Secondary cooling system

Another letter and a number are added after the above to describe the secondary cooling system. 

Secondary Loss ot Coolant Accident A leak rn (lie light water secondary cooling system wliidi leads to a Loss of Pumping Accident. 

Loss of Heat Sink Failure of heat transfer between the primary and secondary cooling systems requiring emergency cooling. 

Loss of River Water Failure of the river water supply to the once-through secondary cooling system requiring conservation of the water on hand. 

Figure 15.30 A typical closed-circuit secondary cooling system...

H. Sato (JAEA) presented a paper discussing detection methods and system behaviour assessments for a tube rupture of the intermediate heat exchanger (IHX) for a sulphur-iodine based nuclear hydrogen plant. A rupture could be detected by monitoring the secondary helium gas supply using a control system that monitors the differential pressure between the primary and secondary helium gas supply. Isolation valves would be used to reduce the helium flow between the primary and secondary cooling systems. The study showed that the maximum temperature of the reactor core does not exceed its initial value and that system behaviour did not exceed acceptance criteria. 

Secondary cooling system IS process inlet temperature 880°C... 

In the current HTTR, the whole secondary cooling system is involved in the CV so that there is no need for considering radionuclide transportation outside of the CV in case of IHXTR. Also, the safety analysis result showed that there are no significant impacts on reactor cooling ability and therefore the scenario is not identified as a selected event in the safety evaluation. 

On the other hand, the IS process in the HTTR-IS system should be built outside and set at an appropriate distance from the reactor building in order to prevent flammable and toxic gas inflow to the building from the reactor safety point of view (Sakaba, 2008). Hence, the secondary cooling system will be modified to penetrate the CV and reactor building in order to provide heat to the IS process. The IHXTR in the HTTR-IS system would result in radionuclide transportation to the outside of the CV. 

In order to reduce radionuclide transportation from the primary to the secondary cooling system, containment isolation valves should be installed. These valves will be automatically closed by the signal detecting the heat transfer tube rupture of IHX. Meanwhile, the current engineered safety features actuating system is not designed to detect the rupture since the scenario did not impact on the reactor safety of the original HTTR. Therefore, a detection method of heat transfer tube rupture of IHX should be established. 

Monitoring differential pressure between the primary and secondary cooling systems would be one of the candidates for detecting the heat transfer tube rupture. Meanwhile, the pressure varies even at normal operation and therefore there is a concern of malfunction of CV isolation valves. In case of primary helium gas recovery flow rate, primary helium pressure control system only covers the rated operation since the system activates when the primary coolant pressure reaches about 3.95 MPa. On the other hand, the primary-secondary differential pressure control system covers start-up, shutdown and rated operations. The control system keeps supplying helium gas to the secondary cooling system during the scenario. Thus, monitoring the entire secondary helium gas supply would be an effective way to detect the tube rupture. 

Detection method evaluations are conducted using the HTTR-IS system model. CV isolation valves are set to actuate when the integral of secondary helium gas supply exceeds the actuation conditions. In this calculation, the value 0.2% of secondary cooling system mass is selected as a reference case. Regarding the actuation time of the isolation valves, we selected the previous design value of 30 seconds for the high temperature isolation valve (HTIV) (Nishihara, 1999) and 5 seconds, which is feasible in the utilised temperature range (Tanaka, 1999), for the actuation time of low temperature isolation valve (LTIV) as a reference. 

Based on the reference case, parametric calculations were conducted specifying the actuation condition and time of isolation valves as a parameter. Table 2 shows the actuation times of isolation valves, calculation results of secondary cooling system isolation time and amount of helium gas transportation from primary to secondary. 

Run no. Actuation time of isolation valves (s) (HTIV/LTIV) Detection time of rupture (s) Secondary cooling system isolation time (s) Amount of helium gas transportation from primary to secondary cooling systems (kg)... 

Interfacing isolation valves simultaneously could reduce the helium gas transportation from the primary to the secondary cooling system. 

Fig. 3.6. A capillary on-column injector including the secondary cooling system. 1, microsyringe 2, valve lever 3. valve seal 4, stainless-steel rotating valve 5, column seal 6, cooling jacket 7, capillary column. Reproduced from (42).

Fig. 7 Modified design concepts for the thermocouple wells on the secondary cooling system.

Using an oil/gas heat-exchanger This is similar to the above system but more expensive because of the need for a secondary cooling system. The heat-exchanger is generally cooled with a circulation of mineral oil. Cooling with a water/gas heat-exchanger is not (or only very rarely) practised... 

A portion of the cold water leaving pump 55 is bypassed from the secondary cooling system through a pipe 62 feeding condenser coils 52 and is returned to the secondary cooling system through pipe 63. 

The HCIHX for the HTTP is a vertical helically-coiled counter flow type heat exchanger in which the primary helium gas flows on the shell side and the secondary in the tube side as shown in Fig.2. The major specification is shown in Table 1. The primary helium gas of the maximum 950°C enters the HCIHX through the inner tube In the primary concentric hot gas duct. It is deflected under a hot header and discharged around the heat transfer tubes to transfer primary heat to the secondary cooling system. It flows to the primary circulator via an upper outlet nozzle and returns between the inner and outer shell in order to cool the outer shell. 

Motor with secondary cooling system protection against most solid material and splashing water... 

Be capable of providing post-tranalent shutdown flow requirements Indefinitely> assuming Instantaneous loss of power to the primary cooling system and concurrent loss of flow In the secondary cooling system. 

Be independent of both the primary and secondary cooling systems Including piping to the reactor coolant manifold. 

Secondary cooling system One sodium loop heat transport from intermediate heat exchanger (IHX) to steam generator (SG) ... 

Secondary cooling system - Coolant temperature - Pressure 310/485°C Non-pressurized (slightly higher pressure than in the primary system) The same as for the 10 MW(e) plant... 

Barriers to fission products include fuel matrix, fuel cladding, primary coolant, primary coolant boundary, secondary coolant, secondary coolant boundary and containment boundary. The ENHS module walls provide boundary between the primary and secondary cooling systems. The pool vessel walls and steam generators provide boundary between the secondary cooling system and reactor building. 

Emergency boron injection after the loss of reactor shutdown functions with core cooling by diversified secondary cooling system powered from modified non-safeguard buses. 

Six items are started when placing the Primary-Secondary Cooling system in operation. The sequence in which the items are started has no safety significance except that ... 

---