Prediction of Nongaseous Fission Products Behavior in the Primary Cooling System of High Temperature Gas-Cooled Reactor

In high temperature gas-cooled reactors (HTGRs), some amounts of fission products (FPs) are released mainly from fuel with failed coatings and are transported in the primary cooling system with the primary coolant during normal operation. In that case, condensable FPs plateout on the inner surface of components in the primary cooling system. On the other hand, since the HTGRs use helium gas as primary coolant, the primary coolant is not activated itself and very small amount of corrosion products is generated. Then, γ-ray emitted from the FPs becomes main source in shielding design of the HTGRs, and not only release amount from fuel but also plateout distributions of the FPs should be properly evaluated. Therefore, prediction of plateout behavior in the primary cooling system of HTGRs was carried out based on the calculation result of plateout distribution in High Temperature Engineering Test Reactor. Before the calculation, analytical model was verified by comparison with experimentally obtained plateout distributions and the applicability of the model to predict the plateout distributions in the primary cooling system of HTGR was certified.

This report describes the predicted result of plateout distribution in the primary cooling system of HTGR together with the verification result of the analytical model.     

Safety design

JAERI established the safety design philosophy of the HTTR based on that of current reactors such as LWR in Japan, considering inherent safety features of the HTTR. The strategy of defense in depth was implemented so that the safety engineering functions such as control of reactivity, removal of residual heat and confinement of fission products shall be well performed to ensure safety. However, unlike the LWR, the inherent design features of the high-temperature gas-cooled reactor (HTGR) enables the HTTR meet stringent regulatory criteria without much dependence on active safety systems. On the other hand, the safety in an accident typical to the HTGR such as the depressurization accident initiated by a primary pipe rupture shall be ensured. The safety design philosophy of the HTTR considers these unique features appropriately and is expected to be the basis for future Japanese HTGRs.This paper describes the safety design philosophy and safety evaluation procedure of the HTTR especially focusing on unique considerations to the HTTR. Also, experiences obtained from an HTTR safety review and R&D needs for establishing the safety philosophy for the future HTGRs are reported.     

Small PWRs using coated particle fuel for district heating, PFPWR50

It has been said that nuclear energy is an important option for especially developing countries to satisfy their increasing energy demand. However, it will be difficult to deploy first of a kind nuclear power plant in developing countries because extensive safety demonstration has to be conducted in industrialized countries. On the other hand, it will be essential to present rigid proof of reliable operational experience to develop proper understanding of the safety features of new reactor systems among the people around the demonstration plant sites. One of the ways to solve the issue is to integrate existing technologies supported by a great deal of data and experience into a new reactor design. Based on the consideration, a small-sized district heating reactor system based on the pressurized water reactor (PWR) technologies combined with the fuel concept of high temperature gas cooled reactors (HTGRs) has been studied. The purpose of the combination of these two existing concepts is to take the best advantages of both excellent operational experience of PWRs and the integrity of HTGR fuel, coated particle fuel, against fission products release even at high temperature. We expect that this approach will help create a breakthrough to the current stagnation of nuclear power deployment.     

Modeling spent TRISO fuel for geological disposal: corrosion and failure under oxidizing conditions in the presence of water

