* . I OFT ORNL P 140 NE O o - - - 1 EE EEEEEEEE DIT 1.1:25 114 16 IT- MICROCOPY RESOLUTION TEST CHART NATIONAL DUREAU OF STAMOANOS - 1963 . T - - = ir;-* This paper was submitted for publication in the open literature at least 6 months prior to the issuance date of this Micro- card. Since the U.S.A.E.C. has no evi- dence that it has been published, the pa- per is being distributed in Microcard form as a preprint. : - 1 1 . . : A si JAKO ' '*! W . ' . ' . . no T ! !. 1 LEGAL NOTICE This report was prepared as an account of Government sponsored work. Neither the United States, nor the Commission, nor any 1 person acting on behalf of the Commission: A. Makes any warranty or representa- tion, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, appa- ratus, method, or process disclosed in this report may not infringe privately owned rights; or B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this report. As used in the above, “person acting on behalf of the Commission” includes any em- ployee or contractor of the Commission, or employee of such contractor, to the extent that such employee or contractor of the Commission, or employee of such contractor prepares, disseminates, or provides access to, any information pursuant to his employ- ment or contract with the Commission, or his employment with such contractor. . SALVA 2 ON hf 140 DHE AUG 1 1 1964 ASTER Effect of Floor-Reflected Neutrons on Rossi-a Measurements with the iad. 1 Health Physics Research Reactor* . .. .... J. T. Mihalczo . Oak Ridge National Laboratory Oak Ridge, Tennessee zit ! ... -*ji - LEGAL NOTICE The report wupper mm room of Governo m om wort, Katthor the Mulind mate, mor me Calle, my more acting on whole of the Commons A. Makina nywarmy armprentation, empresord or implied, mu roepect to the secr. mory, cupletno, o tolewa h ilaration contained two report, or that the a way tormation, parta, mehed, or mouse declared than who raport may not intende mirately wed thus 1. Anna my lowedno oth roopact to the wo er for d rood thing from the wa ter , unnt, with a man dochodu w raport Now the whore, porno aztag on heball of the Color marketing my me moramoctor oh C enter, plays a week anto , to do what melk plyn a contructor Cultur, * pleyen wel watracter pemersa, dominated, or pod non ne, ne moretha Wapi oranirect t al or Mewah menur, This paper wus mubmitted for publication in the open literaturo at least onths prior to the issuance date of this Micro- card. Since the 1.8.A.L.C. has no evi- dence that it has been published, the pa- er is being distributed in Microcard form as a proprint. No. of pages 6 No. of tables a No. of figures 2 * Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation. ORNL-p-140 1 ROSSI-Q MEASUREMENTS WITH HPRR Oak Ridge National Laboratory J. T. Mihalczo Post Office Box X Oak Ridge, Tennessee . .*.* = -.* 2 . . . - . . * -*- . 7 1- V . - : 1 3 * . * - * Effect of Floor-Reflected Neutrons on Rossi-a Measurements with the Health Physics Research Reactori J. T. Mihalczo When the Rossi-a, or pulsed-neutron, technique is used for measuring the prompt-neutron decay constant of an unreflected reactor, the effect on the results of neutrons reflected from nearby wall surfaces is always of concern. The ORNL Health Physics Research Reactor (HPRR), an unreflected and unmoderated cylin- drical core of 90 wt % uranium (93.2% 235U) and 10 wt % nolybdenum shown in Fig. 1, is an ideal reactor for determining this effect because it has a small core from which more than 50% of the neutrons leak out and because the time required for a leakage neutron to travel to a reflecting surface and back to the core is large compared with the prompt-neutron lifetime of the reactor. The reactor is housed in a 70- by 30-ft building with 50-ft-high walls of approximately 0.13-in.