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NUCLEAR POWER PLANTS ELECTRICAL RETROFITTING FOR COST EFFECTIVENESS, RELIABILITY AND OPERATING EFFICIENCY

L. CIUFU* and M.O. POPESCU**

Politehnica University of Bucharest – Faculty of Electrical Engineering, Bucharest, Romania,

*Politehnica University of Bucharest, Bucharest, Romania, ciufu.laurentiu@gmail.com

** Politehnica University of Bucharest, Bucharest, Romania, mihaioctavian.popescu@upb.ro

ABSTRACT

In the context of continuous fast growing of the energy demand the current power plants

retrofitting concept may represent an important step in the emission reduction, being able to

offer in the same time a maximum operating efficiency. This desideratum can be obtained by

implementing a rigorous energy management plan, based on an increased energy production

capacity of non-pollutant electrical power plants and future-oriented frame on extending their

lifetime operation. This management is focused on using state-of-art electronic, electrical and

industrial control equipments, which can represent a real key factor. Thus, in this paper an

analysis of the electrical system retrofitting is presented. As a part of this research the authors

propose and simulate ambitious ways to upgrade actual control and command of the electrical

operating systems, by promoting variable speed for large pumps and also computer software,

as SCADA, for an intelligent control and monitoring of these studied processes.

Key words: NPP, Electrical Retrofitting, VSDs, SCADA

Introduction

In the context of the global continuous fast growing of the energy demand, in conjunction with the

climatic changes generated by the greenhouse gas emissions, such as carbon dioxide CO2 pollution

determined by the specific processes of energy production, the current power plants retrofitting concept

may represent a real key factor in the emissions reduction efforts, being able to offer in the same time a

maximum operating efficiency. This desideratum can be mainly obtained by implementing a rigorous

energy management plan within the plant, based on an increased capacity of energy production and a

future-oriented frame of extending their lifetime operation. According to worldwide energy capacity

factor presented in Fig. 1, the greatest share of the of non-pollutant electrical power plants is represented

by the Nuclear Power Plants (NPPs), with 0% carbon dioxide CO2 emissions. In comparison with

classical power plants with fossil fuels as coal, lignite, oil or gas, the NPPs have fully gained their

international reputation of being very clean and capable to produce and sell cheap energy, proving during

time its reliability and dependability on the global energy market.

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Fig. 1. Specific emissions percents of a 1000MW electrical generation capacity of a classical power plant versus a

nuclear power plant [1]

The Nuclear energy now represents 12 percent of total global electricity production and comprises 19

percent of total electrical generation in Romania and in the United States, 29 percent in South Korea, 43

percent in Sweden, and 82 percent in France [3]. The existing NPPs are one of the cheapest sources of

electrical power production in the United States [4], while France boasts the lowest electricity prices in

Western Europe [5]. In general, nuclear power plants have in total, competitive costs when compared with

fossil fuel generation power plants. The investment costs are higher, but today most of these plants only

produce for fuel costs and therefore are attractive.

As the current generation of NPPs have passed their mid-life, increased monitoring of their health is

critical to their safe operation. This is especially true now that license renewal of nuclear power plants has

accelerated, allowing some plants to operate up to 60 years or more. [23] In the meantime, the nuclear

power industry is currently working to reduce generation costs by adopting condition-based maintenance

strategies and automation of testing activities. Thanks to the advances in computer technologies, signal

processing and analytical modeling have provided the nuclear industry with ample means to automate and

optimize maintenance activities and improve safety, efficiency, and availability, while reducing costs and

radiation exposure to maintenance personnel. This paper provides a review of these developments and

makes use of their advantages by setting an internal energy savings strategy plan focused on a rigorous

energy efficiency management and a future-oriented life extension frame.

A rigorous energy management plan

In order to set a target on an increased energy production, based on internal energy savings considerations

and also setting a frame of a future-oriented life extension plan, the authors consider that the rigorous

energy management should be implemented in actual operating NPPs, focusing on the following aspects:

I. The first aspect is subject to maintenance and operating costs and may be achieved by taking into

consideration the possibilities of internal consumption reduction. The tools necessarily for this objective

are the state-of-art electronics, electrical and industrial control equipments of the plant electrical system.

