Keywords

1 Introduction

The steam generator tube is the weakest part of the pressure boundary of the primary circuit [1]. If the tube is ruptured, the radioactive nuclides will leak from the primary circuit to the secondary circuit and cause pollution of the secondary circuit, and the leak of the secondary circuit itself will lead to the release of radioactivity into the environment, which will seriously affect the safe operation of the pressurized water reactor nuclear power plant and pollute the environment, causing harm to equipment and personal safety. In the design of the leak monitoring system for the steam generator tube of the nuclear power plant in my country, two strategies are usually used to monitor the total gamma count rate and N-16 activity in the secondary circuit [2]. The two strategies are redundant with each other and are suitable for different working conditions: the N-16 activity monitor is the main measurement and supplemented by the total gamma count rate when the reactor power is higher than 20%; the N-16 activity monitor is no longer representative when the nuclear power level is lower than 20% [3]. And the rationality of the monitoring alarm threshold directly affects whether the monitoring system can run stably and effectively, so choosing a reasonable alarm threshold is the key to the design [4].

At present, VVER, M310 and ACP1000 units are typically set for leak monitoring alarm thresholds of steam generator tube in domestic nuclear power plants. Small modular reactors are different from the steam generator types of VVER, M310 and ACP1000 units, and the reactor power is also quite different, so the alarm threshold settings for steam generator tube leak monitoring of VVER, M310 and ACP1000 units cannot be directly used. Therefore, in this study, on the basis the principle of setting the leak monitoring alarm threshold for the steam generator tube of the VVER, M310 and ACP1000 units, and according to the design characteristics of the small modular reactor, the leak monitoring alarm threshold of the small modular reactor steam generator tube is carried out.

2 Setting of the Steam Generator Tube Leak Monitoring Alarm Thresholds in Domestic Nuclear Power Plants

2.1 VVER Unit

VVER determines that 1 kg/h is the normal design basis leak rate according to a large number of operating data of the units, and the N-16 first-level alarm threshold of steam generator tube leak monitoring is 2 kg/h, which is twice the normal design basis leak rate. The second-level alarm threshold of 5 kg/h is the safe operation limit of the primary side to the secondary side leak rate. The Russian side did not give the specific calculation process in the setting value report.

The total gamma first and second level alarm thresholds of the steam generator tube leak monitoring of VVER units are 1 × 10−6 Gy/h and 2 × 10-6 Gy/h respectively. There is no specific calculation method in the alarm threshold list provided by the Russian side. The speculation should be mainly based on the following aspects: the primary coolant source term, the secondary coolant radioactivity limit and the damage of the tube.

2.2 M310 Unit

For M310, the N-16 first-level alarm threshold of steam generator tube leak monitoring is 5 L/h, and the second-level alarm threshold is 70 L/h. The method does not give the basis for selecting the alarm threshold. It is speculated that the first-level alarm threshold is about 3 times the design basis leak rate under normal operating conditions, and the second-level alarm threshold of 70 L/h is derived from the operating regulations. The M310 steam generator tube leak monitoring total gamma first and second level alarm thresholds are 200 cps and 300 cps respectively. The French side does not give a specific calculation method, and it is speculated that the design idea is similar to that of the VVER.

3 Leak Monitoring Alarm Thresholds for Steam Generator Tube of Small Modular Reactor

3.1 Small Modular Reactor Design Features

The small modular reactor adopts once-through steam generator, a total of 16 units [5], of which 4 units are in a group, and each group is connected together and shares a main steam pipeline. In the small modular reactor, one N-16 monitor and one total gamma monitor are respectively set on each main steam pipeline, and the two devices are mutually redundant. Under the normal operating conditions of the small modular reactor, the total leak rate of the primary coolant to the secondary coolant is 3.6 kg/h, so the total leak rate of the steam generator corresponding to one main steam pipeline is 1.8 kg /h under normal operating conditions.

3.2 Leak Monitoring N-16 Alarm Thresholds for Steam Generators Heat Transfer Tubes

Due to the N-16 alarm threshold for the leak monitoring design for the steam generator tube of the VVER and M310 units, a consensus has been reached between the review, design department and the owner. In the absence of extensive operational data to support, the practice of linking normal leak rates to operational thresholds is easier to interpret and sufficiently conservative. Therefore, this study attempts to determine the N-16 first and second level alarm thresholds from the following two aspects:

  1. (1)

    Determine the alarm threshold based on the review of similar nuclear power plants, the consensus reached between the design department and the owner on the monitoring alarm threshold.

  2. (2)

    The threshold setting is linked to the normal leak rate. Therefore, the first-level alarm threshold of the steam generator leak monitoring of the small modular reactor is to be considered to be twice the normal design basis leak rate, which is 3.6 kg/h; the second-level alarm threshold is 9 kg/h that set to be 5 times the normal design basis leak rate.

3.3 Total Gamma Alarm Threshold Setting for Leak Monitoring of Steam Generator Tube

Referring to the VVER and M310 units, the setting of total gamma alarm threshold setting for leak monitoring of steam generator tube should be determined by the primary coolant source term, the secondary coolant radioactivity limit and the size of the damage to the tube. Since there is currently no relevant specification for the secondary coolant radioactivity limit, this study only considers two factors, the primary coolant source term and the size of the damage to the tube. Since the N-16 monitor and the total gamma monitor are redundant with each other, the first and second level alarm thresholds of the two should correspond to each other, indicating the same leak level of the steam generator tube under different working conditions. Therefore, according to the N-16 alarm threshold value of steam generator tube leak monitoring, the total gamma of leak monitoring under 20% power level can be calculated through theoretical analysis, so as to guide the setting of the total gamma alarm threshold value of steam generator tube leak monitoring.

