Proposed pebble bed modular reactor

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Error! No text of specified style in document.‑5: DECOMMISSIONING PHASE : INPUT/OUTPUT PARAMETERS (duration about 1 year)

·         Only essential services are provided during the decommissioning phase

·         Detailed plans and options are developed for the dismantling of the Plant


























3.2.5          Dismantling Phase : Input Output Diagramme

Table Error! No text of specified style in document.‑6: DISMANTLING PHASE : INPUT/OUTPUT PARAMETERS (duration 1 – 2 years)

·         This phase will entail the removal of all equipment (radiological and non-radiological)


·         Only essential equipment and services will remain in order to manage the security and safety of the spent fuel


·         This phase resembles the Construction Phase in the reverse e.g. equipment is removed, irradiated parts are decontaminated and building structures are demolished as needed and disposed of.


















3.3.1          Input data

Ü        Nitrogen/Helium

v         Nitrogen for installation and commissioning period

Approximately 1 000 kg Nitrogen is required for the initial clean up of the MPS15 and gas systems. The demonstration module will require additional 3 576 kg Nitrogen during the installation and commissioning period.

v         Helium for normal operations

The expected helium loss during normal operations is specified as 0.1% of inventory per day. The inventory is approximately 7 290 kg and the helium loss is approximately 2 661 kg per year. The inventory loss is based on the THTR/HTR specification, although less inventory loss was experienced. A more accurate figure will be determined during the first year of operation.

v         Helium for planned outages

During each sixth year planned outage, a helium loss of approximately 898 kg is expected. An additional 250 kg is required during the outage when the core reflector is replaced.

Ü        Graphite spheres

The central graphite column in the reactor will be replaced once in about 20 years of the plant life. A total of 110 000 graphite spheres will be removed from the graphite storage tank to replace those in the central column. The spheres removed from the storage tank will have to be replaced to maintain the de-fuelling capability.

The usage of graphite spheres due to normal wear and maintenance activities is expected not to exceed 3 000 spheres over the life of plant.

Ü        Fuel Spheres

The Fresh Fuel Storage Area is designed to store six months’ supply of fresh fuel, i.e. approximately 70 000 fresh fuel spheres in 70 fresh fuel canisters.

The Spent Fuel Storage Area in the basement of the building is to provide storage space for all the spent fuel produced during the lifetime of the reactor plant, which is 35 Full Power Years (FPY). This amounts to about 6.0 x 106 spent fuel spheres. The spent fuel spheres will be stored in 10 Spent Fuel Storage Tanks.

Only two of the 10 Spent Fuel Tanks will be in operation at any given time. Spent fuel is loaded in daily batches into the two tanks, until the tanks are full. Thereafter, the tanks will be sealed and the next pair of tanks is brought into operation. A total of about 425 spheres are loaded into the tanks per Full Power Day (FPD).

Ü        Demineralised water

The initial filling of the module’s closed water circuits will be obtained from Koeberg and transported in tankers. Approximately 500 m³ of demineralised water will be required for the first fill.

For make-up water, decontamination and other cleaning processes, demineralised water at an average of 10 m³ per month is required.

Ü        Potable water system

During normal operation, the volume of potable water required per month is indicated in Table Error! No text of specified style in document. -7.

Table Error! No text of specified style in document.‑7: VOLUME OF EXTERNAL SERVICE WATER REQUIRED DURING NORMAL OPERATION

Plant Configuration

Normal Operation

Potable Water Consumption


Services & auxiliary buildings + 1 module


During an outage about 50m³ of water per day is required.

Ü        Safety

The Final Safety Design Philosophy (FSDP) ensures that the fuel will retain its integrity to contain radioactive fission products under normal and accident conditions and thereby allow radiological safety to be assured. This is achieved by relying on fuel whose performance has been demonstrated under simulated normal and accident conditions. Fuel integrity will not be compromised even under accident conditions. 16

To ensure that the fuel integrity is maintained, the plant design for operating and accident conditions:

v         Includes sufficient heat removal capability such that fuel temperatures will remain in the proven safe region;

v         Limits chemical reaction and physical deterioration of the fuel; and

v         Provides adequate measures to control reactivity and power. 17

Ü        Safety Analyses

Appropriate analysis demonstrates that the design objectives have been met with adequate margins. The design has been systematically analysed to ensure that all normal and abnormal conditions have been identified and considered. When appropriate the design or operation of the plant is modified. This analysis is updated for design changes and reviewed periodically. 18

Ü        Probabilistic Risk Assessment

The comprehensive Probabilistic Risk Assessment (PRA) confirms that the Plant is safe to operate and that the PBMR design meets regulatory risk criteria.

The PRA of the PBMR design provides a systematic analysis to identify and quantify all risks that the Plant imposes to the general public and the worker and thus demonstrates compliance to regulatory risk criteria.

