CONTAINMENT STRUCTURES
© M. Ragheb
4/19/2011
INTRODUCTION
A misconception about nuclear power plants containment structures is that their massive concrete construction is a protection against the release of radioactive products in the case of a postulated accident.  Such a task is achieved by the overall containment system as a collection of the “Engineered Safety Features,” not just by the concrete shell alone.
It must be understood that the concrete component is meant as a biological shield against gamma-ray radiation and a protection of the reactor internals against damage from the effects of the outside elements including missiles such as light posts driven by tornado or hurricane 100-miles per hour winds, and even the direct impact by a massive aircraft such as a Boeing-747.
The concrete shell in fact is strong at its exterior curvature, and weak at its interior curvature.  This is an inherent characteristic of shell structures.  Think about how difficult it is to crush an egg by squeezing it in
one’s hand, yet it is easy for the weak and helpless chick to peck its way out of the interior of an egg’s shell.reactor pressure中文
The concrete shell is designed to withstand the direct impact of an aircraft on its exterior, but miserably fails a buildup of stress at its interior.  An increase of stress by steam release, if unquenched, at its interior will eventually cause it to fail; much like a chain at its weakest link.  The weakest links in that case occur at the coolant inlet and outlet pipes and the instrumentation cabling and electrical power penetrations.
Figure 1.  Large 22 in Liner Tear near a containment scale model piping penetration.
Source: Sandia Laboratory.
PRESSURIZED WATER REACTOR, PWR CONTAINMENT SYSTEM
PWR designs are surrounded by a containment system with multiple Engineered Safety Features, ESFs.  A dry containment design is shown in Fig. 2, and consists of a steel shell containing multiple safety systems.  A concrete biological shield surrounds the steel shell.  The biological shield is meant to protect against the outside elements and is not meant as a barrier against the release of radiation.  For instance it is designed to withstand a direct hit by a Boeing-747 aircraft, and a 100-miles per hour missile such as a light pole driven by tornado or hurricane winds.
Figure 2.  PWR dry steel shell containment surrounded by concrete biological shield.
Another PWR containment design is shown in Fig. 3, where an ice condenser is used to quench any release of radioactivity or steam caused maybe by an earthquake event.
Figure 3.  Sequoia Unit 1 PWR ice condenser containment.
The containment contains the various circuits for emergency core cooling water injection into the primary system.  These include:
1. The accumulators which are large vessels containing water under nitrogen gas pressure.  They are connected to the primary system by automatic valves, which open if an accident occurs that reduces the primary system pressure below 40 bars.
2. The High pressure Injection System (HPIS) allows pumping of water into the system at
a high head or pressure of about 100 bars but at low flow rates.
3. The Low pressure Injection System (LPIS) allows water to be pumped at a low head or pressure below 30 bars at high flow rates.
4. A containment spray system to quench any released steam in the case of an accident.
Figure 4.  Containment spray and system safeguards components.
Table 1.  Functions and types of essential equipment in drywell and the containment
structure.
Figure 5. Controlled diffusion flame burning of hydrogen released in a reactor accident.
hydrogen ignition at the Three Mile Island accident.

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