Boiling water reactor containment

TAEA participated to the OECD/NEA International Standard Problem No 42 (ISP-42) which is hosted by the Paul Scherrer Institut (PSI), Switzerland. The ISP-42 test was performed in the PANDA test facility, at the PSI, as a sequence of Phases A through F, representing typical passive safety system operating modes covering certain specific phenomena. The configuration used for ISP-42 was corresponding to the European Simplified Boiling Water Reactor containment and passive decay heat removal system at about 1 40 volumetric and power scale, and full scale for time and thermodynamic state. 

To prevent such release, off gases are treated in Charcoal Delay Systems, which delay the release of xenon and krypton, and other radioactive gases, such as iodine and methyl iodide, until sufficient time has elapsed for the short-Hved radioactivity to decay. The delay time is increased by increasing the mass of adsorbent and by lowering the temperature and humidity for a boiling water reactor (BWR), a typical system containing 211 of activated carbon operated at 255 K, at 500 K dewpoint, and 101 kPa (15 psia) would provide about 42 days holdup for xenon and 1.8 days holdup for krypton (88). Humidity reduction is typically provided by a combination of a cooler-condenser and a molecular sieve adsorbent bed. 

By contrast, uranium fuels for lightwater reactors fall between these extremes. A typical pressurized water reactor (PWR) fuel element begins life at an enrichment of about 3.2% and is discharged at a bum-up of about 30 x 10 MW-d/t, at which time it contains about 0.8 wt % and about 1.0 wt % total plutonium. Boiling water reactor (BWR) fuel is lower in both initial enrichment and bum-up. The uranium in LWR fuel is present as oxide pellets, clad in zirconium alloy tubes about 4.6 m long. The tubes are assembled in arrays that are held in place by spacers and end-fittings. 

The General Electric simplified boiling-water reactor (SBWR) of lower (600 MWe) power features natural ckculation of the coolant rather than the usual forced ckculation. Use is made of a water reservok and pools for emergency cooling of the reactor and the containment building ak. 

Windscale reactors, and some US reactors of the 1950 period, the fuel was cooled by air blown straight to atmosphere, and no use was made of the heat to produce power. In all power reactors now operating, the coolant is contained in a closed-circuit pressure vessel. Outer containment buildings, which can also withstand some pressure in the event of failure or leakage from the pressure circuit, enclose the US pattern pressurised water and boiling water reactors, but no such provision was made for the Russian boiling water reactor at Chernobyl. All defences (cans, pressure vessel, containment building if provided) must be breached before fission products can be released to atmosphere. 

The fuel rods for boiling and pressurized water reactors are constructed similarly. They are filled with helium to improve the heat transfer from the pellets to the cladding tube and to withstand better the pressure in the reactor and contain no fuel at the top end of the fuel rods to improve fission gas retention. The latter can be ensured by holding the fuel in place with the aid of a spiral spring. Both ends of the cladding tube are welded gas tight. The fuel rods for pressurized water reactors are manufactured with a helium pressure of ca. 23 bar and ca. 5 bar for fuel rods for boiling water reactors. 

Zr-2.5 Nb [zirconium alloyed with 2.5 w/o (weight percent) niobium] has better mechanical properties than zircaloy, but is conoded more rapidly by water containing oxygen, such as is found in boiling-water reactors. It was the material prefened in 1971 [El] for pressure tubes in Canadian pressurized-water reactors. 

The example is a passive type boiling water reactor of 600 MWe, provided with a double containment and a stack. 

Recent boiling water reactor (BWR) designs with internal pumps or natural circulation, which eliminate bnUcy external reactor equipment like jet pumps and steam generators, permit extremely compact containment buildings. Because modular construction technology can be readily applied to these BWR containments, the qnantities of material and constrnction times are reduced substantially. In the absence of snbsidies, BWRs can be expected to increasingly dominate the economic competition in the LWR market. 

This report examines the severe accident sequences and radionuclide source terms at the Sizewell pressurised water reactor with a prestressed concrete containment, the Konvoi pressurized water reactor with a steel primary containment, the European Pressurised water Reactor (EPR) and a boiling water reactor with a Mark 2 containment. 

Table 11-3. Accident sources to the containment for boiling water reactors [S-2]... 

This report examines the severe accident sequences and radionuclide source terms at the Sizewell pressurised water reactor with a piestressed concrete containment, the Konvoi pressurized water reactor with a steel primary contaimnent, the European Pressurised water Reactor (EPR) and a boiling water reactor with a Mark 2 containment. The report concludes that the key accident sequences for European plant designs are transient events and small loss-of-coolant accidents, loss of cooling during shutdown, and containment bypass sequences. The most important chemical and transport phenomena are found to be revaporisation of volatile radionuclides from the reactor coolant system, iodine chemistry, and release paths through the plant. Additional research is recommended on release of fission products from the fuel, release of fission products from the reactor coolant system, ehemistry of iodine, and transport of radionuclide through plants. 

The quintessential engineered system for the removal of aerosol particles is the spray found in the containments of many pressurised water reactors and in the drywells of many boiling water reactors. Sprays remove particles by ... 

The reactor module components are contained within three steel pressure vessels the reactor vessel, a steam generator vessel, and connecting cross-vessel. The uninsulated steel reactor pressure vessel is approximately the same size as that of a large boiling water reactor (BWR) and contains the core, reflector, and associated supports. The annular reactor core and the surrounding graphite reflectors are supported on a steel core support plate at the lower end of the reactor vessel. Top-mounted penetrations house the control-rod drive mechanisms and the hoppers containing boron carbide pellets for reserve shutdown. 

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