A demonstration that regulatory risk criteria are met was achieved through focus on the challenges to fuel integrity. Other systems, structures and components act as further barriers or obstacles to the release of fission products, and are modeled in the PRA. This best estimate approach provides a measures of the levels of defence-in-depth that exist in the design and operation of the PBMR and provided a tool for the optimisation of the design and operating programmes. 19

Ü        Defence-in-Depth

Defence-in-Depth ensures that many barriers are established between the public and the radioactive material. These barriers may be physical or administrative. Defence-in-Depth provides multi barriers for the public and the worker. 20

Ü        ALARA

The design ensures that the dose that might be received by the operators and public as well as releases to the environment in normal operations and accident conditions, not only meets all regulatory limits but is also As Low As Reasonably Achievable (ALARA). 21

Ü        Radiation Protection Programme (RPP) for Normal Operation

The principle of ALARA is embodied in all operating Support Programmes. In particular a Radiation Protection Programme specifies Radiation Protection (RP) limits and conditions. Its operating procedures include measures to control the release of radiological effluent. It minimizes, as low as reasonably achievable, the radiological exposure to the plant personnel, general public and environment. 22

A Radiation Protection Programme controls access to areas where radiation and/or radioactive contamination may be present. This is accompanied by a radiation protection monitoring programme to ensure that no worker will receive undue exposure to radiation and that only authorized radiation workers are allowed to work in controlled areas. A comprehensive plan protects personnel from excess exposure during maintenance activities.

Ü        Test and Commissioning Programme

The Test and Commissioning Programme will demonstrate the performance of all Systems, Structures and Components (SSC) and materials will perform as designed. This programme ensures that any of the SSC and the Plant will operate safely in normal and accident conditions.

A pre start up commissioning programme will test components for sub-system and systems as far as possible, prior to fuel loading.

Due to the unique design of the PBMR, there are features, which exploit natural physical phenomena in ways that have not been used in this application before. The test and commissioning programme will ensure that any physical phenomena that have a unique application to the safety of the PBMR design are adequately demonstrated on the module. Assurance is provided that the assumptions made during design and analysis is valid.

Ü        Operating Procedures

The documentation in place to support the safety operation of the PBMR is in the form of General Operating Rules (GOR). The GOR are interface documents between the PBMR plant design and the actual operating practices. They prescribe the operating rules, within which compliance ensures that the Plant stays within the envelope of its design bases in any operating state, normal or abnormal, and ensures that the main assumptions in the safety assessment, remain valid.

Adherence to the Plant operating procedures ensures that during the normal operation the plant remains within a domain of the design and licensing basis. Operating Technical Specifications (OTS) define the technical rules and regulatory requirements to be observed in order to maintain the plant within licensing basis. They are developed to ensure that the assumptions in the design and analysis remain valid.

Ü        Waste Management Programme

A Waste Management Programme ensures that the generation of radioactive waste is minimised throughout the lifecycle of the plant. Management rules for the processing, conditioning, handling and storage of radioactive waste limits the radiological doses to the plant personnel and general public, and the radiological impact on the environment.

Ü        Maintenance Programme

A Maintenance Programme is developed to keep all the equipment required for plant operation available and reliable. The Programme includes appropriate control, monitoring and management systems, using preventive, predictive and corrective maintenance. The technical basis for the programme is founded on the Safety Case.

Assurance that there are adequate means to monitor the plant, and detect when the Plant is outside of its normal operating envelope is obtained by establishing and an appropriate test and surveillance programmes.

An Emergency Plan appropriate to the level of nuclear hazard that the PBMR poses under abnormal or accident conditions is in place and lays down the level of preparation required both on and off site.

Ü        Security

Site access control is enforced by means of security fencing site access control, Plant access control, and restricting entry to designated security access points, where permits will be issued. Lock out systems and camera surveillance forms part of security systems.

Prior to the loading of the nuclear fuel, further safety measures will be implemented.

Before any nuclear fuel is brought onto site, it will be assured that adequate safety and security plans are in place and that the physical barriers such as fences, grates, doors, walls or ceilings are installed and functional, to deter and deny unauthorized access to the facility.

The following systems must be operational:

v         Physical security systems.

v         Nuclear fuel storage facilities.

v         All safety-critical equipment interfaces.

v         Material control and accountability areas.

v         Non-interruptible power supplies.

v         Area monitoring systems such as lighting and communication.

v         Surveillance, assessment, detection and alarm systems.

v         Vital areas and vital equipment.

v         Material access areas.

v         Safety and security interfaces.

v         Adequate safety and security personnel to continuously operate and monitor surveillance and assessment equipment.

v         Critical plant areas such as computer security areas are contained within clearly defined perimeter barriers and the means of access is limited to entry portals, which are controlled.

