• Nuclear Engineering and Technology
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• 제47권2호
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• pp.187-195
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• 2015

Human-induced initiating events, also called Category B actions in human reliability analysis, are operator actions that may lead directly to initiating events. Most conventional probabilistic safety analyses typically assume that the frequency of initiating events also includes the probability of human-induced initiating events. However, some regulatory documents require Category B actions to be specifically analyzed and quantified in probabilistic safety analysis. An explicit modeling of Category B actions could also potentially lead to important insights into human performance in terms of safety. However, there is no standard procedure to identify Category B actions. This paper describes a systematic procedure to identify Category B actions for low power and shutdown conditions. The procedure includes several steps to determine operator actions that may lead to initiating events in the low power and shutdown stages. These steps are the selection of initiating events, the selection of systems or components, the screening of unlikely operating actions, and the quantification of initiating events. The procedure also provides the detailed instruction for each step, such as operator's action, information required, screening rules, and the outputs. Finally, the applicability of the suggested approach is also investigated by application to a plant example.

• Nuclear Engineering and Technology
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• 제44권7호
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• pp.773-780
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• 2012

The article presented is concerned with an evaluation of the corrosion behavior of SA-508 low alloy steel (LAS) and Type 309L stainless steel (SS) cladding of a reactor pressure vessel under the simulated primary water chemistry of a pressurized water reactor (PWR). The uniform corrosion and galvanic corrosion rates of SA-508 LAS and Type 309L SS were measured in three different control conditions: power operation, shutdown, and power operation followed by shutdown. In all conditions, the dissimilar metal coupling of SA-508 LAS and Type 309L SS exhibited higher corrosion rates than the SA-508 base metal itself due to severe galvanic corrosion near the cladding interface, while the corrosion of Type 309L in the primary water environment was minimal. The galvanic corrosion rate of the SA-508 LAS and Type 309L SS couple measured under the simulated power operation condition was much lower than that measured in the simulated shutdown condition due to the formation of magnetite on the metal surface in a reducing environment. Based on the experimental results, the corrosion rate of SA-508 LAS clad with Type 309L SS was estimated as a function of operating cycle simulated for a typical PWR.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 1992년도 한국자동제어학술회의논문집(국내학술편); KOEX, Seoul; 19-21 Oct. 1992
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• pp.880-886
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• 1992

There has been continuous effort to achieve total automation of the power plants. But due to complexity, nonlinearities and time-varying properties of the system, no success has been reported if conventional and/or modern control theories are applied. In spite of this, the start-up and shutdown operations are successfully performed by skilled human operators who are able to utilize a great wealth of knowledge and past experiences. In this paper, in order to automate the shutdown operation of power plants, it is proposed that the operation be performed by a more efficient method than the current used, through dividing the total process into several subprocesses, introducing checkpoints, and using fuzzy logic. For this, fuzzy logic controllers and fuzzy decision-makers are designed and the validity is shown by simulation via a set of piecewise continuous shutdown models.

• Nuclear Engineering and Technology
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• 제36권6호
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• pp.497-511
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• 2004

A PSV (pressurizer safety valve) popping test carried out in the early phases of a refueling outage may trigger a test-induced LOCA(loss of coolant accident) if a PSV fails to fully close and is stuck in a partially open position. According to a KSNP (Korea standard nuclear power plant) low power and shutdown PSA (probabilistic safety assessment), the failure of a high pressure safety injection (HPSI) accompanied by the failure of a PSV to fully close was identified as a dominant accident sequence with a significant impact on low power and shutdown risks (LPSR). In this study, we aim to investigate and verify a new means for mitigating this type of accident using a thermal-hydraulic analysis. In particular, we explore the applicability of an aggressive cool-down combined with operator actions. The results of the various sensitivity studies performed there will help reduce LPSR and improve Refueling outage safety.

