及控制棒价值等,得出在反应性温度系数及反应性控制方面不会导致安全问题,相反有优化作用。因此,从安全角度分析,在压水堆中嬗变⁹⁹Tc和¹²⁹I是可行的。     

Study on multi-recycle transmutation of LLFP in light water reactor

The effectiveness of transmutation for long-lived fission products (LLFP) in light water reactors (LWR), i.e. both BWR and PWR, considering the large capture cross-section of FPs in thermal region was evaluated. Calculation results of iodine and technetium transmutation in BWR and PWR suggested an effective use of BWR as compared to PWR. To obtain transmutation fraction [TF] of 30 to 40%, the irradiation period needed for ⁹⁹Tc transmutation was estimated as 10 to 15 years, and the period for ¹²⁹I transmutation was estimated as 30 to 40 years, respectively. The evaluations bring a new concept of multi-recycle LLFP transmutation using LWR_TR (LWR for transmutation).     

Study on MA and FP transmutation in fast reactors

Feasibility studies have been performed to develop an optimized fast reactor core for reducing long-term radiotoxicity of nuclear waste by minor actinide(MA) and long-lived fission product(FP) transmutation, taking into consideration fuel cycle technology. Systematic parameter survey calculations were implemented to investigate the basic characteristics of MA and FP transmutation in a fast reactor core. The hybrid MA-loading method, where Np nuclide is dispersed uniformly in the core and target subassemblies containing Am, Cm and rare earth nuclides are loaded into the blanket region, has the potential to achieve the maximum transmutation of MA with no special fuel design considerations. The introduction of target subassemblies using duplex pellets - a moderator annulus surrounding a ⁹⁹Tc core - has a great potential to transmute long-lived fission products in the radial blanket region of the fast reactor core.     

Fusion-driven transmutation of selected long-lived fission products

The long-term radiological burden associated with nuclear power production is usually attributed to long-lived fission products (LLFP). Their lifetime and large equilibrium mass and hence radioactivity accumulated in the course of fission energy generation make their storage a rather formidable task to solve. Therefore the idea of artificial incineration of LLFP through their transmutation has been quite naturally incorporated into the concept of self-consistent nuclear energy system (SCNES) based primarily on fast breeder reactor technologies. However it is now acknowledged that neutron environment of fission facilities including fast breeder reactors does not seem most appropriate for LLFP transmutation. The issue has been then extensively developed within the framework of multi-component self-consistent nuclear energy system (MC-SCNES). Neutrons of specific quality required for LLFP transmutation are proposed there to be of non-fission origin. Given neutron excess available and neutron quality, a fusion neutron source (FNS) is appearing as the candidate No. 1 to consider for LLFP transmutation. Research on LLFP transmutation by means of FNS has very long history and has received an additional boost during the decade passed. In the present study, potential of thermal flux blanket of FNS is exemplified by transmutation of ⁹³Zr and ¹²⁶Sn, the most difficult LLFP to transmute. It is shown that transmutation of ⁹³Zr is effective even with a rather modest neutron loading of 1 MWt·m⁻², typical for ITER project. Transmutation of ¹²⁶Sn, however, requires neutron loading of as high as 3 MWt·m⁻² for DD fusion and is quite unattractive for DT fusion. In the latter case, transmutation through the threshold (n,2n) reaction may be preferable.     

Interpretation of actinide transmutation in thermal and fast reactors

The commonly used transmutation rate of minor actinides in nuclear reactors is decomposed into four components, overall fission rate, Pu production rate, MA production rate, and element production rate. The physical meanings of these factors are described. The transmutation rates of minor actinides in two types of highly-moderated PWRs, a MOX fueled Na cooled fast reactor, and a metal fueled Pb cooled fast reactor are interpreted using the four components. The metal fueled Pb cooled fast reactor can incinerate minor actinides most (79kg/GWth/year), and this amount is about 4 times larger than the thermal reactors. The thermal reactors have large relative overall fission rates for ²⁴¹Am and have a potential for the incineration of ²⁴¹Am.     