High temperature gas reactors (HTGRs) are being considered for near term deployment in the United States under the GNEP program and farther term deployment under the Gen IV reactor design (U.S. DOE Nuclear Energy Research Advisory Committee, 2002). A common factor among current HTGR (prismatic or pebble) designs is the use of TRISO coated particle fuel. TRISO refers to the three types of coating layers (pyrolytic carbon, porous carbon, and silicon carbide) around the fuel kernel, which is both protected and contained by the layers. While there have been a number of reactors operated with coated particle fuel, and extensive amount of research has gone into designing new HTGRs, little work has been done on modeling and analysing the degradation rates of spent TRISO fuel for permanent geological disposal. An integral part of developing a spent fuel degradation modeling was to analyze the waste form without taking any consideration for engineering barriers. A basic model was developed to simulate the time to failure of spent TRISO fuel in a repository environment. Preliminary verification of the model was performed with comparison to output from a proprietary model called GARGOYLE that was also used to model degradation rates of TRISO fuel. A sensitivity study was performed to determine which fuel and repository parameters had the most significant effect on the predicted time to fuel particle failure. Results of the analysis indicate corrosion rates and thicknesses of the outer pyrolytic carbon and silicon carbide layers, along with the time dependent temperature of the spent fuel in the repository environment, have a significant effect on the time to particle failure. The thicknesses of the kernel, buffer, and IPyC layers along with the strength of the SiC layer and the pressure in the TRISO particle did not significantly alter the results from the model. It can be concluded that a better understanding of the corrosion rates of the OPyC and SiC layers, along with increasing the quality control of the OPyC and SiC layer thicknesses, can significantly reduce uncertainty in estimates of the time to failure of spent TRISO fuel in a repository environment.     

Experimental and computational study of the pyrocarbon and silicon carbide barriers of HTGR fuel particle

HTGR safety is secured by a system of barriers limiting the emission of fission products from the core into the surrounding environment during normal operation and postulated anticipated accidents. An experimental-computational analysis of two fundamentally important barriers — fuel kernels and their coating, whose function is to contain radionuclides and to protect workers and the environment, is examined. The function of the barriers and the requirements which they must satisfy are examined for HTGR fuel particles. The results of post-reactor studies are analyzed. Mathematical models and computational codes simulating the behavior of fuel particles are analyzed. Probabilistic-statistical models and the GOLT code are being developed to evaluate the behavior of fuel particles under irradiation. Together with other models, this code is used for comparative test calculations of the behavior of particle fuel under normal irradiation conditions (<1300°C). The first results of such calculations are discussed. __________ Translated from Atomnaya énergiya, Vol. 105, No. 1, pp. 14–25, July, 2008.     

keff uncertainty quantification and analysis due to nuclear data during the full lifetime burnup calculation for a small-sized prismatic high temperature gas-cooled reactor

To benefit from recent advances in modeling and computational algorithms,as well as the availability of new covariance data,sensitivity and uncertainty analyses are needed to quantify the impact of uncertain sources on the design parameters of small prismatic high-temperature gas-cooled reactors(HTGRs).In particular,the contribution of nuclear data to the keff uncertainty is an important part of the uncertainty analysis of small-sized HTGR physical calculations.In this study,a small-sized HTGR designed by China Nuclear Power Engineering Co.,Ltd.was selected for keff uncertainty analysis during full lifetime burnup calculations.Models of the cold zero power(CZP)condition and full lifetime burnup process were constructed using the Reactor Monte Carlo Code RMC for neutron transport calculation,depletion calculation,and sensitivity and uncertainty analysis.For the sensitivity analysis,the Contribution-Linked eigenvalue sensitivity/Uncertainty estimation via Track length importance Characterization(CLUTCH)method was applied to obtain sensitive infor-mation,and the"sandwich"method was used to quantify the keff uncertainty.We also compared the keff uncertainties to other typical reactors.Our results show that 235U is the largest contributor to keff uncertainty for both the CZP and depletion conditions,while the contribution of 239Pu is not very significant because of the design of low discharge burnup.It is worth noting that the radioactive capture reaction of 28Si significantly contributes to the keff uncer-tainty owing to its specific fuel design.However,the keff uncertainty during the full lifetime depletion process was relatively stable,only increasing by 1.12％owing to the low discharge burnup design of small-sized HTGRs.These numerical results are beneficial for neutronics design and core parameters optimization in further uncertainty prop-agation and quantification study for small-sized HTGR.     

keff uncertainty quantification and analysis due to nuclear data during the full lifetime burnup calculation for a small-sized prismatic high temperature gas-cooled reactor

