Development of Safety-related Residual Heat Removal Chains from German Technology Pressure Water Reactors (Light and Heavy Water)

Introduction

The Nuclear Power Plants (NPPs) with Pressure Water Reactor for enriched fuel (PLWR, Pressurized Light Water Reactor) and for natural uranium (PHWR, Pressurized Heavy Water Reactor), developed in Germany, are largely identical in their basic design. However, there is a striking difference in the scope of the main reactor systems. While in PLWR these only consist of Reactor and Reactor Coolant System including Pressurizer and Pressurizer Relief Tank, in PHWR the Moderator System is added. In power operation of a PLWR, the entire thermal reactor power is transferred to the water/steam cycle via the Steam Generators. In PHWR, on the other hand, part of the power (approx. 10%) has to be removed – at a lower temperature level – from the moderator, which is spatially separated from the main reactor coolant within the Reactor Pressure Vessel, but is kept at the same pressure via function-related compensating openings. This portion of power is used to preheat the feed water before it enters the Steam Generators. The Moderator System installed for this purpose can also be used in a second function as the inner link in the Residual Heat Removal Chain (RHRC) for cooling the reactor after it has been switched off.  In PLWR the analog system is operated exclusively for the removal of residual heat from the reactor and, if necessary, the fuel pool. In the following, the development of the RHRC of both NPP lines is shown and the main differences between both NPP-types in this regard are explained by comparing the most recently erected plants, DWR 1300 MW (KONVOI) and Atucha 2.

Residual Heat Removal Chain, Structure and Terms

Figure 1: Residual Heat Removal from the Reactor; Definition of “System” (or “RHR link”), “Sub-System”, “RHR Line” and “RHR Chain”

Fig. 1 shows the basic structure of the three-part RHRC using the example of a plant with four cooling lines, as is the case with DWR 1300 MW and Atucha 2 (CNA 2). The figure also illustrates the terms „system“ (or “RHR link”), „subsystem“, „RHR line” (resp. “redundancy”) and „RHR Chain”.  While a “system” contains the entirety of all “subsystems” of an RHR link (horizontal unit), each “RHR line” is made up of three adjacent subsystems, from heat source to heat sink (vertical unit). All RHR lines together form the “RHR Chain” (although in normal usage a chain is understood to mean what is referred to here as a line).

The RHRC thus consists of three procedural systems, namely

• a circulation system for reactor coolant or moderator, connected to the reactor cooling loops or directly to Reactor Pressure Vessel *,
• an intermediate cooling system, which takes heat from it in heat exchangers, and
• transfers it in another heat exchangers via the so-called Secured Service Cooling Water System to the external heat sink. „Secured“ expresses that the system – like the entire RHRC – has a fail-safe design and that its electrical units can be operated via the NPPs Emergency Power Supply, if necessary.

In power plants where the service cooling water cannot be taken directly from a body of water (river, lake, sea), but in turn must be re-cooled e.g. in a cooling tower, the RHRC is in fact expanded by a fourth link. However, this is not explicitly included in the thermodynamic RHRC calculation. As with open circuit water cooling, it is then assumed that there is a fixed service cooling water inlet temperature which the re-cooling system has to implement or fall below on the basis of assumed external climatic conditions.

*   In the case of sump operation after loss of coolant, the extraction does not take place          from the reactor system, but from the floor (sump) of the reactor building interior. With          the PLWR, this is achieved by switching to a separate suction line in the intake to the             Residual Heat Removal Pump. The PHWR uses the Safety Injection Pump for this – possibly          in parallel operation with the Moderator Pump – which thus becomes part of the RHRC.

     In the further explanations and figures these RHRC special variants are not considered.

Temporal Development of the RHRC

The development steps up to the latest version of the RHRC for PLWR and PHWR go hand in hand with the chronological growth of the unit sizes of both NPP variants from the second half of the 1960s to the end of the 1980s (Fig. 2).

