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Penetration of heavy concrete shields by high-energy neutrons
Letters to the editors
315
S 4 f
FIG. 2.--Dependence of = on Z : O--141Ce;
-'-""--Q
A--~03Hg, 51Cr; D--gSNb.
3 2 i
0
tO
2g
30
/~0
Z
FIG. 3.--Dependence of ~ and ~/~ on Ee f o r i r o n : o-----~; A--c~E. F o r E e = 1.25 MeV the results of FuJI-rA et al. ~' for e~ B°Co are introduced. ~"
2 0
0"5
bO
Ey,
15
MeV
The results of FUJITA et al. ~='are in good agreement with this dependence. The dependence is of the same character for other Z. It was established that the results of this and a previous experiment (2' can be described with an accuracy of 4-15 70 by the empirical formula A(x) = A(oo){1 -- exp [(l'9Ey -- 5.3)(1 -- e-°'°TZ)x]}
(3)
(where Ee is the energy of the primary y-rays in MeV), which was obtained by analysis of the dependences of ~ on Z and Ee given above, taking (1) into account. It should be noted that ~E/~ = 1 + k Z (for all the energies studied), where k = 9 x 10 -3. The results of FuJrrk et aL '2~ for 6°Co also satisfy this empirical formula. Using this information, we can calculate the differential albedo for 0 = 0, in the given geometry, and in the energy range of the primary y-rays studied, for any homogeneous scatterers of any thickness. Acknowledgment--In conclusion the authors t h a n k L. E. GOmtSTErs for his help in processing the data by computer. D. B. POZDNEYEV S. A. CHtmrN REFERENCES 1. BULATOVB. P. and GARYCHSOV E. A. Atorrm. Energ. 5, 631 (1958). 2. FuJrrA H., KOBAYASr~ K. and HYODO T. Nacl. Sci. Engng 19, 437 (1964). 3. R.ASO D. Radiation Res. 19, 384 (1963). 4. POZDNEYEVD. B. Atomn. Energ. 20, 317 (1966).
Penetration of heavy concrete shields by high-energy neutrons* (Received 18 November 1965) IN TInS paper we study the distribution of neutron fluxes of different energy groups in heavy concrete shields with varying water content. The experiments were carried out using neutron beams supplied * Translated by .L STUART from Atomnaya Energiya 20, 355 (1966).
316
Letters to the editors
c
? 6 5 4 3 ~AN 0
I000
2000
5000
Thickness of shield,
4000
kg/m~
FIO. 1.--Distribution of neutron fluxes of different energy groups in concrete: @----highenergy neutrons (E > 20 MeV), threshold reaction l~C(n, 2n)lxC; + - - f a s t neutrons (2-20 MeV), threshold reaction 81P(n, p)zlSi; m--intermediate neutrons (E ~ 1.44 eV), reaction nSIn(n, 7)n~mln; in these groups the hydrogen content is 0'35% by weight; A--intermediate neutrons, hydrogen content 0 . 7 ~ by weight; V--intermediate neutrons, hydrogen content 1'07oo by weight. by the Joint Nuclear Research Institute (JINR) synchrocyclotron. Descriptions of the experimental apparatus in the synchrocyclotron hall and of the method have been given by ZAITSEVtt~ and SYCttEv e t aL ~
The shield was constructed from an assembly of 53-ram thick slabs of heavy (haematite) concrete. The chemical composition of the concrete at a density of 3480 kg/m 3 was as follows (in percentages by weight): hydrogen 0"35, carbon 0.55, oxygen 34'7, sodium 0.9, magnesium 0'9, aluminium 0.7, silicon 2.6, calcium 4-3, iron 54-0 and other elements 1'0. The hydrogen content was varied by altering the distance between the slabs, which were set up in a water tank.* The experimental data characterizing the attenuation of neutron fluxes of different energy groups are shown in Fig. 1. Table 1 gives the attenuation lengths of high energy fluxes in the concrete under investigation for thicknesses of 1500-5000 kg/m ~. It also shows the calculated values of ;t/2in, where TABLE
1.--EXPERIMENTAL
V A L U E S OF T H E R E L A X A T I O N L E N G T H
(kg/m ~)
Energy of protons striking beryllium target (MeV)
;t
;t/;tln
170 250 350 480 660
12504-45 13504-45 15204-45 16804-54 1620_-k54
1.05-I-0.04 1.144-0-03 1.28 :k0"03 1-42£:0.03 1.374-0.03
;tin is the inelastic interaction length of high-energy neutrons (E > 100 MeV) with nuclei. We can conclude from the data of Table 1 and of the work of ZATISEV e t al., cl) SYCREV e t al. (~ and KOMOCHKOVand SYcI-mVTM that the attenuation in a shield of neutron fluxes with energies of a few hundred MeV is characterized by a relaxation length ;t given by the relation ;t ~ (1-3 4- 0"l);tin. * To prevent the water percolating into the concrete, the slabs were coated with epoxy resin.
