E-Journal of Advanced Maintenance Vol. 11 No. 2 (2019) 86-91
Japan Society of Maintenology

Operation and Maintenance experience of Sodium Leak Detector in Monju
Keitaro MUTO1,*, Daisuke HAMANO1, Mamoru KAWABATA1, Yasuhiro TABAKOYA1
and Chisato YANAI1
1

Japan Atomic Energy Agency (Monju), 2-1, Shiraki Tsuruga-shi, Fukui pref, 919-1279, JapanABSTRACT
Gas sampling type sodium small leak detector, SID (Sodium Ionization Detector), a gas sampling detector
installed to monitor small sodium leak in the Prototype Fast Breeder Reactor Monju, had been operated since its
system start-up test through to the 40% reactor’s output test. It’s use was terminated in April, 2018. SID showedsome indication variations during its operation period, and necessary measures were implemented.
As a result, the SID system maintained its functions without any critical malfunction until the end of its
operation.
KEYWORDS

ARTICLE INFORMATION

proto type fast breeder reactor, Monju, sodium leak detector, sodium
ionization detector, SID, DPD, gas sampling

Article history:
Received 23 October 2018
Accepted 26 June 20191.

Introduction

In the Prototype Fast Breeder Reactor Monju, the small sodium leak detectors are placed in thecooling system which contains liquid sodium as a coolant. Because of the characteristic of FBR thatthe coolant pressure is low, a small leak happens from a crack on pipe before it breaks. So it ispossible to shut down the reactor safely before coolant piping rupture by detecting a small sodiumleak.
Sodium Ionization Detector (SID) is a type of a small sodium leak detector for the reactor vesseland primary cooling system. The scope of the sodium leak Detector, SID is shown in Fig. 1.
Primary Cooling System(atmosphere : nitrogen)

Scope of the SID:surround by red lines

Secondary Cooling System(atmosphere : air)Fig. 1. Scope of the sodium leak detector, SID system in Monju

The SID system samples the gas from the nitrogen atmosphere around the cooling pipes, machines
and rooms by sampling pump.
The sampling lines have many sampling nozzles and sampling flow rate of each nozzles areadjusted to be constant. The sampled gas flow into the detector and detects sodium. Then, the sampled
gas is returned to the sampled room again. The outline of the sampling system is shown in Fig. 2.
*Corresponding author, E-mail: muto.keitaro@jaea.go.jp

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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Operation and Maintenance experience of Sodium Leak Detector in Monju

The SID system was designed and manufactured by two companies (MHI, Ltd, around the rector;
Hitachi, Ltd, around the primary cooling system) to obtain diversified technical expertise.
The SID ionizes the gas with the filament kept at a high temperature of about 1000°C. The ioncurrent increase depending on the ionized aerosol concentration, so SID can detect a sodium leak. TheSID can detect 10-10 g/Na cm3 of the aerosol concentration in the sampling gas. The outline of the SID
is shown in Fig. 3.

Fig. 2. Outline of the gas sampling system

Fig. 3. Outline of the SID

SID is placed in parallel with Differential Pressure Detector (DPD). DPD traps sodium aerosol by
the filter, and detects a small sodium leak by measuring the differential pressure of the filter. Also
DPD’s filter is used to judge a sodium leak by chemical analysis when a sodium leak is suspected.

2.

Operation and Maintenance Experience of Sodium Leak Detector(SID)

2.1. Failure contents of sodium ionization detector (SID)
SID was operated between 1995 and 2018, beginning from the test run of Monju until its
decommissioning. Regular maintenance, reviews of alarm set point, updating and remodeling of thesystem were carried out after the 40% output test.
Table 1 shows the maintenance reports issued during the operation period in the maintenancemanagement of the SID detector, classified according to failure contents and causes. Table 2 shows
the fault classification of Table 1 applied to the machines constituting the SID system.
Table 1. SID - Issued Maintenance Reports - (Failure Contents and Causes)
No.

Failure contentsCauses

12345678

Abnormal soundBlack out
Criteria deviation
Decline in sensitivity
Dent on the bellows
Disconnection
High suction pressure
Indication variation

9101112Malfunction
Misalignment (switch)
Pump trip
Seat leakAged deterioration
External factor（Lightning)
Aged deterioration , Instrument drift
Aging
Human error
Deterioration
Filter Clogging
Aged deterioration, Design problems, Externalfactor（dust, static electricity, Temperature change,etc.）, Initial variation
Aged deterioration, Initial variation
Aging
Aged deterioration, wear
Aged deterioration
Total87

Number ofreports
11101242101438157

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Operation and Maintenance experience of Sodium Leak Detector in Monju
Table 2. Fault classification of Table 1. According to Machines
Machines

Parts(Failure contents Table 1. No.)

Control equipment

Numberof reports

Number ofreports(SID unmonitored*)
1

Internal parts (3,8), Relay (9), Logic unit (9),7Logic program (8)Control switch
Internal parts (9)2None
Detector
SID Filament (6,8), Internal parts (4,8)
127FilterDepth filter (7)2NonePower supply
Power supply unit (2), Internal parts (8), NFB (9)31Preamplifier
Internal parts (3)2None
Pressure indicator Internal parts (3), Micro switch (9)3NoneRecorderInternal parts (3,9), motor (9)9None
Sampling pump
Bearing (11), Carbon vane (11), Internal parts (1,9)
10
8ValveBellows for solenoid valve (5), Limit switch (10),7None
Lubricant (9), Seat parts (12)Total―5717* SID unmonitored: Including the occurrence of a short time (a few seconds) unmonitored condition

From Table 1, 57 maintenance reports on the SID system occurred during the operation period.
Among them, there were 17 events were unmonitored by the SID detector (underlined in Table 2.).
Of these 17 cases, disconnection of SID filament (4 cases) and trip of sampling pump (7 cases) wereincluded and occurred during the period of 1993-2000.
After the new maintenance programs were applied in 2008, regular parts replacement andequipment inspection based on the maintenance schedule started. Since the continuous improvementof facilities and the introduction of the maintenance programs, the SID unmonitored failure due tointernal component failures was not occurred.
2.2. SID problems and countermeasures
SID detects sodium leakage by sensing a fine current increase (0.01～100nA). Therefore, the SID
system is susceptible to external factors (thermal, dust, moisture, and so on). The ion current by theSID fluctuated during its operation period, and necessary measures were implemented.
2.2.1. Filament material affecting SID indication values
On October 25th, 2004, after replacing the SID sensors installed to primary cooling system, theSID ion current increased from 0.5 nA, under normal conditions to 6 nA, exceeded the alarm setting
value of 5 nA. We immediately confirmed that a sodium leak did not actually occur by analyzing theDPD filter.
After flushing the SID sampling line, the SID was restarted, after that, the SID ion current
increased to maximum 20 nA after 13.5 hours from the restart of measurement. Therefore, wereplaced the SID sensor with spare parts of old stock (made before 1998). As a result, we confirmedthat the SID ion current was stable. Thus, we investigated to identify the cause of the ion current
increased.
We analyzed the SID sampling gas (sample No. 1, No. 2) to identify the cause of the increasein the ion current values. The sampled gas was dissolved in pure water or hydrogen peroxide solution
and analyzed to measure impurities concentration by ion chromatography. We found that the gas
included chlorine, NH3, NOx and SOx, which is shown Fig. 4, 5.
For the reproduction test, the test gas containing the identified substance was reproduced.
However, the test gas did not increase SID ion current significantly.
Based on previous sampling gas experiments, it was presumed that the characteristic of the sensor
causes the indication variation. Both SID sensors (made before 1998 and made 2004) were made with88

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Operation and Maintenance experience of Sodium Leak Detector in Monju

same manufacturing and speciation, although there are different years.

Fig. 4 Analysis of SID sampling gas byion chromatography (pure water)

Fig. 5 Analysis of SID sampling gas by ion
Chromatography (hydrogen peroxide solution)

The SID sensors have filament using high purity platinum (99.99%). We investigated whether
there is any difference in impurities in these filaments. We analyzed the impurities in the SID
filaments using ICP-MS (high frequency inductively coupled plasma mass spectrometry). The results
are shown in Table 3. From Table 3, it was confirmed that the SID sensor made in 2004 contains moreiron and aluminum in comparison to the sensor made before 1998. The purity of the platinum portionof both SID filaments was specified as 99.99%, but the products manufactured before 1998 were infact purity close to 99.999%.
Table 3. Impurities contained in the SID filaments. (ICP-MS)
SID sensor with spare parts
SID sensor with ion currentof old stock (Made 1998)
increasing (Made 2004)
Filament purity
99.99%
99.99%
Impurities
Evaluation value wtppb
Evaluation value wtppb
Na
1350
93
K
1830
20
Ca
2850
23Mg
152
204
5670
27600
FeCu
1230
1330
Ni
2210
1100
Cr
323
441
Zn
213
486
Mn66
445
Al30112300
Ti
136
78Mo
28
39Co
178
26Sn
61
117
Ce
<14W
1765
Pb
742
925
Total
17457
45926

test SID sensor (improvedsensor ) (Made 2006)
99.999%Evaluation value wtppb
66
82
37
46
1630
386
686
103
70
20
247
30
24
649
<126634147

The gas sampled by the SID from the nitrogen atmosphere included 0.5% oxygen and moisture
equivalent to dew point -10°C. In this condition, no significant correlation between low oxygen
concentration variation and SID ion current has been confirmed. Therefore, we investigated theinfluence of moisture on SID sensitivity. The nitrogen atmosphere was added to moisture andmeasured by SID to confirm the change of SID ion current. The results are shown in Table 4.
The SID sensor made 2004 had the characteristic that the SID ion current increased when it waspassed through an atmosphere containing moisture.

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Operation and Maintenance experience of Sodium Leak Detector in Monju
Table 4. Dew point and SID ion current in N2 atmosphere
Dew point (Td/°C)
Ion current (nA)
SID sensor with ion current
Measurement condition(N2 atmosphere)
increasing (Made 2004)-55 (dry)
0.30-10 (SID sampling gas)
0.38
00.44
100.52

Ion current (nA)test SID sensor (improved sensor)(Made 2006)
0.06
0.07
0.07
0.08

A sensor containing a large amount of aluminum or iron as an impurity is more sensitive tomoisture than a normal sensor, and it is inferred that compounds on the sensor surface react withaerosols more than platinum, resulting in increased ion current.
Based on these results, we then examined countermeasures for SID ion current increase. In theplant environment, it is difficult to reduce moisture, so we tried to reduce impurities of aluminum andiron in the SID sensor.
However, it was difficult to get rid of those impurities selectively, Therefore, high purity platinum
sensors and the test SID sensor made for evaluation (made 2006: Pt 99.999%) were adopted. From
Table 3, we confirmed that impurities of iron and aluminum contained in the test SID filament weresignificantly smaller.
From Table 4, the test SID sensor was not significantly affected by the moisture. Therefore, wereduced impurities by setting the filament purity specification to 99.999%. As a result, we achieved
reducing indicator value variation.
2.2.2. SID ion current increased by temperature change
The liquid sodium is controlled by heaters and kept stable at about 200°C in the piping even when
the reactor is stopped. When sodium is drained for maintenance, these heaters are turned off.
On December, 25th, 2015, the maintenance of machines was completed and the piping around thereactor was heated to 200°C to fill the liquid sodium. Then, the SID ion current increased andexceeded the alarm set value of 1 nA, and a SID leakage alarm was issued. At this time, we analyzed
the DPD filter installed together with SID and confirmed that sodium leakage had not occurred. The
cause of this SID ion current increased was assumed to be similar to the case in 2.2.1.
It was confirmed that the increase in the SID ion current correlated with the temperature change,
so we adjusted the set voltage of the pipe preheating heater as s provisional countermeasure.
After this countermeasure, the SID indication never exceeded the alarm set value of 1 nA, and thefalse alarms was suppressed.
2.2.3. SID ion current variation due to door operation
On September 20th, 2012, it was found that the SID ion current decreased. In the investigation, wechecked the accuracy of the related instruments, and we found looseness of the screws, poor contactof the coaxial cable connector, but no abnormality was found. After that, we confirmed the variationof SID indication occurred at the time of the opening and closing of the panel door in the on-site
console panel for SID system.
Placing the electrostatically charged plastic plate close to the coaxial cable, the SID ion current
decreased. The reduced amount of ion current was in the range of leakage current (10 ~ 44 pA) of thepreamplifier circuit. Therefore, it was presumed that the leakage current of the substrate decreased dueto charging of the preamplifier circuit.
As a result of the investigation, the on-site console panel was installed on grating, the pressure
variation inside the on-site console panel was generated by opening and closing the door, and thestructure was such that the air entered and exited from the grating bottom shown in Fig. 6.
It was suspected that the wind generated by the entrance and exit of this air came into contact with thesurface of the coaxial cable and affected the coaxial cable for instance the air stream generates staticelectricity (electric charge) on the surface of the coaxial cable due to the friction between the air andthe sheath (skin), so the weak current flows to the signal path. Static electricity caused by wind90

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Operation and Maintenance experience of Sodium Leak Detector in Monju

friction affects the core material through which weak current flows with the following mechanism.
(1) Wind contacts the surface of the sheath of nonconductor, induced polarization occurs due tofriction(2) Electrostatic induction occurs with the outer conductor(3) Inductive polarization occurs in insulator(4) Electrostatic induction occurs in the inner conductor, which affects the minute current of the SID
Following the occurrence of these issues, the following countermeasures were implemented
shown in Fig. 7.
(1) In order to suppress static electricity on the cable surface from affecting the cable core material, it
was exchanged for a cable with a semiconductor layer between the insulating layer and theconductive layer.
(2) In order to be able to check the influence of indicator fluctuation, we raised the zero point(15→50 pA) and added the alarm set value for decreasing SID ion current.
(3) Vibration was generated due to contact with the coaxial cable, and there was a possibility ofindicator fluctuation. Therefore, a cover was attached to the detector which fixed this problem.
As a result of facility improvement, we succeeded in suppressing SID indication fluctuation.

Fig. 6 SID console panel

3.

Fig. 7 SID improvement cableConclusion

We have implemented measures and modifications to the SID system, as the system detects finecurrents and is influenced by impurities in filaments and electrification caused by draft friction. The
SID system was stably maintained. Any critical malfunctions did not occur in the measuring
instruments, and we were able to obtain valuable technical expertise in operating the SID system inthe actual plant stably for a long time.
In order to operate SID system stably, it is necessary to introduce mechanisms to exclude
unstable elements for fine currents. Also, we need to develop and demonstrate SID system which isable to detect sodium selectively and does not respond to moisture in the future.

Acknowledgement
The authors would like to thank Yoshihisa Kaneko (JAEA), Shin Usami (JAEA), Tsutomu Irie(JAEA), Shuichi Takeda (Hitachi, Ltd) and Naoto Motooka (MHI ,Ltd) for useful discussion andadvice.

References[1]

K. Morizono:, ''Studies on applicability of smoke sensors as a sodium leak monitoring system'',
ICONE-10 (2002/04).

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