_____ PRELIMINARY STUDY OF A MOLTEN-SALT BURST REACTOR 6AjK; RIDGE 'NATIONAL LABORATORY operated by UNION CARBIDE CORPORATION | for the u.s. ATOMIC ENERGY COMMlSSION UNION CARBIDE ORNL TM- 2282 copyno.- F DATE - July 10, 1968 , ,'A M Perry H. F. Bauman | ~ W. B. McDonald J. T. Mihalczo Diétribfifioh X ‘ Dr. Vlctor Rcuevskl ’ ISPRA ] 2. - M. J. Skinner 3.-5. DTIE ' e “0"05 Thxs document contoms mformahon of .a preliminary nature and was prepared pnmanly for internal use at the Oak Ridge National Zchorufory It is subject to revision or correcflon and therefore does not represent a fmal report . : ¥l VoL ' CEWIUTON OF THS DOCUMENS IS UNIMITED —— LEGAL NOTICE- [ Thir report was prepared as an account of Government sponsered work. Kefther the United | States, nor the Commission, nor any pergon acting on behalf of the Commission: i 4. Makes zuy wartanty or representation, expressed or implied, with respect to the accu- “an | racy, completeness. or usefilness of the information contained ir this report, or that the use . S i of any information, apparatus, methad, or process disclesed in thig report may not iniringe | privately owned righis; or g ; B. Assames sny liabilitles with respsct to the uge of, or for damages resuiting froin the . R use of any information, apparstus, method, or process digclosesd in this report. As used in the above, *“person acting on behalf of the Commisglon® includes any sm- ployee or contractor of the Commission, or employee of such contractor, to the extent that such smployes or contractor of the Commigsion, or employeo of such conirzctor prepares, disseminates, or provides access to, any information pursuant to his empivyment or contract 3 witk the Commiesion, or his employment with such contracter. PRELIMINARY STUDY OF A MOLTEN-SALT BURST REACTOR A. M. Perry W. B. McDonald E. F. Bauman J. T. Mihalczo The guestion has been posed whether one could pulse a molten-salt reactor so as to achieve an integrated flux of 10® neutrons/cm® per burst in a test cavity at the center of the reactor, and, if so, what energy yield would be required and what would be the shape of the pulse? The nominal objectives appear to be: 1) 1-2 X 10*® neutrons/cm® per burst, without a tight specification on the neutron spectrum, 2) a burst width in the neighborhood of 10 msec or less. Since it appears that the second objective should be easy to satisfy, ~we have looked at the possibility of achieving the Tirst without much regard for factors which might influence the burst shape, and have sub- sequeptly estimated the burst width for one of several reactor con- figurations one might contemplate using. 1. Selection of Salt ~ The ealculations were based on use of the salt LiF (7% mole %) UF, (27 mole %), and for each reactor considered, the critical enrich- -menfi,of'the uranium was determined. Separated Li7 would of course be used. | Relevant properties of the salt are: Melting point, °C 450 Specific heat of the melt, cal/gm °C 0.217 | Density of the melt, gm/cm’ 5.26 — §,3 X 10°% T(°C) 2. Allowable Energy Input We postulate that the temperature of the salt could be allowed to rise 1000°C, i.e., nominally from 500°C to 1500°C. The properties given above yield the following values at 1000°C (average termperature): et R e TR e} A ‘u:’-?Efif‘nfi.tkrfw L ‘Density’ 4,33 gm/cn”® Specific heat 4 wsec/em® °C Thus, the integrated fiux that can be obtained in the test cavity corresponds to a maximum energy density of h-kwsec/cm3 per burst. 3, Geometry ‘For the purpeses of this preliminary exploration, we postulate spherical geometry, with concentric regions and dimensions as follows: Test cavity {void) 15 enm radius Ni shell C .1 em thick Fueled salt 15 to 60 cm thick Ni shell * 5 ecm thick Graphite shell - 25 em thick An addi%ional case was exanined with the thickness of the outer Ni con- teiner increased to 15 cm, and the-graphite'shell omitted; this was done for a 60-cm-thick salt region. 4. Results The integrated fluxes obtainable in the test cavity are shown for each case in Table 1, along with geometrical specifications and other ~ derived results. ©Some of these results are . also plotted in Fig. 1. . -The neutron spectrum for case #8, with $2-cm-OD core (3 ft) is shown in Fig. 2, as the integral above energy E, as & function of I, i.e., [ at fm(i)(E',t)dE' , E . . where the time integration is taken over the duration of the burst. For s S comparison, the fission spectrum is also given, normalized to 1 X 1016 neutrons. o . B In case #3, which was the same as case #1, except that the S.cm Ni and 25-cm graphite shells were replaced by a single 15-cm Ni shell, both the peak-to-average power density ratio andrfhe'integrated flux in the test cavity were the same as in case #1. i Table 1. Burst Characteristics vs Core Size Integrated Flux in Fuel® , Test Cavity Case Cutside Fuel Critical ax Burst Nurber =~ Diameter Volume Enrichment 235U Mass = Total >100 kev yield ( cm) (liters) (g (kg) avg (neutrons/ecm?