REALISTIC NEUTRON SPECTRA FOR RADIATION PROTECTION
AND OTHER APPLICATIONS AT AERI, BUDAPEST

J. Pálfalvi ¹⁾, L. Sajó-Bohus ²⁾ and M. Balaskó <sup>1)

1)</sup> Atomic Energy Research Inst. P.O.B. 49, H-1525 Budapest, Hungary
²⁾ Simon Bolivár University, Ap. 89000, Caracas, Venezuela

Keywords: neutron spectra, neutron radiography, neutron beam formation, neutron dosimetry

INTRODUCTION

The BRR is a tank type reactor, moderated and cooled by light water. The cylindrical tank made of Al alloy has a diameter of 2300 mm and the height is 5685 mm. The heavy concrete reactor block is situated in a rectangular, semi-hermetically sealed hall, its area is 600 m². The core is arranged in a hexagonal lattice with a pitch of 35 mm and contains 229 fuel rods, the ²³⁵U enrichment is 36%. The core is surrounded by removable and fixed Be reflectors. The nominal thermal power is 10 MW which results in a thermal flux of about 2*10¹⁴ n/cm²s, meanwhile the total flux is 3*10¹⁴ n/cm²s. Three out of the 10 horizontal channels were constructed in a way to facilitate the versatile use of them, providing different irradiation geometries.
The following variable parameters characterize these beams:

• beam diameter: from few mm up to ∼20 cm, depending on the collimation, diaphragms, filtration and shutter system applied;
• sample treatment: samples can be rotated, horizontally and vertically moved (scanning) facilitating the homogeneous irradiation of large and thick objects;
• neutron energy spectra, dose rate, neutron-to-gamma ratio: can be varied by beam couplers, built in fixed and rotatable filters, as well as, by external filters placed inside collimators of different size and shape;

The importance of constructing reference neutron radiation fields with energy spectra that more closely simulate spectra found in workplaces became clear in recent years and inspired us to build up our facilities.

BIOLOGICAL IRRADIATION FACILITY (BIF)
The 5^th horizontal channel was selected for constructing the BIF. It started with the calculation of the leakage spectra from the core at the channel taking into consideration Be and Al materials as reflectors, or beam couplers, with different thickness. It was found that the Be more drastically modifies the spectrum than the Al, decreases the fast fraction and increases the thermal and epithermal flux (see Figs. 1&2). In order to be flexible the following solution was accepted. The Be reflector rods originally positioned in front of the channel were modified: cutting out a window which was replaced by Al. Also the steady Be reflector was drilled through and an Al plug was inserted. Thus, the thickness of the Al beam coupler is 140 mm. This resulted in a quite hard spectrum, but providing the possibility to soften it – if necessary – by different filters placed inside the beam tube. The modified Be-Al rods can be removed and pure Be rods inserted again if a softer starting spectrum is needed.

Fig. 1 The effect of the Be reflector on the shape of the leakage spectrum on the dependence of the thickness. The calculated core spectrum without any reflector is marked as 0 cm.

The 3330 mm long beam tube was divided into 3 segments. The 1^st one, in a distance of 1900 mm from the core is the beam shutter of 715 mm made of iron and heavy concrete. The next one is a rotatable heavy concrete disk having six cylindrical holes for filters: open window, 50, 100, 150 and 200 mm of Bi and 200 mm of Pb. The last unit is a tank of 480 mm long for any liquid which can be pumped in and out. The tank is separated from the filter by a 20 mm thick Pb disk, not removable, the wall of the tank is made of Al exhibiting another not removable filter of 15 mm. There is an additional Fe disk of 20 mm, effective only when the Pb filter is selected. The outer orifice of the beam tube joins to a collimator having a length of 1400 mm. Manually changeable filters can be place inside or in some cases samples can be irradiated here. In front of this collimator there is a variable space for irradiation, surrounded by several layer of shielding materials (borated water, paraffin, heavy concrete and lead).

Fig. 2 Spectrum modifying effect of the Be beam coupler vs. the thickness. O stands for the mean energy of the spectrum; • is related to the transmitted fraction and ∇ marks the fast neutron (> 1 MeV) fraction of the spectrum. Thermal neutrons were neglected.

Many filter combinations can be used to simulate operational spectra at power reactors. In Fig. 3 two such spectra are presented (Sp1, Sp2) together with the leakage spectrum from the Al beam coupler (Sp4). For comparison the lead shielded SILENE spectrum [1] is given (Sp3). Beside the fix filters and the beam coupler the following filters are used. For Sp1: 4 mm Al, 45 mm Pb, 30 mm Fe and 200 mm Bi; for Sp2: 27 mm Al, 45 mm Pb, 20 mm Fe, 200 mm ordinary concrete and 10 mm B₄C. The spectra can be evaluated if some characteristic quantities are compared as shown in Table 1. With increasing mean energy both the ambient dose equivalent conversion factor and the response of the LB6411 survey meter is increasing, meanwhile the Bonner sphere ratio is decreasing [2]. It is should be mentioned that the H*/LB6411 ratio is constant within 2% (343.5 ± 6.1) in this energy range. (Thermal neutrons were neglected)

Table 1 Calculated dosimetric quantities for spectra presented in Fig. 3, The 3“/9.5” Bonner sphere signal ratios were calculated for ³He detectors. The LB6411 response is normalized to 1 for bare Pu-Be spectrum.

Quantities	Sp1	Sp2	Sp3	Sp4
3"/9.5" ratio	1.14	1.17	0.94	1.38
Mean E (keV)	534	510	813	474
H* (pSvcm2)	174	171	201	151
LB6411 Resp.	0.50	0.49	0.59	0.45

Fig. 3 Leakage and filtered spectra at the BIF: Sp4, Sp1 and Sp2, respectively; the lead filtered SILENE spectrum is marked as Sp3 [1].

In the following Table 2 typical fast neutron dose rate and flux values are presented mostly used for biological experiments. Only the changeable filters are mentioned.

Table 2 Fast neutron (E>100 keV) absorbed dose rate in water and flux values for typical filter assemblies at BIF

Filter thickness in mm	Dose rate mGy/s	Flux 106 n/cm2s
200Pb+20Fe+45Pb+10B4C	0.0233	2.17
150Bi+454Pb+10B4C	0.172	16.2
50Bi+454Pb+10B4C	1.32	120
50Bi+10B4C	2.81	260
100 PMMA	2.99	277
45Pb+10B4C	4.36	405
Direct beam	7.58	700

DINAMIC NEUTRON RADIOGRAPHY UNIT (DNRU)

This very versatile unit was built at channel 2, having a fix beam coupler made of 90 mm Be +10 mm Al. This arrangement provides softer spectrum than it is at the BIF, with a large amount of thermal neutrons. The beam shutter of 965 mm was made of iron and heavy concrete. In the beam tube there is a pin-hole collimator with a diameter of 25 mm, the collimation ratio is 170. The beam tube joins to a beam formation and filtering unit shown in Fig 4. It is equipped with movable Cd and In filters, both of them are 2 mm thick and can be used separately or simultaneously in order to vary the thermal to intermediate flux ratio meanwhile the fast flux remains unchanged. In Table 3 the flux values are presented for several beam formation arrangements. The maximum beam diameter is limited to 20 cm but the irradiation area is equipped with a special, remote controlled sample holder system which allows to move the samples horizontally and also vertically, covering an area of 60 by 80 cm². In this way, large objects as phantoms, can be irradiated [3].