Corresponding author: Aleksandr V. Antonov ( antonov@iate.obninsk.ru ) Academic editor: Yury Korovin
© 2020 Aleksandr V. Antonov, Gennady A. Yershov, Olga I. Morozova.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Antonov AV, Yershov GA, Morozova OI (2020) Justification of the nuclear power plant safety and operating efficiency in selection of maintenance and repair parameters. Nuclear Energy and Technology 6(1): 5561. https://doi.org/10.3897/nucet.6.51782

Nuclear power plants (NPP) are subject to stiff requirements as to the cost effectiveness of their operation. However, since the NPP operation may be associated with the occurrence of severe environmental, social, political, material and other consequences as the result of nuclear and radiological accidents at NPPs, ensuring the specified level of the NPP safety is an operational task of the utmost importance. The current practice of the NPP design and operation suggests that the objectives involved in improving the efficiency of operation and those of ensuring the NPP safety level required by federal standards and regulations (NP001) are achieved in isolation: no issues of ensuring high efficiency of the NPP operation are taken into account when addressing the issues involved in ensuring the safety of the NPP and, vice versa, it is a priori assumed that the required level of safety is ensured at any time when high efficiency of operation is achieved. The reason for this is the absence of procedures that make it possible to assess, in an integrated manner, the interdependence of economic and technological factors. The paper describes some of the methods to raise the efficiency of the NPP operation by selecting the best possible NPP maintenance and repair (M&R) strategies leading to high utilization and capacity factor values. A distinctive feature of the proposed techniques is that the efficiency of the NPP operation is proposed to be improved while ensuring at the same time the required level of the NPP safety with any NPP configuration.
Risk, safety level, emergency release, probability, frequency, nuclear fuel damage, process regulations, maintenance and repair
Nuclear power plant (NPP) units are industrial structures designed for generation of electricity having which as part of the country’s fuel and energy complex (FEC) should be cost effective and justified (
For the general public, the safety of an NPP is defined by the acceptability of the damage from the NPP operation (
A practicable way to determine the NPP safety level in terms of resultant damage is to use probability safety indicators (PSI): total probability of severe accidents (in an interval of one year) or total probability of a major emergency release (in an interval of one year). More than that, these characteristics make it possible to compare the safety level of various NPPs since other “traditional” units for measuring the safety level with regard for the severity of damage (exposure dose, amount of radioactive contamination, etc.) are extremely dependent on many parameters of the environment as such.
Using NPPs as a source of electricity require such conditions to be simultaneously fulfilled as ensuring the safety of NPPs as complex engineering systems involving, in principle, the potential for the occurrence of destructive events characterized by radioactive contamination of the biosphere and human exposure (
A variety of indicators are used to estimate the economic efficiency of the NPP operation, the most usable of which is utilization factor, K_{UF}:
${K}_{\mathrm{UF}}=\sum _{i=1}^{n}{t}_{i}/\left(\sum _{i=1}^{n}{t}_{i}+\sum _{i=1}^{m}{\tau}_{i}+\sum _{j=1}^{k}{\tau}_{j}\right)$, (1)
where i is the NPP operation cycle number; n is the number of operating cycles for the considered period of operation; t_{i} is the time for which the NPP is in the serviceable condition in the ith cyclе, h; τ_{i} is the duration of the ith M&R (scheduled or unscheduled) requiring the NPP to be stopped to be used for the intended purpose; m is the number of failures (recoveries) for the period of interest; j is the M&R number; k is the number of M&Rs requiring the NPP shutdown in the considered period; and τ_{j} is the duration of the jth M&R requiring the NPP to be stopped to be used for the intended purpose, h.
