Upgrade and Shakedown Test of a High Temperature Fluoride Salt Test Loop

A FLiNaK high temperature test loop, which was designed to support the Thorium Molten Salt Reactor (TMSR) program, was constructed in 2012 and is the largest engineering-scale fluoride loop in the world. The loop is built of Hastelloy C276 and is capable of operating at the flow rate up to 25m3/h and at the temperature up to 650°C. It consists of an overhung impeller sump-type centrifugal pump, an electric heater, a heat exchanger, a freeze valve and a mechanical one, a storage tank, etc. Salt purification was conducted in batch mode before it was transferred to and then stored in the storage tank. The facility was upgraded in three ways last year, with aims of testing a 30kW electric heater and supporting the heat transfer experiment in heat exchanger. Firstly, an original 100kW electric heater was replaced with a 335kW one to compensate the overlarge heat loss in the radiator. A pressure transmitter was subsequently installed in the inlet pipe of this updated heater. Finally, a new 30kW electric heater was installed between the pump and radiator, the purpose of which was to verify the core’s convective heat transfer behavior of a simulator design of TMSR. Immediately after these above works, shakedown test of the loop was carried out step by step. At first the storage tank was gradually preheated to 500°C so as to melt the frozen salt. Afterwards, in order to make the operation of transferring salt from storage tank to loop achievable, the loop system was also preheated to a relatively higher temperature 530°C. Since the nickel-base alloy can be severely corroded by the FLiNaK salt once the moisture and oxygen concentration is high, vacuum pumping and argon purging of the entire system were alternatively performed throughout the preheating process, with the effect of controlling them to be lower than 100ppm. Once the salt was transferred into the loop, the pump was immediately put into service. At the very beginning of operation process, it was found that flow rate in the main piping could not be precisely measured by the ultrasonic flow meter. Ten days later, the pump’s dry running gas seal was out of order. As a result, the loop had to be closed down to resolve these issues.

Dynamic Modeling of the NSSS Based on NHR200-II Nuclear Heating Reactor

The nuclear heating reactor (NHR) is a typical integral pressurized water reactor (iPWR) developed by the institute of nuclear and new energy technology (INET) of Tsinghua University, which has the safety advanced features such as the primary circuit integral arrangement, full-range natural circulation, self-pressurization. Power-level control is crucial for the operational stability and efficiency of the NHR, and the dynamic modeling is a basis for control system design and verification. From the conservation laws of mass, energy and momentum, a lumped-parameter dynamical model is proposed for the nuclear steam supply system (NSSS) based on the 200MWth nuclear heating reactor II (NHR200-II). The steady-state model validation is given by the comparing the parameter values of this model and that for plant design. Then, both the open-loop responses under the disturbances of reactivity and coolant flowrates as well as the closed-loop responses under the case of power ramp are given, where the rationality of the responses are analyzed from the viewpoint of plant physics and thermal-hydraulics. This model can be utilized for not only the control system design but also the development of a real-time simulator for the hardware-in-loop control system verification.

Verification of Alarm Displays for the Nuclear Power Plant With Two Modular High-Temperature Gas-Cooled Reactors

The nuclear power plant with two modular high-temperature gas-cooled reactors (HTR-PM) is under construction now. The control room of HTR-PM is designed. This paper introduces the alarm displays in the control room, and describes some verification and validation (V&V) activities of the alarm system, especially verification for some new human factor issues of the alarm system in the two modular design. In HTR-PM, besides the regular V&V similar to other NPPs, the interference effect of the alarm rings of the two reactor modules at the same time, and the potential discomfort of the two reactor operators after shift between them are focused. Verifications at early stage of the two issues are carried on the verification platform of the control room before the integrated system validation (ISV), and all the human machine interfaces (HMIs) in the control room, including the alarm system are validated in ISV. The test results on the verification platform show that the alarm displays and rings can support the operators understand the alarm information without confusion of the two reactors, and the shift between the two reactor operators have no adverse impact on operation. The results in ISV also show that the alarm system can support the operators well.

Proposed License Structure of Small Modular Reactor in China

Small Modular Reactor (SMR) is getting more and more attention due to its safety and multi-purpose application. License structure is an important issue for SMR licensing. Modular design, construction and operation, shared or common structure, system and components (SSC) challenge existing large light water reactor license structure. Existing nuclear power plant license structure, characteristics of SMR and its effect on license structure, and research progress of U.S Nuclear Regulatory Commission (NRC) are analyzed, SMR license structure in China are proposed, which can be used as a reference for SMR R&D, design and regulation.

