All foreseeable internal hazards and external hazards for a research reactor, including the potential for human induced events directly or indirectly to
affect the safety of the research reactor, shall be identified and their effects, both individually and in credible combinations, shall be evaluated. Hazards shall be considered in designing the layout of the facility and in determining the postulated initiating events and generated loadings for use in the design of relevant items important to safety for the reactor facility.
6.45. Items important to safety shall be designed and located with due consideration of other implications for safety, to withstand the effects of hazards or to be protected, in accordance with their importance to safety, against hazards and against common cause failure mechanisms generated by hazards. This also applies to non-permanent equipment.
Internal hazards
6.46. An analysis of the postulated initiating events shall be made to establish all those internal events that could affect the safety of the research reactor facility.
These events may include equipment failures or malfunctions.
6.47. The potential for internal hazards such as fires and explosions, flooding, missile generation, pipe whip, jet impact or the release of fluid from failed systems or from other installations on the site shall be taken into account in the design of the research reactor facility. Appropriate preventive and mitigatory measures shall be taken to ensure that nuclear safety is not compromised. Some external events could initiate internal fires or floods or lead to the generation of missiles. Such combinations of external and internal events shall also be considered in the design where appropriate.
Fire and explosion
6.48. Structures, systems and components important to safety shall be designed and located, subject to compliance with other safety requirements, so as to minimize the effects of fires and explosions. A fire hazard analysis and an explosion hazard analysis shall be carried out for the research reactor facility to determine the necessary ratings of the fire barriers and means of passive protection and physical separation against fires and explosions. The design shall include provisions:
(a) To prevent fires and explosions;
(b) To detect and extinguish quickly those fires that do start, thus limiting the damage caused;
(c) To prevent the spread of those fires that are not extinguished, and of fire induced explosions, thus minimizing their effects on the safety of the facility. Internal fires and explosions shall not challenge redundant trains of safety systems.
6.49. Firefighting systems shall be automatically initiated where necessary.
Firefighting systems shall be designed and located so as to ensure that their use or rupture or spurious or inadvertent operation would not increase the risk of criticality23, would not harm personnel, would not significantly impair the capability of structures, systems and components important to safety, and would not simultaneously affect redundant safety groups and thereby render ineffective the measures taken to comply with the single failure criterion (see paras 6.76–6.79).
6.50. Non-combustible or fire retardant and heat resistant materials shall be used wherever practicable throughout the research reactor facility (including for tests and experiments), in particular in locations such as the reactor building and the control room. Flammable gases and liquids and combustible materials that could produce or contribute to explosive mixtures shall be kept to the minimum necessary amounts and shall be stored in adequate facilities to keep reactive substances segregated.
6.51. Fires and explosions shall not prevent achievement of the main safety functions as well as monitoring the status of the facility. These shall be maintained by means of the appropriate incorporation of redundant structures, systems and components, diverse systems, physical separation and design for fail-safe operation.
External events
6.52. The design basis for natural and human induced external events shall be determined. The events to be considered shall include those that have been identified in the site evaluation (see Section 5).
6.53. Natural external events shall be addressed, including meteorological, hydrological, geological and seismic events, and all credible combinations thereof (see para. 6.69). Human induced external events arising from nearby
23 This aspect is important in particular for critical assemblies and subcritical assemblies and dry fuel storage facilities, which shall be designed to be safely subcritical following activation of the fire protection system and during firefighting activities.
industries and transport routes shall be addressed. In the short term, the safety of the facility shall not be dependent on the availability of off-site services such as the electricity supply and firefighting services. The design shall take due account of site specific conditions to determine the maximum delay time by which off-site services need to be available.
6.54. A research reactor facility located in a seismically active region shall be equipped with a seismic detection system that actuates the automatic reactor shutdown systems if a specified threshold value is exceeded.
6.55. Features shall be provided to minimize any interactions between buildings containing items important to safety (including power cabling and instrumentation and control cabling) and any other structure as a result of external events considered in the design.
6.56. The design shall be such as to ensure that all items important to safety are capable of withstanding the effects of external events considered in the design, and if not, other features such as passive barriers shall be provided to protect the reactor facility and to ensure that the main safety functions will be achieved.
6.57. The design shall provide for an adequate margin to protect items important to safety against levels of external hazards more severe than those selected for the design basis, derived from the site hazard evaluation.