HIGHLY SELF-SHIELDED BURNABLE POISONS. Thesis

Thesis/Dissertation · Tue Jan 01 04:00:00 EST 1963

OSTI ID:4129189

Holl, R J

A new burnable poison concept is proposed and evaluated for use in nuclear reactors. The unique features of the concept are the use of an element with a very high neutron capture cross section as poison material and the utilization of the microscopic self-shielding factor of individual poison disks. The large cross section allows the poison to be highly self-shielded over most of its design lifetime and still to burn out rapidly at the end-of-life. The large cross section also provides high spatial selfshielding with small particle sizes (on the order of 100 microns for the poison selected). This allows the poison particles to be dispersed either in the fuel material or in another material as a discrete poison element. With disk geometry having a large diameter-to-thickness ratio and high initial self-shielding, neutron capture by the poison causes only a small decrease in the effective cross section of the disk until nearly fill depletion. Thus, the endurance of the burnable poison (irradiation lifetime) is determined by the thickness of the disk. The rate of decrease of the effective cross section with irradiation, for a constandt thickness, is determined by the diameter-tothickness ratio. Any decreasing curve for the burnable poison's effective cross section can be approximated by selection of the appropriate disk dimensions. Better approximations can be made by using mixtures of several disk sizes. After reviewing the potential elements that could be used as the burnable poison, gadolinium was selected as most promising. The only significant uncertainty found was the possible presence of an excessive poison residue (poison cross section remaining at the end of the design irradiation lifetime) due to the unknown cross section of gadolinium-158. An evaluation of the performance of natural gadolinium as the burnable poison material indicated that a thermal cross section of less than 1000 barns was required for the 156 isotope. A review of cross section measurements for gadolinium indicated that this upper limit for the 156 cross section was probable. A measurement of the reactivity of an irradiated sample of natural gadolinium, in which most of the high cross section material was burned out, was made to determine the poison residue. Attributing this residue to gadolinium156 gave an effective thermal cross section of 300 plus or minus 300 barns for the isotope. This is sufficiently low to establish the feasibility of using gadolinium as the burnable poison element. Self-shielding factors are given for gadolinium metal disks of varying dimensions in several thermal neutron spectra. Measurements were made of the self-shielding factors for a set of disk sizes. The calculated and measured values were in satisfactory agreement. The effective poison cross sections versus irradiation are given for a range of disk sizes. A measurement for a single disk size following a nominal irradiation (nvt = 6 x 10/sup 19/) was made. This was in good agreement with the predicted value. Although this provides some support for the model employed for the calculations, additional measurements of irradiated samples are recommended. The effects of using the burnable poison on the temperature coefficient and temperature defect of the reactor were calculated. lt was found that the temperature defect caused by the poison partially offsets its usefulness in holding down excess reactivity. The temperature coefficient due to the poison has the potentially useful characteristic of being negative and large at low temperature, decreasing in magnitude with increasing temperature, and finally becoming positive at high temperature. The final result is that the burnable poison concept should be able to be employed advantageously in many thermal reactor designs. (Dissertation Abstr., 24: No. 4, Oct. 1963)

• Research Organization: Originating Research Org. not identified
• NSA Number: NSA-18-004697
• OSTI ID: 4129189
• Country of Publication: Country unknown/Code not available
• Language: English

Related Subjects

ABSORPTION
BURNUP
CAPTURE
CONFIGURATION
CONTROL
CROSS SECTIONS
DENSITY
DISPERSIONS
DISTRIBUTION
EFFICIENCY
ELEMENTS
ENERGY
ENERGY RANGE
EQUATIONS
ERRORS
FUELS
GADOLINIUM 156
GRAIN SIZE
HIGH TEMPERATURE
IRRADIATION
ISOTOPES
LOW TEMPERATURE
MEASURED VALUES
NEUTRON FLUX
NUCLEAR REACTIONS
NUMERICALS
PERFORMANCE
PLANNING
POISONING
POWER PLANTS
REACTIVITY
REACTOR CORE
REACTOR SAFETY
REACTOR TECHNOLOGY
SHIELDING
SPECTRA
SPECTRAL SHIFT
TEMPERATURE
THERMAL NEUTRONS
THICKNESS