FMEDA and FIT-Based Safety Assessment of NPP I&C Systems Considering Expert Uncertainty

The complexity of modern safety critical systems is becoming higher with technology level growth. Nowadays the most important and vital systems of automotive, aerospace, nuclear industries count millions of lines of software code and tens of thousands of hardware components and sensors. All of these constituents operate in integrated environment interacting with each other — this leads to enormous calculation task when testing and safety assessment are performed. There are several formal methods that are used to assess reliability and safety of NPP I&C (Nuclear Power Plant Instrumentation and Control) systems. Most of them require significant involvement of experts and confidence in their experience which vastly affects trustworthiness of assessment results. The goal of our research is to improve the quality of safety and reliability assessment as result of experts involvement mitigation by process automation. We propose usage of automated FMEDA (Failure Modes, Effects and Diagnostic Analysis) and FIT (Fault Insertion Testing) combination extended whith multiple faults approach as well as special methods for quantitative assessment of experts involvement level and their decisions uncertainty. These methods allow to perform safety and reliability assessment without specifying the degree of confidence in experts. Traditional FMEDA approach has several bottlenecks like the need of manual processing of huge number of technical documents (system specification, datasheets etc.), manual assignment of failure modes and effects based on personal experience. Human factor is another source of uncertainty. Such things like tiredness, emotional disorders, distraction or lack of experience could be the reasons of under- and over-estimation. Basing on our research in field of expert-related errors we propose expert involvement degree (EID) metric that indicates the level of technique automation and expert uncertainty degree (EUD) metric which is complex measure of experts decisions uncertainty within assessment. We propose usage of total expert trustworthiness degree (ETD) indicator as function of EID and EUD. Expert uncertainty assessment and Multi-FIT as FMEDA verification are implemented in AXMEA (Automated X-Modes and Effects Analysis) software tool. Proposed Multi-FIT technique in combination with FMEDA was used during internal activities of SIL3 certification of FPGA-based (Field Programmable Gate Array) RadICS platform for NPP I&C systems. The proposed expert trustworthiness degree calculation is going to be used during production activities of RPC Radiy (Research and Production Corporation). Our future work is related to research in expert uncertainty field and extension of AXMEA tool with new failure data sources as well as software optimization and further automation.

We Never Built Small Modular Reactors (SMRs), but What Do We Know About Modularization in Construction?

The key characteristics of small modular reactors (SMRs), as their name emphasized, are their size and modularity. Since SMRs are a family of novel reactor designs, there is a gap of empirical knowledge about the cost/benefit analysis of modularization. Conversely, in other sectors (e.g. Oil & Gas) the empirical experience on modularization is much greater. This paper provides a structured knowledge transfer from the general literature (i.e. other major infrastructure) and the Oil & Gas sector to the nuclear power plant construction world. Indeed, in the project management literature, a number of references discuss the costs and benefits determined by the transition from the stick-built construction to modularization, and the main benefits presented in the literature are the reduction of the construction cost and the schedule compression. Additional costs might arise from an increased management hurdle and higher transportation expenses. The paper firstly provides a structured literature review of the benefits and costs of modularization divided into qualitative and quantitative references. In the second part, the paper presents the results of series of interviews with Oil & Gas project managers about the value of modularization in this sector.

Status of District Heating Reactor and its Development Prospects in China

Nuclear heating reactor is a new type of power plant that uses nuclear energy as heat source. Low temperature nuclear heating reactor should be the forerunner and main force for developing nuclear heating plant in China. Due to the lower water temperature required by the heating system, this dedicated, non-power generating nuclear reactor works at low temperatures and pressures with inherent safety features. The design, construction and operation of the nuclear heating reactors in various countries in the world were reviewed in this paper, and China’s new demonstration nuclear heating project and NHR-200 low-temperature heating reactor which would be used was discussed in the paper. We put forward the developing route and suggestion for the development of low-temperature heating reactor in China.