Along with K_{UF}, installed capacity utilization factor (ICUF) is widely used (
$ICUF={W}_{\mathrm{act}}/{W}_{\mathrm{max}}=\left({N}_{\mathrm{AAP}}\times {T}_{\mathrm{act}}\right)/\left({N}_{\mathrm{norm}}\times T\right)$, (2)
where W_{act} is the amount of energy actually generated by the NPP for the given period of operation Т, MW×h; W_{max} is the maximum possible amount of energy the NPP could generate for the specified period of operation Т provided it continually operated at the rated power level (with no idle periods), MW×h; Т is the duration of the specified period of operation, h; T_{act} is the actual time of the NPP operation in the electricity generation mode, h; and N_{AAP} is the actual average power the NPP operated at for the actual period of being used for the intended purpose (electricity generation).
The following formula is used to switch from the K_{UF} factor to ICUF
$ICUF={K}_{\mathrm{OR}}\times {K}_{\mathrm{UF}}$, (3)
where K_{OR} is the operating rate.
It has been found that the key contributors to reducing K_{UF} and ICUF are the NPP idle periods during scheduled and unscheduled M&Rs (
There is no extensive experience in revising the NPP M&R requirements. However, the NPP idle times during M&R can be cut by revising the M&R strategies from the point of view of providing the possibility for:
The proposed methods are based on analyzing the NPP safety level both qualitatively (using a deterministic safety analysis) and quantitatively (using a probabilistic safety analysis which makes it possible to estimate the NPP safety level in terms of PSIs).
It is important to note that the proposed methods do not serve exactly to confirm the fulfillment of the PSIs defined in NP001 (NP00197, NP 00115). On the one hand, using these (especially, the unscheduled M&R method) allows verifying the requirements of the NPP process regulations (PR) from the point of view of the following question: if the fulfillment of the NP001 requirements is ensured, as far as the PSIs are concerned, during scheduled and unscheduled M&Rs in accordance with the PR rules and limits (the methods make it possible to prove, in quantitative terms, the validity of the effective PR provisions or to demonstrate that these are possibly invalid from the point of view of ensuring the fulfillment of the NP001 requirements (NP00197, NP 00115) as far as the PSIs are concerned). On the other hand, using these methods allows “maneuvering” in the PSI intervals (limits) specified in NP001 such that to reduce the M&R durations raising so the NPP economic efficiency while not violating the legally defined NPP safety level requirements.
Speaking about M&R, one needs to understand that this procedure necessarily involves changes in the NPP equipment configuration (
$RIF=FD{F}_{i}\left({Q}_{i}=1\right)/FD{F}_{\mathrm{b}}$, (4)
where Q_{i} is the probability of the ith component to fail to perform its function; FDF_{i} is the frequency of nuclear fuel damage during the outage of the ith component for M&R for the analyzed length of time (PRspecified or expected); and FDF_{b} is the respective frequency of nuclear fuel damage with the considered equipment being serviceable.
This method is based on a hypothesis that the NPP operating efficiency can be improved by mitigating the PR requirements to the PRspecified NPP equipment outage allowed time (OAT) after the expiry of which the NPP is shut down administratively (certainly, if the serviceability of the failed components was not restored for the specified OAT). In a general case, the application of the method consists in the following: one needs to make initially sure that the logical and probabilistic model (LPM) of the NPP used for the PSI quantitative calculations is adequate to the NPP actual state (
In the event the LPM cannot be recognized as being adequate to the actual NPP state and fit for the subsequent quantitative analysis (NP09515), it needs to be respectively updated, e.g., by new base events or logic switches to be added to it to enable simulation of components under unscheduled M&R, and to simulate the emergency sequences, other than considered earlier, or to simulate in more details the existing emergency sequences, involving potentially the components in question.
Then the proposed OAT is represented in the LPM.
It is important that simulation takes into account the lifecycle stage the NPP is at, since each of the stages has various mechanisms of equipment degradation (
Various distribution laws, including exponential, normal, logarithmically normal, WeibullGnedenko, Relay, beta, and gamma distribution laws, are used to take into account, in a correct manner, the nature of the considered equipment failures and the NPP’s current lifecycle stage.
