Primärbericht

Test of In-Core Flux Detectors in KNK II

0 1 « 0 2 « 1 8 p 2 4 B Oktober / 1979

Kernforschungszentrum Karlsruhe

Primärbericht

Test of In-Core Flux Detectors in KNK II

0 1 » 0 2 . 1 8 p 2 4 B Oktober / 1979

PSB- Ber. IZ IZ9 (Kl.

P. Hoppe, F. Mitzel

Institut für Neutronenphysik und Reaktortechnik

INR 968

Note: This Primary Report contains preliminary information only for internal briefing ofthe Institutes and external partners in cooperation of the Karlsruhe Nuclear ResearchCenter. The report or its contents may not be transmitted to third parties without theconsent of the KfK Patents and Licences Department.

Kernforschungszentrum KarlsruheGesellschaft mit beschränkter Haftung

Test of In-Core Flux Detectors in KNK II

Contents

1. Introduction

2. Description of the Experimental Set-up

3. Performance Tests in the Reactor

3.1 lonization Chambers

3.1.1 Saturation Current-Curves

3.1.1.1 Comparison of all Detectors

3.1.1.2 Variation of the Plateau Characteristics

with Increasing Irradiation Time

3.1.2 Absolute Detector Sensitivities

3.1.2.1 Neutron Sensitivity

3.1.2.2 Influence of the Gamma Sensitivity

3.1.2.3 Influence of the Leakage Current

3.1.3 Linearity of the Detector Response and its

Dependence on the Burn up

3.2 Self Powered Detectors

3.2.1 Detector Lifetime

3.2.2 Comparison of Evaluated and Measured Detector Currents

4. Conclusions

Zusammenfassung

Die Entwicklung von In-Core Flußdetektoren zum Einsatz in SchnellenNatriumgekühlten Brutreaktoren befindet sich noch im Anfangsstadiumund es gibt für solche Detektoren nur wenig Betriebserfahrung. Daherwurden "Self-Powered" Neutronen- und Gamma-Detektoren und neutronen-empfindliche Ionisationskammern, welche spezieJLl für den Einsatz innatriumgekühlten Reaktoren entwickelt wurden, in der KompaktenNatriumgekühlten Kernenergieanlage KNK II erprobt.

Insgeamt wurden sieben Flußdetektoren in den KNK-II-Kern mittelseines speziellen Testeinsatzes eingebaut. Fünf von ihnen wurden schonwährend der ersten Betriebswoche im Reaktor defekt. Aufgrund von elek-trischen Widerstands- und Kapazitätsmessungen scheint dies durch Natrium,welches in die Detektoren und die Kabel eindrang, verursacht worden zusein. Da durch Vorprüfungen die Dichtigkeit der Detektoren und der Kabelzum Zeitpunkt des Einbaus nachgewiesen wurde, wird deshalb Rißbildungin den Detektor- oder Kabelmänteln während des Einsatzes in Natriumvermutet. Nur zwei Ionisationskammern zeigten nicht solche Defekte.Aber eine von ihnen versagte ebenfalls, was sich durch Verlust dertypischen Strom-Spannungscharakteristik bemerkbar machte. Bei der anderenverringerte sich die Empfindlichkeit während der Testperiode auf ca. 20 %vom Anfangswert. Es wird vermutet, daß in beiden Fällen Veränderungendes Füllgases eine Rolle spielen.

Summary

The development of In-Core Detectors for Liquid Metal Fast Breeder Reactors(LMFBRs) is still in an early stage and little operating experience isavailable. Therefore "Self-Powered" Neutron and Gamma Detectors and neutronsensitive ionization chambers -especially developed for LMFBRs- have beentested in the Fast Sodium Cooled Test Reactor KNK II.

All in all seven flux detectors have been installed in the core of KNK IIby means of a special test rig. Five of them failed already within the firstweek during operation in the reactor. Due to measurements of electricalresistances and capacities, sodium penetrating into the detectors or cablesprobably seems to be the cause. As tests prior to the installation in thecore proved the tightness of all detectors at the beginning of the tests/it is suspected that small cracks have developed in the detector casingsor in the outer cable sheaths during their exposure to the hot coolant.Only two ionization chambers didn't show such faults. However, one of thosefailed because the saturation current plateau disappeared and the otherone's sensitivity decreased by a factor of five during the test period. Itis suspected that in both cases changes of the filling gas might be involved.

