https://youtu.be/ceQQE59M9UQ
Inspector Slag looks up as Marmaduke the Radiation Protection Technician begins the entry briefing. “It is late in this refueling outage,” he says, then pauses and slowly sips his coffee, “and we need to inspect the Sand Pocket Drains, deep inside the Reactor Vessel Containment Building and Torus cavity…” Okay, being an engineer is sometimes very much like “The Maltese Falcon” detective stories where Sam Spade investigated the scene, gathered evidence, researched the facts, solve the mystery and apprehend the culprit.
In 1980 the Nuclear Boiler Water Reactor industry discover wall thinning of the drywell bio-shied liner after 11 years of operation, due to leakage from the cavity seal during refueling operations. This condition was not unexpected by the General Electric original design team for the “Sand Pock Drains”. However, the “Sand Pocket” functions as one of the structural supports for the drywell, while the air gap provides some independence between the reactor building structure and drywell biological shield wall. The drywell annulus (air gap) is primarily provided to assure no unrelieved deflection resulting from the temporary concrete placement loading during construction.
The Drywell Sand Pocket functions as one of two primary load transfer points to the Reactor Building structure. These two locations are designed to minimize stress concentration, either by reinforcement rings such as on the Drywell cylinder around the stabilizer support or by a sand cushion zone at the Drywell bottom support. In addition, the sand pocket was specifically designed to prevent water logging based on the depth and width provided. The approximate volume of the sand pocket is 867 ft3, but is occupied by a sand filter (per ASTM G-33 grading requirements) that is 44% void (or 2854 gals) and 55% solids, representing a void ratio of 80%. In the case of leakage into annulus between the Drywell and Reactor Building, this gap also functions as a drain into the sand pocket area through 1” diameter holes spaced at 18” centers (92 holes, total flow area 73 in2). The sand pocket has eight 4”diameter, schedule 40 drain lines (total area 101.84 in2) that discharge into the Reactor Building Torus area with a slope of 1/8.75, which have a permeability coefficient of 0.84 in/min and represents a flow rate conservatively at 9.8 in3/min (0.3 gals/hr).
In the case of a single Drywell sand pocket drain line is found to be obstructed, then the potential exists for a build up of moisture in the immediate sand pocket area. However, because of the concentric continuity of the sand pocket design and as long as adjacent Drywell Sand Pocket Drain line(s) are functional, any significant moisture would be drained by these nearby lines, removing leakage water and precluding any potential impact the Drywell liner. As previously demonstrated, having four of the eight Drywell sand pocket drain lines functional was sufficient to preclude an environment that could lead to loss of material to these inaccessible areas of the drywell bio-shield liner.
The worst-case condition is when all lines are plugged and subsequently the air gap and sand pocket is full of pure water. This condition does not challenge the primary design function for stress distribution of the Drywell liner. However, this pure water environment condition for a non-alloyed material (ASTM SA 516) does result in an active corrosion mechanism and has the potential for material loss of about 25 mills/year, excluding any additional mechanisms.
When considering the industry experience, it would suggest that even under a worst-case general area material loss condition of about 25 mils (0.025 inches) assumed to occur over an operating cycle, there would be an insignificant challenge to the integrity of the Drywell liner shell, which has a nominal thickness of 1.5 inches. Therefore even if the worst-case rate of corrosion were assumed since the last inspection, the loss of material of the drywell shell would not reach minimum wall thickness or threaten structural integrity.
Inspector Slag slumps into the briefing room chair and turns to Marmaduke. “Thank you for guiding us through the wells of the reactor cavity area. The mystery of the Sand Pock Drains has been solved and the bio-shield liner-thinning perp has been detained.”