Corresponding author: Anatoly V. Zrodnikov ( suvorov@ippe.ru ) Academic editor: Yury Korovin
© 2019 Petr P. Dyachenko, Anatoly V. Zrodnikov, Oleg F. Kukharchuk, Alexey A. Suvorov.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Dyachenko PP, Zrodnikov AV, Kukharchuk OF, Suvorov AA (2019) Problem of nuclear-laser power engineering and methods of their solution. Nuclear Energy and Technology 5(3): 257-263. https://doi.org/10.3897/nucet.5.46381
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The concept of a high power reactor-laser system based on a nuclear pumped optical quantum amplifier (OKUYaN) was formulated at IPPE in the mid-1980-ies. The idea amounted to the use of wide-aperture OKUYaN as an amplifier within the already well-known “master laser – two-pass amplifier with phase conjugation” scheme.
The structure of such an amplifier includes a system of two neutron-coupled units – an ignition reactor (RB) and a nuclear pumped laser amplifier (LB). The ignition unit is a compact multi-core pulsed fast neutron reactor. The laser amplifier unit operates on thermal neutrons and, with regard to the neutronics, it is a subcritical booster zone of the ignition reactor unit.
Unique reactor-laser complex incorporating demonstration sample of a pulsed reactor-laser system based on OKUYaN (test facility “Stand B”) having no analogues anywhere in the world, was developed and put into operation at IPPE in 1999 for the purpose of substantiation of basic principles of the OKUYaN concept and demonstration of the possibility of its practical implementation, as well as verification of calculation codes and development of relevant equipment elements.
Problems overcome in the development and construction of “Stand B” test facility, the choice and justification of the neutronics and laser characteristics of the OKUYaN demonstration sample are discussed in the present paper. Provided are the results of a detailed computational-experimental study of the demonstration sample characteristics, the data from systems studies of direct conversion of nuclear fission energy into laser radiation energy in complex reactor-laser devices and the results of examination of prospects for the development of nuclear-laser power engineering.
Multi-core nuclear reactor, neutrons, fission fragments, nuclear pumping, laser, optical quantum amplifier with nuclear pumping
Generation of nuclear excitation plasma, i.e. the state of the matter emerging during deceleration in it of products of nuclear reactions as the result of excitation and ionization of atoms, is the primary process of interaction of charge nuclear reaction products with matter.
Recombination non-equilibrium represents an important specific feature of nuclear excitation plasma. This means that inverse density of population of quantum levels of its components develops in the process of relaxation of such plasma, i.e. the necessary conditions for direct conversion of energy from nuclear reactions into laser radiation energy are satisfied. The device, in which such direct conversion is achieved, is called direct nuclear pumped laser (NPL). Design and principle of operation of such device is easy to understand using the example of one of the simplest NPL types, layout of which is presented in Fig.
Until recently, investigation of nuclear pumped lasers was conducted in a number of laboratories in Russia (RFNC VNIIEF, SSC RF – IPPE, RFNC VNIITF, Prokhorov General Physics Institute, MEPhI), USA (Sandia National Laboratories, University of Illinois, NASA Langley Research Center, Los Alamos National Laboratory, University of Missouri – Columbia and others), Germany (Technishe Universität München) and China (Institute of Nuclear Physics and Chemistry of CAEP). Seven scientific conferences on different levels dedicated to the subject in question were held and five monographs were published (
By the present moment laser generation was obtained in nuclear pumping of about thirty different gaseous laser-active media with products of different nuclear reactions including 10B(n,α)7Li, 6Li(n,α)3T, 3He(n,p)3T, 235U(n,f)f and others. (
When sufficiently large amount of uranium-containing LAEL and neutron moderating materials are assembled to form the unified compact system the latter begins to multiply neutrons and may become critical or close to criticality. Such systems are called the reactor-laser systems. The interest to these systems is explained by the unique properties of the nuclear reactor as the source of laser pumping, such as its high-energy capacity, compactness and self-sufficiency. Thanks to the high penetrative capacity of neutron in neutron-multiplying media, the possibility emerges to pump laser-active media in practically unlimited volumes. All the above opens the perspective for the development of super-powerful, compact and autonomous sources of laser energy, i.e. the development of principally new direction of the use of nuclear energy – nuclear laser power generation necessary for implementation of a whole series of the newest laser technologies, for instance, such technologies as the inertial thermonuclear fusion, beam power generation, power supply and launching space missions, remotely controlled deep cutting and welding, energy intensive chemical synthesis, isotopic separation, 3D metal ceramics lithography and many others.
