Laboratoire de Physique des Réacteurs et de Comportement des Systèmes (LRS)
FACULTÉ DES SCIENCES de BASE
PH-ECUBLENS - CH - 1015 LAUSANNE /
swissnuclear / PSI
Fortbildungskurs Kerntechnik 2010
A.3 Reaktorpraktikum
Data sheets for the experiments
A3.1 Radiation Measurements
Date: …29.. January 2010
Morning/Afternoon [please underline]
Group Number: 6B
Names1. André Scheidegger
2. Katerina Stanova
3. Jürg Studer
These data sheets can be downloaded and filled out electronically (including plotting of the graphs, etc.) from the following link: http://lrs.epfl.ch, “Fortbildungskurs download”.
After completion of your group report, please send this (pdf only), within a week’s time, at the following address: .
Kindly, at the same time (and also as a group), submit your feedback on the Reaktorpraktikum by filling out the questionnaire attached at the end.
Please note that your final grade for the Reaktorpraktikum (Course Module A.3) will be based on two, equally weighted criteria, viz.
(i) your activity as a group (conducting the measurements, laboratory report, etc.), for which the same grade will be given to each member of the group.
(ii) your individual performances in the written examination for the module.
A3.1.1 Gamma Detectors
I) Gamma dose measurements:
The measurements employ a gamma dose-meter (Automess, type 6150 AD 6; essentially a Geiger-Müller counter) and a 60Co gamma source (ID No. 10353).
Procedure:
1. Report the dose rate values for the various distances given in the table below:
Source-to-detector distance, d (cm) / Dose rate (mSv/hr)~0 / 38,30 – 41.10
2 / 9.50 – 10.90
5 / 2.95 – 3.51
10 / 1.10 – 1.50
II) Gamma spectroscopy using a NaI(Tl) detector system:
Apart from the sodium iodide scintillator detector itself, the counting system employs a high-voltage unit (+1000V), a pre-amplifier, an amplifier, a multi-channel analyser MCB-926 with Maestro-32 software, and a PC for data acquisition and analysis.
Procedure:
1. Obtain the NaI(Tl) gamma spectrum of the provided 60Co source by acquiring data for 300 s. ü
2. Select the 60Co photopeaks by using the option “Mark ROI”. ü
3. Click on the function “calculate” to get the required information about the photopeaks – location (in terms of channel number) and energy resolution (expressed as “full-width-at-half-maximum (FWHM)”). Enter the values obtained in the table below
Source / Gamma energy(keV) / Channel
number / FWHM
(keV)
60Co
(ID No. 10353) / 1173 (1171) / 2048 / 53.03
1332 (1322) / 2312 / 58.27
III) Gamma spectroscopy using a HP(Ge) detector system:
A high-purity germanium semiconductor detector, cooled by liquid nitrogen, is used here for high-resolution gamma spectroscopy. The counting system consists of a pre-amplifier, a DSA-1000 unit containing a high-voltage supply (+4000V), an amplifier and a discriminator (all integrated into a single unit), and GENIE-2000 software on a PC for data acquisition and multi-channel analysis.
Procedure:
1. Acquire data for 400 s with the 60Co source, using a source-to-detector distance of 5 cm. ü
2. Carry out the energy calibration of the detector system on the basis of the channel numbers corresponding to the 60Co photopeaks. ü
3. Determine the energy resolution (FWHM) of the 60Co photopeaks. ü
4. On the basis of the energy calibration carried out, determine the gamma-ray energy of the second provided source (137Cs). ü
5. Report your results in the table below:
Source(ID No.) / Gamma energy
(keV) / Channel
number / FWHM
(keV)
60Co
(ID No. 10353) / 1173 (1175) / 6146 / 1.55
1332 (1334) / 6982 / 1.57
137Cs
(ID No.16170) / 662 / 3464 / 1.31
Important Instructions:
On the software window:
a) Click analyze > peak locate > VMS standard peak search, for searching peaks.
b) Click analyze > execute > peak analysis report, for detailed analysis of peaks.
Questions related to Experiment A3.1.1 (gamma detectors)
Briefly respond to the following questions:
1. Measurements I): Comment on your measured variation of the gamma dose rate with distance.
The dose rate decreases proportional to the reciprocal square distance (1/r2).
2. Measurements II): What is the average energy resolution (FWHM) of the NaI(Tl) detector system for the 60Co gammas, expressed as a percentage?
Average energy resolution: = Æ FWHM /Æ Gamma energy
= (53.03 + 58.27)/2 / (1171+1332)/2
= 4.46%.
