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Slow Burning in a Subcritical Reactor Driven by a Proton Beam* P.O

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Slow Burning in a Subcritical Reactor Driven by a Proton Beam* P.O SLOW BURNING IN A SUBCRITICAL REACTOR DRIVEN BY A PROTON BEAM* P.O. Demchenko, Ye.V. Gussev, L.I. Nikolaychuk National Science Center “Kharkov Institute of Physics and Technology”, Kharkov, Ukraine E-mail: [email protected] PACS: 28.52.-s; 52.59.-f 1.INTRODUCTION are wastes of the fuel cycle of power fission reactors, especially of neptunium, plutonium, americium, curium The experience of nuclear power reactor operation isotopes (Minor Actinides) with half-life of hundred and with uranium and plutonium isotope fuel fissioned by thousand years [2]. At present, thermonuclear reactor neutrons has shown that future extensive nuclear power applications, in particular with magnetic confinement usage is impossible without solutions of some scientific, (Tokamak), are considered for transmutation of technological, and ecological problems. They are safety radionuclides, which are produced in the fuel cycle of and reliability of modern reactors operating in the critical power fission reactors [8]. Thus, in future the general mode; utilization of radioactive waste produced by them; nuclear power industry on the base of nuclear fusion inclusion into the fuel cycle thousands of tons of fertile reactors as well as fission ones may be created. In this uranium-238 that is the cleaning rejects at enrichment case, the fission reactors will supply with power the plants as well as thorium-232 [1,2]. thermonuclear reactor systems. The most dangerous accidents of nuclear reactors are Unfortunately, engineering and technological results of the uncontrolled growth of the effective criticality difficulties and high costs of scientific developments do factor Kef and subsequent heat reactor run away which leads not permit to create powerful fusion neutron sources on to the following burst out and ejection of the great amount of the base of magnetic or inertial plasma confinement in the radioactive products in the environment. To ensure the nearest future. nuclear reactor operation for a long time, the abundant Now some progress has been reached in creation of amount of fissile material is loaded in it. In this case the powerful fast neutron sources using cascade processes positive reactivity (Kef-1)/Kef is compensated by control rods, (spallation) in collisions of high-energy protons with fabricated from the material absorbing abundant neutrons. heavy nuclei [6]. In collisions of the intermediate and Thus, all modern power reactors both with thermal and fast high-energy protons of W≥0.5-3 GeV with heavy nuclei up neutrons are potentially dangerous objects [3,4]. to 40 neutrons per one proton may by generated in The most dangerous accidents with uncontrolled dependence on the proton energy and nuclear mass [9]. growth of Kef can be excluded if a nuclear reactor operates Simultaneously, the technology advance is observed in in a subcritical mode, i.e. under condition of Kef<1 [5]. developments of high current proton accelerators of high However, to get a stationary in time neutron flux when energy, in particular the linear ones [10]. As a result, the Kef<1, it is necessary to introduce an external neutron projects of high-current linacs for average currents up to source in a multiplication reactor medium. However, the 100 mA and proton energy of 1-1.6 GeV are being initial neutrons must not be generated due to fuel nuclide developed in some countries [11,12]. On the basis of these fission by neutrons. Now, it is assumed that the most accelerators powerful neutron sources are being powerful external neutron sources may be created on the constructed both for investigations in the neutron physics, base of both thermonuclear reactions, and at collisions of and for solution of nuclear power problems. ≥ high-energy protons (W 1 GeV) with nuclei of heavy In this paper a burning process was studied in a elements (tungsten, lead, bismuth, thorium, uranium) subcritical cylindrical reactor driven by an external [6,7]. neutron source with an intensity spherically distributed in As it is known about 80% of released fusion energy is space. The reactor blanket was considered to consist of removed from D-T plasma by the fast neutrons with fertile uranium-238 with different plutonium–239 energy of E≈14 MeV. The kinetic neutron energy may be enrichment for which the effective neutron multiplication converted into electrical power with 30-35 % efficiency factor varied in limits of 0≤Kef<1. only due to the neutron moderation in a substance and The calculations are presented for the spatial neutron released heat removed by a coolant into a stream source generated in the reactor blanket by a proton beam generator. More effective thermonuclear neutron of definite energy, current and radius. However, the utilization is their absorption in a subcritical