To benefit from recent advances in modeling and computational algorithms,as well as the availability of new covariance data,sensitivity and uncertainty analyses are needed to quantify the impact of uncertain sources on the design parameters of small prismatic high-temperature gas-cooled reactors(HTGRs).In particular,the contribution of nuclear data to the keff uncertainty is an important part of the uncertainty analysis of small-sized HTGR physical calculations.In this study,a small-sized HTGR designed by China Nuclear Power Engineering Co.,Ltd.was selected for keff uncertainty analysis during full lifetime burnup calculations.Models of the cold zero power(CZP)condition and full lifetime burnup process were constructed using the Reactor Monte Carlo Code RMC for neutron transport calculation,depletion calculation,and sensitivity and uncertainty analysis.For the sensitivity analysis,the Contribution-Linked eigenvalue sensitivity/Uncertainty estimation via Track length importance Characterization(CLUTCH)method was applied to obtain sensitive infor-mation,and the"sandwich"method was used to quantify the keff uncertainty.We also compared the keff uncertainties to other typical reactors.Our results show that 235U is the largest contributor to keff uncertainty for both the CZP and depletion conditions,while the contribution of 239Pu is not very significant because of the design of low discharge burnup.It is worth noting that the radioactive capture reaction of 28Si significantly contributes to the keff uncer-tainty owing to its specific fuel design.However,the keff uncertainty during the full lifetime depletion process was relatively stable,only increasing by 1.12％owing to the low discharge burnup design of small-sized HTGRs.These numerical results are beneficial for neutronics design and core parameters optimization in further uncertainty prop-agation and quantification study for small-sized HTGR.     

弥散型燃料裂变产物释放行为分析研究

为了获得弥散型燃料裂变产物向一回路冷却剂的释放特性,开展了弥散型燃料裂变产物释放行为研究,开发了适用于弥散型燃料的裂变产物源项计算程序,并对裂变产物源项进行了影响分析。结果表明:沾污铀和起泡破损后裂变产物的核素谱存在一定差异;裂变产物的释放与起泡当量直径的平方成正比;对于弥散型燃料而言,起泡破损中通过反冲释放的占比较低;相同破口条件下的弥散型和陶瓷型燃料中裂变产物的释放存在量级的差别。本文开发的程序能够用于分析弥散型燃料的裂变产物源项,为后续相关研究工程设计奠定基础。      

世界核电发展趋势与高温气冷堆

核能的发展面临经济竞争力、核安全、核废物的最终处置及防止核武器材料扩散的挑战。为改善公众的可接受性 ,核电厂的安全性进一步改进。电力市场体制的非管制化改革加剧了电力技术的竞争。环境保护意识增强使核废物的处置倍受关注。 80年代中期以来发展的先进轻水堆核电厂如ABWR ,System 80 ,EPR ,AP60 0等是今后一段时期内商用核电的主力堆型。进入 2 0 0 0年之际 ,美国能源部正在规划发展第四代先进核能系统 ,目标是在 2 0 2 0年或之前 ,向市场提供经过验证的成熟的第四代核电厂技术 ,以替代美国退役的核电容量。球床高温气冷堆被认为是第四代先进核能系统的优选技术。南非ESKOM电力公司选择了球床高温气冷堆作为今后核电发展的堆型。清华大学承担设计和建设的 10MW高温气冷实验堆计划在 2 0 0 0年内临界。通过10MW高温气冷堆的建造 ,我国已形成了高温气冷堆技术的自主知识产权 ,初步具备了自主设计、制造和建造的能力     

Model development for the estimation of fission product release under normal and accident conditions in a HTGR