3. Measurements III): What is the factor of improvement in the energy resolution (FWHM) of the HP(Ge) detector system, relative to NaI(Tl)?
Factor of improvement: = Æ FWHM (NaI(T1)) / Æ FWHM (HP(Ge))
= (53.03+58.27)/2 / (1.55+1.57)/2
= 35
A3.1.2 Neutron Detectors
I) Neutron dose measurements:
The measurements employ a neutron dose-meter (Berthold, type LB-6411) and a Pu-a-Be neutron source. The gamma dose-meter used earlier is also available.
Procedure:
1. Report neutron dose rate values for the distances listed in the table below.
2. Report the gamma dose rate for at least one of the distances.
Source to detector distance, d(cm) / Neutron dose rate
(mSv/hr) / Gamma dose rate
(mSv/hr)
50 / 640 / 0
100 / 165 / 0
200 / 42 / 0
300 / 18 / 0.64
II) Moderating properties of polyethylene:
This experiment is mainly to demonstrate the fact that a neutron detector is generally much more sensitive to slow neutrons than to fast neutrons – e.g. 1/v cross-section of the B10(n,a) reaction. The measurements are performed using the following pre-installed equipment:
· A BF3 detector system, consisting of a high-voltage unit (+1650V), a pre-amplifier, an amplifier, a timer/counter unit and a single channel analyser.
· Several blocks of polyethylene.
· A Pu-a-Be neutron source.
Procedure:
1. Report, in the table below, the integral counts (counting time: 100 s), as obtained with the BF3 counter for different thicknesses of polyethylene moderator placed between source and detector. (Note that each moderator block is ~4 cm thick.)
Description / BF3 detector count/100 s1. Source / 2169
2. Source + 1 block of moderator / 6375
3. Source + 2 blocks of moderator / 5277
4. Source + 3 blocks of moderator / 3621
III) BF3 detector response, neutron propagation in water:
The measurements are carried out in the CARROUSEL water tank, containing a Pu-a-Be neutron source. A large-diameter BF3 detector system is employed for the experiment. This consists of a high-voltage unit (+1200V), a pre-amplifier, an amplifier, a single channel analyser and a timer/counter unit. A multi-channel analyser MCB-926, with MAESTRO-32 software and a PC for data acquisition and analysis is also available.
Procedure:
1. Carry out a multi-channel analysis of the BF3 detector response to obtain the energy spectrum of the secondary charged particles of the B10(n,a) reaction.
2. Use the detector in the single-channel counting mode to obtain the integral detector counts as a function of distance from the source in the moderating medium (water). Use 5 to 16 cm as the range of source-to-detector distances for your measurements.
Important Instructions:
To simplify the task, a Microsoft Excel file has been prepared. Enter, in the first column, your measured data (taken between 5 and 16 cm) after having normalised all the counts such that the value at 5 cm is unity. The plot will be created automatically, and the following two “standard function” curves will appear directly for comparison: and .
Questions related to Experiment A3.1.2 (neutron detectors)
1. Measurements I): Comment on your measured variation of the neutron dose rate with distance.
The neutron dose rat does decrease with distance. The decrease corresponds as expected pretty much to a 1/r2 relationship. The influence of scatted neutrons on i.e. walls seems to be small
Measurements I): What would be the total radiation dose for someone standing for 10 minutes at the position at which you carried out both neutron and gamma measurements?
The dose received from neutrons is much higher. Furthermore the quality factor for neutron is much higher then for gamma radiation. Total dose received is the sum of the neutron dose (18 mSv/hr) and the gamma dose rate (<1 mSv/hr). At the distance where the measurements are done (3m), the total dose in 10 min would be slightly higher then 3 mSv. Thus at our distance (6m) a person would receive less then 1 mSv.
2. Measurements II): Why does the BF3 detector count rate, as function of the added polyethylene blocks, first increase and then later decrease?
Detector system: The BF3 neutron detector counts thermal neutrons due to the reaction 10B(n,a)Li7. The neutron capturing of Boron is strongly dependent on the energy of the neutron (cross section for capturing is 104 higher for thermal then fast neutrons). For this reason the BF3 detector must we well wrapped with parafin that acts as a moderator.
The moderation of neutrons (scattering effect) and the absorption (in structural materials) are parallel processes. When a block of moderator (polyethylene 4 cm thick) is added, the BF3 detector count rate first goes up due to moderation. However, the addition of 1 and 2 additional blocks decrease the count rate. The absorption of especially thermal neutrons in the polyethylene becomes now more dominate then the additional moderation effect.