A Core Design of Innovative Breeder BWR

High breeding with light water cooling has been studied for decades, though is not easy to be achieved. The main obstacle is the moderating effect of light water, which softens the neutron spectrum. To harden the neutron spectrum and thereby to enhance the fuel utilization or even to achieve breeding with light water cooling, the tight-lattice assembly was proposed and applied to High Conversion LWRs. Nonetheless, none of them achieved high breeding. Until recently, the tightly packed fuel assembly (TPFA) is designed for the purpose of high breeding. The ratio of hydrogen atoms to heavy metal atoms (H/HM) in this assembly is significantly reduced to be less than 0.1. Super Fast Breeding Reactor (Super FBR) adopts TPFA and achieves breeding performance with compound system doubling time (CSDT) of 43 years. In this study, the breeder BWR core also applies TPFA and achieves CSDT of 50 years. BWR is one type of the most extensively built reactors in the world, with abundant operation experience and mature technologies. Breeder BWR is considered to be capable of being incorporated into the current BWR plants with a handful of modifications, thus obtaining optimal economy.

Study on the Operating Strategy Optimization of Moving to Remote Shutdown Station When Main Control Room Is Un-Inhabitable

For PWR, remote shutdown station (RSS) is a redundant control mean to shut down the reactor when main control room (MCR) inhabitation is challenged (e.g. fire, smoke...). Nowadays, due to nuclear power plants control measures were improved with DCS system, a full function DCS RSS was equipped and more essential equipment could be controlled on RSS. Under operating conditions that prohibit nuclear power plant operators to stay in the main control room, the operators should move to RSS and shutdown the reactor to ensure plant safety following <Moving to remote shutdown station when main control room is un-inhabitable operating strategy> (RSS strategy for short) to fallback the plant from power operation to cold shutdown. The original operating strategy by nature circulation is no longer the best choice both for operation safety and economy efficiency, and an optimized new strategy should be raised. Based on the former reason, an optimized operation strategy was raised in this paper. In the optimized strategy, all plant normal standard operation modes were considered as initial conditions, rather than only considering power operation condition in the original one. The fallback mode and fallback strategy for each initial condition was also designed and optimized. To accelerate the depression and heat removal process, a forced circulation operation strategy is adopted when the reactor coolant pumps are available, and less local operation was included by taking advantages of the full function operating measures on RSS. To simplify the whole procedure structure, the operation modules of other general operating procedures are reused. To validate the effectiveness of the optimized operating strategy, a full scope PWR simulation tool was employed to make thermo hydraulic calculation validation of the reactor response and also the remote control station HMI supporting validation. By simulating the original strategy and the optimized one and related analysis, we found that the optimized strategy is effective, and able to be executed based on the remote control station hardware. By executing the optimized strategy, the unit can fall back to the cold shutdown condition safely and a few hours were saved compared with the original strategy. The optimized strategy had already been implemented on real PWR nuclear power plant.

Multi-Diversity for FPGA Platform Based NPP I&C Systems: New Possibilities and Assessment Technique

Diversity approach is used to decrease risk of common cause failure (CCF) of Nuclear Power Plant (NPP) Instrumentation and Control systems (I&Cs). Application of a multi-diversity, i.e. a few different types of version redundancy allows minimizing CCF risk. On the other side, implementation of diversity increases cost and complicates maintenance of multi-version I&Cs. Hence, it is important to find optimal solution according with criteria “required level of diversity (safety) / minimal cost and maintenance complexity. Modern FPGA technology creates additional possibilities to meet requirements of the standards (such as NUREG/CR-7007, IEEE Std 7-4.3.2-2016, IAEA SSR-2/1:2016, IAEA NP-T-3.17:2016 and others) by developing main and diverse subsystems on the basis of the same FPGA platform. Existing diversity normative base should be enhanced in three directions — scope, depth and rigor to provide more detailed description of possible applied techniques and tools for quantitative assessment. The goals of the paper which overviews practical issues of diversity application are the following: - present extended classification of diversity considering additional types of version redundancy for FPGA platform based I&Cs (logical processing equipment, life cycle, logic/algorithm etc.) in comparing to NUREG7007; - describe the modified technique of diversity assessment taking into account three and more levels of diversity classification; - illustrate and discuss variants of assurance of the required degree of diversity by use of the RadICS FPGA platform to develop main and diverse subsystems. The classification is specified considering diversity of hardware and FPGA designs. In particular, diversity of hard logic and soft processors, interfaces and buses, self-diagnostics means and others are described and embedded into NUREG/CR-7007 classification. The NUREG7007-based diversity assessment techniques supporting all stage of analyzing options are discussed, and algorithms for versions choice are described. This technique takes into account more detailed specification of diversity classification (for types, subtypes and sub-subtypes of diversity for logic diversity, logic processing equipment diversity and others) and options to evaluate weight coefficients. Case study is based on description of two options of RadICS FPGA platform application to develop two-version NPP I&C, which meets standard requirements to diversity.