Two approaches to the development of powerful reactor-laser systems are known as of today. The first approach (
The concept of powerful reactor-laser system based on the optical quantum amplifier with nuclear pumping (OKUYaN) was formulated in 1986 at the SSC RF – IPPE (
It is fairly easy to demonstrate that in ideal (limiting) case of one neutron undergoing leakage per one fission event in the RU when the direct neutron coupling RU → LU reaches maximum (fission in the laser unit is initiated with probability close to unity by neutron escaping from the reactor unit), the inverse LU → RU neutron coupling is missing and ∆tRU1/2 ≤ tLU, it follows from the definitions of the neutron multiplication factor kLUeff and neutron lifetime τLU in the laser unit that
Е LU = ЕRU /(1 – kLUeff) ν; (1)
∆tLU1/2 = –0.7 tLU / ln kLUeff, (2)
where ЕLU, ЕRU is the energy released in the LU and RU, respectively; ν is the average number of prompt fission neutrons; ∆tRU1/2, ∆tLU1/2 is the energy release pulse width at half maximum in the RU and LU, respectively.
It follows from the above that for parameters realistic as of today ЕRU = 15 MJ, kLUeff = 0.95, tLU = 100 µs, ν = 2.45 and the efficiency of conversion of fission energy into laser radiation energy equal to ~1% we obtain the value of laser energy equal to ~1.2 MJ with duration at half maximum of the pumping pulse ~1.3 ms.
Unique reactor-laser complex “Stand B” - demonstration sample of the OKUYaN reactor-laser system having no analogues anywhere in the world, was developed and put into operation at IPPE in 1999 for the purpose of practical demonstration of feasibility of the OKUYaN concept, as well as verification of calculation codes and working out relevant hardware elements. (
Brief description of the design of the device and results of experimental investigation of neutronics and laser characteristics of the demonstration sample of the reactor-laser system based on the OKUYaN concept (“Stand B”), as well as the development methods for upgrading the pilot facility for the purpose of increasing energy and power of the laser beam are discussed below.
Main results presented in the paper were published in the reference sources and are patented (
Picture of the “Stand B” OKUYaN is presented in Fig.
Each of the cores is equipped with controls that allows to control the reactor when it is in criticality for delayed neutrons, and also very quickly (with a speed of up to 220 βeff/s) put the reactor in supercriticality for instantaneous neutrons to generate a pulse and return by the end of the pulse at a speed of about 200 βeff/s from the critical state at delayed neutrons to the state of deep subcriticality (up to -20 βeff). Transition of the reactor from the conditions of instantaneous supercriticality to the conditions of delayed criticality in the process of pulse generation takes place automatically due to the negative temperature reactivity factor.
Test stand is designed in a way allowing the reactor performing alternately the following two functions. Firstly, generating neutron burst for irradiating not neutron-multiplying elements of laser unit hardware on the first workplace (WP) corresponding to Phase 1 of the test facility put into operation in 1996. Secondly, it can serve as the ignition reactor unit during operation with neutron-multiplying laser unit in the composition of the OKUYaN test sample on the second WP constituting Phase 2 of the test facility put into operation in 1999.
Laser unit represents cylindrical structure with 2.5-m length and 1.7-m diameter with lateral cavity for arrangement of ignition reactor cores. Laser unit is assembled from standard elements of the following five types: laser-active element (LAEL), LAEL imitator, element of inter-tubular neutron moderator, element of neutron reflector and the power multiplication channel (PMC). Arrangement of elements in the LU is determined by the loading map.
LAEL and its imitator are the main elements of the LU (Fig.
Polyethylene and paraffin are used as the materials of element of neutron moderator and reflector, respectively. The PMC represents aluminum tube standard for the LU design under discussion, filled with alternating fuel and neutron moderator pellets. Mass of fuel (uranium dioxide with 235U enrichment of 90%) in the PMC amounts to approximately 150 g; that of neutron moderator is equal to 3.5 kg.
OKUYaN is activated as follows. Laser unit is raised to the upper position using special lifting device. Reactor is moved from the shielding along the rails to be positioned under the laser unit occupying strictly fixed position. Laser unit is lowered into the lower position. As the result, the RCs appear to be positioned inside the LU. Reactor pulse is ignited by the command from control panel and measurements of required neutronics and laser characteristics of the facility are performed.
The purpose of investigation of OKUYaN neutronics characteristics essentially amounted to the search for such configuration of the LU and reactor operational mode when the direct RU → LU coupling would be maximum, while the inverse LU → RU coupling would be minimum. Three LU loading maps and two operational modes of the ignition reactors were investigated.
Results of studies are shown in Fig.
It is clear from the results presented in Fig.
Values of ЕLU ~7 MJ and ∆tLU1/2 ~2 ms are evidently close to the limiting values of these variables in the case of use of Bars-6 reactor as the ignition reactor of the OKUYaN. It is clear that these values significantly differ from the values following from expressions (1) and (2). This is explained by high sensitivity of reactors of this type to neutrons pertaining to LU → RU inverse coupling.
