Cernavoda CANDU severe accident evaluation

The papers present the activities dedicated to Romania Cernavoda Nuclear Power Plant first CANDU Unit severe accident evaluation. This activity is part of more general PSA assessment activities. CANDU specific safety features are calandria moderator and calandria vault water capabilities to remove the residual heat in the case of severe accidents, when the conventional heat sinks are no more available. Severe accidents evaluation, that is a deterministic thermal hydraulic analysis, assesses the accidents progression and gives the milestones when important events take place. This kind of assessment is important to evaluate to recovery time for the reactor operators that can lead to the accident mitigation. The Cernavoda CANDU unit is modeled for the of all heat sinks accident and results compared with the AECL CANDU 600 assessment.     

Severe accident codes status and future development

New demands for acceptance of nuclear power require full deterministic evidence of nuclear power plants (NPP) safety. From this point of view, the role of precise deterministic analysis of NPP safety plays a very important role for both the existing and future generation of NPPs. Considering the current status of existing severe accident codes, one may conclude that their capabilities are quite limited and not sufficient to prove NPP safety. This conclusion is based on the experience of usage of these codes, analysis of models and experimental database supporting codes and used for their validation. At the same time, the modern level of development of computer techniques and numeric methods allows the use of equations based on first principles rather than correlation. The transition to physical modeling appears to be more effective in the cases of designing and validation of codes using both separate effect and integral tests, and allows predictive power of codes to be increased and the range of uncertainties to be reduced. Moreover, physical modeling allows critical points of models and codes to be understood, and permits the planning of integral tests to resolve severe accident and accident management issues.     

CANDU核电机组的特点与发展

简述了CANDU反应堆的优点 ,如使用天然铀燃料、容量因子高、某些方面固有安全性高、能廉价大量生产同位素 :问题是压力管寿命只有 2 5a、重水管理复杂、氚排放量偏大。还概述了CANDU反应堆的近期发展 ,如燃料设计、燃料通道设计、提高热传输系统参数、建造优化和强化非能动安全性等     

Improvements of the reactivity devices modeling for the advanced CANDU reactor

In the context of the ACR™ (Advanced CANDU Reactor), 3D transport calculations are required in order to simulate the reactivity devices located perpendicularly to the fuel channels. The computational scheme that is usually used for CANDU-6 and ACR reactors is based on a simplified supercell geometry in which the fuel clusters and devices are replaced by annuli. Recently, an exact modeling of 3D supercell configurations was introduced within the framework of the ACR calculations. However, with such a model, fine meshing requirements lead to problems that are very demanding in terms of computational resources.

In this paper, we present improvements introduced in the ACR context to reduce the cost of the 3D supercell calculations. Two avenues of investigations are reported. First, the introduction of an accelerated characteristics method permits to reduce the computational burden of such calculations involving a large number of regions. In addition, contrarily to CANDU-6 supercell configurations, the ACR 3D geometry is prismatic and consequently a special tracking procedure can be used. This approach introduces no approximation and is significantly faster than the general 3D tracking technique. Thanks to these modifications in the computational procedure, 3D supercell calculations with a level of mesh discretization comparable to 2D cell configurations become affordable for industrial applications.     

Severe accident mitigation programme for the forsmark and ringhals plants

In February 1986 licensing requirements regarding severe accidents in nuclear power plants were given by the Swedish Government. This regulation constitutes conditions for operation of the plants beyond 1988. The requirements are based on the conditions previously given for the Barsebäck plant including construction of the filtered venting system, which was completed at Barsebäck in 1985.For the Forsmark and Ringhals plants a strategy is being implemented to meet the new requirements. A strong emphasis is put on both hardware and procedural measures to bring the reactor core back to stable cooling - even if it is severely damaged - and maintain the containment integrity during an accident. The hardware modifications include measures to prevent temperature or pressure induced early containment failure for the BWRs, reliable back-up water sources for containment spray and means for filtered venting of all plants to prevent late containment failure by overpressure. The ultimate aim is to minimize the environmental impact of a severe accident and meet a release limit set at 0.1% of the core fission product inventory excluding noble gases.     