This article has attempted to estimate the radioactivity release from fuel materials during normal and transient conditions by coupling the TRISO fracture and the fission product (FP) diffusion. Two calculation models, named TRISO Fracture Analyzer (TRIFA) and DIFfusion Analyzer (DIFA), are developed. TRIFA is initially used to calculate the fraction of fractured fuel particles, thus determining the amount of fission gas release. The obtained particle fracture function is then used as input for the diffusion rate calculation. DIFA simulates with a single spherical fuel element, a pebble, irradiated under normal and accident conditions. It describes the diffusive transport of fission products by numerically solving the diffusion equation. The finite difference method is applied to obtain fission product release rates from a pebble to coolant. The model comparisons show that the new developed models are reliable, fast, and correspond with previous results of other models. As for HTR-10, the coupled models, TRIFA and DIFA, are applied to calculate the level of fission product release after accidents. The following conclusions can be drawn. First, the mitigation should be carried out until the maximum fuel temperature reaches under transient. Second, the mitigation should be intensively considered if the burn-up exceeds 5%FIMA (∼48 GWd/MTU) when transient happens. Additionally, it is found that there is the threshold burn-up where the rapid FP release occurs due to the numerous TRISOs fractured. Further investigations are needed to extend the use of the method developed in this work to the safety assessments for high-temperature gas-cooled reactors (HTGRs). This article will hopefully serve as a platform for designing the advanced TRISO that can minimize the activity release, and providing the rationale of development of the intensive accident mitigation system in future.     

Design of coated fuel particles for a hybrid fusion-fission system

Designs have been developed for coated ThO₂ fuel particles to be used in a hybrid fusion-fission system that could be operated without reprocessing. The fresh fertile fuel particle would first be cycled through the blanket of a fusion reactor to breed ²³³U, which would then be ‘burned’ in a thermal fission reactor. The depleted fuel would then be refreshed in a second pass through the fusion reactor, and the process above repeated as many times as feasible. Designs of coated particles for up to three cycles through the hybrid system of reactors have been developed. The outer structural layer for these particles is made from vapor-deposited silicon carbide, because of its remarkable dimensional stability under fast neutron irradiation, and an inner layer of porous pyrocarbon is used to accommodate the buildup of gaseous reaction products inside the particle. The production of gaseous emission products from the interaction of high-energy fusion neutrons with coating materials and with the oxygen in the kernel contributes significantly to pressure vessel stresses in these coatings, whereas gaseous fission products alone dominate in conventional thermal reactors. The most stringent design for the three-cycle particle is identical in fuel loading to the reference fertile particle for an HTGR, which would constitute an ideal hybrid partner for the fusion reactor. Consideration is also given to coated-particle designs for the containment of the bred tritium used to fuel the D-T fusion reactor.     

The high temperature gas-cooled reactor as a source of high temperature process heat

The nuclear reactor has established itself as a future major supplier of electrical energy. The industrial market for other forms of energy, however, is almost as large and represents a new potential for the use of nuclear reactors. The high temperature gas-cooled reactor (HTGR) has been developed for commercial application in the electric power generation field. Since the HTGR is capable of delivering process heat in the temperature range of 1000–1500°F, it has many applications that would not be possible at the lower operating temperatures of water-cooled reactors. This paper briefly summarizes the development of the HTGR and outlines its salient technical features. Modifications to the reactor that enable it to be used as a process heat source are discussed. Specific applications are developed for coal gasification, steelmaking, and hydrogen production.     

安全经济高效的先进能源

具有第四代安全经济特性的核电应该是人们期待的先进的清洁低碳能源。高温气冷堆是当今研发的第四代核电堆型之一,但现有的设计还存在需要排除的严重的安全隐患。堆芯不熔化,不等于说不会有严重事故发生。需要吸取国外球床高温堆和柱状高温堆两种实验堆型运行的经验教训、扩展安全观念和应对安全低概率事件,确保反应堆不出现后果极其严重的放射性释放事故。当热电转换系统采用与燃气蒸汽联合循环耦合应用的技术以后,会发挥高温堆所长,更大地提升转换效率,形成一种高安全低投资和高效率的双燃料清洁能源,可用于大堆或小堆的应用环境,可满足电力系统基本负荷和调锋负荷的需要。在工程设计上采取一系列改进和创新措施,包括釆用规则床模块化及地下反应堆设计以后,可在提高反应堆核心部位安全防卫能力的同时,防范低概率事件,成为一种新的安全经济高效的先进能源。     

