Operating margin for radioactive effluent released to the environment from korean pressurized water reactors compared to the effluent control limits, obtained by monitoring the concentration of radioactive effluent with radioactive effluent monitors

ABSTRACT

Korean pressurized water reactors (PWRs) generally use radioactive effluent monitors for monitoring the concentration of radioactive effluents released to the environment. In this study, the operating margins for radioactive effluent monitors were analyzed to determine the levels of real-time concentration of effluents compared to effluent control limits (ECLs), the regulatory limits. The results show that the concentration of radioactive effluents released from Korean PWRs complied with the ECLs during the years 2012–2016. It was also found that outages at Korean PWRs did not impact the operating margins for radioactive effluent monitors; that is, there was no remarkable difference of the concentration of effluents between normal operation and maintenance periods. In terms of simultaneous effluent releases, the results demonstrate that exceeding the ECLs is unlikely to occur even under the hypothetical condition of coincident effluent releases from multiple discharge points at a Korean PWR.

KEYWORDS:

• Radioactive effluent
• radioactive effluent monitor
• margin
• pressurized water reactor
• alarm setpoint
• effluent control limit

Introduction

Radioactive effluents discharged from Korean nuclear power plants (NPPs) are controlled by three criteria: concentration and radioactivity in radioactive material and dose to members of the public depending on the types of nuclear power reactors [1]. In particular, radioactive effluent monitors are generally used for monitoring the concentration of radioactive effluents released from Korean pressurized water reactors (PWRs). These monitors aim at providing an early warning of rapid increases in the concentration of radioactive effluents released to the environment using alarm (high) and warning (low) setpoints [2]. Alarm setpoints are determined so as not to exceed effluent control limits (ECLs), the Korean regulatory limits on the concentration of radioactive materials. The values given in ECLs are equivalent to the radionuclide concentrations which, if inhaled or ingested continuously over the course of a year, would produce an effective dose of 1 mSv y⁻¹ for members of the public living around NPPs [3]. Although warning setpoints are not mandatory requirements, they are widely used by Korean PWRs to take preemptive action for abnormal increases in concentrations [2].

Radioactive effluent monitors measure the gross counting rate for beta or gamma rays using a single-channel analyzer, which has one microprocessor assembly for each radiation detector, and then the algorithm of the monitor integrates the radiation readings to obtain a value for the effluent concentration [4]. Three types of detector assemblies are commonly used for gaseous effluent monitors: particulate and noble gas for beta detection and iodine for gamma detection. Liquid detectors monitor the overall gamma radiation levels in liquids. If radioactive effluent monitors were equipped with a multi-channel analyzer, it would be possible to monitor all individual radionuclides in gaseous or liquid effluents; however, it would take a great deal of time to analyze the samples, which makes real-time monitoring impossible. Thus, the purpose of radioactive effluent monitors is not to calculate the accurate effluent concentration but to make it possible to take prompt action in response to an abnormal surge of the concentration of effluents released to the environment using real-time monitoring [5]. The accurate effluent concentration is usually calculated at NPPs by sampling before discharging effluents to the environment [5].

In terms of radioactive effluent control, Korean NPPs have reported annually on the total amount of radioactivity discharged from NPPs and the resulting radiation doses to members of the public to the regulatory body and have disseminated this information to the public [6]. In particular, the annual effective doses to the public resulting from radioactive effluent releases during the years 2006–2015 were approximately on the order of 10⁻³ mSv or 10⁻² mSv [7,8]. On the other hand, there is not much information about the concentration of radioactive effluents or actual levels of radiation readings of effluent monitors compared to the corresponding alarm setpoints because the concentration of effluents varies in real time, depending on the NPP operating conditions. This study analyzed the margins of radiation readings of radioactive effluent monitors compared to the corresponding alarm setpoints to indirectly determine the levels of real-time concentration of effluents compared to the corresponding ECLs. In particular, the difference in the margins between routine operation and maintenance periods at NPPs was investigated to identify the effect of outage on the concentration of radioactive effluents. Furthermore, the possibilities of exceeding the ECLs were evaluated under the hypothetical condition of simultaneous releases of effluents from multiple discharge points at Korean PWRs taking into account the operation of multiple reactors at a single site.

