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XYLENE POWER LTD.

FNR BLANKET

By Charles Rhodes, P.Eng., Ph.D.

BLANKET OVERVIEW:
The function of the FNR blanket is to absorb almost all the surplus neutrons from the nuclear chain reation in the FNR core zone and to use these neutrons to convert U-238 into Pu-239 and Pu-240. The fundamental question from a practical reactor engineering perspective is: "How thick must the blanket be?" A related question is:"Is there any merit in use of different size fuel tubes in the reactor blanket than in the reactor core?"

A secondary and related question is relasted to neutron activation of the liquid sodium. That question is: "Does it make economic sense to increase the blanket thickness to reduce neutron absorption by liquid sodium in the liquid sodium guard band? That neutron absorption produces Na-24 which spontaneously decays to Mg-24. That decay produces a Mg-24 sludge which must be filtered out and also makes the primary liquid sodium surface radio active.

An important technical issue that must be addressed to answer the aforementioned questions is: "What is the ratio of neutron random walk path length to blanket thickness?"

Neutrons diffuse through the blanket by scattering. At each scatter a neutron loses a small fraction of its kinetic energy. Between successive scatters the number of neutrons reduces due to neutron absorption. Our first concern is that at least 99% of the neutrons that diffuse out of the core zone must be absorbed in the blanket. Hence the neutron random walk path length in the blanket must be long enouch to ensure 99% absorption. Otherwise the fuel breeding efficiency will be poor. As a neutron proceeds along its random walk path it loses kinetic energy, so the neutron absorption cross section changes. Hence the neutron random walk path length in the blanket must take this change in absorption cross section into account.

Neutrons that are not absorbed in the blanket must be totally absorbed in the liquid sodium guard band.

The combination of the core zone diameter, the blanket thickness, the liquid sodium guard band thickness and the intermediate heat exchanger thickness set the primary sodium pool dimensions, which set the reactor cost. Thus from an overall system cost perspective the required blanket thickness is an important reactor design constraint. The required blanket thickness is relatively independent of reactor power. Hence from a cost perspective it is not practical to make very small fuel efficient breeding FNRs.

The average concentration of U-238 atoms in the blanket is a function of the blanket fuel design. This concentration determines the rate of absorption of neutrons along a neutron random walk path. Since the heat dissipation in the blanket zone is much less than in the core zone it is feasible to make the perimeter blanket fuel tubes larger in diameter which potentially increases the average U-238 concentration in the perimeter portion of the blanket. Whether or not there is any real economy in having different fuel tube sizes remains to be determined. Different fuel tube sizes complicate automation of the fuel manufacturing and fuel reprocessing.

This web page last updated November 23, 2018

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