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Software systems and computational methods
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The search for baseline events for assessing the safety of nuclear refueling operations at nuclear power plants

Maltseva Nadezhda Konstantinovna

PhD in Technical Science

associate professor of the Department of Technogenic Security Systems and Technologies at ITMO University (Saint Petersburg National Research University of Information Technologies, Mechanics and Optics)

197101, Russia, St. Petersburg, str. Kronverkski Prospect, 49

nkmaltseva@hotmail.com
Other publications by this author
 

 
Popova Valeriya Olegovna

Master's Degree, the department of Systems and Technologies of Techtogenic Safety, ITMO University; Engineer "Diakont" Ltd

197701, Russia, g. G sankt-Peterburg, ul. Kronverkskii Prospekt,, 49

lerapopova236@gmail.com
Syrov Aleksandr Aleksandrovich

Engineer, "Diakont" Ltd

197101, Russia, g. G sankt-Peterburg, ul. Uchitel'skaya, 2

stts@diakont.com

DOI:

10.7256/2454-0714.2022.1.19323

Received:

29-05-2016


Published:

05-01-2022


Abstract: The relevance of the topic of the safety of nuclear refueling operations is associated with the specificity of exploitation of RBMK units. One of the most hazardous, from the perspective of accidents at modern nuclear power plants, is the process of nuclear fuel reloading. The operations on rearrangement of fuel cartridges entail the risk of fuel damage, and thus, the likelihood of the release of radioactive substances exceeding the permissible limits. The process of reloading RBMK, if the reactor is at full capacity, consists of the vast number of complex operations characterized by a range parameters. Non-observance of the criteria for carrying out operations, or if the parameter values exceed permissible limits, with high probability leads to an accident. This article explores the possibility of application of formalized approach towards determination of the baseline events that may cause accidents for the purpose of the development of essential protection instruments. The formal approach would allow detecting the excessiveness in protection instruments on the existing blocks, as well as revealing the accident situations that cannot be prevented using these protection instruments. The author formulated systemic approach towards comprehensive assessment of the accident rate of structurally complex systems. Adaptation of this method relative to REM allows systematizing the search for baseline vents that entail uncontrolled situations, as well as optimizing the protection instruments that would ultimately enhance reliability of the system, simplify the exploitation process, and reduce the time of operating cycle of the controller for processing of the protection.


Keywords:

refueling, unit operation, Water-Water Energetic Reactor, High Power Channel-type Reactor, initiator, expert commentary, Atomic Power Station, token approach, plant safety analysis, HAZOP method

This article is automatically translated. You can find original text of the article here.

         Currently, two types of reactor installations are mainly operated in our country: water-water power reactors (VVER) and high-power channel reactors (RBMK). The two types of reactors have fundamental differences in terms of the basic principles of operation [1, 7].

         The relevance of the chosen issue, namely, the safety of nuclear fuel transshipment operations (RBMK), is related to the specifics of the operation of RBMK units. One of the most dangerous, from the point of view of accidents at modern nuclear power plants, is the process of overloading nuclear fuel. During the operations on the rearrangement of fuel cartridges, there is a risk of fuel damage, and, as a result, the probability of the release of radioactive substances beyond the permissible limits.

         Fuel overload on RBMK-type reactors occurs, among other things, while the reactor is at capacity (on average, 2-3 cassettes are replaced per day), unlike VVER, where the reactor is stopped for this, which makes the process of rearranging cassettes on RBMK even more dangerous. The process of overloading the RBMK when the reactor is at capacity consists of a very large number of complex operations characterized by a variety of parameters. Failure to comply with the criteria for performing operations, the output of parameter values beyond the acceptable ranges is likely to lead to an accident.

         To increase safety and reduce the likelihood of fuel damage, there are protections. Currently, in the practice of NPP operation, protections are formed on the basis of expert opinion [2, 3, 5].

         This article discusses the possibility of using a formalized approach to identify the initial events that can lead to accidents, in order to form the necessary and sufficient set of protections. A formal approach will allow you to determine redundancy in the protection sets on existing blocks, as well as help identify emergency situations for which protection is not provided.  

         In order to achieve these goals, a systematic approach is needed to fully assess the accident rate of structurally complex systems.

         Most methods of safety analysis of technically complex systems are based on the fact that possible types of equipment failures or dangerous deviations are formed based on the knowledge and experience of the researcher. In some cases, this approach introduces significant subjectivity into the results of the security analysis.

         To reduce subjectivity when performing security analysis and formalizing the procedure for determining initial events, it is proposed to use the HAZOP method.

         The HAZOP method [8] was developed in the 60s of the last century and is a systematic search for dangerous equipment conditions. The HAZOP method is based on the concept of "path of change" and "leading word". The path of change refers to the physical movement of equipment mechanisms from point A to point B or the transition from one state to another (see Figure 1).

Figure 1 - Concepts of the path of change

         A leading word is a word or a group of words that allow you to stimulate the search for possible deviations in the parameters of the technological process on any particular path of change. Examples of suggestive words and their meanings are given in Table 1 [1].