3. Measurements III): Why does one observe two distinct energy peaks in the pulse height spectrum of a BF3 detector?
The absorption of neutron in boron leads to the following reactions:
10B + n => 7Li + 4He (a) + 2.792 MeV (6%)
10B + n => 7Li* (excited state) + 4He (a) + 2.31 MeV (94%)
Thus 2 distinct peaks with corresponding intensities are observed.
4. Measurements III): Explain, in qualitative terms, why the spatial variation of the neutron detector response is different from the standard functions provided for illustration?
The provided functions do no encounter for moderation of the fast neutrons. Water is a very good moderator. Therefore, the count rare in the near vicinity of the Pu-a-Be source goes up within the first 1 and 2 cm. Water is also a good absorber. Thus, the absorption of the slow neutrons by water becomes thereafter the dominant process (Bremsentfernung in Wasser ca. 13 cm, absorption length ca. 6 cm). For the above reasons, in heterogeneous light water reactor technology one needs to find a optimal geometry for fuel and moderator arrangement.
Institut de Physique de l'Energie et des Particules (IPEP)Laboratoire de Physique des Réacteurs et de Comportement des Systèmes (LRS)
FACULTÉ DES SCIENCES de BASE
PH-ECUBLENS - CH - 1015 LAUSANNE /
swissnuclear / PSI
Fortbildungskurs Kerntechnik 2010
A.3 Reaktorpraktikum
Data sheets for the experiments
A3.2 Reactor Experiments on CROCUS
Date: …29. January 2010
Morning/Afternoon [please underline]
Group Number:6B
Names1. André Scheidegger
2. Katerina Stanova
3. Jürg Studer
A3.2.1 Approach-to-critical by variation of control rod position /different water level
Apart from the CROCUS reactor itself, two neutron counting systems (consisting in each case of a detector, a high-voltage supply, a pre-amplifier, an amplifier and a scalar unit) are used. The two neutron detectors are a U235 fission chamber and a BF3 ionisation chamber, both of which form part of the safety CROCUS reactor instrumentation. All changes on the reactor will be carried out by the reactor operator.
Procedure:
1. Carefully observe the procedure followed by the supervisor to bring the reactor to the sub-critical state required for the start of your measurements, e.g. insertion of the external source, withdrawal of the cross-shaped safety rods, ensuring that the South and North control rods are inserted appropriately, filling of the moderator in the reactor tank, etc.
2. The sub-criticality of the reactor at the start of your measurements is due to the fact that the South control rod is fully inserted. The moderator level and the position of the North control rod correspond to the values needed for the final critical state and, as such, will remain fixed throughout your measurements.
Note these values below:
Moderator level: 920 mm and Position of the North and South control rod: are fully outside due to a blocked control rod.
3. Obtain the fission chamber and BF3 detector responses for three different (positions of the South control rod) water levels indicated in the table below:
water level (mm) / Fission chamber(cps) / Ionisation chamber
Current (A)
920 / 28.6 / 2.80E-10
930 / 35.6 / 4.00E-10
945 / 87.8 / 1.10E-08
4. For each set of neutron detector responses, employ the inverse-counts extrapolation method for estimating the South control rod position for criticality.
5. Ask the operator to make the reactor critical via withdrawal of the South control rod to the predicted value. With the external source in its shielded position, the neutron flux should be stabilised corresponding to a reactor a power of 0.1 W.
6. Note the South control rod position for criticality.
The water level is at 954 mm
7. Ask the operator to increase the reactor power, stabilising its value of
18 W. Note, once again, the South control rod position for criticality.
(Water level is brought to 990 mm)
8. Ask the operator to bring the reactor back to 18 W. Note, once again, the South control rod position for criticality.
Water level is brought back to 954, while the power remains constant
Period 1 (in graph above): Reactor at point of criticality, water level at 954 mm.
Period 2: Water level is brought to 990 mm -> reactor power is increasing fast.
Period 3: Water level is brought back down to 954 mm -> reactor stable at 18 W after a short phase of stabilization.
Period 4: reactor shut down -> the reactor power and neutron flux is rapidly going down.
9. Fill out the table below:
Position of the South control rod for criticality, from extrapolation of the U235 fission chamber measurements (mm) / ------Position of the South control rod for criticality, from extrapolation of the BF3 detector measurements (mm) / ------
Actual water level for a reactor power of 0,1 W (mm) / 954
Actual water level for a reactor power of 18 W (mm) / 990
Actual water level for criticality, with the return to 18 W (mm) / 954
Important Instructions: