I—24. The term ‘negative pressure containment’ is used to describe a containment system that typically consists of the following subsystems (Fig. I—7):

(a) A containment envelope that comprises the reactor buildings, the connecting pressure relief duct, vacuum ducts, the vacuum building and all the containment extensions.

(b) A pressure relief system which comprises the pressure relief blowout panels that isolate the reactor buildings from the connecting pressure relief duct and the pressure relief valves that isolate this relief duct from the vacuum building.

(c)

A vacuum system that maintains a subatmospheric pressure inside the vacuum building, so that when this building is connected to the

FIG. I-7. Schematic diagram of a negative pressure containment system for a pressurized heavy water reactor: 1, reactor buildings; 2, vacuum building; 3, pressure relief duct; 4, blow-out and blow-in panels; 5, pressure relief valve; 6a, upper chamber; 6b, evacuation system; 7, vacuum building evacuation system; 8, vacuum building spray system; 9, dousing tank; 10, filtered air discharge system.

containment the atmosphere from the containment passes into the vacuum building.

(d) An energy suppression system, comprising a dousing tank, upper chamber vacuum system and spray header, which is housed inside the vacuum building and which can absorb all the energy released to the vacuum building.

(e) An atmospheric control system that controls the atmosphere within the reactor buildings.

(f) A filtered air discharge system to help to maintain subatmospheric pressure within the containment envelope in the long term after an accident. The reactor buildings are maintained at slightly negative gauge pressures in both operational states and post-accident conditions.

I—25. Energy management is achieved by relieving the peak pressure in the reactor building to the vacuum building via the pressure relief system, which is actuated by a small increase in pressure in the reactor building. Additional energy suppression takes place when the steam drawn into the vacuum building is condensed by the spray system, which is automatically actuated by a change in pressure in the vacuum building. Long term heat removal from the containment is achieved by the atmospheric control system that cools the building air and by the heat exchangers in the recirculation system of the emergency core cooling system. Radionuclide management is accomplished by plate-out on the internal surfaces of the containment envelope, by washout afforded by the spray and by the leaktightness of the containment envelope.

1.25. The containment structures and systems should be so designed that all those components of the reactor coolant pressure boundary that cannot be safely isolated from the reactor core, as well as the safety systems located inside the containment that are necessary to keep the core in a safe state, are protected against the external events included in the design basis.

BIOLOGICAL SHIELDING

1.26. In operational states and in accident conditions, the containment structures contribute to the protection of plant personnel and the public from undue exposure due to direct radiation from radioactive material contained within the containment and containment systems. Dose limits and dose constraints as well as the application of the ‘as low as reasonably achievable’ principle (for the optimization of radiation protection) should be included in the design basis of the structures [1, 9, 10]. The composition and thickness of the concrete, steel and other structural materials should be such as to ensure that the dose limits and dose constraints for operators and the public are not exceeded in operational states or in the accident conditions that are considered in the design.

4.120. In some containments with a steel shell, heat released in the containment under accident conditions can be removed passively through the containment walls. The secondary containment is designed to remove the heat by providing a natural circulation path for air (the chimney effect) and a means for passive spraying of the outside of the primary containment. Other contain­ments introduce passive cooling condensers that transfer the heat by means of natural convection to a water pool. If such passive containment cooling is adopted, the following aspects should be considered:

(a) The area of the cooling surface should be sufficient to transfer the heat generated in the containment and to cool down the atmosphere and the structures inside the containment. The heat transfer coefficient should be conservatively determined for all operational states.

(b) The necessary natural circulation within the containment and that to the outside heat sink should be ensured for all relevant design basis accidents.

(c) The entire system should be well validated by means of tests and analyses. A thorough search should be conducted for possible harmful effects and failure modes, in order to achieve a high degree of confidence that the safety functions will be fulfilled in all design basis accidents.

MANAGEMENT OF RADIONUCLIDES

5.28. Where it is technically feasible, the design should provide for the visual inspection of containment structures (including the tendons for prestressed concrete containments), penetrations and isolation devices.

5.29. Visual inspection of the containment envelope, including appurtenances and penetrations, should be made in conjunction with each of the tests specified in paras 5.18-5.24. Visual inspections are important for the proper monitoring of ageing effects.

Availability tests

5.30. The design should provide a capability for monitoring or testing all items of equipment in containment systems at intervals that reflect their importance to safety, or for otherwise demonstrating the necessary reliability for the containment systems individually or as a whole.

5.31. A capability for testing isolation valves during plant operation, such as by actuating them to function with a partial stroke, may contribute greatly to the assurance of the reliability of the system.

1.86. When the containment system is challenged, there should be no need for any action to be taken by the operator within a certain ‘period of grace’[5]. For any necessary manual intervention, the operator should have sufficient time to assess the conditions in the plant before taking any action. The plant design should not prevent the operator from initiating appropriate actions in response to clear and unequivocal information.

Performance of the secondary containment

1.87. The secondary containment should be able to withstand the possible pressurization of the volume between the primary and secondary containments in the event of an accident or a malfunction of the ventilation system, and should be able to withstand external loads either alone or in combination with the primary containment.

1.88. To ensure that the pressure between the primary and secondary contain­ments is maintained below atmospheric pressure, the secondary containment and its air extraction system should be operable in the event of a loss of off-site power.

1.89. Safety of Nuclear Power Plants: Design (Ref. [1], para. 5.57) limits the sharing of structures, systems and components in multiunit plants to exceptional cases. For such exceptional cases of the sharing of structures, systems and components between units, all the safety requirements for all the reactors will apply and must be met under all operational and accident conditions.

