Long-term cooling

Ice water baths are commonly used to keep reaction temperatures around 5 to 30 Celsius. Ice water baths are used in place of cold-water baths where long term cooling, but a slight colder temperature is needed. 

USNRC Branch Technical Position ASB 9-2, "Residual Decay Energy for Light-Water Reactors for Long Term Cooling," USNRC 1981. 

The core geometry remains coo I able for long term cool ing... 

The PDHR system is employed for both the hot stand-by and long-term core cooling modes. This system can operate at full reactor coolant system pressure and places the reactor in the long-term cooling mode immediately after shutdown. 

The operation in the long-term cooling mode is automatic. 

The core is re-flooded in some tens of seconds (when the fuel reaches its worst conditions in the transient) then, the core cools steadily. The operators then initiate the long-term cooling procedure. 

A mixture condenser of the indicative volume of about 1500 m, of which 500 m are initially occupied by borated water at 2000 ppm boron and the rest by nitrogen. The function of this component is the collection of the fluid discharged by the primary system and the confinement of the fission products contained in it, the dissipation of its thermal energy to the outside environment and the formation of an additional reserve of water for the long-term cooling of the core, that is beyond the 10 hours (by natural or forced circulation, using the low power pumps, according to the... 

The residual heat removal system (RHRS) is used for controlling the water temperatures in both the RPV and CV for long term cooling after LOCA and is also used for long term decay heat removal during scheduled reactor shutdown... 

Unresolved Safety Issue (USI) A-31 in NUREG-0933 (Reference 1), addresses the safe shutdown of the reactor, following an accident or abnormal condition other than a Loss of Coolant Accident (LOCA), from a hot standby condition (i.e., the primary system is at or near normal operating temperature and pressure) to a cold shutdown condition. Considerable emphasis has been placed on long-term cooling which is typically achieved by the residual heat removal system which starts to operate when the reactor coolant pressure and temperature are substantially lower than the hot-standby values. 

Even though it may generally be considered safe to maintain a reactor in a hot-standby condition for a long time, experience has shown that there have been abnormal occurrences that required long-term cooling until the reactor coolant system was cold enough to perform inspection and repairs. For this reason, the ability to transfer heat from the reactor to the environment, after a shutdown resulting from an accident or abnormal occurrence, is an important safety function. It is essential that a power plant be able to go from hot-standby to cold-shutdown conditions subsequent to any accident or abnormal occurrence condition. 

The CSS is physically separated from the shutdown cooling system in that a dedicated containment spray pump and heat exchanger are provided in each CSS train. This feature and the IRWST eliminate the need to automatically realign the CSS as recommended by SRP Section 6.5.2 to accomplish long-term cooling, since the CSS short-term injection and long-term recirculation are accomplished with the same CSS line-up. 

Adequate long-term cooling capability of the fuel following the LOCA shall be ensured. 

The ECCS must be capable of providing long term cooling such that core temperature remains at an acceptable low value as decay heat is generated. 

The analyses of severe accidents are used to identify shortcomings in the provisions for prevention and mitigation in plant s defence in depth in case the engineered safety features fail to control accidents to be coped within the design basis envelope. Accident management measures are considered the main tools to monitor the plant status, to ensure long-term cooling to prevent recriticality and to confine radioactivity as far as possible. 

The evaluation of containment flooding events addresses the impact of flooding on the SSCs. The APIOOO passive core cooling system, the internal containment compartments, and the equipment locations are designed for internal flooding to maintain post accident long-term cooling flow to the reactor core from the flooded volumes. 

The large-break LOCA methodology considers uncertainty in the decay power level. The small-break LOCA events and post-LOCA long-term cooling analyses use 10 CFR 50 Appendix K, deeay heat, which conservatively assumes infinite irradiation time before the eore goes suberitieal to determine fission product decay energy. 

The in-contaimnent refuelling water storage tank can provide sufficient injection until the contaimnent sump floods up high enough to initiate recirculation flow. In this long-term cooling mode, the core is covered and the steam boils off into the contaimnent. The water in the containment sump provides recirculation flow to the core through the direct vessel injection line. 

After condensation of steam occurs on the steel containment vessel, which is cooled by the passive containment cooling system, the condensed water drains down through the containment wall and it is collected by the containment recirculation, thus providing sufficient makeup water for long term cooling. 

The depressurization phase is followed by the long term cooling phase where the RV and CV pressure is slowly reduced as the core decay heat decreases. 

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