1. Field of the Invention
The present invention relates to a nuclear reactor cavity radiation streaming shield which is mounted near the top of the reactor pressure vessel so as to continuously attenuate radiation streaming up the annulus between the external wall of the pressure vessel and the reactor cavity wall while preventing pressure buildup after a loss of coolant accident. More particularly the invention relates to a radiation shield which is porous to vapor escaping from a reactor following a loss of coolant accident (LOCA) and which continuously attenuates the gamma and neutron radiation from the reactor.
2. Description of the Prior Art
At page 111 of the August 1976 issue of Nuclear News, the advertisement of Permali, Inc., Mount Pleasant, Pa. 15111 provides an excellent example of the status of the present art.
"With the existence of a large annulus between reactor vessel and primary containment shield wall in many pressurized water reactors (PWRs), an unacceptably high dose rate of neutrons can occur due to scattering or streaming. By the utilization of a Permali shield collar at the elevation of the reactor vessel head, however, such neutron streaming can be held within permissible limits.
Constructed of an efficient composite of both neutron-attenuating Permali NH and boronated/neutron-capturing Permali JN phenolic-impregnated densified laminates, shield collars can be fabricated to extremely precise tolerances. In addition, the Permali collars are manufactured in large segments to facilitate their removal during refueling."
The key phrase of this statement is, "By the utilization of a Permali shield collar at the elevation of the reactor vessel head . . ." It is obvious that prior to head removal from the pressure vessel, this segmented shield collar must be removed.
There are other forms for these shield collars. Some shields generally resemble the Permali shield. They may be formed of blocks or bags of shielding material which are stacked on platforms above the annulus and at the level of the reactor vessel head. In whatever form, the units of radiation-scattering material interfere with the removal of the pressure vessel head when gaining access to the internals of the pressure vessel.
The shield structure should be positioned in the top of the annulus so the pressure vessel head may be removed without first moving the shield. However, this position includes the problem of compensating for a loss of coolant accident (LOCA). Upon a LOCA, the buildup of pressure beneath present shield structures could create large projectiles of the shield material and possibly the reactor pressure vessel.
The problem then is to form a shield at the top of the annulus, and below the pressure vessel head, which will attenuate radiation streaming up the annulus and readily pass vapor released in the annulus during a LOCA.