Starting with MZFR (multi-purpose research reactor Karlsruhe) as a prototype NPP of a PHWR and KWO (Obrigheim nuclear power plant) as a PLWR demonstration plant, the unit power outputs increased with almost constant gradients,

• at the PLWR version via KKS (Stade nuclear power plant) to the KWB-A plant (Biblis nuclear power plant, Unit A). This was followed by a consolidation phase with the construction of several (1200 to) 1300 MWel class NPPs before the step towards an EPR size of ≥ 1600 MWel was taken,
• at the PHWR with a significantly flatter course via Atucha 1 (CNA 1) to CNA 2 plant as the last NPP of this type to date.

Hereinafter, the RHRC concepts of all of the above power plants (without EPR) are shown in their original version. Later retrofittings, e.g. as adaptation measures to tightened safety regulations are not considered.

Figure 2: Temporal Development of unit net power of german-type PLWR and PHWR plants

In the first plants – both PLWR and PHWR – the single-line concept or multi-line in meshed construction was common for the systems of the RHRC. Here e. g. cross-connections between individual subsystems of an NKK element are established, via which, if necessary, a standby pump can optionally be connected to several circuits.  However, this design presupposes that failure of passive system parts, such as piping, does not have to be assumed. The extension of event scenarios to be controlled, initially concentrated on the double-ended rupture of a reactor coolant line, in particular the postulate that in the event of an accident, in addition to the maintenance or repair of a component, a single failure also occurs on any system part, led to the transition to the completely line-separated concept with three or four RHR-lines for the RHRC, depending on the unit size. This change took place step by step for both types of NPP, with the line separation developing from the inner to the outer link of the cooling chain, i.e. starting with the Moderator System (PHWR) resp. the Residual Heat Removal System (PLWR) up to the Secured Service Cooling Water System.

The following descriptions are exclusively in the present form, also for the plants that have already been decommissioned.

NPPs with Pressurized Light Water Reactors (PLWR)

Immediately after switching off a PLWR power plant, cooling of the reactor system basically takes place via the Steam Generators (exception: loss of coolant accidents above certain leak sizes). At the time, when cooling is taken over by the RHRC, the pressure and temperature of the reactor cooling circuit have already been reduced to such an extent, that the necessary design values ​​for the Residual Heat Removal System are significantly lower than those of the reactor system. The heat to be removed has sunk so far, that an intermediate cooling system, designed for low temperature and low pressure, as well as open to the surrounding room atmosphere, can be used on the secondary side of the Residual Heat Exchanger. This intermediate cooling system (called “Component Cooling System”) supplies further safety-related and operational cooling points in parallel to the Residual Heat Exchanger. If the NKK has a multi-line structure at least up to and including the Component Cooling System, then two component cooling subsystems are designed so that – alternating – they can supply cooling water to all of the operational cooling consumers (e.g. of Reactor Coolant Pumps and nuclear auxiliary systems) in addition to their line-associated safety-related cooling points.

NPP Obrigheim (KWO), 283 MWel

Figure 3: KWO, Reactor Coolant System and RHR Chain

The NKK is formed from one line, i. e. each NKK system from one circulation circuit.

The Residual Heat Removal System here includes two Residual Heat Removal Pumps connected in parallel and two Residual Heat Exchangers, both of which are integrated on their secondary side in the single component cooling circuit [1].

Special features of KWO are:

• The additional use of the Residual Heat Exchangers as low-pressure coolers within the Volume Control System (not shown in Figure 3),
• Two Emergency Secured Service Cooling Water Pumps (in addition to the regular

two Secured Service Cooling Water Pumps).

NPP Stade (KKS), 630 MWel

Figure 4: KKS, Reactor Coolant System and RHR Chain

The Residual Heat Removal System as the inner link of the RHRC is carried out in two subsystems, but is still mesh-designed with one Residual Heat Exchanger und two Residual Heat Removal Pumps each [2], [3], [4]. The other two NKK links consist – like at KWO – of only one circulation system each, but with special features.  

These are:

• Three Component Cooling Pumps,
• Three Component Cooling Heat Exchangers connected in parallel (which were probably activated as required),
• Two additional Emergency Component Cooling Pumps (not shown in Fig. 4),
• Three Service Cooling Water Pumps.