Methodology

Currently, 21 PWRs, including a permanent shutdown reactor, are in operation in Korea. Multiple reactors are located at a single site due to the geographical features of Korea [8]. According to the regulation of the Nuclear Safety and Security Commission, the Korean regulatory body, a site boundary is defined as the perimeter of the overlapping exclusion areas of adjacent single reactors [9]. Korean NPPs in operation and in permanent shutdown, as of the end of 2018, are summarized in Table 1 [10]. Although Kori unit 1 has been permanently shut down since June 2017, the data of radioactive effluent monitors at Kori unit 1 were included to analyze the previous margins of radiation readings.

Table 1. Nuclear reactors in operation in Korea

Site	Reactor	Type	Net capacity (MWe)	Commercial operation
Kori	Kori 1	PWR^a,b – Westinghouse	587	April 1978
	Kori 2	PWR – Westinghouse	650	July 1983
	Kori 3	PWR – Westinghouse	950	September 1985
	Kori 4	PWR – Westinghouse	950	April 1986
	Shin-Kori 1	PWR – OPR1000^c	1000	February 2011
	Shin-Kori 2	PWR – OPR1000	1000	July 2012
Saeul	Shin-Kori 3	PWR – APR1400^d	1400	December 2016
Wolsong	Wolsong 1	PHWR^e – Candu	679	April 1983
	Wolsong 2	PHWR – Candu	700	July 1997
	Wolsong 3	PHWR – Candu	700	July 1998
	Wolsong 4	PHWR – Candu	700	October 1999
	Shin-Wolsong 1	PWR – OPR1000	1000	July 2012
	Shin-Wolsong 2	PWR – OPR1000	1000	July 2015
Hanbit	Hanbit 1	PWR – Westinghouse	950	August 1986
	Hanbit 2	PWR – Westinghouse	950	June 1987
	Hanbit 3	PWR – OPR1000	1000	March 1995
	Hanbit 4	PWR – OPR1000	1000	January 1996
	Hanbit 5	PWR – OPR1000	1000	May 2002
	Hanbit 6	PWR – OPR1000	1000	December 2002
Hanul	Hanul 1	PWR – Framatome	950	September 1988
	Hanul 2	PWR – Framatome	950	September 1989
	Hanul 3	PWR – OPR1000	1000	August 1998
	Hanul 4	PWR – OPR1000	1000	December 1999
	Hanul 5	PWR – OPR1000	1000	July 2004
	Hanul 6	PWR – OPR1000	1000	April 2005
	Total: 25		23,116

^a Pressurized water reactor

^b Kori unit 1 has been permanently shut down since June 2017.

^c Optimized power reactor

^d Advanced-pressurized water reactor

^e Pressurized heavy water reactor

The total number of gaseous and liquid effluent monitors in Korean PWRs is 360. Scintillators are used mostly as effluent monitor detectors. This study analyzed the radiation readings of all individual monitors and collected data up to 5 years (60 months). The reason for a 5-year data collection period is that Korean PWRs normally take outages every 18 months and a third of the fuel is replaced during this maintenance period. Thus, the data collection period was determined with regard to three refueling cycles (18 months × 3 cycles = 54 months), where all fuels are replaced by new ones. Since Shinkori unit 3 started commercial operation at the end of 2016, the data collection period for Shinkori unit 3 was less than 5 years; however, the data were collected as many as possible including the test operation period.