Letters to the editors
317
Moderated neutron fluxes (E < 20 MeV) attenuate with the same relaxation lengths as high-energy neutron fluxes (E > 20 MeV). The ratio of the fast neutron flux (2-20 MeV) to the high-energy neutron flux does not depend on the amount of hydrogen in the concrete (within the range investigated it is about 0'7). TABLE 2.--CALCLrLATED VALUES OF BUILD=UP FACTORS OF INTERMEDIATE NEUTRONS
Build-up factors
Bao Bn
0.05 5"0 65
Quantity of hydrogen in the concrete (% by weight) 0"1 0.2 0.4 0.6 3'5 1"7 1.0 0.8 35 17 9"0 6.5
0-8 0'6 5.0
Note: Initial energy of intermediate neutrons E0 = 1.5 MeV. Average number of evaporated neutrons of excited nuclear elements in concrete ti = 1-9. The experimental values for the build-up factor of indium resonance neutron fluxes (about 1.44 eV) agree within limits of 10-15 per cent with the values calculated by the method described by SYcrmv et al. ~ Table 2 gives the calculated values for the dose and flux build-up factors (Bao and Bn) of the intermediate neutrons. Using the data of Table 2, we can show that when the amount of hydrogen in the concrete varies within the technologically admissible limits (0.2-0.6 % by weight), the thickness of the shield decreases by 25-30 cm in all. B. S. Svcrmv V. V. MAL'KOV M. M. KOMOCHKOV L. I. ZAITSEV REFERENCES 1. ZAI'rSEVL. N. et aL Atonm. Energ. 12, 525 (1962). 2. SYCHEVB. S. et al. Atomn. Energ. 20, 323 (1966). 3. KOMOCHKOVM. M. and Svcl-rEv B. S. Atomn. Energ. 15, 325 (1963).
A device for making Gscillator measurements on a nuclear reactor* (Received 7 July 1965) IN CERTA~'~experiments in connexion with research on the reactivity effects and kinetic characteristics of a nuclear reactor, use has been made of the oscillator method m. With this method, usually the reactivity in the core is caused to oscillate by introducing a special oscillator device consisting of a strongly absorbing substance and an actuating mechanism which periodically varies the effective area of absorber c-"~. But it is not always possible to introduce such a device into the core. In practice, particularly in the case of experiments on power reactors, a control rod is often used as the oscillator, its actuating mechanism being operated in a reciprocating regime by an experimental device. An oscillator device was designed and constructed for making physical measurements of the I. V. Kurchatov reactor of the first block of the Beloyarsk Nuclear Power Station, and was used, together with the regular manual control system and with ionization chambers, to measure the differential and integral efficiency of manually controlled rods in different operational conditions, and also to establish the frequency characteristics of the reactor. The device could be connected with the activator of any of the manually controlled rods by means of their relay control system, the choice of rod being made by the operator at the control desk. It proved to be a very convenient instrument for mass measurements of the efficiency of a large number of rods over periods of insertion of 60-90 min without disrupting the normal operation of the reactor. It should be noted that the amplitude of the periodic power oscillations during the time of the measurements did not exceed 0.5-1 per cent of the constant level. The device consists of a master oscillator and harmonic analyser (see Fig. 1). The master oscillator includes a frequency divider and a two-position relay controlled by the output pulse of the frequency divider. A sinusoidal voltage with a frequency of 50 cps is applied to the input of the frequency divider, which is a conversion circuit. The coefficient of conversion may be 128, 256, 512, 1024 or 2048. Periodic switching of the master oscillator relay determines the operation of the relay control * Translated by J. STUARTfrom Atomnaya Energiya 20, 437 (1966).