The above actions are followed by the quantitative estimation of the effects the analyzed configurations of the NPP equipment have on the PSIs, using the RIF factor based on formula (4). The proposed criteria of the RIF factor values and the recommendations on the qualitative estimation of the obtained quantitative results and on generation of further practical proposals for the NPP economic efficiency are presented in Table
It is especially important to note that using the unscheduled M&R method makes it possible to check the fulfillment of the PR requirements to the range of the components for the unscheduled M&R outage and for the OAT in terms of ensuring the fulfillment of the NP001 requirements (NP00197, NP 00115) as to the PSIs.
Estimated level of the NPP safety during scheduled and unscheduled M&R.
Qualitative evaluation of the quantitative analysis results  Practical recommendations as applied to the M&R strategies for considered equipment  
NPPs in operation  Newly commissioned NPPs  
Range of RIF values  RIF ≥ 1×10^{–3}/FDF_{b}  RIF ≥ 1×10^{–4}/FDF_{b}  
Unscheduled M&Rs  Unacceptable decrease in safety level  1. Implementation of the PRspecified requirements to the OAT length (or the expected OAT length) is not acceptable in terms of the NPP safety level since no NP001 requirements (NP00197, NP 00115) are fulfilled as to the PSIs.  
2. The considered PR requirements to the OAT length (or the expected OAT length) are too optimistic and should be revised by being toughened (the considered OAT length requires to be reduced).  
3. The NPP operating efficiency cannot be improved by increasing the considered OAT length.  
Scheduled M&Rs  The implementation of the proposed method to improve the NPP operating efficiency is not acceptable in terms of ensuring the NPP safety level since no NP001 requirements (NP00197, NP 00115) are fulfilled as to the PSIs.  
Range of RIF values  1×10^{–3}/FDF_{b} < RIF < 1×10^{–4}/FDF_{b}  1×10^{–4}/FDF_{b} < RIF < 1×10^{–5}/FDF_{b}  
Unscheduled M&Rs  Major decrease in safety level  1. The implementation of the PRspecified requirements to the OAT length is acceptable provided compensating measures are in place to improve the NPP safety level during unscheduled M&Rs of the considered equipment. Otherwise, the analyzed OAT length should be revised by being reduced to make the PR requirements not excessively optimistic.  
2. The NPP operating efficiency cannot be improved by increasing the considered OAT length since no NP001 requirements (NP00197, NP 00115) are fulfilled as to the PSIs.  
Scheduled M&Rs  The implementation of the proposed method to improve the NPP operating efficiency is acceptable in terms of ensuring the NPP safety level provided compensating measures are in place to improve the NPP safety level when implementing the proposed changes and the implementation of which makes it possible to fulfill the NP001 requirements (NP00197, NP 00115) as to the PSIs.  
Range of RIF values  RIF ≤ 1×10^{–4}/FDF_{b}  RIF ≤ 1×10^{–5}/FDF_{b}  
Unscheduled M&Rs  Acceptable decrease in safety level  1. The implementation of the PRspecified requirements to the OAT length (or the expected OAT length) is acceptable since the NP001 requirements (NP00197, NP 00115) are fulfilled as to the PSIs.  
2. The PR requirements to the OAT length (or the expected OAT length) are too conservative and can be revised by being mitigated.  
3. The NPP operating efficiency can be raised by increasing the considered OAT length.  
Scheduled M&Rs  The implementation of the proposed method to improve the NPP operating efficiency without any constraints in terms of ensuring the NPP safety since the NP001 requirements (NP00197, NP 00115) are fulfilled as to the PSIs. 
The method is based on reducing the unscheduled M&R times in conditions of the required level of the NPP safety ensured a priori (when the NPP is in operating modes with reduced power levels or in shutdown modes). In a statement of the kind, this is a classical problem of scheduling theory – the arrangement of a work system, with regard for process and resource constraints, providing for as short schedule length as possible (the shortest possible length of the scheduled M&R process in the proposed method) (
As applied to all of the works forming the scheduled M&R process, it needs to be stressed that all of these are interconnected through rigid resource and process dependences among which their two basic types are identified:
If the optimization of the existing strategies for unscheduled NPP M&Rs is considered in terms of the process sequence for the manipulations made (R_{1} dependences), the NPP idle times during scheduled M&Rs can be reduced by mitigating the existing resource constraints. Since it is not possible to change the dependences R_{1} (preventive maintenance schedules take into account all of the R_{1} dependences), it is proposed to revise the R_{2} dependences by the simultaneous outage for scheduled M&Rs of either more than one safety channel for any system or some of the components in one channel of the given system and some of the components in another channel.