1. Introduction

In-Core flux detectors which have been developed for thermal reactors

cannot be used in liquid metal fast breeder reactors (LMFBRs) mainly

because in the latter case higher operating temperatures (up to 600 C)

are required. Therefore special in-core flux detectors for LMFBRs

have been or are being developed. They will be necessary for large

cores, especially for large heterogeneous cores. So far only very

little operating experience is available for those detectors. There-

fore self-powered neutron and gamma detectors and neutron sensitive

ionization chambers have been tested in KNK II in the frame of the

experimental program for this reactor. The following report describes

the results.

2. Description of the Experimental Set-up

The following flux detectors have been tested in the core of KNK Ii;

Three neutron sensitive ionization chambers (Fl, F2, F3), two neutron

sensitive self-powered detectors (S3, S4) and two gamma sensitive

self-powered detectors (SI, S2). Table 1 and Table 2 at the end of the

report show the specifications of these detectors as provided by the

manufacturers. With one exception (F3) all of them were commercially

available.

All detectors were installed in the reactor by means of a special

test rig at the outer boundary of the reactor driver zone. The axial

detector position was the core mid-plane. The test rig is described

in /!/. All detectors were in direct contact with the coolant. Fig. 1

shows schematically the electtrical circuits which were used for the

performance tests of the detectors.

In order to reduce the leakage current from the central conductor

(No. 1 in Fig. 1) at high temperature the detector F3 was equipped

with an integrated triaxial cable. The intermediate screen (No. 2 in

Fig. 1) had always the same voltage as the central conductor to the

detector anode. Thus a leakage current from the central conductor to

- 2 -

- 2 -

the cable sheath through the insulator could be avoided which would

erroneously increase the anode-current I- due to the neutron flux.

All other detectors were also provided with integrated cables, SI

through S4 with a twinax . design but Fl and F2 only with a coax

design.

The voltage supply could be varied from 0 to 200 Volt in order to

measure the saturation current curves.

The resistances .and capacities of the detectors and the cables were

measured after installation of the detectors in the core but before

power ascension /2/. These measurements confirmed that their electri-

cal properties had not changed during the mounting and that a proper

functioning could be expected.

3. Performance Tests in the Reactor

3.1 lonization Chambers

3.1.1 Saturation Current Curves

3.1.1.1 Comparison of all detectors

A sensitive indication for a properly operating ionization chamber

is the current versus voltage diagram. It should show a saturation

current (plateau) within a wide voltage range. The slope of the plateau

tangent is determined by the leakage current and should be small.

Fig. 2 shows the saturation curves for all ionization chambers at

52 % power level and 90 hours after their installation in the reactor.

The coolant temperature at the position of the detectors was about

400 C. It did not vary significantly with the reactor power because

the coolant flow was changed in proportion to the reactor power.

The ratio of the saturation currents between the detectors Fl, F2

and the detector F3 agrees with a deviation of 8 % with the ratio

of the nominal sensivities. The plots show also that the plateau of

F3 is flatter than those of Fl and F2. This was expected because of

the different cables (triaxial for F3 and coaxial, for Fl, F2; see

para. 2). Though F3 reaches its saturation current only at 60 V

- 3 -

- 3 -

(compared to 15 V for Fl and F2) its plateau width is not significantly

smaller than those of. the other ones as it extends to 200 V (see e.g.

Figs. 5 and 6).

3.1.1.2 Variation of the Plateau Characteristics with

Increasing Irradiation Time

The Figs. 3 and 4 show how the plateau characteristics of the detectors

Fl and F2 deteriorated after a few days. As the reactor power level

changed between 30 % and 50 % during this period, all measurements

were normalized to a power level of 40 %.

After 3 days in the reactor (corresponding to an integrated flux of

nvt = 1.5 • 1020—2) tne detector Fl had completely lost its plateau

characteristics and could not be used any more. The same happened to

the detector F2 after 14 days (corresponding to nvt = 3.2 * 1020—2)-

However at the very beginning (measurements No. 1 and 2 for Fl and

No. 1 - 4 for F2) the tendency was different, i.e. the plateau became

flatter. This is probably due to the fact that the leakage current,

due to temperature and irradiation, decreases to an equilibrium level

during operating voltage supply (see also para 3.1.2.3). But afterwards

the plateaus became always steeper until they completely disappeared.

The first idea that this might be caused by a decreasing insulation

resistance of the detector or of the cable or of both turned out to

be wrong. When the reactor was shut down again the leakage currents

and insulation resistances could.be directly checked! (measurement ,'•.

