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Monitoring fast neutron flux inside spent reactor fuel by CR-39 detector
Radiation Measurements 34 (2001) 601–603
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Monitoring fast neutron ux inside spent reactor fuel by CR-39 detector N.C. Tam, L. Lakosi ∗ Institute of Isotope and Surface Chemistry, PO Box 77, H-1525 Budapest, Hungary Received 28 August 2000; received in revised form 14 January 2001; accepted 21 March 2001
Abstract A model experiment was carried out by CR-39 detectors using 252 Cf neutron sources in an intense gamma 2eld obtained by Co irradiation. Based on this calibration and introducing corrections due to the angle, -dose, and etching time dependences of the detector response, fast neutron ux inside spent fuel assemblies was found to be in the range of 102 –106 n cm−2 s−1 , depending on burnup and cooling time. The e4ect of adjacent assemblies can decrease substantially, if the detector surface is c 2001 Elsevier Science Ltd. All rights reserved. perpendicular to the axis of the fuel assembly. 60
Keywords: ND assay; Spent fuel assemblies; Fast neutron ux; Neighbour e4ect
1. Introduction In our early works, a technique of shape selection has been used to eliminate unwanted disturbing events on CR-39 detectors exposed inside spent fuel assemblies (Tam et al., 1995). During chemical etching, track forms change from conical to hemispherical (Fleischer et al., 1975; Benton et al., 1979; Fromm et al., 1991). Only conical tracks appearing as dark round events corresponding to neutrons of energy above 0:5±0:1 MeV (Tam et al., 1996) have been evaluated. Inside spent fuel assemblies, fast neutrons arrive at the detector from all directions in a huge gamma 2eld. Hence, the angular and gamma-dose dependence of CR-39 neutron response has also been studied (Tam et al., 1997, 1999). This paper deals with the angular dependence including the e4ect of neutrons from back directions, the e4ects of -dose and etching time in order to calculate neutron ux and analyze the e4ect of adjacent assemblies. 2. Angular dependence at various distances by neutron irradiation
252
placed into a large vessel 2lled with borated water containing 12 g=l boric acid. The 252 Cf source, whose neutron spectrum is similar to that of spent fuel, was put into the central channel of the mockup, while the CR-39 track detectors in stainless steel capsules were suspended in place of fuel rods at various distances (1.3–14:5 cm) from the source, with various angles of incidence (0◦ ; 30◦ ; 60◦ ; 90◦ ). The detectors were of 4 × 10 mm size, covered with 1 mm thick polyethylene foils on both sides. After chemical etching (6 h; 6 N NaOH; 70◦ C); tracks of protons recoiled by fast neutrons from 252 Cf were observed. The angular dependence was reported in the last work (Tam et al., 1999). Tracks observed on the rear side of the detector were now additionally evaluated. The results are displayed in a polar diagram (Fig. 1) as iso-eDciency curves characterizing the angular and distance dependence of detector response, i.e. the curves represent given track density rates due to fast neutrons incident from various directions and source-todetector distances.
Cf
As reported earlier (Tam et al., 1999), a mockup of the hexagonal fuel assembly of VVER-440 type reactors was ∗ Corresponding author. Fax: +36-1-395-9002. E-mail address: [email protected] (L. Lakosi).
3. Spatial resolution and neighbour e ect A model calculation of the neutron ux was carried out for the CR-39 detector positioned in the middle of the central dosimetric channel (for in-core instrumentation) in the fuel assembly, surrounded by spent fuel rods as fast neutron
c 2001 Elsevier Science Ltd. All rights reserved. 1350-4487/01/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 1 ) 0 0 2 3 7 - 2
602
N.C. Tam, L. Lakosi / Radiation Measurements 34 (2001) 601–603
Fig. 1. Iso-eDciency curves representing given track density rates (data in tracks=2eld h) due to neutrons incident from various directions and source-to-detector distances.
