Tsuruga Head Office ■Development of Experimental Techniques / Facilities at JAEA R&D Centers14-1 Evaluation of Radioactive Inventory in Nuclear
Facility for Its Decommissioning–The Applicability of a Radioactive Inventory Evaluation Technique for “FUGEN” Verified–
Control rod(a)Measurement data of Activation foil

Extension portionof Pressure tube

Evaluation at the Extension portionof Pressure tube(▲position)(Bq/g)

Iron-water shield

Neutrontransport
calculation(ANISN, DOT-3.5)
Neutron flux
evaluation

Radioactivation
calculation(ORIGEN)

Biologicalshield(Plant investigation)
Neutron fluxmeasurement

Takingsamples・Activation foil
・Spectrometer, etc

Comparative
estimation

Water weightmeasurement in
the Biological shield

Materialcomposition
analysismeasurement
data
calculation
data

Heavy
water

measurement
datacalculation
dataRadio
activation
measurement

Control rod

Extension portionof Pressure tube

Radio activation
evaluation

Fig.14-1 The estimation technique of radioactivation in “FUGEN”
The neutron flux is estimated from measurements
and calculations, and then, the radio-activation
is calculated with the ORIGEN code using it.
We had been acquiring data on neutron flux,
material composition, and water content inconcrete during the reactor operation period
in order to calculate the radio-activation. We
also had been measuring radio-activation.
We verified the validity of our technique bycomparing those data with calculation data.

Biologicalshield

Fuel
Pressure
tube
Calandria
tube
Heavy
water

3E+04

1E+03

4E+04

Co-58

Co-60

Au-198

6E+01

1E+04

3E+05

2E+02

1E+04

7E+05

(a)Measurement
data ofActivation
foil
Co-58
Co-60
Au-198Iron-water shield
Comparative
estimation

Au-198

Evaluation at the biological shield’s surface(◎position)(Bq/g)

Input data
Input data

Co-60
8E+02

Evaluation at the Iron-water shield’s surface(●position)(Bq/g)

Cross-section of reactor core of “FUGEN”
Note; ▲●◎ indicate the measurement
positions of activation foil, andindicates the sampling point of
concrete specimens.

measurement
Evaluation
Extension1E+04
portion
8E+01
data at theof Pressure
calculation tube(▲position)
2E+02
1E+04
data
(Bq/g)

Co-60

tube

Calandriatube

When we carry out the decommissioning of the “FUGEN”
reactor, it is important that the safety assessment of the public
exposure dose, the evaluation of radioactive waste amountand the planning of an appropriate method and procedure fordismantlement are based on an accurate radioactive inventory.
The radioactive inventory evaluation consists of (1)evaluation of the radio-activation on the structural materials
such as reactor cores, caused by the neutron irradiation during
reactor operation and (2) evaluation of the corrosion layer
adhering to the equipment and piping, etc. in which there aresubstances which were in the coolant water and were radioactivated in the reactor core.
The radio-activation of many parts can be evaluated byanalysis following the flow of “Evaluation by analysis” inFig.14-1. However, “FUGEN” is a complicated pressure tube
type reactor different from a light water reactor, and its neutron
behavior near the reactor can be complicated. Thus, it isnecessary to understand the neutron flux distribution accurately
before analyzing the radio-activation, and to evaluate theappropriateness of the result. We acquired as much data as
possible of the element composition, neutron flux, and the radio
activation in investigation of “FUGEN”, and we comparedthose with the evaluations made by analysis (Fig.14-1).

measurement
data
calculation
data

JAEA R&D Review 2008

concrete

Co-58

Co-60

Au-198

6E+01

1E+04

3E+05

2E+02

1E+04

7E+05

Evaluation at the biological shield’s surface(◎position)(Bq/g)
Co-58

Co-60

Au-198

8E+01

1E+04

3E+05

2E+02
1E+04
7E+05
Calculation
data by ORIGEN

Measurement data
Measurement data(less than ND)

Distance from inside of theBiological shield (cm)In “FUGEN”, the radio-activation calculation was analyzed
by dividing the reactor into the core structure area and theshielding area surrounding reactor core.
The radio-activation measurement was carried out as follows.
A high pressure tube test piece irradiated for a certain period
in the core structure area during the reactor operation wasmeasured. Also, many Activation foils set around the reactor
core in the shielding area and irradiated for a certain period
during the reactor operation were gathered and measured.
Moreover, after reactor shutdown, pieces of concrete weretaken from the Biological shield and were measured.
The analytical data values were almost equal or moreconservative than the measurement data in all cases. Thus, thevalidity of our evaluation method was confirmed (Fig.14-2).
This technique was applied to the radioactive inventory
evaluation in the “Decommissioning plan of FUGEN”(authorized on February 12, 2008), and the public exposureand the amount of generation of waste were evaluatedappropriately from these results. Moreover, this technique will
also be applicable to the safety clearance evaluation of thewaste surrounding the nuclear reactor after dismantling which
had been exposed to neutrons.

Reference
Kitamura, K. et al., Evaluation of Radioactive Inventory for Fugen’s Decommissioning, Hoshasen, vol.34, no.1, 2008, p.53-63 (in Japanese).

102

7E+05

specimenin the BiologicalshieldEvaluation
at the Iron-water
shield’s surface(●position)(Bq/g)

measurement
data
calculation
data

FuelComparison between calculated
Fig.14-2
Pressure radio-activation values
and measured

3E+05

Au-198measurement
3E+04
8E+02
data
calculation
4E+04
1E+03
(b)Measurement
data ofdata

Radio activation：Co-60 (Bq/g)(Evaluation byanalysis)