Strugar et at.: Conversion of the RB Reactor Neutrons by High-Enriched tTranium Fuel
Conversion of the RB Reactor Neutrons by High-Enriched Uranium Fuel and Lithium Deuteride
by P. Strugar, 0. Sotić, M. Ninković, M. Pešte, D. AUiparmakov ("Boris Kiđrič" Institute of Nuclear Sciences, Nuclear Engineering Laboratory, Vinca-, Yugoslavia)* A converter of thermal to fast neutrons has been constructed at the RB reactor. The material used for the conversion of thermal neutrons is highly enriched uranium fuel of Soviet production utilized in Yugoslav heavy water experimental re actors RA and RB. Calculations and preliminary measurements show that the converted neutrons spectrum only slightly differs from the fission one. The basic characteristics of converted neutrons canbe expressed by neutron radiation dose giv ing 800 rod (8 Gy) for the reactor operation during 1 hat the power level of 1 kW. This dose is approximately 10 times higher than the neutron dose at the same place without the converter. At the same time, thermal neutron and gamma radiation doses are negligible. The constructed neutron converter offers wide possibilities for applications in the reactor and nuclear physics and similar disciplines, where neutron spectra of high energies are required, as well as in tlie domain of neutron dosimetry and biological irradiations in homogeneous fields of larger dimensions. We have also considered the possibility to convert thermal reactor neutrons to neutrons of energy about 14 MeV by lithium deuteride which is made from a natural lithiuvt.
(E 21; £ W) IMS DESCRIPTORS: r-b reactor; neutron converters: Ml; fast neutrons; highly enriched uranium :Ql; lithium deufcerides:Ql; neutron spectra; radiation doses.
1. introduction biological and similar experiments, it is necessary to have a homogeneous source of pure fast neutrons in a larger The RB is a bare research reactor of low power using heavy experimental space. Having in mind mixed character of water as a moderator and natural metallic uranium, 2% reactor radiations of wide energy range, it seems difficult enriched metallic uranium or 80% enriched uranium to realize such neutron source in the core of any reactor. dioxide dispersed in aluminium, as a fuel[l]. In a fairly large Since the RB reactor is a very flexible system, it is pos experimental space in the reactor core and around it, the sible to provide conditions for conversion of thermal to neutron spectrum is predominantly thermal. In the case fast neutrons of relatively high intensities by suitable of enriched uranium and uranium dioxide fuel segments, arrangement of fuel in the reactor core and its surrounding. the fast neutron flux is comparable to the thermal one Analysis of this possibility, construction of the neutron inside the fuel without presence of heavy water. However, converter and measurement of its basic parameters is the besides other disadvantages such experimental space is subject of this paper. very limited. In a whole series of investigations in the nuclear and 2. Model of the Process reactor physics, in neutron dosimetry, and also for some The neutron spectrum at the converter outlet was calcu • Address: 11001 Beogrrad — Yugoslavia, P. 0. Box 522. lated both by the first collision probability and Monte
XuriLCnei-Blu, Bd. 24, H. 3 101 Strugar et al.: Conversion of the RB Reactor Neutrons by High-Enriched Uranium Fuel
Carlo methods. The neutron transport process at the equi chosen according to Kef. [5], Nuclear parameters of the librium state in the case of isotropic scattering of neutrons core, the heavy water reflector, the reactor tank and is described by Boltzmann'e equation [2]. the neutron converter were calculated after previous homo- If neutron fluxes and sources are developed into ia- genization of their composition. Good agreement of con grange's type polynomials separately for each material zone verted neutron spectra calculated by the collision proba and by applying multigroup approximation of the neutron bility and Monte Carlo methods shows high reliability of energy spectrum with H energy groups [3], the BoUzmunn the obtained results. equation for the long cylinder composed of K homogeneous As it can be seen in Fig. 1 the spectrum of neutrons coaxial zones will be transformed into a system of K x H leaking out from the converter only slightly differs from linear algebraic equations according to neutron flux values the fission neutron spectrum. Sometimes it is suitable to in space-energy points: have the neutron source with similar spectral form but slightly shifted towards lower energies. For this reason we investigated in detail the transport of converted neutrons
1-lJW-l through the screens of certain materials. These materiuls where: were chosen according to their characteristics with respect 0*(r„ +) is the neutron flux of the energy group A {h = 1, to elastic and non-elastic scattering effects and neutron 2, ... , H) in the material zone g (g = 1, 2, ... , K) capture taking into account also their physioo-ohemicul and the space point r^ (i = 1, 2, ... , M) . properties here. The analysis has shown that natural k H t is the neutron scattering cross section for the ener uranium, iron and some other materials are more suitable gy group h in the material zone k (k = 1, for these purposes because of their high non-elastic scatter 2, ... , K). ing of the neutrons of fission energies. The effect of the
