Graphite reflectors

The Los Alamos water boiler served as a prototype for the first university training reactor, started in September 1953 at North Carolina State College. The cylindrical reactor core used uranyl sulfate [1314-64-3] UO2SO4, and cooling water tubes wound inside the stainless steel container. A thick graphite reflector surrounded the core. 

The Arbeitsgemeinschaft Versuchsreaktor (AVR) and Thorium High-Temperature Reactor (THTR-300) were both helium-cooled reactors of the pebble-bed design [29,42,43]. The major design parameters of the AVR and THTR are shown in Table 10. Construction started on the AVR in 1961 and full power operation at 15MW(e) commenced in May 1967. The core of the AVR consisted of approximately 100,000 spherical pebble type fuel elements (see Section 5). The pebble bed was surrounded by a cylindrical graphite reflector and structural carbon... 

The 211 control rods were moved in and out of the core by winches driven by electric motors. Power and neutron flux distribution were measured by in-core self-powered ion chambers, which were inaccurate at lower power. At low power, ion chambers in the graphite reflector were used. 

Blanket structure coolant Chamber wall Graphite reflector Vessel wall... 

Around the outside of the hexagonal replaceable side reflector is a permanent side reflector which provides a transition from the core periphery to a cylindrical outer boundary. Interfacing of the graphite reflector with the reactor vessel is provided by a core lateral restraint structure which is composed of a core barrel and seismic keys. 

The Reactor Core Subsystem (RCSS) consists of fuel elements, hexagonal graphite reflector elements, plenum elements, startup sources, and reactivity control material, all located inside a reactor pressure vessel. The RCSS, together with graphite components of the Reactor Internals Subsystem, constitutes a graphite assembly which is supported on a graphite support structure and restrained by a core lateral restraint structure. (See Figures 4.1-1 and 4.1-2). 

The hexagonal fuel elements are stacked in columns that form an active core annulus with columns of hexagonal graphite reflector elements in the central region and surrounding the active core, as shown in Figure 4.1-2. The core produces a power of 350 MWt at a power density of 5.9 MW/cu m. 

To channel the coolant flow, metal plenum elements containing radiationshielding material are placed on top of the upper graphite reflector, one per column. Hexagonal graphite reflector elements are beneath the active core. These lower reflector elements initially continue the coolant hole pattern from the active core. Flow in these channels exits into the core support blocks. 

The annular reactor core consists of fuel elements, graphite reflector elements, plenum elements, reactivity control material, and neutron startup sources. Each of these components is described below. 

The hexagonal H-451 graphite reflector elements have similar size, shape, and handling hole to the fuel elements (except that some are half-height or three-quarter height). Differences exist in the hexagonal reflectors, depending on their locations in the core, i.e., top, bottom, side, and central reflectors which are described below. 

The fast (E > 0.18 MeV) neutron flux exiting the active core is attenuated by the graphite reflectors. The attenuation of the total neutron flux exiting the permanent side reflector is enhanced by inclusion of borated steel pins (a neutron absorber material) in the outer portions of the reflector. The shielding poisons are located as far as practical away from the active core boundary to limit their impact on the core reactivity. 

The graphite reflector, consisting of a replaceable pebble zone aiid a permanent block graphite zone, both air cooled. 

Two discharge chutes in the pebble support plate can be opened into a discharge bin, thus allowing for removal of the pebbles at any time. This easy removal of the pebbles carries out the MTR feature which provides for removable B, C, and D tank sections (see Section 2.3.1). Furthermore, since the pebbles are in the. highest flux region of the graphite reflector, any radiation damage will affect them first and they can be replaced. 

Briggs. R. B., Cooling of Graphite Reflector for UTR, ORNL CP-50-3-102, March 20, 1950 Temperature in Bottom Thermal Shield Proposed for MTR, ORNL CF-50-1-45, January 10, 1950. 

Briggs, B. B., Neutron Fluxes and Heat Production in. Top and Side Thermal Shields of MTR, ORNL CF-50-1-18, January 6, 1950 Effect of Conduction of Heat from Graphite Reflector Upon Temperature in East Side Thermal Shield of MTR, ORNL CF-50-7-125, July 24, 1950. 

Briggs, R. B., Comparison of Design Neutron Fluxes and Heat Production in the Graphite Reflector and Thermal Shield of the Materials Testing Reactor with Values Measured in the MTR Mock-Up, OHIL CF-SO-7-86, July 20/ 1950. 

The air flows in direct contact with the graphite reflector. 

Measurements of neutron and gamma fluxes inside and above the fuel, in the graphite reflector outside the tank, and in the thermal shield. 

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