Based on the examined experimental data it is possible to suggest two methods for modernization of the ignition pulsed reactor for reducing the effects of LU → RU inverse coupling and, consequently, for enhancing the OKUYaN energy characteristics (
The first method refers to the reactor loaded with conventional fuel (235U, 239Pu) and amounts to the development of fast enough (with time comparable with self-quenching) mechanism of forced transition of the RU to deep sub-criticality (Dkeff ≥ 3beff) for the purpose of ensuring nuclear safe reactor excursion on prompt criticality exceeding the contribution of the LU in its reactivity. It is evident that this represents the task quite difficult from the technical viewpoint, and, probably, practical implementation of this task is even impossible.
The second, more cardinal, method is the application of threshold fissionable element, for instance, 237Np, as the ignition reactor fuel. Fission threshold amounts for this nuclide to about 0.25 MeV. Therefore, neutrons from the LU with spectrum close to thermal spectrum entering the ignition reactor core will not initiate fissions of reactor fuel nuclei, i.e. the value of the inverse neutron coupling coefficient KNpinc in the case of fuel made of 237Np will be much less that the corresponding value of KUinc coefficient in the case of reactor core fuel made of 235U.
In order to estimate the coefficient let us use experimental data on the value of cadmium ratio in the laser unit. Its value averaged over the LU volume amounts to approximately 30 (
Maxwellian distribution of neutrons n(E) corresponding to such cadmium ratio is shown in Figure
i.e. replacement of 235U with 237Np in the ignition pulsed reactor results in the decrease of LU → RU inverse neutron coupling by about five orders of magnitude.
The question of use of 237Np as reactor fuel was discussed in (
The following two important conclusions can be drawn from the above discussion. Firstly, development of ignition reactor with fuel load consisting of 237Np appears, at last from the viewpoint of neutronics, to be a feasible task. Secondly, application of such ignition reactor in the OKUYaN would allow bringing energy balance of such reactor significantly closer to theoretical limit increasing, in particular, energy and power of the pumping pulse in the laser unit of the pilot sample of the OKUYaN by approximately 10 – 20 times.
Two gas mixtures He-Ar-Xe (l = 1.73; 2.03 µm) and He-N2-H2 (l = 0.391; 0.428 µm) most thoroughly investigated by the respective time period and easiest from the viewpoint of technology (can be operated at room temperatures) were selected as LAEL laser-active media for demonstration of operability of the laser unit. The media in question were used in the studies of laser characteristics of the OKUYaN for different LU configurations and different laser and ignition reactor operation modes.
In particular, the mode of free generation on one LAEL in the composition of laser unit equipped with internal and external resonators was investigated. Free generation mode on seven LAEL, as well as modes of one- and two-pass amplifier on seven or 19 LAEL with uranium LAEL used as the driving generator operated in the free pumping mode and LAEL with volume pumping with 3He(n,p)3H reaction products were investigated for He-Ar-Xe medium. Here, flat mirror was used in the two-pass mode instead of SBS cuvette, and Fresnel rhomb was used for rotating radiation polarization. Some of the obtained results are shown in Fig.
Laser characteristics of the OKUYaN pilot facility: а) – laser beam “signature” at the output from the amplifier of the “Stand B” reactor-laser system (l = 2.03 µm); b) – pumping power (1) and total power along the whole laser generation channel (Egen = 43 ±2 J; Espec > 1.5 J/l) on the 2.03-µm transition (2) in He-Ar-Xe medium; c) – pumping power (3) and total power along the whole laser generation channel (one LAEL; Egen = 0.7 ±0.1 J) on the 391.4-nm transition (4) in He-N2-H2 medium.
It is evident, for instance, that for He-Ar-Xe medium efficiency of conversion of energy of fission fragments imparted to the gas into laser radiation energy amounts to about 0.5 and to 1% for energy and power, respectively. This is approximately two times lower than the values following from the calculation kinetic model of this laser and values obtained on the first workplace of the test facility.
Studies (
Studies for optimizing the LAEL design, selecting and investigating active media for specific practical applications of the installation were conducted on the first workplace of the “Stand B” test facility. Refs. (
It was demonstrated in (
Interest in laser liquid is associated with the fact that because uranium is contained in the liquid in the form of solution, pumping efficiency for Nd3+ upper laser level (relative to the fission energy in the LAEL) is approximately 5 – 10 times higher as compared with gas laser and amounts, in accordance with experimental data (
Transmission of laser beam with wavelength of 1.054 µm for POCl3 – SnCl4 – 235U2+ – Nd3+ liquid in cuvettes with different aperture with excitation by uranium fission fragments was investigated in (
Solution of these problems will ensure achieving calculated parameters of reactor-laser systems and will allow initiating the development of full-scale pilot samples of optical quantum amplifier with nuclear pumping for practical implementation of the newest technologies.