Radioactive effluent monitors at Korean PWRs have their own identification numbers and send the radiation readings of instantaneous effluent concentration to the plant information system of Korea Hydro & Nuclear Power, the Korean nuclear utility company. This study used 5-year data produced by 360 effluent monitors, which were recorded electronically in the plant information system every few seconds, and a total of approximately 2.3 billion radiation readings were taken. This enormous body of data included not only ordinary radiation readings, but also abnormal readings, such as errors with negative values or zero and incorrect values resulting from calibration or maintenance. In particular, the values produced during calibration and maintenance were relatively higher than those resulting from the routine operation and were likely to exceed the alarm setpoints of effluent monitors because a high-level radiation source is typically used for inspection. It was necessary to remove the errors and incorrect values in the data set prior to analysis of the radiation readings. After investigating the apparent cases exceeding alarm setpoints of radioactive effluent monitors at Korean PWRs, there have been no real cases exceeding alarm setpoints for the past five years. Thus, this study removed the incorrect values exceeding alarm setpoints using these findings. In addition to negative values, zeros, and values exceeding alarm setpoints, outliers also should be removed from the data. This study used the interquartile range (IQR) to detect outliers and to moderate their effects in the data. The IQR is used to build box plots, a rectangle with top and bottom sides at the levels of the quartiles, for a simple graphical representation of a probability distribution [11]. The IQR is defined as the range between third and first quartiles (IQR = Q₃ – Q₁), and points that fall outside 1.5 times the IQR are normally treated as outliers [11]. This study removed any values more than 1.5 times the IQR above the third quartile (Outliers > Q₃ + 1.5 × IQR). However, positive values less than 1.5 times the IQR are treated as ordinary radiation readings and were used for data processing. Because NPPs aim to keep levels of radioactive materials in effluents released to the environment as low as reasonably achievable (ALARA), radiation readings are typically slightly above the background level. Figure 1 presents box-and-whisker plots and the IQR. Finally, the total number of data for the analysis was approximately 2.2 billion, and SAS 9.3 software was used for data processing.

Figure 1. Box-and-whisker plots and interquartile ranges.

The margins of radiation readings of individual effluent monitors were analyzed in this study using the data recorded in the plant information system. The margin was calculated using the ratio of the maximum reading of the effluent monitor to its alarm setpoint and can be described by the following equation:

(1) $M_i=\frac{S{P}_{Alarm}-R_{Max}}{S{P}_{Alarm}}=1-\frac{R_{Max}}{S{P}_{Alarm}}$(1)

where M_i is the margin of radiation readings of the effluent monitor i, SP_Alarm is the setpoint (Bq m⁻³) of the effluent monitor i, and R_Max is the maximum radiation reading (Bq m⁻³) of the effluent monitor i during the years 2012–2016. Eq. (1)(1) $M_i=\frac{S{P}_{Alarm}-R_{Max}}{S{P}_{Alarm}}=1-\frac{R_{Max}}{S{P}_{Alarm}}$(1) was also used to analyze the difference of margins between routine operation and maintenance periods at NPPs; however, the average radiation readings instead of the maximum readings of the effluent monitors were used to find the effect of outage on the average concentration of radioactive effluents.

In addition to the margins, this study evaluated whether the concentration of radioactive effluents exceeds the ECLs under the hypothetical condition of simultaneous effluent releases from multiple discharge points at Korean PWRs. It was assumed that radioactive effluents were released at the same time from all of the discharge points at a reactor with the maximum radiation readings of the gaseous or liquid effluent monitors. The sum of the ratios of the maximum radiation readings of individual effluent monitors to their corresponding alarm setpoints was calculated using the data recorded in the plant information system and can be written as Eq. (2)(2) $S={\sum }_i\left(\frac{R_{Max}}{S{P}_{Alarm-single}}\right)$(2) :

(2) $S={\sum }_i\left(\frac{R_{Max}}{S{P}_{Alarm-single}}\right)$(2)

where S is the sum of the ratios of the maximum readings of the effluent monitor i to its alarm setpoint, R_Max is the maximum radiation reading (Bq m⁻³) of the effluent monitor i during the years 2012–2016, and SP_Alarm-single is the alarm setpoint (Bq m⁻³) of the effluent monitor i at a single discharge point, which does not apply administrative allocation factors for safety margins [2]. Finally, if the sum is less than 1, it means that the concentration of radioactive effluents does not exceed the ECLs under the hypothetical condition of simultaneous effluent releases from multiple discharge points [2,5].