315
S 4 f
FIG. 2.--Dependence of = on Z : O--141Ce;
-'-""--Q
A--~03Hg, 51Cr; D--gSNb.
3 2 i
0
tO
2g
30
/~0
Z
FIG. 3.--Dependence of ~ and ~/~ on Ee f o r i r o n : o-----~; A--c~E. F o r E e = 1.25 MeV the results of FuJI-rA et al. ~' for e~ B°Co are introduced. ~"
2 0
0"5
bO
Ey,
15
MeV
The results of FUJITA et al. ~='are in good agreement with this dependence. The dependence is of the same character for other Z. It was established that the results of this and a previous experiment (2' can be described with an accuracy of 4-15 70 by the empirical formula A(x) = A(oo){1 -- exp [(l'9Ey -- 5.3)(1 -- e-°'°TZ)x]}
(3)
(where Ee is the energy of the primary y-rays in MeV), which was obtained by analysis of the dependences of ~ on Z and Ee given above, taking (1) into account. It should be noted that ~E/~ = 1 + k Z (for all the energies studied), where k = 9 x 10 -3. The results of FuJrrk et aL '2~ for 6°Co also satisfy this empirical formula. Using this information, we can calculate the differential albedo for 0 = 0, in the given geometry, and in the energy range of the primary y-rays studied, for any homogeneous scatterers of any thickness. Acknowledgment--In conclusion the authors t h a n k L. E. GOmtSTErs for his help in processing the data by computer. D. B. POZDNEYEV S. A. CHtmrN REFERENCES 1. BULATOVB. P. and GARYCHSOV E. A. Atorrm. Energ. 5, 631 (1958). 2. FuJrrA H., KOBAYASr~ K. and HYODO T. Nacl. Sci. Engng 19, 437 (1964). 3. R.ASO D. Radiation Res. 19, 384 (1963). 4. POZDNEYEVD. B. Atomn. Energ. 20, 317 (1966).
Penetration of heavy concrete shields by high-energy neutrons* (Received 18 November 1965) IN TInS paper we study the distribution of neutron fluxes of different energy groups in heavy concrete shields with varying water content. The experiments were carried out using neutron beams supplied * Translated by .L STUART from Atomnaya Energiya 20, 355 (1966).
316
Letters to the editors
c
? 6 5 4 3 ~AN 0
I000
2000
5000
Thickness of shield,
4000
kg/m~
FIO. 1.--Distribution of neutron fluxes of different energy groups in concrete: @----highenergy neutrons (E > 20 MeV), threshold reaction l~C(n, 2n)lxC; + - - f a s t neutrons (2-20 MeV), threshold reaction 81P(n, p)zlSi; m--intermediate neutrons (E ~ 1.44 eV), reaction nSIn(n, 7)n~mln; in these groups the hydrogen content is 0'35% by weight; A--intermediate neutrons, hydrogen content 0 . 7 ~ by weight; V--intermediate neutrons, hydrogen content 1'07oo by weight. by the Joint Nuclear Research Institute (JINR) synchrocyclotron. Descriptions of the experimental apparatus in the synchrocyclotron hall and of the method have been given by ZAITSEVtt~ and SYCttEv e t aL ~
The shield was constructed from an assembly of 53-ram thick slabs of heavy (haematite) concrete. The chemical composition of the concrete at a density of 3480 kg/m 3 was as follows (in percentages by weight): hydrogen 0"35, carbon 0.55, oxygen 34'7, sodium 0.9, magnesium 0'9, aluminium 0.7, silicon 2.6, calcium 4-3, iron 54-0 and other elements 1'0. The hydrogen content was varied by altering the distance between the slabs, which were set up in a water tank.* The experimental data characterizing the attenuation of neutron fluxes of different energy groups are shown in Fig. 1. Table 1 gives the attenuation lengths of high energy fluxes in the concrete under investigation for thicknesses of 1500-5000 kg/m ~. It also shows the calculated values of ;t/2in, where TABLE
1.--EXPERIMENTAL
V A L U E S OF T H E R E L A X A T I O N L E N G T H
(kg/m ~)
Energy of protons striking beryllium target (MeV)
;t
;t/;tln
170 250 350 480 660
12504-45 13504-45 15204-45 16804-54 1620_-k54
1.05-I-0.04 1.144-0-03 1.28 :k0"03 1-42£:0.03 1.374-0.03
;tin is the inelastic interaction length of high-energy neutrons (E > 100 MeV) with nuclei. We can conclude from the data of Table 1 and of the work of ZATISEV e t al., cl) SYCREV e t al. (~ and KOMOCHKOVand SYcI-mVTM that the attenuation in a shield of neutron fluxes with energies of a few hundred MeV is characterized by a relaxation length ;t given by the relation ;t ~ (1-3 4- 0"l);tin. * To prevent the water percolating into the concrete, the slabs were coated with epoxy resin.
