Decay heat removal

LQCAs with failure of fang-term decay heat removal (LTDHR) Inc. event V 59%... 

Functional and hardware relationships between systems are considered in selecting the order of event tree headings. Systems that depend on the operation of other systems in order to perform their function should be listed after the other systems. For example, the decay-heat removal system... 

Issue Resolution, JCOs, Plant Modifications EOOS performs sensitivity analyses to determine the significance of e. Ihe removal of offsite power or decay heat removal equipment. 

The only human actions important in more than 50% of the BWR IPEs are manual depressurization, containment venting, initiation of standby liquid control (SLC), and system alignment for decay heat removal. In PWRs, only switch over to recirculation, feed-and-blecd, and the actions associated with depressurization and cooldown are important in more than 50% of the... 

Av.iil.ihility of an isol.ilion condenser in older BWRs for sequences with loss of decay heat removal (DHRl... 

The in-conlainmeni refueling water storage tank provides 500,000 gallons (about 1900 cubic meters) of water with a gravity head above the core. This water inventory is sufficient to flood the containment above the level of the reactor core and provide decay heat removal by natural circulation. 

There were studied two types of SNF canisters storage in steel tubes, placed in the air medium ( CASCADE type) and in steel tubes, placed in a concrete body inside a built-in structure. The modeling of radioactive decay heat removal was implemented based on the initial integral value of RHR of NP SNF by 2010. 

A two-zone core design is adopted, with one central movable column of graphite spheres surrounded by pebble fuel elements. Tlie purpose of using the two-zone core design is to increase the power output of a single reactor module while maintaining the passive decay heat removal capability. 

Decay heat in fuel elements is assumed to be dissipated by means of heat conduction and radiation to the outside of the reactor pressure vessel, and then taken away to the ultimate heat sink by water cooling panels on the surface of the primary concrete cell. Therefore, no coolant flow through the reactor core would be necessary for the decay heat removal in loss of coolant flow or loss of pressure accidents. The maximum temperature of fuel in accidents shall be limited to 1 bOO C. 

The hot gas duct is represented by coaxial pipes between core and reformer. The iimer hot gas pipe is insulated on the inside and cooled by counterflow on the outside. Since the steam generator has to be on the alert in case of a decay heat removal action, the pipe system has to be safely constructed. In particular, fast pressure transients which could be destructive should be avoided by means of flow limitation. 

Thermal loads upon the process heat exchanger do not allow rapid temperature change rates. Therefore in case of the demand for a decay heat removal system after a fast shutdown of the reactor, an auxiliary cooling system should be used rather than the main cooling system. In addition, much of the decay heat would be removed via the core surfaces to the liner cooling system [10]. 

In 1982, the Research Center Jiilich presented the conceptual design of a 50 MW(th) nuclear process heat plant with a pebble-bed HTGR, named AVR-II, for which a safety-related study has been conducted [29]. Its characteristic features are a slim steel pressure vessel, no separate decay heat removal system, shutdown and control system via reflector rods, surface cooling system, and a simplified containment. The safety of the reactor is principally based on passive system feamres. 

Accident sequences investigated were decay heat removal without / with main cooling system. In the former case, both core and reformer are cutoff from the coolant circulation and a gradual temperature balance is obtained by heat conduction, radiation zind small internal convection. The activation of the main cooling system delayed by 1 h encounters a merely changed temperature distribution allowing for a smooth and gentle restart. 

The safety concept considers two nuclear shutdown systems, a set of six reflector rods for reactor scram and power control and a KLAK system of small absorber balls for cold and long-term shutdown. Decay heat removal is made via the heat exchanger, an auxiliary cooling system, and the panel cooling system inside the concrete cavern, or, in case of a failure of these systems, passively by heat transfer via the surface of the reactor vessel. 

The AHTR 500 is a further development of the HTR-MODUL design with 500 MW(th). The helium coolant is heated up from 330 to 950 °C. The system pressure is 2 MPa. The new feature of this reactor design is a central graphite column to provide an additional heat sink. It contains a passive decay heat removal system on the basis of natural convection which runs also during normal operation. No intermediate circuit is foreseen for the connection with a coal gasification system [41]. 

Two concepts of a He - He intermediate heat exchanger for a heat rating of 125 - 170 MW have been selected. For both, a 10 MW test plant has been operated in the KVK loop verifying the operation of reformers with convective helium. A 10 MW decay heat removal system cooler, hot gas ducts including insulation and liner, hot gas valves, and a steam generator were other components of the KVK loop. Furthermore, a helium purification system was operated in a bypass of the main system. Starting in 1982, the KVK facility was operated for 18,400 h with approx. 7000 h above 900 C [28]. Hot gas duct with internal insulation was operated at temperatures up to 950 °C. The KVK experimental loop has demonstrated reliability and availability even of newly developed components. 

PR on sodium circuits (secondary, auxiliary, decay heat removal)... 

The top shield includes roof slab and two rotatable plugs. Warm roof concept is adopted for top shield to minimise sodium deposition in the annular gaps. The roof slab is a box type structure filled with concrete as the shielding material. It supports the main vessel, primary sodium pumps, IHX, and direct reactor heat exchangers (DHX) of the decay heat removal system. Use of liquid metal seals has been avoided in order to reduce the rotatable support... 

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