It is important to note that a change in the NPP equipment redundancy rate leads to a still greater (as compared with “standard” scheduled M&Rs) deterioration in the reliability of the considered system. And the advantage of using this method is the quantitative justification (or rejection) of the validity of the PR requirements as to the R_{2} dependences since there are no special quantitative calculations broadly used to prove the validity of the PR requirements for the existing requirements to the R_{2} dependences.
The method application procedure is as follows. The key task is to determine what additional equipment can be removed out of service for M&R. This requires identifying the R_{2} dependences the implementation of which can be combined with each other without violating the R_{1} dependences. It is important to differentiate between the equipment units which must be available for use in emergency, and the equipment units which are defined in the NPP design as redundant. Following the selection of the equipment the repair of which permits it to be combined with the repair of other equipment, it is required to analyze the changes caused in the NPP safety level. To this end, the existing NPP LPM is altered so that to have it reflecting reliably the proposed operations (the actions taken to determine the suitability of the existing LPM in terms of the analysis conducted, and, where required, to update same, are similar to the actions described as applied to the unscheduled M&R method). It is further required to note in the LPM which equipment exactly is removed out of service for scheduled M&Rs (with regard for the considered NPP lifecycle stage). Following this, the effects of the proposed M&R strategy on the NPP PSIs are estimated quantitatively using the RIF factor.
The criteria of the RIF factor values, and the recommendations on the qualitative estimation of the obtained quantitative results and on generation of practical recommendations based on the results obtained are presented in Table
Special attention needs to be given to the following: if, based on the results of using any of the methods, it has been found to be necessary to generate compensating measures aimed to improve the NPP safety level, then the RIF factor values need to be governed by to evaluate, in quantitative terms, the efficiency of such measures; the intervals of the RIF values are presented in Table
The identification of the criteria different for the newly commissioned NPPs and the NPPs in operation is explained by the fact that the nonexceedance of the total probability of severe accidents in an interval of one year should be equal to 1×10^{5} for newly commissioned NPPs (NP00115), and it is required to seek to ensure the safety level described in (NP00197) for NPPs in operation.
As far as the acceptability of the criteria proposed for the RIF factor values is concerned, the following needs to be noted (
Therefore, the proposed criteria of the RIF factor values are acceptable since they take into account the accumulated experience in using both nuclear power facilities and conventional complex engineering systems, and do not fall beyond the limits of the commonly accepted approaches to managing risks from various types of human activities.
Methods have been developed for selecting strategies of the NPP equipment operation to raise the operating efficiency of NPPs and ensuring the required safety level. Using these methods allows making justified changes to the existing strategies of scheduled and unscheduled M&Rs for the NPP equipment based on information on the NPP safety level at the time of interest with any NPP configuration. The unscheduled M&R method also makes it possible to verify the PR requirements to the OAT from the point of view of the acceptability of the NPP safety level during unscheduled M&Rs in the process of the NPP power operation.
The methods are based on analyzing the effects of various lengths of times, during which the considered equipment is inspected or its serviceability is restored (that is, the OAT during scheduled M&Rs and the time during which scheduled M&Rs of equipment are undertaken), on the NPP safety level during scheduled and unscheduled M&Rs of the NPP equipment.
The proposed approaches to the generation of practical recommendations for improving the NPP operating efficiency by reducing the idle time during scheduled and unscheduled M&Rs of the NPP equipment are based on the principle of the required NPP safety level being ensured unconditionally. It is proposed that the practical recommendations are generated using the results of analyzing data on the change of the risk increase factor during scheduled and unscheduled M&Rs of the NPP equipment.