Neutron Streaming

From the early years of this century, the only type of radiation used to find long-range order in solids and short-range order in liquids was X-rays. However, neutrons can also be used to carry out diffraction experiments, and with certain advantages, as will be seen. The trouble is that while it is fairly easy (hence, economically attractive) to use an X-ray source, onehas to have a nuclear reactor handy to achieve a neutron stream. The advantages of using neutrons rather than X-rays in diffraction arise from a difference in how diffraction (i.e., interference) occurs for the two forms of radiation. 

Primary coolant enters the upper plenum above the reactor core and flows downward through the coolant channels in the plenum element, then to the top reflector elements above the active core. The metal plenum element on top of each core column contains a small flow plenum. The coolant holes in the plenum elements and the coolant channels in the fuel and top reflector columns below are offset horizontally to minimize the neutron streaming effect. 

The PSR graphite and boron pins also are required to limit neutron streaming... 

Experimentation and, caIcnlation. have shown that to prevent neutron streaming through the annulUs formed by the dummy plug and the hole liner, two steps in the annulus are required.One of these is as close to the thermal, shield as practical and the second is near the outer. face of the concrete shield. The tapered portion of the hole forms the first step, and the removable steel shield at the outer face forms the second step. ... 

In addition to providing a shield for the cooling-water lines, this steel billet forms the second step to prevent neutron streaming through the plug annulus and as a step to prevent neutron and y-ray streaming down the annulus surrounding the radiation door and the concrete shield. 

Lane, J. A., Neutrons Streaming Through Pebble Discharge Chute, ORNL CF-50-1-157, January 30, 1950. 

These consist of materials which have been In the path of the neutron stream emanating from the reactor core during the normal operational life of the reactor. These materials absorb the... 

The RAPID system has a central 300 mm diameter channel which is normally filled by a small sodium flow. In the event of sodium boiling in the core, the channel fills with sodium vapour providing a neutron streaming path to offset the positive sodium void effect. Positive sodium void reactivity effects present one of the major safety issues in LMRs since they could cause an overheating event to escalate. There has been much attention to ways of reducing the effect, such as the RAPID proposal. 

An improved oscillator system, having a sensitivity an order of magnitude better than other techniques, permits relative reactivity measurements in the nuclear environment of the test region. A mechanical oscillator alternately inserts one sample while withdrawing another from the core. A stainless steel rod, with two pockets for samples, fills the passage through the core and prevents neutron streaming. 

Layers 7 and 6 bricks are slightly large and overlap those of the inner neutron shield to prevent neutron streaming This means that layers 7 and 6 of the outer neutron shield require removal prior to the removal of layers 2 and 1 of the inner neutron shield The bricks, when assembled, have a minimum of 1 3 mm (0 05 ) clearance between adjacent bricks m the same layer... 

First, in regard to location 1, the fuel sections are included within the cadmium cover to eliminate neutron streaming and to insure that the resonance flux seen by the thin uranium foil is that seen by the nearby fuel. Otherwise, the thin-foil Np activation would in part be a function of its own effective resonance integral which is characteristic of its thickness. The aluminum foils are fission fragment barriers. 

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