Table 1. Example of suggestive words and their meanings

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less

Suggestive word

Meaning

NO (or NOTHING)

None of the goals of the project has been achieved

MORE (more, higher)

Quantitative increase

Quantitative reduction

ALSO AS (more than)

There is a qualitative change or additional work

PART (of something)

Only some of the project's goals have been achieved

refund

Logically contradicts the goals of the project

OTHER

Complete replacement – another job takes place

WHERE ELSE

Refers to flows, transitions, sources, destinations

BEFORE / AFTER

Refers to the order of the sequence

SOONER / LATER

Time estimates that differ from expected

FASTER / SLOWER

Step completed faster or slower than planned

         The combination of suggestive words with specific equipment parameters characteristic of the path of change under consideration allows you to generate possible deviations in the condition of the equipment (see table 2).

Table 2. Suggestive words applied to process parameters

 and allowing to describe real deviations [4, 8]

padding:0cm 5.4pt 0cm 5.4pt'>

Composition

Equipment Parameter

Suggestive words that can give a meaningful combination

Flow rate (flow)

No more than; less than; return; somewhere; also as.

Temperature

Above, below

Pressure

Above, below, vacuum

Level

No, higher, lower

Offset

Less, more, no

Reaction

Above (speed), below (speed), missing, reverse; same as

Phase

Other, reverse; same as.

Part; same as.

Transmission of information

No, part, more, less, different; same as.

         The HAZOP method was adapted to the REM. Unloading and loading machine (REM) (see fig. 2) designed to overload a working or stopped RBMK reactor.

Figure 2 - Unloading and loading machine

         Horizontal guidance of the REM is carried out using a bridge and a trolley. The bridge moves along the central hall on crane tracks placed at a height of 11 meters from the floor of the hall. The crane trolley moves along the bridge tracks across the central hall. The combination of the mutual movement of the bridge and the trolley allows you to bring the REM to any point of the hall. A fixed biological protection is installed on the trolley, made in the form of a container in which a REM spacesuit is installed. A movable biological protection covering the gap between the container and the floor of the central hall is mounted in the lower part of the container.

         Performing an overload using REM can be represented as follows. The REM with a loaded cassette for staging in the TC is aimed at the channel, docked with it. Then the pressure rises in the spacesuit with the help of a pump. Next, the channel is depressurized and the cassette is removed from the TC into the free pencil case of the store. With the help of the magazine rotation mechanism, a pencil case with a loaded cassette is installed above the channel in the TC. After placing the cassette in the channel, the TC is sealed with a cork.

         Upon completion of the sealing quality check, the REM is undocked with the channel and transferred to the fuel holding pool, where the cassette extracted from the reactor is unloaded.

         HAZOP analysis in relation to REM includes the solution of the following tasks.

         Task 1. Analysis of technological operations performed using REM, determination of the state of REM and possible ways of change.

         When determining the conditions of REM, it is necessary to adhere to the following rules:

- immutability of the physical parameters of the system (pressure, temperature, etc.);

- immutability of the state of the mechanisms in terms of the presence of movement;

- immutability of the system state in terms of the parameters and control modes set by the operator.

         When the current state changes, the system moves to the next state, characterized by a change.

         When determining possible ways of change, the following rules must be followed.

         The transition of REM from one state to another is characterized by a smooth or abrupt change in one or more parameters. As part of the HAZOP analysis, one should strive to consider the shortest possible ways of change. In general, it is recommended to consider the ways of change connecting neighboring states in the order of execution of the technological process. An example of possible ways to change the state of REM:

 - moving the bridge, trolley to the seat of the training stand (TS);

 - docking with the vehicle socket;

 - empty store rotation;

 - moving the empty grip down;

- unscrewing the cork;

 - moving the grab with fresh fuel up

 - undocking with the channel.

- moving the empty grip down and grabbing the cassette

 - unscrewing the plug (depressurization of the TC)

 - moving the spent fuel gripper up, etc.

Task 2. Analysis of the ways of change and the formation of possible deviations.

         To solve this problem, it is proposed to group the parameters and determine the characteristic deviations for them. In general, the set of REM parameters should be divided into groups. For example, the "Pressure" group includes pressure in the spacesuit, drum separators and in the reactor channel. The analysis of the working conditions of the REM made it possible to identify characteristic and most dangerous deviations for each group of parameters (see Table 4).

Table 3.

Parameter Group

Possible deviations

Pressure

Exceeding the permissible value, no change, deviation from the set value

Effort

Non-compliance with the specified capture state, exceeding the permissible value

….

...

Task 3. The results should be presented in the form of table 4.

Table 4.

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Moving the spent fuel gripper up

The path of change

Possible deviations

Consequences of deviations

Unscrewing the plug (depressurization of the TC)"

Unauthorized emptying of the connecting pipe (pressure drop in the connecting pipe)

A violation of the pressure balance in the channel and the connecting pipe can lead to damage to the cassette and the REM

Unauthorized increase of the force on the grip

Damage to the cassette as a result of "mashing" in the channel

….

         On modern RBMK units, about 63 technological operations are provided in the REM automatic control systems, which allow the overload process to be carried out. Such operations consist of many actions with a large number of parameters. The adaptation of the method in relation to the REM allowed us to systematize the search for initial events leading to undesirable events and, as a result, allows us to optimize the number of protections, which will lead to an increase in the reliability of the system, as well as simplifies the operation process and can reduce the operating cycle time of the controller for protection processing.

 

 

 

 

 

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