Letters to the editors
317
Moderated neutron fluxes (E < 20 MeV) attenuate with the same relaxation lengths as high-energy neutron fluxes (E > 20 MeV). The ratio of the fast neutron flux (2-20 MeV) to the high-energy neutron flux does not depend on the amount of hydrogen in the concrete (within the range investigated it is about 0'7). TABLE 2.--CALCLrLATED VALUES OF BUILD=UP FACTORS OF INTERMEDIATE NEUTRONS
Build-up factors
Bao Bn
0.05 5"0 65
Quantity of hydrogen in the concrete (% by weight) 0"1 0.2 0.4 0.6 3'5 1"7 1.0 0.8 35 17 9"0 6.5
0-8 0'6 5.0
Note: Initial energy of intermediate neutrons E0 = 1.5 MeV. Average number of evaporated neutrons of excited nuclear elements in concrete ti = 1-9. The experimental values for the build-up factor of indium resonance neutron fluxes (about 1.44 eV) agree within limits of 10-15 per cent with the values calculated by the method described by SYcrmv et al. ~ Table 2 gives the calculated values for the dose and flux build-up factors (Bao and Bn) of the intermediate neutrons. Using the data of Table 2, we can show that when the amount of hydrogen in the concrete varies within the technologically admissible limits (0.2-0.6 % by weight), the thickness of the shield decreases by 25-30 cm in all. B. S. Svcrmv V. V. MAL'KOV M. M. KOMOCHKOV L. I. ZAITSEV REFERENCES 1. ZAI'rSEVL. N. et aL Atonm. Energ. 12, 525 (1962). 2. SYCHEVB. S. et al. Atomn. Energ. 20, 323 (1966). 3. KOMOCHKOVM. M. and Svcl-rEv B. S. Atomn. Energ. 15, 325 (1963).
A device for making Gscillator measurements on a nuclear reactor* (Received 7 July 1965) IN CERTA~'~experiments in connexion with research on the reactivity effects and kinetic characteristics of a nuclear reactor, use has been made of the oscillator method m. With this method, usually the reactivity in the core is caused to oscillate by introducing a special oscillator device consisting of a strongly absorbing substance and an actuating mechanism which periodically varies the effective area of absorber c-"~. But it is not always possible to introduce such a device into the core. In practice, particularly in the case of experiments on power reactors, a control rod is often used as the oscillator, its actuating mechanism being operated in a reciprocating regime by an experimental device. An oscillator device was designed and constructed for making physical measurements of the I. V. Kurchatov reactor of the first block of the Beloyarsk Nuclear Power Station, and was used, together with the regular manual control system and with ionization chambers, to measure the differential and integral efficiency of manually controlled rods in different operational conditions, and also to establish the frequency characteristics of the reactor. The device could be connected with the activator of any of the manually controlled rods by means of their relay control system, the choice of rod being made by the operator at the control desk. It proved to be a very convenient instrument for mass measurements of the efficiency of a large number of rods over periods of insertion of 60-90 min without disrupting the normal operation of the reactor. It should be noted that the amplitude of the periodic power oscillations during the time of the measurements did not exceed 0.5-1 per cent of the constant level. The device consists of a master oscillator and harmonic analyser (see Fig. 1). The master oscillator includes a frequency divider and a two-position relay controlled by the output pulse of the frequency divider. A sinusoidal voltage with a frequency of 50 cps is applied to the input of the frequency divider, which is a conversion circuit. The coefficient of conversion may be 128, 256, 512, 1024 or 2048. Periodic switching of the master oscillator relay determines the operation of the relay control * Translated by J. STUARTfrom Atomnaya Energiya 20, 437 (1966).