Results and discussion

The data analysis results show that there have been no cases exceeding alarm setpoints for the past five years and most effluent monitors had margins exceeding 90%. In other words, the instantaneous radiation readings of radioactive effluent monitors were less than 10% of the alarm setpoints of the effluent monitors. This indicates that Korean NPPs control the concentration of radioactive effluents released to the environment more rigorously than the level required by the regulation. The averages of 5-year margins of gaseous effluent monitors for particulates, iodine, and noble gases were 95.4 ± 4.3%, 98.0 ± 3.4%, and 92.0 ± 9.4%, respectively. Liquid effluent monitors showed 96.2 ± 5.1% average 5-year margins. It was also found that there were no coincident effluent releases with the maximum radiation readings of the gaseous or liquid effluent monitors at a reactor. Table 2 shows the results of margins of radioactive effluent monitors for each Korean PWR, including the total numbers of discharge points and effluent monitors. In Korean NPPs, two reactors commonly share some radioactive waste processing facilities; the margins of effluent monitors using the same discharge points were thus classified as belonging to both reactors in Table 2.

Table 2. Margins of radioactive effluent monitors at Korean PWRs during the years 2012–2016

Sites	Units	Categories	No. of effluent discharge points	Channels	Detection radiation	No. of monitors	Margins (%)
Kori	Kori 1	Gas	1	Particulate	Gross beta	1	99.4
				Iodine	Gross gamma	1	99.4
				Noble gas	Gross beta	1	100.0
		Liquid	1	Liquid	Gross gamma	1	98.4
	Kori 2	Gas	1	Particulate	Gross beta	1	100.0
				Iodine	Gross gamma	1	100.0
				Noble gas	Gross beta	1	55.8
		Liquid	1	Liquid	Gross gamma	1	99.6
	Kori 3	Gas	6	Particulate	Gross beta	1	97.6
				Iodine	Gross gamma	1	92.1
				Noble gas	Gross beta	6	99.8
	Kori 4	Gas	6	Particulate	Gross beta	1	97.1
				Iodine	Gross gamma	1	95.9
				Noble gas	Gross beta	6	98.5
	Kori 3&4	Liquid	1	Liquid	Gross gamma	2	97.3
	Shinkori 1	Gas	3	Particulate	Gross alpha & beta	3	99.9
				Iodine	Gross gamma	3	100.0
				Noble gas	Gross beta	4	89.4
		Liquid	1	Liquid	Gross gamma	1	89.9
	Shinkori 2	Gas	3	Particulate	Gross alpha & beta	3	99.5
				Iodine	Gross gamma	3	100.0
				Noble gas	Gross beta	4	79.3
		Liquid	1	Liquid	Gross gamma	1	94.5
	Shinkori 1&2	Gas	1	Particulate	Gross alpha & beta	3	98.5
				Iodine	Gross gamma	3	100.0
				Noble gas	Gross beta	3	87.7
		Liquid	2	Liquid	Gross gamma	2	99.5
Saeul	Shinkori 3	Gas	8	Particulate	Gross beta	7	93.8
				Iodine	Gross gamma	7	99.9
				Noble gas	Gross beta	8	82.7
		Liquid	2	Liquid	Gross gamma	3	89.7
Wolsong	Shinwolsong 1	Gas	3	Particulate	Gross alpha & beta	3	100.0
				Iodine	Gross gamma	3	100.0
				Noble gas	Gross beta	4	84.2
		Liquid	1	Liquid	Gross gamma	1	97.3
	Shinwolsong 2	Gas	3	Particulate	Gross alpha & beta	3	89.8
				Iodine	Gross gamma	3	100.0
				Noble gas	Gross beta	4	90.7
		Liquid	1	Liquid	Gross gamma	1	90.3
	Shinwolsong 1&2	Gas	1	Particulate	Gross alpha & beta	3	97.8
				Iodine	Gross gamma	3	100.0
				Noble gas	Gross beta	3	94.3
		Liquid	2	Liquid	Gross gamma	2	99.4
Hanbit	Hanbit 1	Gas	6	Particulate	Gross beta	1	88.5
				Iodine	Gross gamma	1	100.0
				Noble gas	Gross beta	6	99.9
	Hanbit 2	Gas	6	Particulate	Gross beta	1	98.5
				Iodine	Gross gamma	1	99.9
				Noble gas	Gross beta	6	99.9
	Hanbit 1&2	Gas	2	Noble gas	Gross beta	2	97.6
		Liquid	1	Liquid	Gross gamma	2	100.0
	Hanbit 3	Gas	14	Particulate	Gross beta	5	96.5
				Iodine	Gross gamma	5	100.0
				Noble gas	Gross beta	14	98.6
		Liquid	3	Liquid	Gross gamma	3	99.9
	Hanbit 4	Gas	14	Particulate	Gross beta	5	98.1
				Iodine	Gross gamma	5	100.0
				Noble gas	Gross beta	14	99.0
		Liquid	3	Liquid	Gross gamma	3	99.9
	Hanbit 3&4	Gas	1	Noble gas	Gross beta	1	98.8
		Liquid	1	Liquid	Gross gamma	2	99.9
	Hanbit 5	Gas	8	Particulate	Gross beta	5	95.8
				Iodine	Gross gamma	5	98.7
				Noble gas	Gross beta	9	95.0
	Hanbit 6	Gas	8	Particulate	Gross beta	5	94.5
				Iodine	Gross gamma	5	96.4
				Noble gas	Gross beta	9	94.3
	Hanbit 5&6	Gas	1	Particulate	Gross beta	1	95.9
				Iodine	Gross gamma	1	100.0
				Noble gas	Gross beta	1	99.1
		Liquid	3	Liquid	Gross gamma	3	96.2
Hanul	Hanul 1&2	Gas	1	Particulate & iodine	Geiger Muller counter	2	96.9
				Noble gas	Gross beta, Ion chamber	4	96.0
		Liquid	1	Liquid	Gross gamma	3	99.6
	Hanul 3	Gas	10	Particulate	Gross beta	9	90.3
				Iodine	Gross gamma	9	95.4
				Noble gas	Gross beta	13	84.5
	Hanul 4	Gas	10	Particulate	Gross beta	9	88.0
				Iodine	Gross gamma	9	96.2
				Noble gas	Gross beta	13	85.3
	Hanul 3&4	Gas	1	Particulate	Gross beta	2	100.0
				Iodine	Gross gamma	2	99.9
				Noble gas	Gross beta	2	97.9
		Liquid	1	Liquid	Gross gamma	2	81.1
	Hanul 5	Gas	9	Particulate	Gross beta	5	93.7
				Iodine	Gross gamma	5	88.3
				Noble gas	Gross beta	8	88.4
	Hanul 6	Gas	9	Particulate	Gross beta	5	90.5
				Iodine	Gross gamma	5	89.8
				Noble gas	Gross beta	8	90.7
	Hanul 5&6	Gas	1	Particulate	Gross beta	2	85.0
				Iodine	Gross gamma	2	99.9
				Noble gas	Gross beta	1	96.5
		Liquid	1	Liquid	Gross gamma	2	99.4
Total			164			360