• 대한전자공학회논문지SD
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• 제41권10호
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• pp.77-83
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• 2004

VLSI 시스템에서 전력 소모를 줄이기 위해서는 메가블록이 동작하지 않는 동안 전원을 차단하여 누설 전류를 억제하는 방법이 효과적이다. 최근 들어 다중 문턱 전압 CMOS를 사용하여 전원을 차단하는 방법이 널리 연구되고 있으나, 동작 주파수가 증가함에 파라 전원 복귀에 필요한 시간이 짧아지게 되고, 짧은 시간에 전원이 복귀되면서 전원선에 대량의 전류가 순간적으로 흐르게 된다. 이에 따라 매우 큰 전원 잡음이 생겨서 전원 전압이 안정적이지 못하고 흔들리게 되며 이는 많은 경우 시스템의 오동작을 초래하게 된다. 본 논문에서는 이러한 문제점을 해결하기 위하여 새로운 전원 복귀 기법을 제안한다. 제안하는 기법은 메가블록의 전원이 차단되었다가 다시 복귀할 때 한꺼번에 전원을 켜는 것이 아니라 파이프라인 방식으로 몇 단계로 나누어 전원을 켬으로서 전원선에 흐르는 최대 전류 및 이에 따른 전원 잡음을 크게 억제한다. 제안하는 파이프라인 전원 복귀 기법을 검증하기 위해서 컴팩트 플래시 메모리 제어기 칩에 본 기법을 적용하여 곱셈기 블록의 전원을 차단하고 복귀할 때의 전원 잡음을 모의실험하고 분석하였다. 모의실험 결과, 제안하는 기법은 기존의 전원 차단 기법에 비해 전원 잡음을 매우크게 줄일 수 있음을 확인하였다.

• 한국압력기기공학회 논문집
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• 제15권1호
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• pp.33-39
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• 2019

An 1,500MW advanced power reactor required the standard design approval by a Korean regulatory body in 2014. The reactor has been designed to have a 4-train independent safety concept and a passive auxiliary feedwater system (PAFS). The full power risk or core damage frequency (CDF) of 1,500MW advanced power reactor has been reduced more than that of APR1400. However, the risk during the low power and shutdown (LPSD) operation should be reduced because CDF of LPSD is about 4.7 times higher than that of internal full power. The purpose of paper is to analysis design alternatives to reduce risk during the LPSD. This paper suggests design alternatives to reduce risk and presents sensitivity analysis results.

• 대한전자공학회논문지
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• 제13권5호
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• pp.17-23
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• 1976

고속열중성자로에서 정상 운전중인 원자로를 운전정지하였다가 재가동할 때 가장 문제가 되는 것은 핵분열 생성물인 Xe135의 독소작용이다. 이것은 Xe135가 원자로 출력에 영향을 주는 열중성자에 대한 흡수단면적이 크고 그의 반감기가 길기 때문이다. 그러므로 원자로의 일시적 운전정지가 요구될 때 이의 재가시에는 반듯이 이 독소를 능과할 수 있는 충분한 초과반응도를 가해 주던지, Xe135가 붕괴되어 그의 농도가 줄어든 이후에야 원자로의 재가동이 가능하게 된다. 위와 같은 문제는 사실상 원자로 운전시 안전성 뿐만 아니라 경제성에도 큰 영향을 주고 있다. 본 논문에서는 이 점을 고려하여 Pontoyagin의 최대원리를 이용하여 운전정지를 최적화시키므로서 언제든지 원자로를 전출력으로 재가동할 수 있도록 운전정지 방법을 개선하였다. 그러나 제어과정에서나 그 이후에도 X, 농도는 제어된 허용치를 넘지 않고 최소시간 이내에 모든 제어를 끝내도록 하였다. The buildup of fission product, i.e. Xe-135 poisoning, is a prime factor in restarting a nuclear reactor from the shutdown, which was under normal operation in the high flux thermal reactor, It is caused by the high absorption crosssection of Xe-135 to thermal neutrons and its long half life, from which the thermal power is affected. It is then possible to restart a nuclear reactor after the sufficient excess reactivity to override this poisoning must be inserted, or its concentration is decreased sufficiently when its temporary shutdown is required. As ratter of fact, these have an important influence not only on reactor safety but also on economic aspect in operation. Considering these points in this study, the shutdown process was cptimized using the Pontryagin's maximum principle so that the shutdown mirth[d was improved as to restart the reactor to its fulpower at any time, but the xenon concentration did not excess the constrained allowable value during and after shutdown, at the same time all the control actions were completed within minimum time from beginning of the shutdown.

