4.2.
The ultimate heat sink is the medium into which residual heat is discharged in the different plant states after shutdown of the reactor, and it normally consists of a large body of water or the atmosphere, or both. The body of water can be a sea, a river, a lake, a reservoir, groundwater or combinations of these, but in general access to inexhaustible, natural supplies of water is preferable to limited capacities. For an ultimate heat sink that relies on the atmosphere, cooling towers or spray ponds, with their associated structures and systems, are the usual equipment designed to transfer heat to the atmosphere. Some passive reactor plant designs also rely exclusively on the atmosphere for dissipating reactor decay heat immediately following plant transients and accident conditions. The medium used as a receptor for the decay heat can also be used as a source of cooling for turbine condensers during power operation; however, the associated heat transfer systems are out of the scope of this Safety Guide.
4.3.
For a site with multiple units, the items important to safety designed as interfaces with the ultimate heat sink medium should be specific to each unit.
4.4.
The capacity of the ultimate heat sink should be adequate to absorb decay heat from all the different reactors and spent fuel pools at the site. This capacity should be designed taking into consideration that all units could be in accident conditions simultaneously.
4.5.
The reliability and capacity of the ultimate heat sink should be ensured for both the short term and the long term, taking into account all the relevant heat loads generated during normal shutdown modes, anticipated operational occurrences and accident conditions, the rates of heat rejection during those conditions and relevant regulations pertaining to environmental protection.
4.6.
The short term and long term capacity of the ultimate heat sink should be preferably achieved by the use of inexhaustible, natural supplies of water or the atmosphere. Where access to an inexhaustible supply of water or the atmosphere at the site is not available:
The capacity of the ultimate heat sink should be ensured by an adequate amount of water always being available at the site. This capacity should be adequate to absorb all heat loads generated at the site until the heat sink can be replenished.5 Account should be taken of factors that could delay the replenishment process. Such factors include evaporation, human induced events, natural hazards, accident conditions at the plant, availability of interconnections and the complexity of the procedures for replenishment.
A minimum amount of water, including a margin for uncertainties, should be immediately available to bring reactors to the safe shutdown state in the event of any postulated initiating event. For each unit, this minimum quantity6 should already be stored in the basins of the cooling towers or spray pounds dedicated to the unit.
Beyond this minimum capacity, the additional water needed prior to replenishment could be stored in an on-site reservoir, with the possibility to transfer this water from the on-site reservoir to the ultimate heat sink. This transfer system should be considered as a support system to help to fulfil the safety function of the ultimate heat sink and should be safety classified accordingly.
In terms of the long term capacity of the ultimate heat sink, the make-up systems to replenish on-site reservoirs should be permanently installed and should be designed with an adequate rate to meet the long term heat removal capacity.
4.7.
To fulfil the design objectives in terms of capacity and reliability and to apply the concept of defence in depth (see Requirement 7 of SSR-2/1 (Rev. 1) [1]), the use of a different ultimate heat sink or different access to the ultimate heat sink might be necessary (see para. 6.19A of SSR-2/1 (Rev. 1) [1]).
4.8.
Structures associated with the ultimate heat sink should be designed to withstand the loads caused by the hazards derived from the hazard evaluation for the site. Recommendations on the consideration of external events (e.g. extreme temperatures and conditions, frazil ice, ice cover, floods, tsunamis, high winds, biological phenomena, collisions with floating bodies, clogging, low water levels, sand and sludge silting, and events involving hydrocarbons) in the design of such structures are provided in NS-G-1.5 [10].
4.9.
The provisions ensuring the effectiveness and availability of the ultimate heat sink with regard to the site’s natural hazards should be designed with adequate margins to cope with levels of natural hazards exceeding those derived from the hazard evaluation for the site (see para. 5.21A of SSR-2/1 (Rev. 1) [1]).
4.10.
In determining the necessary capacity of the ultimate heat sink, design basis environmental parameters should be defined with account taken of the time periods during which those conditions are assumed to exist (see NS-G-1.5 [10]).
4.11.
The effectiveness of the ultimate heat sink should not be unduly affected by short term variations in the environmental parameters.
4.12.
The design basis environmental parameters should include the water temperature of the ultimate heat sink for ‘once-through’ water cooling systems and the dry bulb temperature of the air for dry cooling towers. Both wet bulb and dry bulb temperatures are necessary environmental parameters for wet cooling towers, cooling ponds and spray ponds and other heat transfer systems that use evaporative cooling.
4.13.
It should be ensured that the capability for heat load rejection is maintained following any interruption of power generation or loss of operability of normal heat removal systems.
4.14.
The ultimate heat sink should be designed to be capable of absorbing the relevant heat loads at the maximum peak heat rejection rate for the different plant states, with the time dependent behaviour of the individual heat loads taken into account.
4.15.
In establishing the maximum heat rejection rate, the most severe combination of individual heat loads should be identified for all postulated initiating events for which the system is called upon to perform a normal operation or to fulfil a safety function.
4.16.
In determining the capacities demanded of the ultimate heat sink and its directly associated heat transfer systems, the various heat sources and their time dependent behaviour should be precisely identified to ensure that the temperature of the coolant remains within specified limits. The heat loads that should be taken into consideration include the following:
Residual heat of the reactor coolant system;
Decay heat of the spent fuel with the storages at maximum capacity;
Heat generated by the operation of structures, systems and components to achieve and maintain a safe plant shutdown or to mitigate the consequences of an accident (if heat produced by the components is transported by the residual heat transfer chain);
Heat from other accident related heat sources (e.g. chemical reactions).
4.17.
In establishing the residual heat loads of the reactor (including decay heat, heat due to fission during shutdown, and the energy stored in the reactor coolant system and other operated heat removal systems or structures), it should be assumed that the fuel has been exposed to operation at power for a period of time that would produce the maximum decay heat load. The decay heat should be evaluated consistently with applicable standards.
4.18.
The total heat load and the rejection rate of heat from spent fuel should be evaluated on the basis of the maximum number of spent fuel elements that can be stored at the site at any one time. Either the decay heat curves for the particular fuel (with appropriate individual post-shutdown times applied to the various fuel elements) or a conservative average post-shutdown time for all fuel elements should be used.
4.19.
Accident conditions might produce additional sources of heat, such as the heat emanating from metal–water reactions of the fuel cladding or from other heat producing chemical reactions within the containment. If potential metal–water reactions are determined to be significant as an additional heat source, then they should be quantified as a function of time and included in the sizing criteria.