NPP Biblis Unit A (KWB-A), 1150 MWel

Figure 5: KWB-A, Reactor Coolant System and RHR Chain

With KWB-A, already in 1975 the RHRC took the shape, which subsequently – with a few safety-relevant additions – became the standard and has since been used for all following PLWR plants [5], [6]. The number of RHR lines usually, but not necessarily, corresponds to the number of reactor cooling loops. For this size of units (and also for the EPR concept (≥ 1600 MWel) four Steam Generators and thus four reactor cooling loops are required for heat transfer to the water/steam cycle in power operation. Accordingly, the RHRC also consists of four independent RHR lines with a heat transfer capacity of 50% each, based on the design case. (Note: Even for plants with only three reactor cooling loops, this “one to one” assignment of loop and line number can be obtained without violating the safety philosophy (repair and simultaneously single-failure) when the heat transfer capacity of each line is increased to 100%.) In Figure 5, the two inner Component Cooling Circuits are designed for the alternating supply of operational component points. For this purpose, in addition to the regular Component Cooling Pump, a second pump is connected in parallel, operated in case of a very high cooling water demand.

NPPs of DWR 1300 MW class

Figure 6: DWR 1300 MW, Reactor Coolant System and RHR Chain

The increasing safety-related requirements, set down e. g. in the „RSK Guidelines for Pressurized Water Reactors“ [7] and in „Safety Regulations of the KTA“ [8], [9], in particular

• elevated awareness of the fuel pool inventory as a source of activity, and
• the inclusion of „civilization-related external impacts“ (aircraft crash, explosion pressure waves, third part influences) as cases to be managed,

led to important extensions for the system technology of the steam generator feed as well as for the RHRC [10].

With the Emergency Feed Water System, a possibility of short and medium-term heat removal from the Reactor Coolant System via the Steam Generators was created, independent of the Feed Water Tank and the regular Emergency Power Supply. For the subsequent long-term cooling via the so-called Emergency RHR Chain in this two of the four RHR lines, whose residual heat removal circuits contain a Fuel Pool Cooling Pump, 

• an Emergency Component Cooling Pump within the Safety Component Cooling System*, and
• an Emergency Secured Service Cooling Water Pump in the Secured Service Cooling Water System.

are installed in parallel to the existing pumps.

The Fuel Pool Cooling Pumps themselves act as “Emergency Residual Heat Removal Pumps” as part of the Residual Heat Removal System in this case. If required, all this pumps are supplied with power via the Emergency Generators, which – after the Emergency Feed Water Pumps have been disconnected – are driven by the Emergency Diesel Engines.

For the fuel pool cooling, in addition to the two RHR lines that include the Fuel Pool Cooling Pumps, there is also another fuel pool cooling circuit whose Fuel Pool Cooler is supplied by the Operation Component Cooling System*. In this way, heat removal from the fuel pool is possible in principle via each of the four RHR lines.

* With introduction of the new “Power Plant Labeling System (KKS)” in 1976 the Component

   Cooling System was, without any technical impact, split into “Safety Component Cooling

   System” and “Operation Component Cooling System”. The former includes the Component     

   Cooling Pumps, the Component Cooling Heat Exchangers as well as the supply of all cooling    

   points that are relevant for operation of the RHRC. The latter only consists of the connected    

   pipe network, which distributes and collects the cooling water flows to consumers of    

   nuclear operating systems inside Reactor- and Reactor Auxiliary Building.

NPPs with Pressurized Heavy Water Reactors (PHWR)

The function of the Moderator System in power operation of the plant requires identical pressure and temperature design values as for the Reactor Coolant System itself. However, this also opens up the possibility – by switching over valves inside the Moderator System and with an appropriate design of the RHR Intermediate Cooling System as the middle link of the RHRC – to take over the cooling of the reactor immediately after shut down, even without additional Steam Generator feed. This option has not yet been implemented for the MZFR as the first PHWR plant. Only CNA 1 and CNA 2 are equipped with a high pressure/high temperature designed RHRC and are therefore independent of the function of the main heat sink (steam turbine condenser) for cooling down the plant after all shut-down occasions to be assumed.

Multi-purpose research reactor Karlsruhe (MZFR), 50 MWel

Figure 7: MZFR, Reactor Coolant System and RHR Chain

The shutdown concept of the MZFR basically corresponds to that of PLWR plants, with priority on the Steam Generators [11]. Only when this – below a certain coolant temperature – is no longer thermodynamically possible, switch over to RHRC operation has to be performed for further cooling of the plant. Moderator temperature and heat to be removed at this time are already so low that the Moderator Cooler on its secondary side can be operated with inlet temperatures, which are accepted by the other cooling points without boiling at its outlet; even at the slight overpressure with which the Component cooling System is operated.

A special feature of the MZFR-RHRC is that the operating pressure in the Secured Service Cooling Water is higher than in the Component Cooling System. In the event of a heat tube leak in the Component Cooling Heat Exchanger, transition of possibly radioactive contaminated water to the environment is thereby avoided, but pollution of the deionized water in the component cooling circuit may happen instead. In subsequent plants, the pressure gradation was implemented consistently from the heat source (high) to the heat sink (low).

NPP Atucha 1 (CNA 1), 319 MWel

Figure 8: CNA 1, Reactor Coolant System and RHR Chain

In contrast to MZFR, with CNA 1 one can already speak of an important step towards a multiple-line design of the RHRC. The Moderator System consists of two completely separate loops, each of which adjacent to a circuit of the RHR Intermediate Cooling System [12], [13], [14]. Deviating from MZFR, the task of this system is to be able to take over the reactor cooling already shortly after shut down of the reactor. The associated temperature and pressure values ​​in the system preclude the use of the Component Cooling System for heat removal; this is designed to only supply all other safety-related and the operational cooling points as a single circuit. It is fitted out with two Component Cooling Heat Exchangers and Component Cooling Pumps of full capacity each. The RHR Intermediate Cooling System is equipped with a third RHR Intermediate Cooling Pump. In the event of failure of one of the two regular pumps this additional pump takes over the circulation in the affected circuit. The principle of line separation for the RHR Intermediate Cooling System is not impaired by this. The return lines of the RHR Intermediate Cooling Circuits cannot be shut off to the area around the Moderator Cooler flowed through by the feed water during power operation, so that the feed water pressure is impressed on them in their standby state. After the feed water lines at the outlet of the Moderator Cooler have been shut off and transition to the RHRC cycle operation is completed, the water balance in the RHR Intermediate Cooling Circuits (absorption of expansion water when heating up, recovery of contraction water when cooling down) can be carried out via expansion tanks as well as discharges to the feed water tank on the one hand, and feed from the feed water tank or the deionized water tank by means of system-associated pumps on the other hand.

A line assignment has not yet been made for the outer RHRC link, the Secured Service Cooling Water System. Three parallel Secured Service Cooling Water Pumps can feed a manifold, from which all intercoolers as well as the Fuel Pool Coolers are supplied.

NPP Atucha 2 (CNA 2), 692 MWel

Figure 9: CNA 2, Reactor Coolant System and RHR Chain

A clear line separation concept has been implemented at CNA 2. Although the plant only has two reactor cooling circuits, the Moderator System and the entire RHRC are constructed with four lines, each of them having a capacity of 50 % of the total power to be removed in the design case. Thus the „repair + single-failure“ criterion for accidents is fulfilled. Not only the RHR Intermediate Cooling System, but also the Safety Component Cooling System here consists of four circuits, which supply the respective associated consumers – i. e. pumps and their motors – with cooling water. The circuits of the two outer redundancies in Fig. 9 can also be optionally switched on to cooling points of the fuel assembly transport devices (not shown in Fig. 9). One circuit of the two inner redundancies serves not only its safety-relevant consumers, but also the Operation Component Cooling System, the other one stands by for that.  The design of the RHR Intermediate Cooling System enables – if necessary – a takeover of heat transfer from the reactor cooling system after shut-down of the plant without the aid of steam generator feed. To achieve the maximum possible heat removal capacity, the bypasses inside the RHR Intermediate Cooling Circuit around Moderator Cooler and RHR Intermediate Cooling Heat Exchanger must be closed. If it is necessary for the RHRC to keep the reactor cooling system in a desired temperature state or to cool it down according to a specified shutdown gradient, this is done by opening/closing the bypass around the Moderator Cooler (without intermediate positions) and by controlling the flow rate through the primary side of the RHR Intermediate Cooling Heat Exchanger on the one hand and the bypass around the cooler on the other (Shutdown control).