• Nuclear Engineering and Technology
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• 제53권9호
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• pp.2888-2898
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• 2021

Vital area identification (VAI) is an essential procedure for the design of physical protection systems (PPSs) for nuclear power plants (NPPs). The purpose of PPS design is to protect vital areas. VAI has been improved continuously to overcome the shortcomings of previous VAI generations. In first-generation VAI, a sabotage fault tree was developed directly without reusing probabilistic safety assessment (PSA) results or information. In second-generation VAI, VAI model was constructed from all PSA event trees and fault trees. While in third-generation VAI, it was developed from the simplified PSA event trees and fault trees. While VAIs have been performed for NPPs in full-power operations, VAI for NPPs in low-power and shutdown (LPSD) operations has not been studied and performed, even though NPPs in LPSD operations are very vulnerable to sabotage due to the very crowded nature of NPP maintenance. This study is the first to research and apply VAI to LPSD operation of NPP. Here, the third-generation VAI method for full-power operation of NPP was adapted to the VAI of LPSD operation. In this study, LPSD VAI for a few plant operational states (POSs) was performed. Furthermore, the operation strategy of vital areas for both full-power and LPSD operations was discussed. The LPSD VAI method discussed in this paper can be easily applied to all POSs. The method and insights in this study can be important for future LPSD VAI that reflects various LPSD operational states. Regulatory bodies and electric utilities can take advantage of this LPSD VAI method.

• 산업경영시스템학회지
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• 제37권4호
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• pp.63-71
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• 2014

As power consumption increases, more power utilities are required to satisfy the demand and consequently results in tremendous cost to build the utilities. Another issue in construction of power utilities to meet the peak demand is an inefficiency caused by surplus power during non-peak time. Therefore, most power company considers power demand management with time-based electricity rate policy which applies different rate over time. This paper considers an optimal machine operation problem under the time-based electricity rates. In TOC (Theory of Constraints), the production capacities of all machines are limited to one of the bottleneck machine to minimize the WIP (work in process). In the situation, other machines except the bottleneck are able to stop their operations without any throughput loss of the whole manufacturing line for saving power utility cost. To consider this problem three integer programming models are introduced. The three models include (1) line shutdown, (2) block shutdown, and (3) individual machine shutdown. We demonstrate the effectiveness of the proposed IP models through diverse experiments, by comparing with a TOC-based machine operation planning considered as a current model.

• Nuclear Engineering and Technology
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• 제46권4호
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• pp.513-520
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• 2014

In this paper, three types of operational and industrial absorbers used at research reactors, including Ag-In-Cd alloy, $B_4C$, and Hf are selected for sensitivity analyses. Their integral effects on the main neutronic core parameters important to safety issues are investigated. These parameters are core excess reactivity, shutdown margin, total reactivity worth of control rods, thermal neutron flux, power density distribution, and Power Peaking Factor (PPF). The IAEA 10 MW benchmark core is selected as the case study to verify calculations. A two-dimensional, three-group diffusion model is selected for core calculations. The well-known WIMS-D4 and CITATION reactor codes are used to carry out these calculations. It is found that the largest shutdown margin is gained using the $B_4C$; also the lowest PPF is gained using the Ag-In-Cd alloy. The maximum point power densities belong to the inside fuel regions surrounding the central flux trap (irradiation position), surrounded by control fuel elements, and the peripheral fuel elements beside the graphite reflectors. The greatest and least fluctuation of the point power densities are gained by using $B_4C$ and Ag-In-Cd alloy, respectively.