The coefficients 2^ m(>*Pi i) in equation (1) are integral natural uranium screens on the shifting of converted neu values: tron spectrum is such that the maximum product of energy and neutron spectrum E&(E) shifts from 2 MeV to 1 MeV at the screen thickness of about 3 cm, or to 0-5 MeV where -Pjt,tn(r2) are tne Lagrange type polynomials (at the at the thickness of 10 cm. It is evident in Fig. 1 that tlio shape of the shifted spectrum" is not changed essentially. point m of the zone k) and f*(^,i» »*) dVk is the probability for the neutrons of the energy group h, leaving the ele mentary volume dFt around r in the material zone k, to reach the elementary volume surrounding the point rgi of the material zone g. The total contribution of the fission, scattering and exter nal source neutrons at the space point rb>m of the energy group h in the material zone k is: «{('*.») = p- i *P7<*) ®* + £ 2T*-A (h) ni'*,*) + ,.«) (3) «=1 whore: p'j. is the probability for the fission neutron to have i0~z 10J> 1 10MeV energy within the energy h in the material £ * zone k, i'ig. 1. The spectrum of converted neutrons kvK is the effective factor of neutron multiplica x fission spectrum of uoufcrous; O spectrum of converted neutrons; tion, A spectrum of converted neutrons behind the 10 era screon of natural v v k is the average number of fission neutrons of uranium; O dose spectrum of converted neutrons the energy group v in the material zone k, £?{k) is the fission cross section for the neutrons of 4. Construction of the Neutron Converter the energy group v in the material zone k, The neutron converter consists of 560 segments of 80% 2*>-*h fj^ ja tne cross section for neutron scattering from enriched uranium fuel in the box of aluminum alloy of energy group v into group h, nuclear purity. r s tne Qo *( *,i») * contribution of the external neutron The high-enriched uranium fuel is made in the USSR as source to the energy group h in the space well as 2% enriched standard fuel of the RB reactor. Both point rt m of the material zone k. types of the fuel segments are of the same geometry and 3n some cases of the analysis of the converter neutron have approximately the same content of 286U (Fig. 2). spectra the Monte Carlo method was also used [4]. However, 2% enriched fuel is metallic uranium and 80% enriched fuel is made of uranium dioxide dispersed in aluminum (Fig. 2). The aluminum box of the converter is 3. Neutron Spectra and Neutron Dose Calculation a parallelepiped with the dimensions 1116 mm, 1120 mm, Energy spectra and the total neutron doses at the con and 76 mm. Segments of 80% enriched fuel were arranged verter outlet (Fig. 1) have been calculated using the model in two lines and placed close one by the other (Fig. 3). of the process described in the previous section. Factors of Since the uranium layer does not cover the whole length conversion of the neutron flux to the neutron dose were of the segments, one line was raised by 5 cm using an 102 Kemeocrgio, Bd. 'ii, H 3 Strugar et al.: Conversion of the RB Reactor Neutrons by High-Enriched Uranium Fuel aluminum supporter in order to avoid larger "holes". Thus, electromotor with a reducer and a disc rotating around its the possibility of neutron transit through the converter axis at the speed of about 2 rpm. without conversion is greatly reduced. The walls of the By analyzing safety aspects of the operation of the aluminum box are 3 mm thiek and its internal dimensions reactor-converter coupling it has been shown that such are chosen to enable dense packing of fuel segments. The coupling can be completely controlled by the existing external side of the converter box is made in three parts control system in the same way as in the case of the reactor for easier handling of the fuel segments whereas the other without converter. On the other side the neutron converter sides are welded both mutually and to the supporter. automatically shuts down after termination of the reactor In order to eliminate thermal neutrons in the leaking operation. The converter also does not essentially disturb out neutron spectrum, external side of the converter box the radiation field in the building where the RB reactor can be covered by a cadmium sheet. is located. Finally, the quantity of energy generated by the A special rotary mechanism was constructed with the converter only insignificantly increases its temperature. aim to enable irradiation of a greater number of Bamples under identical conditions. The mechanism consists of an 5. Measurement of Neutron and (Jamnia Doses Neutron radiation doses were measured using a dosimeter with a polyethylene sphere, 25-4 cm in diameter, and an europium activated lithium iodide scintillator. The scin tillator was located in the center of the polyethylene sphere and served as detector of thermal neutrons while the sphere itself served as a moderator of epithermal