emitters. On the basis of the results of the model experiments by the 252 Cf neutron source (Tam et al., 1999), we calculated the function of the track density rate D(R) generated by the spent fuel rods in a sphere of radius R around the CR-39 detector. The fuel rods were considered to be divided into equal elementary parts of 12 mm height (the same as the size of the 252 Cf neutron source). The individual contributions Di (ri ; i ) of these elements with coordinates ri and i were taken from the experimental results mentioned above, and then summed up for all surrounding fuel rods within a distance R (see Fig. 2a), resulting in a function D(R) = Di (ri ; i ) (1) ri 6R i
The results obtained by numerical calculation are shown in Fig. 2b. It is seen that only a sphere of radius R = 9–10 cm is e4ective, which can be considered to be a kind of spatial resolution. Since the minimal distance between the adjacent assemblies is about 9 cm, the neighbour e4ect is practically negligible in general in spent fuel assay. However, when the investigated assembly of too small neutron yield is surrounded by neighbours with high neutron yield (Tam et al., 1999), the e4ect of adjacent assemblies is large. Because track density has a strong angular dependence, the neighbour e4ect can decrease substantially (about 10 times), if the detector surface is perpendicular to the axis of the fuel assembly. In this way, spent fuel assemblies of low burnup and=or long cooling time can be monitored too. 4. Neutron ux inside fuel assemblies Track density rate depends on -dose, etching time, and angle. Earlier, the correction factors for -dose (Tam et al., 1999) and etching time (Tam et al., 1997) have been studied. On the basis of the data of angular dependence shown in Fig. 1, the angular correction Fang can be calculated as the weighted average of the normalized individual contributions
Fig. 2. (a) Calculational model for neutron ux. (b) Relative track density rates due to fast neutrons arriving from a sphere of radius R.
r Fang = i ri
i
i
Di (ri ; i )
Di (ri ; i = 0◦ )
:
(2)
A numerical calculation has been carried out for all parts of the fuel assembly. The value of Fang has been found to be 0.52. Using the parameters: 2eld size 300 × 400 m, eDciency of CR-39 in air is 2:5 × 10−4 track=neutron at normal etching conditions (6 h etching time, 6 N NaOH, 70◦ C, no -dose, = 0◦ ), the neutron ux inside di4erent spent fuel assemblies can be evaluated. The results for our earlier experiments (Tam et al., 1997) are shown in Table 1. The detectors at that time were over-etched, therefore a correction factor of 0.3 was applied. For a freshly discharged VVER-440 type fuel assembly of normal burnup (35 MWd=kgU), the fast neutron ux may reach 106 n cm−2 s−1 , while the lower limit, which can be monitored when an assembly is surrounded by
N.C. Tam, L. Lakosi / Radiation Measurements 34 (2001) 601–603
603
Table 1 Experimental data and calculated results for neutron ux inside spent fuel assemblies, corrected for angular, etching time, and -dose dependence Assembly No. Track density ratea [track/2eld.h] Gamma correction (-dose [kGy]) Neutron ux [104 n cm−2 s−1 ] a Etching
25796 7 0.75 (0.49) 5.5
25280 8.5 0.60 (1.33) 8.3
29590
34128
16
14.4
21
0.62 (1.16)
0.49 (2.42)
0.48 (2.97) 26
16.2
25136 0.41 0.97 (0.15) 0.25
conditions: 18 h etching time, 6 N NaOH, 70◦ C.
normal neighbours, is about 102 n cm−2 s−1 , corresponding to 5 MWd=kgU burnup and 5 yr cooling time. References Benton, E.V., Preston, C.C., Ruddy, F.H., Gold, R., Roberts, J.M., 1979. Proton and alpha particle response characteristics of CR-39 polymer for reactor and dosimetry applications. In: Francois, H. et al., (Eds.), Proceedings of the 10th International Conference on SSNTD, Lyon, Pergamon Press, Oxford, New York, Frankfurt, pp. 459 – 467. Fleischer, R.L., Price, B.P, Walker, R.M., 1975. Nuclear Tracks in Solids. University of California Press, Berkeley and Los Angeles.
Fromm, M., Membrey, F., Chambaudet, A., Saouli, R., 1991. Proton and alpha track pro2les in CR-39 during etching and their implications on track etching models. Nucl. Tracks Radiat. Meas. 19, 163–168. Tam, N.C., Baricza, K., Pavlicsek, I., Lakosi, L., 1995. Investigation of fast neutron emission of spent fuel assemblies with CR-39 track etch detectors. Radiat. Meas. 25, 695–698. Tam, N.C., Baricza, K., Lakosi, L., 1996. Energy discrimination by shape selection in etched track detection technique. Radiat. Prot. Dosim. 66, 339–342. Tam, N.C., Baricza, K., Lakosi, L., 1997. Disturbing e4ects in spent fuel assay using CR-39 detectors. Radiat. Meas. 28, 785–789. Tam, N.C., Baricza, K., Lakosi, L., 1999. Angular and dose dependence of CR-39 neutron response for shape-selected tracks. Radiat. Meas. 31, 419–424.