Generic CANDU 6 plant severe accident analysis employing SCAPSIM/RELAP5 code

The purpose of the present study is to assess the capability of SCDAPSIM/RELAP5 to perform the deterministic analysis for postulated severe accidents for CANDU plant and to gain information for potential improvements in code modelling. SCDAPSIM/RELAP5 is a widespread and detailed computer code for severe accident analysis that can be adapted to benchmark the CANDU dedicated tools, MAAP4–CANDU and ISAAC. Simulations of station blackout (SBO) and large loss-of-coolant accident (LOCA) scenarios, which, through further system failures, may eventually lead to severe core damage (SCD) accident in a CANDU 6, are presented. The paper provides details concerning the methodology and nodalization used, and interprets the results obtained. Comparisons of the SCDAPSIM/RELAP5 simulations with the MAAP4–CANDU code reported results are presented. Also, some insights are given on possible reasons for the discrepancies between the SCDAPSIM/RELAP5 and MAAP4–CANDU code predictions.     

Development of severe accident management guidance (SAMG) for the Canadian CANDU 6 nuclear power plants

This paper discusses the severe accident management guidance (SAMG) development process undertaken for the Canadian CANDU 6 nuclear power plants (NPPs); the customization process of the generic CANDU SAMG for the Point Lepreau NPP is presented. Examples of severe accident management (SAM) guidelines related to containment pressure control are included in this paper. This paper also provides an overview summary of the severe accident analysis program at Atomic Energy of Canada Limited (AECL) that complements the SAM guidelines development process for the CANDU 6 NPPs in Canada. These analyses provided insights into the accident progression and basis to develop the SAM guidelines.     

Advanced technologies for CANDU reactors

The inherent features of a reactor based on multiple pressure tubes, rather than a single pressure vessel, provide CANDU with considerable flexibility for continuous design improvements. Of particular significance is the presence of a large volume of cool heavy-water moderator within the core. This can act as a heat sink for passive heat rejection during high-temperature accidents. We describe elements of our R&D program that are aimed to exploit this benefit. Similarly, safety, reliability and economy are the focus for our exploration of advanced technologies in the areas of advanced fuel and fuel channels, improved heavy-water production and computer-based systems that facilitate reactor design, construction and operation.     

用于CANDU堆换料方案快速评价的线性敏感矩阵方法

为了实现CANDU堆换料方案的快速评价,本文提出了线性敏感矩阵方法(LSM).该方法基于各种扰动引起的堆芯状态变化互不干涉且堆芯对扰动的响应与扰动量成正比的假设,借助于事先针对参考堆芯形成的敏感矩阵,主要的堆芯参数经简单代数运算就可得到,无需耗时的三维扩散方程求解.秦山三期1号机组部分运行历史检验结果表明,该方法不但可大幅提高换料方案评价的速度,而且还具有较高的精度,能满足现场工程应用的要求.     

Severe accident mitigation concept of the EPR

For the EPR great emphasis is laid to gain further improvement in prevention of severe accidents. Nevertheless an additional level of safety with specific engineered safety features is introduced to cope with the potential consequences of a severe accident with an assumed core melt down. To prove and demonstrate the feasibility of these features supporting research and development are performed via close co-operations with national and European research centres.     

Severe accident recriticality analyses (SARA)

Recriticality in a BWR during reflooding of an overheated partly degraded core, i.e. with relocated control rods, has been studied for a total loss of electric power accident scenario. In order to assess the impact of recriticality on reactor safety, including accident management strategies, the following issues have been investigated in the SARA project: (1) the energy deposition in the fuel during super-prompt power burst; (2) the quasi steady-state reactor power following the initial power burst; and (3) containment response to elevated quasi steady-state reactor power. The approach was to use three computer codes and to further develop and adapt them for the task. The codes were SIMULATE-3K, APROS and RECRIT. Recriticality analyses were carried out for a number of selected reflooding transients for the Oskarshamn 3 plant in Sweden with SIMULATE-3K and for the Olkiluoto 1 plant in Finland with all three codes. The core initial and boundary conditions prior to recriticality have been studied with the severe accident codes SCDAP/RELAP5, MELCOR and MAAP4. The results of the analyses show that all three codes predict recriticality—both super-prompt power bursts and quasi steady-state power generation—for the range of parameters studied, i.e. with core uncovering and heat-up to maximum core temperatures of approximately 1800 K, and water flow rates of 45–2000 kg s⁻¹ injected into the downcomer. Since recriticality takes place in a small fraction of the core, the power densities are high, which results in large energy deposition in the fuel during power burst in some accident scenarios. The highest value, 418 cal g⁻¹, was obtained with SIMULATE-3K for an Oskarshamn 3 case with reflooding rate of 2000 kg s⁻¹. In most cases, however, the predicted energy deposition was smaller, below the regulatory limits for fuel failure, but close to or above recently observed thresholds for fragmentation and dispersion of high burn-up fuel. The highest calculated quasi steady-state power following initial power excursion was in most cases approximately 20% of the nominal reactor power, according to SIMULATE-3K and APROS. However, in some RECRIT cases higher power levels, approaching 50% of the nominal power, were predicted leading to fuel temperatures exceeding the melting point, as a result of insufficient cooling of the fuel. Long-term containment response to recriticality was assessed through MELCOR calculations for the Olkiluoto 1 plant. At a stabilised reactor power of 19% of nominal power, the containment failure due to overpressurisation was predicted to occur 1.3 h after recriticality, if the accident is not mitigated. The SARA studies have clearly shown the sensitivity of recriticality phenomena to thermal-hydraulic modelling, the specifics of accident scenario, such as distribution of boron-carbide, and importance of multi-dimensional kinetics for determination of local power distribution in the core. The results of the project have pointed out the importance of adequate accident management strategies to be used by reactor operators and emergency staff during recovery actions. Recommendations in this area are given in the paper.     

CANDU反应堆物理程序和方法

简要介绍了加拿大原子能公司目前用于ＣＡＮＤＵ反应物理设计和分析的计算机程序和方法,对栅元,超栅元和堆芯三种计算方法及相应的计算机程序进行了讨论。对物理分析中每 理论表达和应用的求解方法也作了说明。     

The development and demonstration of integrated models for the evaluation of severe accident management strategies—SAMEM

This study is concerned with the further development of integrated models for the assessment of existing and potential severe accident management (SAM) measures. This paper provides a brief summary of these models, based on Probabilistic Safety Assessment (PSA) methods and the Risk Oriented Accident Analysis Methodology (ROAAM) approach, and their application to a number of case studies spanning both preventive and mitigative accident management regimes. In the course of this study it became evident that the starting point to guide the selection of methodology and any further improvement is the intended application. Accordingly, such features as the type and area of application and the confidence requirement are addressed in this project. The application of an integrated ROAAM approach led to the implementation, at the Loviisa NPP, of a hydrogen mitigation strategy, which requires substantial plant modifications. A revised level 2 PSA model was applied to the Sizewell B NPP to assess the feasibility of the in-vessel retention strategy. Similarly the application of PSA based models was extended to the Barseback and Ringhals 2 NPPs to improve the emergency operating procedures, notably actions related to manual operations. A human reliability analysis based on the Human Cognitive Reliability (HCR) and Technique For Human Error Rate (THERP) models was applied to a case study addressing secondary and primary bleed and feed procedures. Some aspects pertinent to the quantification of severe accident phenomena were further examined in this project. A comparison of the applications of PSA based approach and ROAAM to two severe accident issues, viz hydrogen combustion and in-vessel retention, was made. A general conclusion is that there is no requirement for further major development of the PSA and ROAAM methodologies in the modelling of SAM strategies for a variety of applications as far as the technical aspects are concerned. As is demonstrated in this project, the generic modelling framework was refined to enable a number of applications. Some recommendations have also been made regarding the applicability of these approaches to existing operating reactors and future reactors. The need for further research and development in the area of human reliability quantification was identified.     

秦山CANDU项目的计算机辅助设计

简要介绍了加拿大原子能公司在秦山ＣＡＮＤＵ项目设计中应用计算机辅助设计与工程管理工具的情况。本文介绍了其它专用程序软件及应用。对使用的独立 的非ＣＡＤＤ工具也作了介绍。     

钍基先进坎杜堆子通道分析

利用子通道计算程序ASSERT-PV V3R1计算TACR1000在不同钍装填模式、不同功率、不同寿期下的子通道热工水力学特性.根据对子通道质量流密度、空泡份额和干涸起始功率方面的计算,从功率展平及安全性的角度考虑,钍铀粉末交混装填模式明显好于内8根棒为钍的装填模式.     

Natural-convection studies for advanced CANDU reactor concepts

AECL is studying advanced reactor designs where natural convection is an important design feature in heat removal processes. The use of a flashing-driven, natural-circulation system to remove moderator heat is being considered. Experiments and code simulations have shown that a flashing-driven system is feasible at normal operating power, but is prone to flow instabilities at low powers. Vapor flashing at superheated conditions and the presence of nucleation sites were found to be important for stable operation over the whole power range. A development concept for CANDU® is to increase the primary coolant pressure and temperature to supercritical conditions. With a natural-convection-driven primary flow, the large variations in fluid properties near the critical point introduce the potential for flow instabilities. Analyses have shown that flow instabilities can occur under certain conditions. Experimental and analytical results on the flashing system are described. The experiments and initial analytical results for the supercritical concept are discussed.     

Reanalysis and evaluation of the seismic capacity of the CANDU 700 MW NPP CERNAVODA

The dynamic response of structures of the CANDU 700 MW NPP due to seismic loadings was conventionally analyzed in the time domain using modal substructure procedures. The frequency-independent parameters were tuned to the main frequencies of the soil-structure system. This is a common procedure widely used in the preliminary design of power plant structures and provides conservative results. However, parallel to the rapid progress being made in upgrading the capability of data processing systems, methods and software tools have become available which work in the frequency domain using complex (soil-structure) mathematical models or models in which the soil is represented by frequency-dependent impedances. In order to demonstrate the reserves existing in the design of the CANDU 700 reactor building, frequency-domain calculations were additionally prepared. The analyses were based on appropriate mathematical 3D-models of the coupled vibrating structures of the reactor building and as the soil represented by frequency-dependent impedances. The results obtained by using the time and frequency domain methods were compared and the safety margins of the CANDU design discussed.     

CANDU 6机组重水管理

分析了秦山三期ＣＡＮＤＵ型重水堆重水泄漏、回收和损失的途径,介绍了重水的供应、收集、净化、回收、提浓和监测系统。     

Severe accident management strategy verification for VVER-1000 (V320) reactor

Severe accident analysis of a reactor is an important aspect in evaluation of source term. This in turn helps in emergency planning and Severe Accident Management (SAM). The use of the Severe Accident Management Guideline (SAMG) is required for accident situation which is not handled adequately through the use of Emergency Operating Procedures (EOP), thus leading to a partial or a total core melt. Actions recommended in the SAMG aim at limiting the risk of radiologically significant radioactive releases in the short- and mid-term (a few hours to a few days). Initiation of SAMG for VVER-1000 is considered at two core exit temperatures viz. 650 °C as a desirable entry temperature and 980 °C as a backup action. Analyses have been carried out for VVER-1000 (V320) for verification of some of the strategies namely water injection in primary and secondary circuit. These strategies are analysed for a high and low pressure primary circuit transients. Station Black Out (SBO) is one such high pressure transient for which core heat can be removed by natural circulation of the primary circuit inventory by maintaining the secondary side inventory. This strategy has been verified where the feed water injection to secondary side of SG is considered from external power sources (e.g. mobile DG sets) as suggested in SAM guidelines. The second transient, a low pressure event is analysed for verification of the SG flooding and core flooding strategies. The analysis shows that SG flooding is not adequate to arrest the degradation of the core. In case of core flooding strategy, the analyses show that core flooding is not adequate to arrest the degradation of the core for the large break LOCA where as for small break LOCA the injections through available safety systems are adequate. The assessments are carried out with integral severe accident computer code ASTEC V1.3.     

An evaluation of a general venting strategy in a CANDU 6 reactor building

If the reactor building sprays or local air coolers are not available, depressurization by reactor building venting is considered as a useful mitigation strategy for a severe accident management of the Wolsong plants. As the containment filtered vent system is not established in the Wolsong Units, the reactor building isolation system can be a substitute for reactor building venting. The D₂O vapour recovery system which has a 0.76 m (30 in.) diameter penetration is expected to meet the NRC requirements. To investigate the effectiveness of the Reactor Building Venting Strategy, three kinds of accidents are analyzed: a SBO, a Small LOCA and a Large LOCA. The reactor building pressure behavior was analyzed with the ISAAC computer code for four different cases: without venting, 379 kPa(g)/345 kPa(g) (55 psig/50 psig), 345 kPa(g)/276 kPa(g) (50 psig/40 psig) and 345 kPa(g)/207 kPa(g) (50 psig/30 psig) valve open/close pressures. When the reactor building spray or local air coolers can not be operated, a depressurization strategy by using the D₂O Vapour Recovery System could prevent a reactor building failure and reduce the amount of CsI released to the environment. The present study shows that the operation of valves at a pressure of 379 kPa(g)/345 kPa(g) (55 psig/50 psig) is safe and effective. Based on the current study, the strategy of reactor building venting is involved in severe accident management guidance-5.