Neutronic and thermo-hydraulic analyses of a small,long-life HTGR for passive decay-heat removal

Since the accident at Fukushima Daiichi Nuclear Power Plant in 2011, design concepts for nuclear reactors have been reconsidered with much greater emphasis placed upon passive systems for decay-heat removal. By considering this issue, the design parameter conditions for high temperature gas-cooled reactors (HTGRs) with passive safety features of decay-heat removal were obtained by residual-heat transfer calculation using equations for fundamental heat transfer mechanisms in our previous works. In the present study, the appropriate size of reactor core for a 100 MW_t reactor operating at 1123 K of the initial core temperature was found using the conditions. Consequently, neutronics and thermo-hydraulic analyses for the proposed reactor core were performed and the proper optimizations to control the excess reactivity and flatten the change in power peaking factor during operation were done successfully. By the systematic method to decide the core design which satisfies the condition for passive decay-heat removal, a long-life small HTGR concept whose excess reactivity was small during the operation was shown. The small excess reactivity is a significant advantage from the view point of safety in reactivity accident.     

MELCOR程序在HTGR事故分析中的最新进展

MELCOR程序是美国NRC在安全评审中使用的一体化系统分析程序,早期主要用于轻水堆严重事故分析。近年来,该程序逐渐用于高温气冷堆的石墨腐蚀、裂变产物行为和石墨粉尘等物理现象方面的研究。本文介绍了在最新版本的MELCOR2．1程序中,针对高温气冷堆特点所进行的扩展和开发,以及MELCOR程序在高温气冷堆（HTGR）事故分析中的计算流程。     

A concept of the multipurpose liquid metallic-fueled fast reactor system (MPFR)

Against fossil fuels, the nuclear energy is the only alternative energy source in the next century. Such energy source as the future nuclear power plant is expected to meet the following requirements. First, high temperature output for the multiple energy conversion capability as the electricity generation and the production of alternative fuels (hydrogen), which can be used widely in transportation systems. Second, the capability for siting close to the energy consumption area without onsite refueling. Third, the capability for nuclear fuel breeding and incineration of long-lived fission products, and fourth, the harmonization between active and passive safety features. This paper describes the basic concept of the Multipurpose liquid metallic-fueled Fast Reactor system (MPFR), which satisfies all mentioned requirements with introducing the U-Pu-x (x: Mn, Fe, Co) liquid metallic alloys for the fuel. We can obtain such characteristics as high operational temperature of the reactor (between 550 °C and 1200 °C) and elongation of the core operational lifetime by the inherent fission product separation in the liquid fuel by using these alloys. The enhanced self-controllability is achieved by the thermal expansion of liquid fuel; and the re-criticality phenomenon at the core compaction events can be eliminated by discharging of the liquid fuel from the core.     

Small high temperature gas-cooled reactors with innovative nuclear burning

Since the innovative concept of CANDLE (Constant Axial shape of Neutron Flux, nuclide densities and power shape During Life of Energy producing reactor) burning strategy was proposed, intensive research works have been continuously conducted to evaluate the feasibility and the performance of the burning strategy on both fast and thermal reactors. We learned that one potential application of the burning strategy for thermal reactors is for the High Temperature Gas-Cooled Reactors (HTGR) with prismatic/block-type fuel elements. Several characteristics of CANDLE burning strategy such as constant reactor characteristics during burn-up, no need for burn-up reactivity control mechanism, proportionality of core height with core lifetime, sub-criticality of fresh fuel elements, etc. enable us to design small sized HTGR with a high degree of safety, easiness of operation and maintenance, and long core lifetime which are required for introducing the reactors into remote areas or developing countries with limited infrastructures and resources. In the present work, we report our evaluation results on small sized block-type HTGR designs with CANDLE burning strategy and compared with other existing small HTGR designs including the ones with pebble fuel elements, under both uranium and thorium fuel cycles.     