neutrons [5]. Gamma radiation doses were measured by KAKTUS type instruments having 1 1 effective volume [6]. During neutron and gamma dose measurements the detectors were located at 30 cm distance from the neutron converter and at 70 cm height above the lower basis of the core of the reactor. Series of measurements were performed, with and without the neutron converter. In both cases radiation doses were measured either with the cadmium sheet or without it. The measured space distribution of converted neutrons is shown in Fig. 4. tfcfance frm converter axis •* •- 0-| , , , , , , , , r -, ,—l JTig. 2. Fuel segment oE 80 % and 2 % enriched uranium 0 20 hO 50 30 mem distance from coanriei *- Fig. 4. Space distributions of converted neutrona In the presence of the converter, gamma dose around the RB reactor increases by about 50%, whereas the cad mium sheet increases the gamma dose level by 10% only. This increase of gamma radiation dose is actually only apparent, since in respect to the total radiation dose the part of the gamma dose considerably decreases and amounts to 7% of the total neutron dose. According to calculations performed for the spectrum of converted neutrons the total dose of 1 rad (10 m Gy) corresponds to the total dose of 8-8 rem (88 mSv), while the doses of the first neutron collisions amount to about 75% of the total doses. Taking this value into account and the measured neutron dose rem values, one gets, under the conditions specified above, for the reactor operation during I h at the power of 1 kW, a neutron dose of Fig. 3. Reactor-converter coupling 1 natural uraninm (lattice pitch 11-3 cm); 2 2% enriched uranium (lat Dn = 800 radAWh (8 Gy/kWh) tice pitch 11-3 cm); 3 2% enriched uranium (lattice pitch 8-0 cm); 4 80 % enriched uranium dioxide = 7000 rem/kWh (70 Sv/kWh) . (4) Keriienergio, Bd. 24, H. 3 103 Strugar et al.: Conversion of the RB Reactor Neutrons by High-Enriched. Uranium Fuel ii. LiD Converter The converted neutron spectrum only slightly differs from the fission one. The neutron dose at 50 em from the Nowadays the study of 14-MeV radiobiological effectB reactor tank was 800 rads (8 Gy) in the case of the reactor attends considerable interest, both because of the presence operation at the power level of 1 kW for 1 h. This dose is of such neutrons in -weapons, accelerators and fusion 10 times higher than the thermal neutron dose at the same sources and because of the recent strong interest in their place but without the converter. However, the shape of use for cancer therapy and medical radiography [7], the spectrum of converted neutrons has great significance, Such neutrons may be produced by the conversion of too. reactor neutrons into "14-MeV neutrons" (from 10-8 to It is also possible to obtain the neutron spectra of 19-7 MeV) using lithium deuteride as a converter. For that energies somewhat lower than those of converted neutrons. converter 'LiD is generally employed, the usual material This can be achieved by the transfer of converted neutrons for the thermonuclear weapons, and therefore hardly through natural uranium, iron or other screens. accessible in the quantity needed- That was the reason to analyze such a converter made of LiD with lithium of Complete elimination of thermal neutrons and reductiou natural isotopic content. This analysis starts with the basic of gamma dose to a negligible value makes this neutron conversion relations, takes into account neutrons absorp source quite suitable for research in the field of reactor and tion and tritium generation, and, finally, estimates the nuclear physics where it is required to have neutrons of "14-MeV neutrons" flux and the heat generated in the fission energies. Such a neutron source is especially inter proposed converter, when the converter was coupled to esting for neutron dosimetry, as well as for calibration and each of three Yugoslav research reactors. Results of development of neutron dosimeters and dosimetric methods. calculations show that the converter made of LiD with Wide applications of this neutron source may also be natural lithium is 50% less efficient than the converter expected in biological and similar experiments where hi^ of •LiD (Fig. 5). Intensity of "14 MeV neutrons" is within energy neutrons are required. limits of 5x 10" ... 1010 (n/cm^) for the converter used At many biomedical facilities the only available neutron either as external converter with the reactor RB, within source is a reactor with slow and fission neutrons. For the the thermal column in the reactor TRIGA, or as a "fuel" same purposes the source of "14-MeV neutrons" would be segment at the research reactor RA. welcome, too. This source might be of interest not only for biomedical applications but for physical experiments, fast neutron radiography, fast neutron damage studies, etc. LiD converter can be made from lithium deuterido with natural lithium. Such a converter is only 50% less efficient than the converter made of 6LiD. Received April 15, 1980 References [1] D.Popović et a!., Zero Kucrgy itB Reactor, Bull. "B. KideliS" Institute 9 (10.59) 168, 5/13. 00Z 0 104 Koritoucitfio, Bu. 24, U, 3