- thick aluminum. The sensitivity of Rossi-a measurements with the HPRR to floor-reflected neutrons was investigated with the reactor suspended near the center of the building at heights of 1.15 and 24 ft above a 64- x 64- x 8 in. slab of concrete blocks (p = 2.25 g/cm3) stacked on the building floor. The method of measurement has been described previously.' Pulses from a plastic scintillator (Nuclear Enterprises No. 102) which sampled leakage neutrons and gaüma rays triggered a time-of-flight analyzer, and pulses from a small spiral fission counter positioned in the HPRR burst rod hole at the midplane of the core were sorted in time. The results of the measurements are given in Table 1 and shown in Fig. 2. Although the prompt-neutron decay constants for both heights agree within ex- perimental error, the value for the 1.15-ft height 18 1.5% smaller than that for the 24-ft height. *Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation. - - -2- These measurements indicate that the prompt-neutron decay constant measured by the Rossi-a technique for this type of reactor is not significantly affected by neutrons reflected from materials which are farther than about 1 ft from the surface of the core and which do not change the reactivity of the reactor by more .than 308. RI P . . . . hi Table 1. Prompt-Neutron Decay Constant for the HPRR Mass Adjustment Rodº Position (in.) Constantic Prompt-Neutron Decay (10 6 sec ijc Height Above Floor (pt) 4.6 1.150 3.07 24.0 0.664 + 0.006 0.671 + 0.005 0.577 I 0.005 Reactor configuration: 8-in.-diam by 9-in.-high cylinder containing 96.88 kg of 2350. Burst rod, sample-irradiation-hold plug, and five bolt inserts removed. Regulating rod fixed at 3.25 in. from bottom of core. Distance from bottom core. "Error indicated is the standard deviation obtained in the least-squares analysis of the counter data. "Reactivity associated with floor reflection was +31 cents. -4- References 1. J. D. Orndoff, Nucl. Sci. Eng. 2, 450 (1957). 2. M. I. Lundin, Health Physics Research Reactor Hazards Summary, ORNL-3248 (Aug. 24, 1962). 3. J. T. Mihalczo, "Prompt Neutron Lifetime in Uranium Metal Cylinaers and Annuli," Nucl. Sci. Eng. (in press). . . . . ---... . . - - - - -- 1 List of Figures Fig. No._ Title The ORML Health Physics Research Reactor (HPRR). Detector Response vs Time in Ros81-0 Measurements with the HPRR at Heights of 1.15 and 24 pt. ... n WY UNCLASSIFIED ORNL PHOTO 65365 ** NA , ... . . na . . * . 2 So. OS TA . . . . . .Ona . .. w . OY . .. ** SS. . ? . ' **. U 10 PAPIYOS WATC TOVU S: .. . . . i IS 02 OY Yii OD . YX. . . . . 1. S ... . . . . YO . . o . .. CNP WVAUSV COLO ' '. a SRX W . M . . . . : . .. . TO . ALE . .. . Nr. w . . .. . DOG SOS . . . : . TOWN . O TO .... . . . LOS M K WHERES HUKUA . . 3 . .. ON .. " .. ORI TISTA .. " .. CO24 . . : *** . . : . w MY 1. . 2 . . O . .. DI M SOS CS . 0 ... TO . . . 10. . . 11 . 12 sini : SYYTY MY 26 $ R CORE . . V8 .. CIU WO 0 . . 1 . : C O . . . COS CY . . a * . . OS: . : . os . . .. V . SCINTILLATION COUNTER W . el FISSION COUNTER (IN BURST ROD HOLE) MIL .An.. 1 + Pijl UNCLASSIFIED ORNL DWG 64-5248 - - REACTOR 24 ft ABOVE FLOOR Q * (0.664 $ 0.006) x 10Ⓡ sec- COUNTS OL 11 III REACTOR 1.15 ft ABOVE FLOOR Q* (0.677 + 0.005) x 10 sec OBSERVED BACKGROUND SUBTRACTED A 2.5 7.5 10.0 5.0 TIME (sec) . - - - neg z END DATE FILMED 8 / 24 / 65 . .