This could represent a key factor that once implemented, can successfully put in practice the above

mentioned energy management plan. In order to seek the possibilities to achieve this target the plant

electrical and C&I system should by highlighted. Therefore, the potential parts of these systems must be

taken into account by a proper analysis.

II. The second aspect is subject to a frame setup of a future-oriented life extension plan, which may

be also considered as a part of the above mentioned subject regarding technical retrofitting, concerning

plant improvements as safety upgrades and/or modifications of fuel characteristics and performance as

well as refueling patterns and lead times. It‟s important to mention that NPP lifetime extension affects the

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overall operation and maintenance strategies, decommissioning schedule and strategy, radioactive waste

management and disposal requirements and can have a big impact on a country's overall nuclear energy

program [8]. For example in Romania, where the nuclear energy sector has an important role on the

energy market, with 2 operating CANDU Nuclear Units capable to generate nearly 1413 MW,

representing 19% of the national capacity factor of 7.6TW every hour [9], the implications of extending

Unit 1 lifetime operation with for another 25 years is very important for the Romanian energy grid, being

in this moment one of the Nuclearelectrica‟s2 major investment projects. According to a preliminary

analysis [11], the effective shut down of the Unit 1 it is set to be done in 2025-2026, and the retrofitting

process should start in the early 2020, with an estimated cost of about 1.5 billion euro. This process will

help in maintaining the nuclear energy program at the same capacity for another 25 years which would

mean a considerably contribution of stable energy, able to face the environment needs and to offer an

accessible price to final consumer.

Electrical system retrofitting

I. Background

The safe and economic operation of a NPP requires the plant to be connected to an electrical grid system

that has adequate capacity for exporting the power from the NPP, and for providing a reliable electrical

supply to the NPP for safe startup, operation and normal or emergency shutdown of the plant. Important

characteristic of nuclear power plant is that after reactor shut down, it continues to generate heat. Residual

heat generation can last for days after shutdown and require continuous operation of reactor cooling

system for prolonged period of time. Such system must have robust, reliable and diverse sources of

electrical power. Extended unavailability of cooling systems can have adverse effects on reactor core and

can cause release of radioactivity into environment. [26]

In order to seek the potential energy efficiency parts of the nuclear electrical system a proper analysis of

the system must be performed first. Therefore, a technical layout should be considered first in order to

describe the main system functions. Basically, there are two main functions of electrical systems in a

power plant, and their role is to:

to distribute the electrical power from the power supply to the components;

to provide electrical protection for the power source and also for the components supplied.

In a NPP there are several levels of electrical power distribution (other than from the generator to the

generator transformer):

High AC Voltage (may also be abbreviated VAC) (e.g. 4160, 6900, or 13800 Volts);

Medium AC Voltage (e.g. 480 VAC);

Low AC Voltage (e.g. 120, 240 VAC, 260 VAC).

High voltage systems are used to supply equipment that has motors with high horsepower ratings.

Examples of these are:

Feed water pumps;

Recirculation or Reactor Coolant Pumps;

Circulating Water Pumps;

Condensate Pumps;

Cooling Tower Pumps and Fans;

High Pressure Emergency Makeup Pumps;

Containment Spray Pumps.

2 Nuclearelectrica S.A. is a Romanian state owned company, reporting to the Ministry Energy, Small and Medium

Sized Enterprises and Business Environment, in charge of managing Cernavodă NPP and its Nuclear Fuel Plant,

localized in Piteș ti County.

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Medium voltage systems are used to supply equipment that has motors with moderate horsepower ratings.

Examples of these are:

Auxiliary or Emergency Feed water pumps;

Heating Ventilating Air Conditioning (HVAC) Fans and Chiller Units;

Control Rod Drive Motor-Generator Sets;

Motor operated valves.

Low voltage systems are used to supply equipment that has motors with low horsepower ratings.

Examples of these are:

Lights;

Small pumps.