Reactivity control technique for a pressurized water reactor with an inventive TRISO fuel particle composition

In this research paper a reactivity control technique has been suggested for the conceptual design of a compact sized pressurized water reactor (PWR) core with an inventive tristructural-isotropic (TRISO) fuel particle composition. This conceptual design is a light water cooled and moderated reactor which utilizes TRISO fuel particles in PWR technology. The use of TRISO fuel in PWR technology improves integrity of the design due to its fission fragments retention ability. The fuel provides first retention barrier within fuel itself against the release of fission fragments that makes this design concept safer and environment friendly. The suggested TRISO fuel particle composition has a small amount of Pu-240 with 2.0 w/o in the place of U-238 which acts as reactivity suppressor. Reactor codes WIMS-D/4 and CITATION have been used for simulation and core design modeling. Results reveals that the amount of excess reactivity can be reduced significantly by using a small amount of Pu-240 in TRISO fuel which in turns reduces the number of gadolinia rods in the core required for excess reactivity control and completely eliminates the requirement of soluble boron system. Therefore the effective and optimal use of reactivity suppressor and burnable poison suppresses and flattens the core excess reactivity throughout the core life and hence number of control rods can be reduced without compromising on the shutdown margin.     

Post-test analysis of helium circulator trip without scram at 3 MW power level on the HTR-10

Safety demonstration tests were conducted on the 10 MW High Temperature Gas-cooled Reactor-Test Module (HTR-10) to verify the inherent safety characteristics of modular High Temperature Gas-cooled Reactors (HTGRs) as well as to obtain the transient data of reactor core and primary cooling system for validation of HTGR safety analysis models and codes. As one of these safety demonstration tests, a simulated anticipated transient without scram (ATWS) test called loss of forced cooling by tripping the helium circulator without reactor scram was carried out at 3 MW power level on October 15, 2003. This paper simulates and analyzes the power transient and the thermal response of the reactor during the test by using the THERMIX code. The analytical results are compared with the test data for validation of the code.Owing to the negative temperature coefficient of reactivity, the reactor undergoes a self-shut down after the stop of the helium circulator; the subsequent phenomena such as the recriticality and power oscillations are also studied. During the test a natural circulation loop of helium is established in the core and the other coolant channels and its consequent thermal response such as the temperature redistribution is investigated. In addition, temperatures of the measuring points in the reactor internals are calculated and compared with the measured values. Satisfactory agreements obtained from the comparison demonstrate the basic applicability and reasonability of the THERMIX code for simulating and analyzing the helium circulator trip ATWS test. With respect to the safety features of the HTR-10, it is of most importance that the maximum fuel center temperature during the test is always lower than 1600 °C which is the limited value for the HTGR.     

Measurement of Acoustic Velocity and Attenuation in an Air-Water Two-Phase Medium

Actinides, mainly responsible for the long term risk of spent fuel, are the principal candidates to transmutation due to their large absorption cross sections.

Systems driven by particle accelerators have been investigated in the past to produce fissile material. Recently these systems have been reconsidered to destroy minor actinides (MA) and long-lived fission products (LLFP), reducing the need for the traditional final confinement of radioactive waste.

Two Monte Carlo calculation models have been developped to determine the criticality safety conditions and the burning capability of MAS and of Pu.

A Pu burner, whose core is poisoned with Th to compensate by producing ²³³U the burnup reactivity due to the even Pu isotopes, can operate at a low proton current using perhaps a cyclotron, incinerating 70% of the charged Pu; its burning capability would be the production of about 1.5 PWRs.

Liquid fuel accelerator driven systems can be used in the future (due to the accelerator dimensions) for MA burning using D₂0 as carrier in a homogeneous core; such a system can burn the production of more than 15PWRs.

In the future, also the problem of LLFP burning could be solved definitively using a system with D₂0 as carrier.