www.elsevier.com/locate/radmeas
Monitoring fast neutron ux inside spent reactor fuel by CR-39 detector N.C. Tam, L. Lakosi ∗ Institute of Isotope and Surface Chemistry, PO Box 77, H-1525 Budapest, Hungary Received 28 August 2000; received in revised form 14 January 2001; accepted 21 March 2001
Abstract A model experiment was carried out by CR-39 detectors using 252 Cf neutron sources in an intense gamma 2eld obtained by Co irradiation. Based on this calibration and introducing corrections due to the angle, -dose, and etching time dependences of the detector response, fast neutron ux inside spent fuel assemblies was found to be in the range of 102 –106 n cm−2 s−1 , depending on burnup and cooling time. The e4ect of adjacent assemblies can decrease substantially, if the detector surface is c 2001 Elsevier Science Ltd. All rights reserved. perpendicular to the axis of the fuel assembly. 60
Keywords: ND assay; Spent fuel assemblies; Fast neutron ux; Neighbour e4ect
1. Introduction In our early works, a technique of shape selection has been used to eliminate unwanted disturbing events on CR-39 detectors exposed inside spent fuel assemblies (Tam et al., 1995). During chemical etching, track forms change from conical to hemispherical (Fleischer et al., 1975; Benton et al., 1979; Fromm et al., 1991). Only conical tracks appearing as dark round events corresponding to neutrons of energy above 0:5±0:1 MeV (Tam et al., 1996) have been evaluated. Inside spent fuel assemblies, fast neutrons arrive at the detector from all directions in a huge gamma 2eld. Hence, the angular and gamma-dose dependence of CR-39 neutron response has also been studied (Tam et al., 1997, 1999). This paper deals with the angular dependence including the e4ect of neutrons from back directions, the e4ects of -dose and etching time in order to calculate neutron ux and analyze the e4ect of adjacent assemblies. 2. Angular dependence at various distances by neutron irradiation
252
placed into a large vessel 2lled with borated water containing 12 g=l boric acid. The 252 Cf source, whose neutron spectrum is similar to that of spent fuel, was put into the central channel of the mockup, while the CR-39 track detectors in stainless steel capsules were suspended in place of fuel rods at various distances (1.3–14:5 cm) from the source, with various angles of incidence (0◦ ; 30◦ ; 60◦ ; 90◦ ). The detectors were of 4 × 10 mm size, covered with 1 mm thick polyethylene foils on both sides. After chemical etching (6 h; 6 N NaOH; 70◦ C); tracks of protons recoiled by fast neutrons from 252 Cf were observed. The angular dependence was reported in the last work (Tam et al., 1999). Tracks observed on the rear side of the detector were now additionally evaluated. The results are displayed in a polar diagram (Fig. 1) as iso-eDciency curves characterizing the angular and distance dependence of detector response, i.e. the curves represent given track density rates due to fast neutrons incident from various directions and source-todetector distances.
Cf
As reported earlier (Tam et al., 1999), a mockup of the hexagonal fuel assembly of VVER-440 type reactors was ∗ Corresponding author. Fax: +36-1-395-9002. E-mail address: [email protected] (L. Lakosi).
3. Spatial resolution and neighbour e ect A model calculation of the neutron ux was carried out for the CR-39 detector positioned in the middle of the central dosimetric channel (for in-core instrumentation) in the fuel assembly, surrounded by spent fuel rods as fast neutron
c 2001 Elsevier Science Ltd. All rights reserved. 1350-4487/01/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 1 ) 0 0 2 3 7 - 2
602
N.C. Tam, L. Lakosi / Radiation Measurements 34 (2001) 601–603
Fig. 1. Iso-eDciency curves representing given track density rates (data in tracks=2eld h) due to neutrons incident from various directions and source-to-detector distances.
