A nuclear power station may only be constructed and operated in the UK if the licensing authorities are satisfied that the agreed criteria for nuclear safety have been met. In order to convince the authorities that the necessary approvals may be issued, the li­censee instructs the contractor to write a comprehen­sive description of the plant and the way in which it is to be operated. This document, which also contains detailed arguments supported by any necessary calcu­lations demonstrating the safety of the plant under both normal operating and fault conditions, is known as the Station Safety Report, There has been a general improvement over the years in the degree of detail and the quality of Station Safety Reports for succes­sive stations. This reflects not only the increasing at­tention paid by the Licensing Authorities to nuclear safety, but also the development in the philosophy of nuclear safety.

The Station Safety Report has always been pro­duced in three quite distinct stages although for the most recent stations at Heysham 2 and Torness, this approach has been somewhat modified.

A preliminary safety report is produced by the contractors when the contract is first awarded. This version of the report is produced primarily for the purchaser of the station and gives an outline of the intended safety case. The first stage of the Station Safety Report proper, known variously as the Stage 1 Safety Report or the Preconstruction Safety Report (PCSR), is produced and issued to the licensing au­thority in support of a request for their consent to proceed with construction beyond the initial site pre­paration, i. e., beyond completion of the ‘blinding’ stage of the foundation. Because this stage of the report is produced so early, much of the design detail is unavailable and the report lacks the rigor which would be required before agreement to load fuel or raise power could be obtained. The report does, how­ever, contain an indication of the information which will be provided and a general description of the plant to be constructed. Clearly, if a station to be con­structed is of a proven design with little intention to change that design the detail of the Stage l report can be considerably enhanced.

After the issue of the consent to continue construc­tion beyond the foundations and as the design detail is completed, the second stage of the Safety Report, the Stage 2 Safety Report is written. This issue of the report is intended to provide the licensing authorities with sufficient information and argument to enable them to agree to fuel being loaded into the reactor. It still lacks some information, in particular that which will come from commissioning tests or as a result of late modifications to plant, but it nevertheless forms the basis of the final version of the Safety Report.

The Stage 3 Safety Report or Final Station Safety Report is produced as a series of amendments and additions to the Stage 2 report. For the earlier stations the Final Safety Report appeared as two sets of vol­umes, the Stage 2 Safety Report, and the volumes containing the amendment sheets and addenda. Lat­terly, these amendments and addenda have been in­corporated with the Stage 2 Safety Report into a single set of volumes. Whilst the Final Safety Report contains substantial detail, this is distilled from a vast number of detailed project documents which appear in the list of references. The Final Safety Report gives the base line for the design of the station, and the de­finitive safety arguments, when commercial operation is achieved on the second reactor of a two-reactor station. The Final Safety Report for all stations prior to Heysham 2/Torness was therefore frozen at some point in time, agreed with the Licensing Authorities as being about the time when the second reactor nominally achieved full power. It is from this base line that modifications to plant or to the safety arguments are considered by the Nuclear Safety Committee.

For Heysham 2 power station and Torness a slightly different approach has been adopted. In the first place these stations were very similar to those built at Hinkley Point В and Hunterston В and hence the Preconstruction Safety Report (PCSR) was more de­tailed than had been the ca^e for the earlier stations. In the second place, the concept of the Reference Safety Statement (RSS) was implemented. The RSS is a continuously updated compendium of documents which describe at any time the state of the Safety Case. This compendium of documents is initially simply the PCSR. Further submissions, information supplied in letters and minutes of meetings, are pro­gressively added to the documentation and a record kept in a Schedule of the Reference Safety Statement. The report produced for the fuel loading consent at Heysham 2 is the Station Report and is equivalent to the Stage 2 Safety Report for the earlier stations. At the time of its issue, 6 months before the fuel loading consent is required, it largely represents the total content of the RSS. In other words it replaces the PCSR and most of the information in the other sub­missions, letters and minutes of meetings. The RSS is continually added to in the time following the issue of the Station Safety Report and will include results from the commissioning tests, experience during early operation and any modifications to plant. It is in­tended from time to time to incorporate the infor­mation in the RSS into new versions of the Station Safety Report hence maintaining throughout the life of the station a coherent up to date Safety Case. One further point about the Heysham 2/Torness Safety Reports is that they should be suitable for general publication if required. The Safety Reports for the earlier stations included an extensive list of references containing the detailed calculations supporting the case made. It was decided that it would be quite impractical to have this arrangement if the report were to be made generally available, and hence the infor­mation in the detailed documents was distilled into a number of identified references to be included as extra volumes of the Safety Report itself. Thus, the Station Safety Report, but not necessarily the full com­pendium of documents making up the RSS, is in­tended to be essentially self-explanatory.

For the Sizewell В PWR, a PCSR was also pro­duced and made pubiicaily available for the Sizewell В enquiry. This document went into still greater detail than any previous PCSR. On the basis of this docu­ment, together with additional information requested subsequently by the licensing authority (Nil), the Nil issued a Site Licence. This document contained many references to letters sent to the N11 and is to be up­dated that the substance of the safety arguments made in those letters are included in the text. The updated PCSR is also to be published.

During manufacture, construction and commission­ing the PCSR will be modified, amplified and updated into the Final Safety Report which records the state of the plant in the ‘as-built’ condition. In the past, each of the CEGB nuclear power stations has com­prised two reactors and the Final Safety Report has been produced and issued prior to power raising on the second of the two reactors, enabling experience gained in power raising on the first to be included in the report. For a PWR station with only one reactor, it is probable that the Final Safety Report would be required before the Licensing Authority give consent to raise power.

It is on the Final Safety Report that the Operating Rules, Operating Instructions and general requirements for safe operation of the plant will be based, and it is this report which will be provided to the Station Manager as the basis for the safe operation of his station.

It is noteworthy that Sir Frank Lay field in his report following the Sizewell В Public Enquiry stated that І see no reason why the revised PCSR and FSR should not be made pubiicaily available, and as many supporting documents to the PCSR as practi­cable placed on deposit where they can be inspected.’ He also added that The PCSR, its supporting docu­ments and the FSR are technical in nature. It is im­portant that reports are published at a later stage in the licensing process which are suitable for lay public and parliamentary scrutiny. These documents should not be over-simplified, but should explain the principal safety aspects thoroughly and clearly.’ The CEGB has already taken the decision to publish the PCSR and is likely to follow the remainder of Layfield’s advice.