® X 107*6) - (Mw sec) h 152 1821 17.5 854 1.545 z,L 0.97 L7k 1 122 g3L 19.7 93 1.423 3,1 0.95 2625 6 112 718 21.1 398 1.378 3,0 0.94 2084 7 102 538 - P22 320 1,331 2.8 0.92 1615 8 92 . 301 2L .2 253 1.282 2.7 0.91 121k 9 &o 272 27.3 200 1.231 2.5 0.88 883 10 72 178 72,1 153 1.178 2,2 0.84. 605 11 62 108 414 120 1.125 1.7 0.77 3832 aFor all cases shown, cavity outside diemeter = 30 cm, inner shell thickness = 1 cm, outer shell thickness = 5 cm, grephite shell thickness = 25 cm. ORJL D .y W ~ Ve £5-1200 hearely . S .+ _.ORKL D45, 65-12008 3 4 %S 67891 2 3 4 567891 - 2 3 4 5 6789% .2 3 4 3567891 2 3 4 567891 ~ ~ » W AU O BO. ~ . Q. K”" W b RNNEGED- ~N R “a, D o) W & B ANOD s 8 An estimate of the burst width was made for case #2 (not listed in Table 1) which was the same as case #1 except that the graphite shell thiqkness was 10 cm. The Rossi ¢ at delayed critical, Oy s Was calcu- lated, by use of the DTF transport code, and found to be 9 X 10° sec~t, The burst width was estimated approximately from the expression® O = 3.5/& ’ where =O£‘i (p—L) and p is the reactivity in dollars} Thus,; for p = 2 dollars, the burst width at half maximum is estimated to be 0.4 msec. The reactors smaller than case #2 would have narrower bursts. 5. Discussion v Several interesting features are apparent from these results. a) The reactors are all fast, i.e., from 25 to 40% of the total ~ integrated flux 1s above 100 kev, and essentially all of the flux is abdve 1 kev. b) - The flux of neutrons above 100 kev is qfiite insensitive to reactor size, when the normelization is for & given maximum energy density (e.g,, L kwsec/cm?). This is so because the peak-to-average power density ratio rises with increasing core aize (see Table 1). The total flux falls off fairly sharply with decreasing core size below - .perhaps 30 in. diam. c) A single nickel container shell, perhaps 6 in. thick, appears . to be a satisfactory reflector, yielding quite flat power distributions, Nat least for the case studled. It is possible that some gains could be realized by optimizing the container-reflector regions of the assembly. d) Burst widths. While the burst widths have not been calculated with great care, the results obtalned for one of the largest'cores studied appear to confirm the expectation that the pulses of the desired magnitude will be less than l msec in width, 1T F. Wimett et al., M"Godiva II —-An Unmoderated Pulse Irradiation . Reactor," Nucl. Sci. Eng., 8(6): 691-708 (1%0). R Very little attention has so far been given to engineering aspects of & practical burst reactor. The guenching mechanism, depending on expansion of the liquid fuel, can be significantly affected by details of the geometrical arrangement, such &s the shape of the core (e.gz., spherical or cylindrical), and the location of the free liquid surface. The range of temperatures that can be permitted may be extended -somewhat by pressurizing the cover gas, and in any event the core vessel will have to be capable of withstanding substantial mechanical shocks. A thick vegsel will therefore probably be required. Details of the control mechanisms, and in particular of the devices for introducing reactivity very rapidly, will require considerable thought. 2 L e 6. Summary A first lock at the possibility of using a molten-salt reactor to U produce intense, sharp bursts of neutrons indicates that fluxes of . 2—-3 % 10%® peutrons/cm® per burst can be achieved in s central test . - ' cavity. This can be accomplished with a core perhaps 30 in. in outer | diameter, having a volume of about 200 liters, and with a burst yield of about TO0C Mw sec. The neutrons are essentially all above 1 kev, and a third of the flux is above 100 kev. The estimated burst width is less than 1 msec. 10 Aggendix | One of the above,reacfiers, operating in burst mode with a maximum energy density of L Mw sec/liter, mey be compared with a similar reactor ‘operating at constant power with a maximum power density of 4 Mw/liter and with & coolant,temperature rise, AT, equal to 1000°C times the - residence time of the. fuel in the core, in seconds. ihe implication is - of course that a reactor like case #10, for example, at a power level of ' 600 Mw, would produce a totel (fast) flux in the test cavity of 2 x 1016 - neutrons/cm sec. : fi | The power density of L MM/liter is probably attainable., One is led to speculate that such a reactor, with cooling adequate for 600-Mw operation, could be operated in pulsed mode with & pulse rep;tition rate of 1/sec. Since the pulses could then not be initiated from a very low neutron level; it is doubtful that pulses as short as those cited above could be achieved. We have not yet estimated what pulse shape might be 'obtalned at such a high repetition rate, but there appear to be some interesting possibllities here worth further 1nvestigat10n. _' ' . T wyet | o