blanket from general properties of the burning dynamics are valid also uranium-238 or thorium-232 surrounding a thermonuclear for another nature of initial neutrons, in particular, for reactor [7]. Fissile plutonium-239 or uranium-233 thermonuclear neutrons in installations of the inertial produced in the blanket may be removed and used for the plasma confinement having the spherical symmetry. fuel enrichment of usual power reactors. Moreover, when liquid lithium is used for a blanket cooling, tritium will be 2. BURNING IN A SUBCRITICAL REACTOR produced in it, which is a thermonuclear reactor fuel. WITH EXTERNAL NEUTRON SOURCES The second important application of fusion neutrons is The investigated subcritical reactor is a cylinder of the transmutation of long-lived radioactive nuclides which radius R0 and length 2H0. The cylindrical coordinate Problems of Atomic Science and Technology. 2002. № 5. Series: Plasma Physics (8). P. 33-35 33 neutron flux Ф(r,z) is stationary then in the quasilinear system (r,z) has been chosen in such a way that the z-axis approach the variation in time t of density Ni of the i–kind coincides with a longitudinal cylinder axis, and the system nuclei can be presented as [5]: origin is at the cylinder center. The external neutron = − σ a + ∑ λ + 3 dNi (r, z,t) / dt i Ф(r, z) ij Ni source intensity S0(n/cm s) is supposed uniformly j , (1) distributed in a sphere of radius Rsp with the center at the + ∑ [σ c + λ ] jiФ(r, z) ji N j coordinate origin. For this geometry the analytical j ≠ i expression may be obtained for the spatial neutron flux a where σi is the microscopic cross section of neutron density distribution Φ(r,z) in the reactor [13], solving the c absorption by a i-nucleus; σji is the microscopic cross diffusion neutron equation in one-group approximation section of neutron radiation capture by a j-nucleus with [3,4]. formation of a i-nucleus; λij is the constant of the i-nucleus The analysis of neutron field characteristics in the decay with j-nucleus formation. In the spallation region subcritical reactor [13] shows that the absolute value of one needs to take into account the additional nuclide the neutron flux is Φ(r,z)~S0, and depends essentially on fission by high energy protons, the rate of which is JNiσ the effective neutron multiplication factor sp σ sp 2 i (W)/e, where i (W) is the i-nuclide fission cross section (criticality factor) Kef=K-(LDB10) , where K is the averaged over the path length of protons with the energy multiplication factor in an infinite medium and LD is the W in the blanket. neutron diffusion length in the reactor blanket matter, This paper investigates the temporal dynamics of 2 2 π 2 λ B10 =(2.405/Ra) +( /2H) . Here Ra=R0+0.7 tr, H=H0+0.7 densities of the main blanket actinides: uranium-238, λ tr are respectively extrapolated radius and longitudinal plutonium and americium isotopes, as well as the total λ reactor dimension, at which the neutron flux is zero, tr is concentration of fission nuclides for the simplified the transport neutron length [3,4]. The case Kef=0 uranium-plutonium fuel cycle [4]. → corresponds to usual absorption diffuse medium; for Kef Fig. 1 show variations in the time of densities of 238U, 1 the flux value Φ(r,z)→∞, i.e. the reactor passes into the and plutonium isotopes (239Pu, 240Pu, 241Pu), and also the critical operation mode and does not need an external total fission products (FP) at the center of the burning neutron source [13]. region for the reactor of the concrete dimensions and For numerical calculations of the burning it is beam current I=1 mA (rb=5 cm). The initial blanket 238 necessary to assume the concrete values of external material is fertile U (Kef=0). As it follows from Fig. 1, neutron source intensity S0, and radius Rsp of the region 238U density monotonously decreases in time (burning- occupied by initial neutron source, reactor dimensions R0 out), but fissile 239Pu density increases monotonously at and H0 and nuclear physical characteristics of the blanket. the beginning (breeding stage), at tm≈18 years it reaches For the definition of these characteristics in one-group the maximum value (equilibrium concentration) and approximation it is necessary to know the neutron energy further monotonously decreases. The initial, reduced to distribution [3,4]. 239 the beam current, Pu breeding rate for Kef=0 is The neutron energy spectrum was supposed similar to . 239 ~2.7 kg/mA year. The equilibrium Pu concentration Cm, one of the fast reactor with metal uranium-plutonium fuel i.e. NPu-239/NU-238, outside the spallation region is defined and a sodium coolant, with the spectral density maximum by corresponding neutron cross sections of absorption and near 200 keV [4]. To calculate group constants the energy radiation neutron capture: dependences of microscopic neutron cross sections have c a Cm = (NPu-239/NU-238)m=σ U-238/σ Pu-239, been used from several papers, in particular [14].
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