An important further development compared to CNA 1 is the handling of the water balance in the RHR Intermediate Cooling Circuits. Facilities for absorbing expansion water and re-feeding it when the circuit cools down as well as replacing operational medium losses in the first period after an accident occurs (in the event of failure of operational demineralized water supply) are set up for each circuit self-sufficient and spatially separated from each other in the Reactor Building Annulus.

Each of the four subsystems of the Secured Service Cooling Water System with one Secured Service Cooling Water Pump each, supplies all of the assigned heat exchangers in parallel, that are 

• one RHR Intermediate Cooling Heat Exchanger,
• one Component Cooling Heat Exchanger,
• one Secured Intermediate Cooler,

This heat exchanger removes the heat loss from the line-assigned Emergency Diesel Engine and the Secured Chilled Water System, which is absorbed in the so-called Secured Closed Cooling Water System.

• One Fuel Pool Cooler (Really only a total of two coolers, each of which connected to two Secured Service Cooling Water Pumps for alternating supply)

Comparison DWR 1300 MW – Atucha 2

By comparing the RHRC configurations of the latest PLWR- and PHWR plants in Fig. 10 it is intended to show at a glance their differences in the type and scope of process engineering equipment for the removal of residual heat from the reactor cooling system. Furthermore, it is made clear which or how many subsystems/lines must be active during power operation of the plant.

In order to complete the comparison, the water/steam cycle must also be included. By using the Moderator Cooler for preheating the feed water, the PHWR is considerably simplified in comparison to the PLWR (Fig. 11). In addition to the High Pressure Preheaters themselves, the steam extraction points on the high-pressure section of the steam turbine and the connecting steam pipes are eliminated for the PHWR.

The second above item determines the number of pumps that are to be operated continuously, and thus also the electrical auxiliary power demand as well as the net efficiency. This in turn influences the economic attractiveness of the plant.

In Fig. 10 mean:

• Thick drawn subsystems and components: Used in power operation
• Thin drawn subsystems and components:                Ready for operation readiness
• Thin drawn heat exchanger edging, but

       with thick drawn flow symbol:                                    Flow through its secondary side,

                                                                                            but without heat input

 Figure 10: DWR 1300 MW – CNA 2, Comparison of RHR Chains regarding their necessary 

                  use during power operation of the plant;

                  Explanation of Numbers: see Figures 6 and 9

For DWR 1300 MW, the upper part of Fig. 10 shows the minimum amount of subsystems to be operated.

It is assumed that

• The fuel pool cooling circuit connected to the Operating Component Cooling System is sufficient to maintain the fuel pool water under the desired temperature. Otherwise, one of the outer lines would have to be operated with a Fuel Pool Cooling Pump, additionally or exclusively.
• Operation of one of the four Secured Chilled Water Systems (which are redundantly supplied by the so-called Secured Closed Cooling Water Systems) is sufficient and therefore only one of the Secured Intermediate Coolers (No. 7 in Fig. 6 and Fig. 10 above) has to be flowed through. If this is not the case, then additional subsystems of the Secured Service Cooling Water System (and with that Secured Service Cooling Water Pumps too) must be activated.

With CNA 2, the constantly running Moderator Pumps mean that their cooling points – line-separated  – always have to be supplied with cooling water via the Safety Component Cooling System. Therefore, all its subsystems as well as the entire Secured Service Cooling Water System must always be operated. With regard to the heat removal capacity, actually only the line which is connected to the Operation Component Cooling System with its permanent heat input is utilized, fully or only partially.

The part of the RHR Intermediate Cooling System not flowed by feed water is separated and in stand-by condition.