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

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 ¹⁵N enrichment on nuclear characteristics and the evaluation of toxicity of ¹⁴C generated from ¹⁴N 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.     

Symbiosis system for transmutation on nitride fuel FBR and ADS

A conceptual scheme for mass flow of transmuting Plutonium (Pu), minor actinides (MA) and long-lived fission products (LLFP) is studied. In this feature, the existing light-water reactors (LWRs) cycle will be main stream for nuclear electric generation during a long-term period more than 50 years, and Pu will be utilized in mixed oxide fuel (MOX)-LWRs. In future, when Pu recycling system will be achived by introducing high-conversion LWRs (HCLWRs) and/or fast breeder reactors (FBRs), the accelerator driven transmutation system (ADS) transmutes Pu, MA and Iodine from Purex or Dry reprocessing. This is due to reduce burden for transmuting the excess or remained Pu, MA and LLFP by commercial reactor plants in Pu-recycling system. For this purpose, we introduce a concept of symbiosis system for transmutation based on nitride fuel FBR and ADS. The core design for lead-bismuth (Pb-Bi) cooled FBRs and ADS, Pb-Bi technologies, 15N enrichment and 14C toxicity are studied. And the mass flows for MA and Iodine are discussed based on an estimated scenario for nuclear electric plants introduction in future.     

Feasibility of a critical molten salt reactor for waste transmutation

This study assesses the feasibility of designing a Molten Salt Reactor (MSR) using the salt mixture of LiF (15 mol%), NaF (58 mol%) and BeF₂ (27 mol%) to be critical when fuelled with TRU from LWR spent fuel without exceeding the actinides solubility limit and while extracting fission products at realistic rates. The first part of the study investigated the graphite-to-MS volume ratio on the neutron balance, transmutation characteristics and graphite lifetime. It is found that a core without graphite moderator is the preferred design option; it offers the best neutron balance, most compact design and alleviated graphite lifetime problem. The second part of the study investigated sensitivity of the epithermal spectrum core to the feed composition, power density, fission products residence time and actinides loss fraction. It is found that the transmutation effectiveness improves with increasing power density and that the shorter the LWR spent fuel cooling time is, the better becomes the MSR neutron balance. The optimal MSR design offers a remarkably high transmutation capability – fissioning of as high as 99.8% of the TRU fed. The transmutation capability of the MSR is also rated in terms of final waste radiotoxicity, decay heat, spontaneous fission neutrons emission, fissile and ²³⁷Np inventory.     

Effect of decontamination factor of recycled actinide and FP on the characteristics of SCNES

Effect of the decontamination factor (DF) of nuclides to be confined in the closed fuel cycle was examined in terms of the system characteristics. Two kinds of indices, equilibrium mass and equivalent radiotoxicity, were used to determine the recovery perfectivity of the nuclides. By using the equilibrium mass, extremely high DF had to be attained. The required values of DF were 10⁸ for LLFP and 10¹² for actinides. On the other hand, using the radiotoxicity, inferior perfectivity of recovery, DF=10⁶, could be acceptable for actinides and there is no necessity of LLFP recycling for transmutation because they only have comparable radiotoxicity with the supplied uranium to the system. The required DF which provide ignorable loss of waste to the outside of the system depends on what index we use. Generalization of index to quantify the hazard or nuisance of nuclear waste and setting the criteria remains unsettled questions.     

R&D activities based on fast reactor cycle technologies for transmutation of TRU and LLFP by JNC

Research and development(R&D) activities on partitioning and transmutation of trans-uranium nuclides (TRU) and LLFP and future R&D program in JNC were summarized. Feasibility design studies have been conducting to investigate the characteristics of a fast reactor core with TRU and LLFP transmutation. It was reconfirmed that the fast reactor has a strong potential for transmuting TRU and LLFP, effectively. R&D for establishing partitioning process of TRU apart from the high-level radioactive wastes have been carried out. By several counter-current runs of the TRUEX process using highly active raffinates, a process flow sheet capable of selective partitioning of actinides and fission products was newly developed. JNC has settled a new R&D program concerning partitioning and transmutation of long-lived radioactive waste based on recommendation of check & review for OMEGA program performed by the Ad Hoc Committee under the Atomic Energy Commission of Japan (AEC). The R&D program is composed of the design studies and development of element technologies (fabrication, irradiation) in the “Feasibility Studies” on commercialized fast reactor system and the basic studies with experiments (nuclear data, reactor physics, fuel property, etc.) to establish database and analytical tools for the TRU- and LLFP- containing fuel and core design.     