Ü        Physical Security

The intention is to install the demonstration module PBMR on the existing Koeberg site, between the inner and outer security fence system. In the longer term it is considered that the PBMR plant will function under (conceptually) similar arrangements to existing Light Water Reactors (LWRs). To this end there is provision for on-shift security guards. The key differences between the PBMR and a standard LWR is that due to the physical layout there are less access points to enter the plant, with no external systems which are safety grade (i.e. systems which ensure the safe operation of the Plant). Therefore the current view is that (given the 1m+ outer wall) the only method of damaging the plant is to gain physical access, through the limited access points.

Ü        Aircraft Crash

The PBMR demonstration module design-basis aircraft crash is a Cessna 210, along with the potential for other crashes. The PBMR’s actual civil design has been analysed for impacts including the German KTA aircraft impact (F4 Phantom at 775km/hr) and the citadel was not breached. Note that only the reactor cavity (with its 2 m thick reinforced walls) needs to remain undamaged to prevent fuel damage and fission product releases. Work is currently ongoing investigating the impact of a Boeing 777 (which has the biggest current aero-engine) and all indications are that the same result will be obtained. The PBMR design lends itself to either burying or increasing wall thickness better than classical LWRs.

Ü        Foundation conditions

Ideally, any PBMR plant should be built on sound bedrock. In the case of this site, drilling results show that such bedrock exists at a depth of some 20 to 22 m below ground level. This is particularly appropriate for the PBMR, since the design of the plant is such that it ideally requires to be embedded to a depth of some 22 to 25 m.

The information regarding foundation conditions, was confirmed by information gathered during the construction of the Koeberg NPS.

Ü        Excavation conditions

Drilling results at the proposed site have shown that the local water table is some 5 m below ground level. Since the site is immediately adjacent to the Atlantic Ocean and about 8 meters above sea level, it must be assumed that during the excavation process, continuous dewatering of the area will be required via pumping. This dewatering will continue during construction, until such time as the building walls have reached ground level and backfilling of the excavation is completed.

Except for the bottom one or two meters of partially fractured rock the material to be excavated will be compacted sand.

No decisions have been made on methods of limiting water ingress to the excavation, or to the excavation methodology to be used. These are dependent on the evaluation of proposals to be received from civil engineering contractors and will be environmentally managed to acceptable levels in the EMP.

Ü        Site access

Access to the proposed site would be via the existing normal access routes to Eskom's KNPS, i.e. via the R27 Highway from the directions of either Cape Town or Saldanha Bay. Although a private Eskom owned road from Duynefontein to Koeberg exists, traversing through the Koeberg Nature Reserve, this route would be designated as off-limits to construction traffic, to and from the PBMR site.

Normal shipments of imported equipment for the demonstration plant would be imported via Cape Town harbour. In the case of certain components of extreme mass, route studies have shown that several of the road bridges between Cape Town harbour and the proposed site could not handle these loads. It is therefore proposed that these abnormal components be imported via Saldanha Bay harbour and transported to site by road via the R27 Highway. Surveys of this route show that that there is only one highway bridge, which would not be capable of handling the loads. The bridge can be bypassed via a temporary road.

Ü        Availability of cooling water

It has been established that the cooling water supply for the demonstration plant will be taken from Koeberg's cooling water pump house, and that the cooling water discharge of the demonstration plant will be routed into Koeberg's cooling water discharge channel. This will result in PBMR not having to build new water intake and discharge structures, with a resultant considerable cost saving. Piping connections between the plant and the Koeberg structures will, however, have to be provided.

Ü        Availability of labour

Skilled labour is readily available in surrounding towns such as Atlantis, some 25 km from the site. Unskilled labour is readily available from any number of the townships surrounding Cape Town. In both cases, the mass transporting of this labour to and from site by PBMR contractors will be essential.

Ü        Availability of local nuclear infrastructure

Being immediately adjacent to Eskom's KNPS, the proposed site of the demonstration Plant is ideal from a nuclear infrastructure point of view. Although it is planned that the demonstration plant will be run essentially independent of Koeberg, it will use some of Koeberg's facilities, such as Radiation Medicine and to some extent, the decontamination facilities.

Ü        Impact on 400m exclusion zone

The PBMR requirement for a 400 m radius exclusion zone around the demonstration plant will have no effect at the proposed site, since the entire area defined by such a radius falls well within Koeberg's existing and much larger exclusion zone.

Ü        Employment

During construction about 1 400 job opportunities will be created with emphasis on local recruitment. During operations about 40 permanent employees will be required to operate the proposed Plant.

3.3.2          Output Data

Ü        Sewerage system

The sewerage requirements for the demonstration plant are catered for by the existing Koeberg sewerage reticulation system. The sewerage flow is estimated at 140 m³ per month.

The expected sewerage effluent is as indicated in Table Error! No text of specified style in document. -8.


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