Sample Extended Synopsis

Use of
137
Cs calibration source in evaluation of BWR fuel burnup
G. Ekenstam
a
, M. Tarvainen
b
a
Swedish Nuclear Power Inspectorate (SKI), Stockholm, Sweden
b
Finnish Centre for Radiation and Nuclear Safety (STUK), Helsinki, Finland
E-mail address of main author:G.Ekenstam@ski.se
A method for evaluating the burnup (BU) of BWR spent fuel was investigated by using a
novel type of
137
Cs calibration source. The source is constructed to fit in the fuel handling
fixtures of all the BWR type power plants in Sweden and Finland. It can be used also in the
interim storage facilities for spent fuel, CLAB in Sweden and TVO-KPA-STORE in Finland.
The source is covered by a watertight steel cylinder which is fixed inside a 0.65 m long
section of ASEA-ATOM type BWR fuel channel. Inside the cylinder there is a 37 GBq
137
Cs
pellet fixed to a wagon which can be driven up and down by means of a stepping motor. By
moving the source, the repeatability of the geometrical positioning is attained. The amplitude
and scanning speed are controlled by a remote control unit. The apparatus is easy to handle
and decontaminate. The source can be transported in a custom made box (45 cm × 45 cm ×
70 cm) under category II-Yellow. In repeated measurements the precision of the new
calibration source was found to be 1l.7%. Use of this calibration source makes it possible to
calibrate the whole measurement chain and to compare the data measured in different
geometries. A typical calibration time is 15 min including source handling.
In recent measurements, a Westphal loss free counting (LFC) system was used in connection
with an ND66 multichannel analyser for scanning of fuel assemblies. By use of LFC, real
time correction of counting losses is performed.
For BU verification 21 assemblies with mean BUs from 14 to 31 MW·d/kg U and cooling
times from 200 to 1500 d were scanned on each of the four corners. The total time needed per
assembly was typically half an hour. The
137
Cs measurement data were corrected for
radioactive decay, self-absorption and inhomogeneous Cs distribution. By use of the
arithmetic mean for the four corners and the earlier defined relation between BWR fuel BU
and
137
Cs activity [1], the BU could be calculated.
The BU calculated from the measured data is shown plotted against the declared BU in Fig. 1.
Error bars reflect the precision of measurements for single assemblies. The ±5% deviation
lines are also plotted. A more detailed description of the method will be published
elsewhere [2].
The measurement system has been developed to be used by the Swedish and Finnish national
safeguards authorities for verification of spent fuel BU

REFERENCES
[1] EKENSTAM, G., Cs
137
 measurements of spent BWR fuel”, Safeguards and Nuclear
Material Management (Proc. 6
th
 ESARDA Annual Symp. Venice, 1984), CEC Joint
Research Centre, Ispra (1984) 327-330.
[2] EKENSTAM, G., TARVAINEN, M., Verification of Burnup of BWR-type Nuclear
Fuel by Measurement of the Cs
137
 Acitivity, Rep.STUK-A52, Finnish Centre for
Radiation and Nuclear Safety, Helsinki (in press)