It can be observed from all above electrical power distribution levels that the majority of the supplied equipments in the plant consist of motors, usually asynchronous motors, who‟s task is to drive a pump in order to power the NPP processes. Control of these processes involves the “throttling” of liquids by incremental by-passing pumping action. [12] This is achieved by the use of process control valves of which a typical Generation II

3 type reactor has more than a hundred. For example, in heavy water-

moderated CANDU type NPP, large amounts of cooling agent (heavy water within a concentration of 99.75%) are needed to cool down the reactor core. In order to perform this process, four big (3.5MW) high voltage (6kV) recirculation pumps are necessary. [9] Reported to the internal energy inventory they represent the greatest share (26.4%) of the internal services power consumption, with a nominal power of 14 MW every hour.

II. Introducing state-of-the-art variable frequency drives (VFD) According to one of the authors assessment study regarding the implementation of the energy efficiency concept in Nuclear Power Plants [10] a new control method for the nuclear unit‟s high voltage large pumps, using state-of-art variable frequency drives, represents the solution for capital cost savings which can be materialized in energy efficiency concept at the NPP internal consumption level. In comparison to classical technology, state-of-art VFDs technology can purely simplify the control circuit algorithm, and to provide at the same time an increased and precise speed control of the pumping process with proven and enhanced reactor control performance. Also its vast experience in industrial applications has proven to be a reliable, safe and advantageous technology, which once used for driving the nuclear units main recirculation pumps, can create a real energy savings opportunity in CANDU NPPs (as presented in Fig.

2).

3 Generation II type reactors include the class of commercial reactors built up to the end of the 1990s as

PWR, CANDU, BWR, AGR, and VVER.

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Fig. 2. Power rating (P) and flow ratio rate (Q) characteristic curve generated by the SINASAVE ENERGY

EFFICIENCY TOOL software as a comparison between current control system (red line) and VFD control method

(blue line)[10]

In order to prove the effectiveness of this method specific computational simulation software was carried out as SINASAVE ENERGY EFFICIENCY TOOL [19], using CANDU reactor specific data. The final impact of this innovative solution consists in a considerable reduction up to 53% of the pump energy consumption, depending of specific parameters regarding the core reactivity and the nominal reactor power set point. Hence, significant decrease in overall NPP internal services energy consumption may occur, with a reduction of 12.4 percent.

Reported to the international research [13][14][15][16][17][18], especially the American (U.S.) research,

was shown [13] that by introducing a VFDs in a motor-generator (MG) retrofitting process of a several

NPPs, MGs used for driving the recirculation pumps respectively, significant consumption reduction may

occur in nuclear units internal services, presenting also the advantage of more precise control of nuclear

reactor. The applicative potential of this research was recovered in the technological retrofitting of the „50

years old operating U.S. NUs with MV VFDs, as Perfect Harmony, provided by SIEMENS [20]. The

measurements made at Browns Ferry NPP and Hatch NPP [13] after this process revealed that for various

loads, superior efficiency (95.7%-96.6) can be achieved, in comparison with the classical MG efficiency

(70%).

On this research line, further efforts were made by the authors [21], by designing a prototype schematic

diagram topology of a Pulse Width Modulation (PWM) Variable Frequency Drive designed for a

CANDU type reactor 6kV medium voltage asynchronous motor, taking into consideration its technical

characteristics. In order to analyze and compare the frequency response characteristics at the motor

terminals, various computational simulations were carried out using PowerSIM (PSIM) software [22].

Various attempts regarding the improvement of the developed schematic diagram topology have finally

revealed that using a model of a constructive topology consisted by a Variable Frequency Drive with

intermediary circuit and with a PWM control of the IGBT semiconductors (as shown in Fig. 3) present a

relatively low ripple of the motor supply current waveforms. Simulations results of the proposed topology

prove and validate at the same time the effectiveness of the new developed schematic diagram.

a) b) c)

Fig.3. Simulation results of the prototype schematic diagram topology consisting of a Variable Frequency Drive

with intermediary circuit and with PWM control of the IGBT semiconductor. a)current spectrum on supply line –

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before rectifier; b)motor stator current spectrum; c)Fast Fourier Transform (FFT Analysis) of the supply motor

current (at the terminals). [21]

