as shown in fig1 a conventional pressurized - water reactor steam generator comprises the pressure vessel or housing 1 having its lower portion or primary header provided with an inlet nozzle 11 and outlet nozzle 12 by which the generator is connected so it forms part of the reactor &# 39 ; s coolant loop . a feed - water nozzle 13 feeds into the pressure vessel or housing above the horizontal tube sheet 14 for heating by the u - tube bundle 15 , the steam exhausting through the steam output nozzle 13a in the steam dome or top of the vessel or housing 1 . beneath the tube plate 14 , a partition 11a separates the incoming coolant from the outgoing coolant , the coolant flowing through the insides of the tubes of the tube bundle 15 . it is to be assumed that as a first step a rough examination of the interior of the tubes of the tube bundle , is made during a reactor shut - down . this may be done by the use of optical equipment , magnetic test methods and the like . if the rough examination provides phenomena which is not clearly recognizable , a detailed investigation of the interior wall condition of the tube zone arousing suspicion , can be made by the practice of the present invention . such a zone is , for example , indicated at a in fig1 . in fig2 the apparatus of the present invention is shown . the two inflatable dams 2 , made of an elastic material adequately impervious to the elastomer used , have first been blown into the central portion of the zone a , the dam 2a being the leading tube and the dam 2b being the trailing tube . the dams are respectively provided with inflating tubes 4a and 4b , each is shaped like a donut and each has its inner periphery provided with a closing disk 21a and 21b , respectively . both inflating tubes are flexible and the tube 4a functions to flexibly interconnect the two dams by a tube length proportioned to slightly exceed the extent of the suspected zone a . when pneumatically blown into position by equipment described hereinafter , the two dams are lightly inflated and when in position , their inflating pressure , using air , for example , can be increased . the disk 21b of the trailing dam is penetrated by the elastomer transport tube 3 , which is a flexible tube , and like the tubes 4a and 4b , fixedly connects with the dam members . the disks 21a of the dam 2a is provided with vent tube 5 . as shown by fig2 the elastomer under pressure has been transported through the open end of the tube having the zone a , through the end of the tube opening below the tube plate 14 , it being understood that below this tube plate the bottom portion or primary header of the steam generator , has the usual manholes through which personnel can gain access to the open end of the cavity formed by the heat - exchanger tube under inspection . the introduction of the pressurized elastomer through the tube 5 into the space between the two dams has forced them apart within the limits permitted by their flexible interconnection in the form of the tube 4a , and in fig2 the apparatus is now confining the elastomer between them , for the curing period . in fig2 the elastomer itself is not shown , to avoid confusing the illustration . the inflating pressure within the two dams , although possibly increased to provide fluid - tight sealing , is not adequate to anchor the two dams against being displaced by the pressurized elastomer . in fig3 the leading dam 22a is shown as comprising a single inflatable balloon - like dam , venting being possible by leakage around its periphery and using inflating pressure having a value permitting this effect as required for venting of air displaced by the injected elastomer . in the modification of fig4 the inflatable dams are replaced by solid elastically deformable members 23a and 23b , such as may be formed by using a material having the consistency of rubber . also , in this case , it is the elastomer transport tube 31 which provides the interconnection between the two dams for limiting their separation distance displaced by the injected elastomer , the tube 31 having a port 32 opening between the two dams . in fig5 the open end of the heat - exchanger tube 15 under inspection , is closed by a plug 6 through which a compressed gas tube 61 extends from a blower 62 or other source of compressed gas , this being used to blow or pneumatically convey the two dams to within the zone a of the heat - exchanger tube . a blower 41 is connected with the dual inflating tube 4c , which may contain the tubes 4a and 4b , for inflation of the two dams when they are of the inflatable type . finally , a pump 33 supplies the elastomer transport tube 3 with elastomer contained in a supply tank 34 . in operation , the blower 62 is operated first to blow or pneumatically convey the two dams to the location where the casting is to occur . the blower 41 is used to adjust the pneumatic dams to the desired and possibly higher sealing pressure . in all cases , the dams are radially elastically deformable so that they can be blown to position and thereafter provide sealing . finally , the pump 33 is operated so that elastomer injected between the two dams , forces the dams apart so that the tube fills with the elastomer and is cast against the inner surface of the tube throughout the zone a . after curing , which preferably is one at room temperatures or at least at no higher temperatures than permits the use of the apparatus , one or another of the tubes 4c or 3 is pulled to remove the cured casting , the plug 6 being , of course , removed at that time . if the elastically hardenable plastic used , such as in the case of the low - consistency silicone elastomer , might possibly bond to the interior surface of the tube , that surface is first coated with a parting material , such as glycerin , prior to the casting of the elastomer . this may be done by a suitable swab introduced to the zone a with the swab saturated with glycerin . as previously indicated , the casting elastomer may be combined with reinforcing fillers to increase the strength of the casting when it is pulled from the casting location . the inflatable dams described have the advantages that when it is time to pull the cured casting from the tube , their inflating pressure can be released so that the pulling tension is applied substantially entirely to the casting itself , causing the casting to stretch and release itself from any depressions or cracks in the inside of the tube , which the elastomer may have penetrated , the tube which interconnects the two dams to limit their separation , usually itself being somewhat elastically deformable and stretching with the casting . the technology of compounding elastically hardenable plastics , including silicone elastomers of the low - consistency type , is well known . the requirements for the present invention are that when uncured , the elastomer should be flowable so that it can be transported through its transport tube to the casting location , and it should cure at the normal temperatures existing during inspection of a pressurized reactor steam generator , when the reactor is shut down , such temperatures nominally being room temperatures . the curing time is not of great importance other than that it should be short enough for convenience .