1.90. External events such as earthquakes that could simultaneously challenge systems serving all units, or events such as the loss of off-site power that could cause the failure of systems common to the units, should be identified and considered in the design.

1.91. Compliance with safety criteria for redundancy, independence and the separation of safety systems should always be considered and any exceptions should be justified.

1.92. In the design of a multiunit plant with a shared or partly shared containment system, appropriate emergency response procedures should be followed for all units in the event that an accident in one unit necessitates the use of the containment function.

4.162. Passive means such as passive autocatalytic recombiners and/or active means such as igniters should be provided for removing hydrogen.

4.163. If it is determined by means of analysis that the hydrogen concen­tration would increase slowly over a long period of time, the actuation of active means of hydrogen removal may be by manual means. In this case, it may be assumed that off-site power is available for active means. If analysis shows with sufficient certainty that the accumulation of hydrogen under LOCA conditions is slow, a mobile control system for combustible gases (i. e. a mobile recombiner) may be used. In this case, appropriate provisions should be made in the design and in the procedures for the use of such a system. The provisions for shielding should be such as to permit connection of the mobile system without causing any undue exposure of operators to radiation.

4.164. A single failure during the use of active hydrogen control systems need not be postulated provided that:

— Repair or means of substitution can be shown to be practicable.

— The generation of hydrogen is slow enough that hydrogen concentration limits will not be exceeded, either during the predicted repair time or during the time necessary to introduce substitute means (such as by putting a mobile recombiner into operation).

Homogenization

4.165. The containment design either should incorporate active means (such as sprays and mixing fans qualified for operation in a combustible gas mixture) or should facilitate the action of mechanisms (such as large volume dispersion

or natural circulation) to enhance the uniform mixing of the containment atmosphere within and between compartments. This is to ensure that localized hydrogen concentrations do not reach combustion limits following an accident. Alternatively, it should be shown by analysis either that uncontrolled local ignition will not occur or else that safety systems and components can survive local ignition.

Inerting

4.166. One possible way to avoid hydrogen combustion is to inert the containment atmosphere during reactor operation (usually with nitrogen). This is mainly applicable to small containments such as those of boiling water reactors.

A.11. Activity measurements are especially useful for detecting breaches that could otherwise go undetected by the measurement of other parameters. Activity should be measured to detect breaches in, and releases from, any of the multiple protective barriers. Hence, measurement locations should include:

— The reactor cooling circuit, to detect fuel failures;

— The containment atmosphere and drains, to detect failures in the primary circuit and connected circuits inside the containment;

— The secondary side circuit, to detect primary to secondary side leaks.

A.12. To detect leaks from the containment structure, the activity in the stack or in connected ventilated buildings should also be measured. Measurements of activity in the stack can be used to detect releases into the containment atmosphere before isolation and to detect leaks from the valves following isolation.

A.13. For double wall containments, it should be considered whether to make measurements of activity in the annulus ventilation system to detect leaks of radioactive material from the primary containment.

A.14. Provisions should be considered for obtaining samples of the containment atmosphere from outside the containment building, to be used for radiochemical analysis.

A.15. In addition, activity measurements in the following areas should be considered:

— In or around systems into which high energy contaminated fluids could enter owing to a lower functional pressure;

— In or around parts of systems connected to the primary circuit or the containment atmosphere but extending outside the containment.

4.81. For containment systems, a set of representative loads and load combina­tions, as well as a set of adequate acceptance criteria, should be established by a similar procedure as for the containment structures, with account taken of the relevant design basis accidents.

4.220. Appropriate instrumentation should be incorporated inside the containment for conducting the periodic leak tests. This should include instru­mentation for monitoring pressures, temperatures, humidity and flow rates. For steel containments, the temperature of the steel should also be measured. The number and the locations of instruments should be specified by the designer in accordance with the environmental conditions to be expected. Guidance on leak rate testing is given in Section 5.

4.221. Means for monitoring major leaks (e. g. by assessing the mass of the containment atmosphere by the use of devices for measuring pressure and temperature) should be incorporated to detect any major openings in the containment boundary caused by equipment failure or operator error. Guidance on monitoring for major leaks is given in para. 5.21.

4.222. Any containment leaks that are not collected in a building equipped with filtration devices, so-called direct leaks, should be carefully monitored to ensure that any leakage directly to the atmosphere would be detected.

I-26. The pressurized containment system (Fig. I-8) used in pressurized heavy water reactors for single unit plant designs typically consists of the following subsystems:

(a) A containment envelope comprising a prestressed, post-tensioned concrete reactor building and its extensions;

(b) An energy suppression system that consists of a dousing tank and a spray system that suppress the initial peak pressure;

(c) A reactor building cooling system to depressurize the containment in the longer term;

(d) A filtered air discharge system to help to maintain subatmospheric pressure within the containment envelope in the long term after an
accident, and an atmospheric control system to aid in cleanup operations for the containment.

I-27. Upon the detection of radioactivity or high pressure in the reactor building, the isolation system closes the appropriate penetrations of the containment envelope.

_v

FIG. I-8. Schematic diagram of a pressurized containment system for a pressurized heavy water reactor: 1, containment; 2, dousing tank and spray system; 3, filtered air discharge system; 4, emergency core cooling system.

I—28. When high pressure is detected in the reactor building, the dousing system is also activated. The initial peak pressure following a LOCA is suppressed by the condensation of steam through the dousing spray system. Long term energy management is provided by the atmosphere control system (building air coolers) and by the heat exchangers in the recirculation system of the emergency core cooling system. Radionuclide management is accom­plished by plate-out on the internal surfaces of the containment envelope, by washout afforded by the dousing spray system, by the leaktightness of the containment envelope and, in some plants, by pH control buffers in the sump.