  1. (1)

    Total gamma count rate calculation

The calculation formula of the count rate is as follows:

$$ C = \sum\limits_{i} {k_{i} \times \eta_{i} \times A_{vi} } $$
(1)
$$ A_{vi} = \frac{{A_{pi} \times q \times \rho_{v} \times {\text{e}}^{{ - \lambda_{i} t}} }}{{Q \times \rho_{p} }} $$
(2)

  • where \(C\) is the total gamma count rate, cps;

  • \(k_{i}\) is the detection efficiency of the nuclide \(i\) in the Monte Carlo model, cps/(Bq/m3);

  • \(\eta_{i}\) is the carrying coefficient of the nuclide \(i\);

  • \(A_{vi}\) is the radioactive concentration of the nuclide \(i\) in the steam at the probe, (Bq/m3);

  • \(A_{pi}\) is the radioactive concentration of the nuclide \(i\) in the primary coolant, Bq/kg;

  • \(q\) is the leak rate, L/h;

  • \(\rho_{v}\) is the steam density of the main steam pipeline at 20% power level, kg/m3;

  • \(\lambda_{i}\) is the decay constant of the nuclide \(i\), 1/s;

  • \(t\) is the transit times between leak location and probe at the 20% power level, s;

  • \(Q\) is the steam flow rate at the 20% power level, L/h;

  • \(\rho_{p}\) is the average density of the primary coolant, kg/m3.

  1. (2)

    Calculation of detection efficiency factor

The efficiency factor of the detector is related to the structure of the detector, the structure of the pipeline, and the relative position of the detector and the pipeline. Small modular reactor steam generator monitoring uses NaI detector. The steam pipe is filled with secondary side steam, the steam density is 19.07 kg/m3, and the steam pipe is wrapped with a layer of thermal insulation material. The MCNP calculation result is a normalized result, and the result needs to be processed:

$$ k_{i} = {\text{F}}8_{i} \times V_{{s{\text{ource}}}} \times I_{i} $$
(3)

where, \({\text{F}}8_{i}\) is the output result of pulses number of the MCNP that emits gamma-rays by the nuclide \(i\), cps/γ;

  • \(V_{{s{\text{ource}}}}\) is the volume of the steam pipe, m3;

  • \(I_{i}\) is the probability that the nuclide \(i\) emits gamma-rays.

  1. (3)

    Analysis of the effect of N-16 on the total gamma count rate

For M310 and VVER units, the transit time from the leak location and probe at low power is longer than that at full power, the N-16 decay share is large, and the contribution to the total gamma count rate is small. However, the transit time from the leak location to the outlet of the once-through steam generator tube at low power is still shorter than the half-life of N-16, and the transit time is shown in Table 1. The total gamma count rate still has a large contribution.

Table 1. N-16 transit times between leak location and outlet
Full size table
  1. (4)

    Retention of radionuclides on the secondary side of steam generator

For radioactive gas nuclides, such as N-16 and nobla-gas, due to their insolubility in water, 100% of the gas nuclides leaking from the primary coolant to the secondary coolant are instantly released into the vapor phase of the secondary side. For iodine and particle-type fission product nuclides, after leaking to the secondary side of the steam generator, they will be retained by the water on the secondary side. The water on the secondary side of the steam generator evaporates in the form of water vapor and droplets. Iodine and particulate fission product nuclides are entrained into the vapor phase. For iodine, due to the once-through steam generator used in small modular reactor, the once-through steam generator has to go through the process from supercooled water to hot steam in the secondary side working medium. According to the characteristics of the boiling phase change of the working medium on the secondary side, the secondary side is divided into a preheating section, an evaporation section and a superheating section. In the superheating section of the once-through steam generator, the iodine in the water will flash quickly into the gas phase, so it is conservatively assumed that the iodine carrying coefficient on the secondary side of the steam generator is 1. The droplet entrainment fraction for particulate nuclides is equal to the water content level in the steam, i.e. 0.25%.

  1. (5)

    The total gamma count rate corresponding to the leak rate of the steam generator tube

In summary, according to formula (1), under low power, when the leak rate is 3.6 kg/h, the total gamma count rate is about 35.8 cps; the leak rate is 9 kg/h, and the total gamma count rate is 89.6 cps.

  1. (6)

    Environmental background

The energy response range of the total gamma monitoring channel for leak of the steam generator tube is wide, and it is easily disturbed by the environmental background. The selection of the threshold value needs to consider this aspect. In this study, the environmental background count rate was considered to be 5 cps

  1. (7)

    The total gamma alarm threshold for leak monitoring of the steam generator tube.

Considering the environmental background, and considering the design margin of  −20% from an engineering point of view, it is recommended that the total gamma count rate is 50 cps as the first-level alarm threshold and 110 cps as the second-level alarm threshold.

4 Conclusions

In this study, referring to the setting of the leak monitoring alarm threshold of the VVER and M310 steam generator tube, and combined with the design value of the leak rate of the small modular reactor steam generator tube, the leak monitoring of the small modular reactor steam generator tube is proposed. Based on the principle of threshold setting, the N-16 alarm threshold for leak monitoring of the steam generator tube is given. The first-level alarm threshold is 3.6 kg/h, and the second-level alarm threshold is 9 kg/h. The calculation method of the total gamma count rate in the case of the small modular reactor for steam generator tube leak monitoring is established. According to the leak rate level of the leak monitoring N-16 alarm threshold, the total gamma alarm threshold of leak monitoring is set through theoretical analysis and calculation. The first-level alarm threshold is 50 cps, and the second-level alarm threshold is 110 cps.