No» 5 for Fl and. Nor. 8 for i'2).-,At -that time there was...no change between

the values before and after irradiation. When the reactor operation

was resumed the detector currents did still further increase. Thus

any responsible effect for this behaviour had not changed the insula-

tion resistance (at least at lower temperatures of about 200 C) butit

must have changed such characteristics which are directly connected

to the detection process. This suggests that the state of the gas

filling may have changed (e.g. the pressure or the composition) during

the test period in the reactor. The gas pressure can change when small

- 4 -

- 4 -

cracks develop in the detector case and the composition can change

if outgassing occurs within the detector due to the high temperature.

When the insulation resistances were checked again seven months later,

a decrease from more than 2 - 1 0 ft to 134 ft was observed for Fl /2/. *)

This additional failure was then probably caused after the first one

by a defect of the chamber casing or of the outer cable sheath allowing

sodium to penetrate into the insulator.

The detector F3 did not show such a drastic change of its plateau

characteristics as can be seen from Figs. 5 and 6. The plateaus were

generally very flat during the whole test period independent of the

reactor power, except for a temporary change (Measurements No. 9 and

10 in Fig. 6).

3.1.2 Absolute Detector Sensitivities

3.1.2.1 Neutron Sensitivity

The absolute detector sensitivities were determined by the measured

detector current I,.... , ... i i _ j ~ ,_ c^ j - * .IM: and the calculated neutron flux corresponding to

the reactor power during the measurement and the detector position.

F A i[ nv J

Using the total neutron flux 0

.0 = J0(E) dE

obtained by integration over the whole energy spectrum E, gives the

actual detector sensitivity in KNK II.

For a comparison with the sensitivities e , to the thermal flux 0_,th th

which are specified by the manufacturers these values have also been

determined.

I 0 ae _ IM • wth • th

, th rf J0(E) a(E)dE

After one year the detector F2 showed s. similar effect. - 5 -

- 5 -

with a = g(T)293 a(0,025eV)T

and g(T) bring the Westcott correction factor.

The following table shows a comparison between evaluated and

specified neutron sensitivities.

Detector

Fl, F2

F3

£«ff A ]

L H

1,015 • 10~19

4,2 - 10"19

£«*,thevaluated

[A/nVttJ

4,77 - 10"18

3,11 • 10"17

£«*,thfrom specification

[A/nvthl

7 - ID'18

2,7 - IQ'17

A good agreement was obtained between evaluated and specified thermal

sensitivities.

3.1.2.2 Influence of the Gamma Sensitivity

The contribution /I.. of the gamma sensitivity (e .) and the gamma

flux (0~) to the measured detector current I., was estimated by means

of the specified e (from the manufacturer) and the calculated flux

0 . The next table shows that this contribution is equal or smaller

than 1 %.

Detector

Fl, F2

F3

fA 1ViR/hj

7.0 • lo"15

1,3 - 10~14

0y[R/h]at full power

8,47 • 107

8,47 • 107

T1M,-YI1M, total

1 %

0,45 %

- 6 -

- 6 -

3.1.2.3 Influence of the Leakage Current

As discussed in para. 2 the cable of detector F3 was equipped with

a special intermediate screen (No. 2 in Fig. Ic) in order to reduce

the leakage current from the inner conductor (No. 1 in Fig. 1) to

the outer sheath. The relatively flat plateaus obtained for this

detector (see Figs. 2 and 5) proves the efficiency of this device.

The measured "Leakage current from the intermediate screen to the

outer sheath !„ was rather high (- 30 yA at 100 V) immediately after

applying the voltage but decreased to an asymptotic value after several

hours. This effect was probably due to a surplus of electrons pro-

duced by irradiation in the insulator whose density decreases when

an external voltage is applied. The asymptotic value of 5 yA at

100 V corresponds to a resistance of 2 • 10 tt for ambient tempera-

tures from 400 C to 550 C for the detector and the upper part of the

cable respectively. However this effect had of course no influence

on the detector current I,,,.IM

By mismatching intentionally the voltages of the guard screen (U )

and of the anode (U,M) a possible residual leakage current has been

checked. It turned out to be negligible as long as both voltages

agreed within less than 1 %.

The detector currents of Fl and F2 were however disturbed by leakage

currents as their cables were not delivered with a guard screen. This

can be seen directly from the slope of the plateau characteristic.

The insulation resistance at operating temperature was estimated from

the plateau tangent:

=7,3

which agrees well with the value given by the manufacturer (see Tab.l).

The resulting leakage current for 100 V operating voltage is I = 13 yA

which is 40 % of the total detector current at 50 % reactor power.