It is regarded that the concentration of radioactive effluents released to the environment during outages at NPPs would probably be higher than that during normal operation because more radioactive materials are released due to system opening during maintenance periods. This study investigated the difference of the margins between normal operation and maintenance periods at NPPs. Contrary to expectations, the results demonstrate that there were no remarkable differences in the margins of radiation readings of effluent monitors between routine operation and outages from 2012 to 2016. The average margins for gaseous effluent monitors during normal operation and maintenance periods were 99.7 ± 0.2% and 99.1 ± 0.7%, respectively. The results for liquid effluent monitors also showed similar outcomes: 99.4 ± 0.3% for normal operation and 99.3 ± 0.3% for outages. Thus, it is found that outages at Korean PWRs do not impact the concentration of radioactive effluents released to the environment. Table 3 shows the margins of radiation readings of radioactive effluent monitors between normal operation and outages during the years 2012–2016.

Table 3. Margins of radioactive effluent monitors at Korean PWRs between normal operation and outages during the years 2012–2016

NPP Sites	Categories	Margins (%)	Outage
		Normal operation
Kori	Gas	99.9	98.1
	Liquid	98.9	98.9
Saeul	Gas	99.8	NA^a
	Liquid	99.5	NA^a
Wolsong	Gas	99.4	99.1
	Liquid	99.1	99.2
Hanbit	Gas	99.9	99.9
	Liquid	99.8	99.8
Hanul	Gas	99.5	99.5
	Liquid	99.4	99.4
Average	Gas	99.7 ± 0.2	99.1 ± 0.7
	Liquid	99.4 ± 0.3	99.3 ± 0.3

^a There were no available data for the Saeul site since Shinkori unit 3 started commercial operation at the end of 2016.