emitters. On the basis of the results of the model experiments by the 252 Cf neutron source (Tam et al., 1999), we calculated the function of the track density rate D(R) generated by the spent fuel rods in a sphere of radius R around the CR-39 detector. The fuel rods were considered to be divided into equal elementary parts of 12 mm height (the same as the size of the 252 Cf neutron source). The individual contributions Di (ri ; i ) of these elements with coordinates ri and i were taken from the experimental results mentioned above, and then summed up for all surrounding fuel rods within a distance R (see Fig. 2a), resulting in a function D(R) = Di (ri ; i ) (1) ri 6R i
The results obtained by numerical calculation are shown in Fig. 2b. It is seen that only a sphere of radius R = 9–10 cm is e4ective, which can be considered to be a kind of spatial resolution. Since the minimal distance between the adjacent assemblies is about 9 cm, the neighbour e4ect is practically negligible in general in spent fuel assay. However, when the investigated assembly of too small neutron yield is surrounded by neighbours with high neutron yield (Tam et al., 1999), the e4ect of adjacent assemblies is large. Because track density has a strong angular dependence, the neighbour e4ect can decrease substantially (about 10 times), if the detector surface is perpendicular to the axis of the fuel assembly. In this way, spent fuel assemblies of low burnup and=or long cooling time can be monitored too. 4. Neutron ux inside fuel assemblies Track density rate depends on -dose, etching time, and angle. Earlier, the correction factors for -dose (Tam et al., 1999) and etching time (Tam et al., 1997) have been studied. On the basis of the data of angular dependence shown in Fig. 1, the angular correction Fang can be calculated as the weighted average of the normalized individual contributions
Fig. 2. (a) Calculational model for neutron ux. (b) Relative track density rates due to fast neutrons arriving from a sphere of radius R.
r Fang = i ri
i
i
Di (ri ; i )
Di (ri ; i = 0◦ )
:
(2)
A numerical calculation has been carried out for all parts of the fuel assembly. The value of Fang has been found to be 0.52. Using the parameters: 2eld size 300 × 400 m, eDciency of CR-39 in air is 2:5 × 10−4 track=neutron at normal etching conditions (6 h etching time, 6 N NaOH, 70◦ C, no -dose, = 0◦ ), the neutron ux inside di4erent spent fuel assemblies can be evaluated. The results for our earlier experiments (Tam et al., 1997) are shown in Table 1. The detectors at that time were over-etched, therefore a correction factor of 0.3 was applied. For a freshly discharged VVER-440 type fuel assembly of normal burnup (35 MWd=kgU), the fast neutron ux may reach 106 n cm−2 s−1 , while the lower limit, which can be monitored when an assembly is surrounded by
N.C. Tam, L. Lakosi / Radiation Measurements 34 (2001) 601–603
603
Table 1 Experimental data and calculated results for neutron ux inside spent fuel assemblies, corrected for angular, etching time, and -dose dependence Assembly No. Track density ratea [track/2eld.h] Gamma correction (-dose [kGy]) Neutron ux [104 n cm−2 s−1 ] a Etching
25796 7 0.75 (0.49) 5.5
25280 8.5 0.60 (1.33) 8.3
29590
34128
16
14.4
21
0.62 (1.16)
0.49 (2.42)
0.48 (2.97) 26
16.2
25136 0.41 0.97 (0.15) 0.25
conditions: 18 h etching time, 6 N NaOH, 70◦ C.
normal neighbours, is about 102 n cm−2 s−1 , corresponding to 5 MWd=kgU burnup and 5 yr cooling time. References Benton, E.V., Preston, C.C., Ruddy, F.H., Gold, R., Roberts, J.M., 1979. Proton and alpha particle response characteristics of CR-39 polymer for reactor and dosimetry applications. In: Francois, H. et al., (Eds.), Proceedings of the 10th International Conference on SSNTD, Lyon, Pergamon Press, Oxford, New York, Frankfurt, pp. 459 – 467. Fleischer, R.L., Price, B.P, Walker, R.M., 1975. Nuclear Tracks in Solids. University of California Press, Berkeley and Los Angeles.
Fromm, M., Membrey, F., Chambaudet, A., Saouli, R., 1991. Proton and alpha track pro2les in CR-39 during etching and their implications on track etching models. Nucl. Tracks Radiat. Meas. 19, 163–168. Tam, N.C., Baricza, K., Pavlicsek, I., Lakosi, L., 1995. Investigation of fast neutron emission of spent fuel assemblies with CR-39 track etch detectors. Radiat. Meas. 25, 695–698. Tam, N.C., Baricza, K., Lakosi, L., 1996. Energy discrimination by shape selection in etched track detection technique. Radiat. Prot. Dosim. 66, 339–342. Tam, N.C., Baricza, K., Lakosi, L., 1997. Disturbing e4ects in spent fuel assay using CR-39 detectors. Radiat. Meas. 28, 785–789. Tam, N.C., Baricza, K., Lakosi, L., 1999. Angular and dose dependence of CR-39 neutron response for shape-selected tracks. Radiat. Meas. 31, 419–424.