This big amount has been corrected in the evaluation of £M.

-. 7 -

- 7 -

3.1.3 Linearity of the Detector Response and its Dependence

on the Burn up.

Because of the early failure of the detectors Fl and F2 these tests

could only be performed with the detector F3. Fig. 7 shows the measured

detector current I.. in dependence of the reactor power. Only the

first measurements (No. 1 to 5) show a linear relationship as indicated

by the line in Fig. 7. The subsequent three measurements (No. 6 to 8)

are slightly below this line. All other detector currents measured

later at almost full power (No. 9 to 16) are far below the expected

values. This means that the detector efficiency has decreased gradu-

a'lly. Fig. 8 shows this explicitely. Here the detector efficiency e

is plotted versus time integral of the reactor power(burn up) and the

neutron flux starting from the installation of the detectors in the

core. The definition of e isP

detector current I1we - = IMp reactor power

The first measurements (No. 1 to 5) being made with an integrated20 1

flux nvt < 3 • 10 —j are obviously scattered around a constant

value. With increasing integrated reactor power the detector efficiency20 1

decreased first continuously until nvt = 14 • 10 —7 was reached.

(Measurements No. 6 to 8). During a small further increase of the

reactor fuel burn upsa very abrupt decrease of e by more than a

factor 2 was observed (between measurements No. 8 and 9). Then ?

still decreased continuously with increasing burn up until the

reactor was shut down after measurement No. 12. At the next measure-

ment (No. 13) following a reactor shut down period of

more than 3 months the ep had increased a little bit but then again

continuously decreased. At the end of this test period £ seemedP

to have reached an asymptotic value (Measurement No. 16). So far

no explanation can be given for this behaviour.

Fuel burn up must be excluded even for the first period (up to measure-

ment No. 8) where e only slightly decreased. The relative change ofp . .

g due to fuel burn up is given by~ "~~_ ________ , _

(— - De o,. - 8 -£- = (1 - a) t

£ >

- 8 -

where a is the reactor fuel burn up

a,, the fission cross section of the fuel and

a the neutron cross section of the sensitive

detector material.

A numerical calculation showed that during this test E should

remain practically constant.

It is interesting that after the sudden decrease of e (measurementP

No. 9) the plateau characteristic also deteriorated (see Fig. 6) but

within 3 days resumed its original flat slope. This seems to indi-

cate that a change of the fission gas which occured temporarily could

be related to this behaviour. However the only possibility to find

a definite explanation for it will probably be a direct post irradiation

inspection as discussed in /2/.

3.2. Self Powered Detectors

3.2.1 Detector Lifetime

The first measurement with the self-powered detectors has been made

four days after their installation in the core corresponding to an

integrated power of 78 MWd. The results are discussed in the next

paragraph. It is the only useful measurement which would be obtained

with these detectors because afterwards the detector current decreased

rapidly within a few days to zero. Resistance measurements during a

following reactor shut down showed that the insulation resistance R

had decreased from originally R > 2000 Mft to 14 tt /2/. This indicates

that the detector casing or the cable sheaths became leak and sodium

pentrating through the insulation has probably caused the damage.

3.2.2 Comparison of Evaluated and Measured Detector Currents

The measured detector currents are compared with expected values

in the following table;

Detector Measured Detector Evaluated Detector

Current I,,, \A\ Current IlA, \A\1 M L - L l M i» J,,,IM

SI

S2

S3

S4

3,2 •

3,15-

3,0 •

2,5 •

IQ'8 7

lo'8 J-1 •>

10 1ID'7 J

1,3 • 10"8

1,15« 10~?

For this comparison it has to be considered that the evaluated

detector currents are affected with relatively big errors:

For the neutron sensitive detectors S3 and S4 with Rhodium-emitters

I1 is given to about 95 % by the (n,g) reaction. The rest is due

to the reactions (y,e ) and (n,y}e ). From the manufacturer no

information of the corresponding detector sensitivity was available

(for the Rh- as well as for the Pt-detectors). The values given in

the literature /3,4,5/ for the (n,3) reaction in the Rh-detectors vary from•™ 91

about 1 to 2*10 ' £A/( ({»thermal)] for 1 cm emitter length. The detector

current !,„ due to the thermal flux was evaluated with an intermediate-21

value of 1.5'10 A/((j)'cm). Another inaccuracy comes from the fact

that in KNK II the thermal flux <j> contributes only with about 5.4 %.

to the total reaction rate in the detectors S3 and S4. The total de-

tector current I, was therefore determined by calculating the ratio

of the reaction rates due to the thermal and the total neutron flux

using a 26-group neutron flux calculation and the corresponding (n,3)

cross-sections.