As shown in Table 2, Korean PWRs have multiple discharge points of radioactive effluents. In terms of simultaneous effluent releases from multiple discharge points, concerns over exceeding the ECLs have been raised by the regulatory body. The ECLs are applied for each reactor and for each gaseous or liquid material [1,3,5]. This study evaluated the possibilities for exceeding the ECLs under the worst case scenario that radioactive effluents were discharged at the same time from all of the discharge points at a reactor with the maximum radiation readings of the effluent monitors. As shown in the previous results of the margins, there were no simultaneous effluent releases with the maximum radiation readings of the gaseous or liquid effluent monitors at a reactor; this scenario is thus impossible in practice at Korean NPPs. The analysis results of the possibilities for exceeding the ECLs proved that the sum of the ratios of the maximum radiation readings of individual effluent monitors to their corresponding alarm setpoints was less than 1 for gaseous or liquid effluents, as summarized in Table 4. This indicates that the concentration of radioactive effluents at Korean PWRs does not exceed the ECLs even under the worst case scenario of simultaneous effluent releases from multiple discharge points. Assuming a more conservative situation, the sum of the ratios for each single site was also less than 1 except the gaseous effluents at Hanul NPP site; thus, exceeding the ECLs is unlikely to occur even for a single site.

Table 4. Sum of the ratios of the maximum radiation readings of individual effluent monitors to their corresponding alarm setpoints

Sites	Categories		Units	Categories
	Gas	Liquid		Gas	Liquid
Kori	0.86	0.21	Kori 1	0.00	0.02
			Kori 2	0.11	0.00
			Kori 3	0.23	0.01
			Kori 4	0.16	0.01
			ShinKori 1	0.14	0.11
			ShinKori 2	0.21	0.06
Saeul	0.24	0.21	Shinkori 3	0.24	0.21
Wolsong	0.30	0.14	Shinwolsong 1	0.15	0.03
			Shinwolsong 2	0.15	0.10
Hanbit	0.56	0.12	Hanbit 1	0.14	0.00
			Hanbit 2	0.05	0.00
			Hanbit 3	0.04	0.01
			Hanbit 4	0.03	0.00
			Hanbit 5	0.13	0.06
			Hanbit 6	0.17	0.06
Hanul	1.76	0.40	Hanul 1	0.11	0.01
			Hanul 2	0.11	0.01
			Hanul 3	0.41	0.19
			Hanul 4	0.42	0.19
			Hanul 5	0.36	0.01
			Hanul 6	0.34	0.01

Conclusion

Korean NPPs monitor concentration and radioactivity in radioactive effluents and dose to the public in order to keep the quantity of both effluent releases and resulting doses as low as reasonably achievable. Many papers and reports have presented sufficient information of the radioactivity in effluents and the resulting doses to members of the public living around NPPs. On the other hand, relatively little information about the concentration of effluents is available because the concentration of effluents varies in real time, subject to the operating conditions. Korean PWRs generally use radioactive effluent monitors to monitor the instantaneous concentration of effluents released to the environment. This study investigated whether the concentration of effluents complies with the ECLs, the regulatory limits, using the data of radiation readings produced by effluent monitors at Korean PWRs.

The results show that there were no cases exceeding alarm setpoints during the years 2012–2016 and the instantaneous radiation readings of radioactive effluent monitors were only less than 10% of the alarm setpoints of the effluent monitors. It was also found that there were no significant differences in the margins of radiation readings of effluent monitors between normal operation and maintenance periods. In terms of simultaneous effluent releases, the results proved that the concentration of effluents is unlikely to exceed the ECLs under the hypothetical condition of simultaneous releases from multiple discharge points at a Korean PWR.

Disclosure statement

No potential conflict of interest was reported by the authors.