In contrast to the DWR 1300 MW, the fuel pool cooling in CNA 2 is completely independent from the heat removal via the RHRC. Here, the fuel pool cooling circuits transfer the heat to be removed directly to the Secured Service Cooling Water via their own Fuel Pool Coolers (No. 9 in Fig. 9 and Fig. 10, below). Each one of the coolers can alternatively be supplied by two subsystems of the Secured Service Cooling Water System; this is why in Fig. 9 and Fig. 10 below – only to illustrate the availability – four pool coolers are drawn.

Figure 11: DWR 1300 MW- CNA 2, Comparison of Water/Steam Cycles (simplified)

Tab. 1 summarizes the most important information from the RHRC systems of DWR 1300 MW and CNA 2. In addition to their design (high/medium/low pressure resp. temperature), their functions in normal operation and assumed accident cases of the plant are listed.

It is also indicated at which points of the RHRC the heat to be removed can be controlled.

This is done by dividing the total flow of the pumps (Residual Heat Removal Pumps/Fuel Pool Cooling Pumps resp. RHR Intermediate Cooling Pumps) between heat exchangers (Residual Heat Exchanger resp. RHR Intermediate Cooling Heat Exchanger) on the one hand and the bypasses around the coolers on the other.

Table 1: DWR 1300 MW – CNA 2, Comparison of RHR Chain functions

Summary

Development of the Residual Heat Removal Chain (RHRC) in NPPs with pressurized water reactors of german design, from the prototype plant MZFR (heavy water) and the demonstration power plant KWO (light water) to the last plants erected, was carried out on three mutually independent areas:

• Pressurized Light- and Heavy-Water DWR (PLWR and PHWR):

Increasing requirements concerning plant-internal damage assumptions

The assumption of failing of passive components and system parts as well as the postulate of simultaneity of “repair case and single failure” led to the (step-by-step)

• transition from the single-line to the multi-line RHRC-design with several functionally independent redundancies of the same heat transfer capacity. In plants with four RHR lines – as is the case with the DWR 1300 MW class as well as with Atucha 2 – each of the lines has to be equipped with a heat removal capacity of 50 %, based on the thermodynamic design case of the entire RHRC.
• This goes hand in hand with abandoning meshing technology in which, for example, if a pump fails, a reserve unit can be switched to various subsystems of an NKK element.
• Pressurized Light Water Reactors (PLWR of new design):

Introduction of emergencies „civilization-related external impacts“

For the first time after the occurrence of accidents, in which it is no longer possible to feed the Steam Generators from the Feedwater Tank, the Emergency Feed Water System was created to remove the residual heat via Steam Generators, for the long-term range via the Emergency RHR Chain, both of them operated by self-sufficient diesel engines/generators. Since the simultaneous repair of a system part is not to be assumed for such emergencies, two Emergency Cooling Lines of a thermal capacity of 100 % each, with respect to the max. power to be removed, are sufficient. For this ECC, subsystems of the middle link (Safety Component Cooling System) and the outer link (Secured Service Cooling Water System) of the existing RHRC are equipped with additional, less powerful pumps – parallel to the main pumps. By using this two lines as ECC which contain a Fuel Pool Cooling Pump in their inner link, this aggregates are also deployed as „emergency residual heat removal pumps“. Both the reactor and the fuel pool can thus be cooled via these lines.

• Pressurized Heavy Water Reactor (PHWR after MZFR):

Transition to a high-pressure / high-temperature RHRC

The residual heat removal concept of the MZFR as the first plant of this type of NPPs is largely identical with that from PLWR. In the first time after reactor shut-down cooling is performed exclusively via the secondary side (Steam Generator) before the RHRC takes over with the Moderator System as the inner link. Only in the subsequent plants Atucha 1 and Atucha 2 the fact has been utilized, that with the Moderator System a “Residual Heat Removal System” is available, which is similar to the Reactor Cooling System regarding its pressure/temperature design values​​. By also designing the middle RHR link, the

RHR Intermediate Cooling System, as a high-pressure/high-temperature circuit it was possible to create a divers residual heat removal option for the Steam Generators, with which reactor cooling is possible from the beginning – without further Steam Generator feeding.

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