For the y sensitive detectors SI and S2 with Platinum-emitters, I.. is

given to about 76% by the reaction (y,e ). The corresponding sensitivity

e found in the literature |5,6| varied between 1 to 2'10 [A/(R/h)]

for 1 cm emitter length. Using a calculated dose rate of 8.47 R/h

(at full reactor power) for the detector position and e =1,5 • 10

[A/(R/h) cm] gives 76% of the calculated detector current. The rest is

due to (n,3) and (n,y,e ) processes and has also been evaluated using a

26-group neutron flux calculation and the corresponding reaction cross

sections.

- 10 -

- 10 -

The conversion of the(n,ß) reaction rate to the detector current is done

in the same way as for the Rhodium-Detectors S3,S4. The conversion of the

(n,Y>e )reaction rate to the detector current contribution has been cal-

culated by using the value given in | 7

4. Conclusions

From the seven tested detectors all failed except, for F3 within the

first week after their installation in the reactor being operated with an

average power level of only 43%. This happened though the operating con-

ditions were well within the limits given by the manufacturers. In the

most cases the failures are probably caused by defects of the detector

casing or the cable sheaths. This allowed sodium to penetrate into the

insulation material thus reducing the resistances as observed.

Prior to this damage the composition of the filling gas must have changed

in the detectors Fl and F2. Only the ionization chamber F3 could be

operated during the whole test period. However the big reduction of its

sensitivity i a facor of five during this time cannot yet be explained.

These observations show that the performance of all In-Core flux

detectors which have been texted so far in KNK II were unsatisfactorily.

Further tests are only reasonable for versions being improved with

respect to the observed faults. This requires at least a full understan-

ding of the reasons for these faults. This would require post irradia-

tion investigations.

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References

III Artz, Kathol, Schlipf

Experimentierstopfen für KNK II

PSB Bericht Nr. 691 01.01. 06P03A

/2l P. Hoppe, R. Machts, F. Mitzel

Untersuchungen zum Betriebsverhalten der Meß we r t auf nähme n

im KNK II-Versuchseinsatz 5.11/2

Primärbericht

/3/ H. D. Warren

Calculational Model for Self-Powered Neutron Detector Nucl. Sei

. and Eng. 48 (1972) p. 331-342

/4/ Self-Powered Flux Detectors Advertised for Sale by Reuter-Stokes

Canada Limited, Box 45, Preston, Ontario

Brochure (1968).

/5/ Data Sheets for SPN-Detectors of the Following Manufacturers:

20 TH Century, Studsvik, Westinghouse

ARI-Indus tries

/6/ C.C. Price, J.R. Karvinen

Performance Testing of Self-Powered Detectors in EBR-II

Instrumented Subassemblies

ANL 8030 (9.1973)

R.B. Shields: A Platinum In-Core Flux Detector.

IEEE Transactions on Nuclear Science Vol. NS-20 N.I

Febr. 1973

c- 14 -

' IM 1

Detector CableJ I2J2M

RU UR= R ( J M

M

Fig. 1a Detectors 5 1 , 5 2 , 5 3 , 5 4J1M-J2M = Flux Sensitive Detector Current

BDttector Cable

Jzfu1M

M

Fig.lb Detectors F1, F2J1M = Flux Sensitive Detector Current

cDttector Cable

U s U2M 1M

•• tf2M

2M

M

Fig. 1c Detector F 3J1M= Flux Sensitive Detector Current

uIM

Fig . 1 Block Diagram of the Electrical Circuits

- 15 -

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CJ

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'S oQ >

O)

ÜL

- 16 -

Date Ref. Power

3. 11.78

6. 11.78

7. 11.78

9. 11. 78

20 .11 .78

20 40 60 80 100 120 UO 160 180

- u1M [v]

Fig. 3 Detector Current J1M for Detector F1 VersusOperating Voltage U1M Normalized to 40 Vo PowerLevel.

- 17 -

JIM [M A]steadilyincreasing

MeasurementNo

3.11.786.11.787. 11.789.11.78

15.11.7816.11.7817. 11.7820.11.78

20

20 40 60 80 100 120 140 160 180

»• u,M[v]Fig. A- Detector Current J1M for Detector F2 Versus

Operating Voltage U1M . Normalized to 40 Vo PowerLevel.

- 18 -

CO

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