Rapid Transmutation of High-Level Nuclear Wastes in a Catalyzed Fusion-Driven System

The aim of this study is to investigate the high-level waste (HLW) transmutation potential of fusion-driven transmuter (FDT) based on catalyzed D–D fusion plasma for various fuel fractions. The Minor actinide (MA) (²³⁷Np, ²⁴¹Am, ²⁴³Am and ²⁴⁴Cm) and long-lived fission product (LLFP) (⁹⁹Tc, ¹²⁹I and ¹³⁵Cs) nuclides discharged from high burn-up pressured water reactor-mixed oxide spent fuel are considered as the HLW. The volume fractions of the MA and LLFP are raised from 10 to 20% stepped by 2% and 10 to 80% stepped by 5%, respectively. The transmutation analyses have been performed for an operation period (OP) of up to 6 years by 75% plant factor (η) under a first-wall neutron load (P) of 5 MW/m² by using two different computer codes, the XSDRNPM/SCALE4.4a neutron transport code and the MCNP4B Monte Carlo code. The numerical results bring out that the considered FDT has a high neutronic performance for an effective and rapid transmutation of MA and LLFP as well as the energy generation along the OP.     

Neutronic Analysis for Transmutation of Minor Actinides and Long-Lived Fission Products in a Fusion-Driven Transmuter (FDT)

This study presents the transmutations of both the minor actinides (MAs: ²³⁷Np, ²⁴¹Am, ²⁴³Am and ²⁴⁴Cm) and the long-lived fission products (LLFPs: ⁹⁹Tc, ¹²⁹I and ¹³⁵Cs), discharged from high burn-up PWR-MOX spent fuel, in a fusion-driven transmuter (FDT) and the effects of the MA and LLFP volume fractions on their transmutations. The blanket configuration of the FDT is improved by analyzing various sample blanket design combinations with different radial thicknesses. Two different transmutation zones (TZ_MA and TZ_FP which contain the MA and LLFP nuclides, respectively) are located separately from each other. The volume fractions of the MA and the LLFP are raised from 10 to 20% stepped by 2% and from 10 to 80% stepped by 5%, respectively. The calculations are performed to estimate neutronic parameters and transmutation characteristics per D–T fusion neutron. The conversion ratios (CRs) for the whole of all MAs are about 65–70%. The transmutation rates of the LLFP nuclides increase linearly with the increase of volume fractions of the MA, and the ⁹⁹Tc nuclide among them has the highest transmutation rate. The variations of their transmutation rate per unit volume in the radial direction are quasi-concave parabolic.     

压水堆高放长寿命裂变产物堆内嬗变平衡循环设计

基于压水堆多燃料循环管理计算,进行长寿命裂变产物（LLFP）核素堆内嬗变分析。基于长寿命裂变产物核素在乏燃料中的比重及核素的放射毒性,¹²⁹I和⁹⁹Tc作为当前嬗变研究的主要裂变产物。为避免碘同位素分离,参照乏燃料中¹²⁷I和¹²⁹I的组分比例,设计当前的碘化物嬗变靶件。将嬗变核素均匀弥散在惰性慢化材料ZrH₂中,放置在控制棒导向管内进行嬗变分析计算。基于该嬗变组件设计方案,对不同的换料方案进行评价和比较,进而搜索嬗变平衡循环。计算显示,当前带有靶件组件的布料方案可达到平衡循环,并能实现LLFP的嬗变。进一步嬗变优化方案设计受限于当前嬗变组件设计。