III. Implementing latest SCADA and online monitoring (OLM) technology

An online monitoring (OLM) system is made up of a data acquisition module involving hardware and

software and a data processing module involving software implemented on a fast computer. The system

can be built into the design of new plants or deployed as an add-on feature to the existing generation of

plants. A fundamental requirement for an OLM system is a fast data acquisition module. In the current

generation of nuclear power plants, data from process sensors is normally sampled by the plant computer

at rates of one sample per second or slower. This is adequate for applications such as calibration

monitoring but not for dynamic analysis. For analysis of dynamic signals, at least 100 to 1000 samples

per second are normally required. In the new generation of nuclear power plants, this requirement can be

accommodated simply by bringing the data into the plant computer through a fast analog-to-digital (A/D)

converter and providing adequate storage to save the data for subsequent retrieval and analysis. However,

in the existing generation of reactors, it is not simple to retrofit the plant with a fast data acquisition

system and new storage provisions. In fact, even recent digital retrofits in nuclear power plants have not

provided the necessary means for fast data collection and storage. As such, in the current generation of

nuclear power plants, a separate data acquisition system must be installed for collection of dynamic data.

[23]

Compared to OLM a Supervisory Control and Data Acquisition (SCADA) is referring to a more complex

measurement and a control system. [25] This “brain” of field data acquisition, revolutionary within

nuclear sector, represents a courageous concept of digitization of nuclear facilities, which in order to be

implemented must prove first its robustness towards classical relay logic and to systematic accomplish a

series of requirements (such as fail safe criteria & redundancy) and Nuclear safety criterion (e.g. SIL –

Safety Integrity Level4). Once this criterion has been respected the current plant can become a state-of-

the-art digitized system, with a distributed control by programmable logic controllers (PLCs) supplied by

remote terminal units (RTUs).

Currently the analogical and/ or digital signals from the field (plant) are collected from various existing

transducers of temperature, pressure, level, flow and so on, and directly sent to redundant computers,

situated near the Main Control Room, where the operators must monitor them on two large displays

(CRTs). This process can be simplified by reading and processing them locally and after forwarded via

Modbus (secured internal network) to decisional units: PLCs. Using SCADA installed on control room

operators can easily compare many measured values to set reference parameters adjusting precisely the

speed/ valves positions in order to achieve default set points. Data can also be restructured in a convenient

form that uses an operator interface HMI, projected to dedicated monitors.

It is a common sense to conclude that using SCADA software technology and also updated online data

monitoring (OLM) together with the late advances in electronic, electrical, industrial control equipments

technologies can significantly improve and boost the actual operating efficiency of the current NPP and

also change of nature of the processes by becoming fully integrated and intelligent systems.

Conclusions

In this paper an overview of applicable state-of-art electronic, electrical, industrial control equipments and

software technologies for plant retrofitting process was presented. Also an analysis of the electrical

system and the control and instrumentation system was included to give the reader a better understanding

4 Safety integrity level (SIL) is defined as a relative level of risk-reduction provided by a safety function, or to

specify a target level of risk reduction. In simple terms, SIL is a measurement of performance required for a safety

instrumented function (SIF) [24]

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of the importance of the retrofitting process. As a part of this research the authors proposed and simulated

ambitious ways to upgrade actual control and command of the electrical operating systems, by promoting

variable speed for large pumps with significant decrease impact in overall NPP internal services energy

consumption, up to 12.4 percent, and also computer software, as SCADA, for an intelligent control and

monitoring of these studied processes. In order to prove its effectiveness further efforts were made by the

authors, by designing a prototype schematic diagram topology of a Pulse Width Modulation (PWM)

Variable Frequency Drive designed for a CANDU type reactor 6kV medium voltage asynchronous motor,

taking into consideration its technical characteristics. This concept was possible thanks to the advances in

computer technologies and simulation software, signal processing and analytical modeling that have

provided the nuclear industry with ample means to automate and optimize maintenance activities and

improve safety, efficiency, and availability, while reducing costs and radiation exposure to maintenance

personnel. This paper provided a review of these developments and put together their means and their

advantages by defining a strategy of internal energy savings focused on a rigorous energy efficiency

management plan and aiming on a future-oriented life extension frame.

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