58 bracing means which are in turn connected at 60 to strut means 62 supported by the cavity wall and flange preferably by two struts per basket the neutron attenuating lumps 44 are preferably made of type 277 shielding material manufactured by reactor experiments inc of san carlos calif the material is heat resistant borated hydrogenated and can operate continuously at 350 u00b0 f it maintains its structural integrity up to 2000 u00b0 f the density is about 60 higher than water the lumps are preferably spherical and may range in size from about one half inch to about two inches in diameter the depth within the baskets 42 is determined by the shielding requirements but typically is in the range of 24 to 30 inches other materials such as type 236 boro silicone shielding available from the same company are also suitable the above materials have a range of hydrogen content of about 3 3 3 9 u00d7 10 22 atoms per cc and a boron content in the range of about 1 0 to 1 5 u00d7 10 21 atoms per cc the macroscopic thermal neutron cross section ranges from about 0 7 to about 1 1 cm 1 with an overall density of about 1 7 grams per cc the compressive strength ranges from about 5 50 psi to about 1500 psi and the tensile strength ranges from about 59 psi to about 100 psi the desirable characteristics of these and equivalent materials include not only a high hydrogen content and preferably boron content for attenuating neutrons but also small size and relatively low mass this permits the baskets to absorb a great deal of any energy that may be released during a nozzle break as previously described with the.