Winfrith SGHWR

Appendix 12D Visit to the Atomic Energy Establishment Winfrith SGHWR Station... 

A project having the size of the Winfrith SGHWR was not, of course, embarked upon lightly. The decision to proceed was. the outcome of detailed consideration of a number of alternative reactor systems backed by development work on key feasibility Items. 

Fig.4. General view of the Winfrith SGHWR with the reactor at power...

Therefore, so far as the site construction programme is concerned, there are few truly novel components in the basic SGHWR system. This applies to a commercial plant of this type as well as the Winfrith SGHWR. 

SYNOPSIS This paper describes the Winfrith SGHWR and its subsequent development for oommercial power stations. Sufficient detail is given to provide an introduction to and overall appreciation of subsequent detailed papers on particular aspects. 

Vented containment was adopted for the Winfrith SGHWR. It comprises a primary containment situated within a secondary containment and connected to it but sealed from it by water lutes. The concrete biological shield surrounding the core and primary circuit is utilised to form the primary containment but the boundary is extended to envelop that part of the refuelling machine above the rotating shield. The power hall, which houses the reactor, turbo-alternator and fuel pond is utilised to form the secondary containment. 

It is shown by comparison with experimental data from zero energy assemblies that the interplay of these parameters is well understood and that in particular the coolant void coefficient of reactivity can be predicted accurately. The representation of the complete core of the Winfrith SGHWR is discussed and it is shown that the validity of the techniques used have been experimentally demonstrated. Finally the nuclear design aspects of larger SGHWRs including those designed for operation with natural uranium fuel are reviewed. 

The onset of instability may be studied with the SPLOSH code. Pig. 4 shows predicted responses of typical Winfrith SGHWR channels to perturbations when they are operated at various power levels. It can be seen that the disturbances are well damped at the channel powers of about 5 MW used in current designs. It is, therefore, concluded that hydraulic instability does not become an effective constraint on performance. 

The random deviations lead to a statistical distribution for the estimated dryout margin. Knowing the power and flow distributions for the core, the probability of dryout for individual channels may be calculated and combined statistically to obtain the dryout probability for the reactor as a whole. Once the probability of dryout has been established as a function of reactor power, it is, in principle, possible to proceed to a discussion of the effect of steady deviations in parameters and of appropriate transients. Table 3 shows an example of probabilities of dryout at overpower estimated with a typical Winfrith SGHWR core distribution of channel powers. In assessing dryout probabilities, the values for individual channels have been combined statistically to obtain a probability of dryout appropriate to the reactor as a whole. From this table, it is clear that the reactor is highly protected against dryout at nominal full power conditions. 

SYNOPSIS This paper describes the bases for control of steam generating heavy water reactor stations and gives some detail of the system adopted for the Winfrith SGHWR. 

A description is provided of the station power, dmm pressure and drum level controls on the Winfrith SGHWR together with the principal features of the emergency protection system. 

Reference is made to commercial SGHWR designs from the point of view of control. The effects of increased size compared with the Winfrith SGHWR on overall dynamic properties and spatial reactor modes are discussed together with control system developments to Improve load following capabilities. 

A mock-up control desk can be connected to the simulator and Is used for training operators. It includes the principal manual controls such as speeder gear and moderator level, together with metered outputs and auto/manual changeover, similar to the Winfrith SGHWR,... 

Fig. 4 Diagram of Winfrith SGHWR drum level and feed control system...

Load following. The rate at which the moderator can be raised on the Winfrith SGHWR corresponds to a reactivity rate of nearly 3xlO 5 k/s at the slowest condition (when the moderator is near full height) giving an Increase of power, relative to. full power, of nearly 20 per min, l.e., approximately twice that specified. This is normally an acceptable rate for dally variations and anticipated load changes. 

Detailed simulation shows that a 500 MW(e) SGHWR could provide a grid load following characteristic in the form of a clean exponential with time constant 15 s. For this, a reactivity rate of 3 mN/s is assumed and the drum size is proportionally similar to the Winfrith SGHWR. Following step changes in grid frequency which corre ond to an additional power demand... 

The simple expedient of using two fuel batches of lower enrichment for the initial loading of the chessboard has been adopted in the Winfrith SGHWR. A 3 batch basic core loading using fuels of 1.24, 1.56/ and 2.28 U-235 has, therefore, been adopted. 

The choice of annual refuelling Implies that the Winfrith SGHWR must operate with a higher mean excess reactivity (and hence fuel Inventory) than the later commercial designs where purely economic considerations are paramount. Moreover the reactor carries a reactivity load in the form of experimental facilities and suffers a higher neutron leakage than commercial reactors due to Its smaller size. All these factors conspire to raise... 

The dual role of the Winfrith SGHWR as a demonstration power producer and irradiation facility for more advanced fuel designs, has led to the adoption of a special fuel management scheme. This involves initial 3-cycle operation with annual major refuelling shutdowns and so will provide large batches of irradiated fuel elements for post-irradiation analysis. An on-load machine has, however, been provided and more advanced fuel management schemes will be demonstrated in the future. 

Attention has been drawn to the accelerating effect of neutron irradiation on the creep of zirconium alloys. The present evidence indicates that this will not cause a problem with cold-worked Zlrcaloy-2 pressure tubes in SGHWR designs. Considerably more information on this aspect of Zircaloy-2 and heat-treated Zr-2- Nb alloy behaviour will arise from current experimental work in research reactors and in the operation of the Winfrith SGHWR. 

The present paper briefly surveys the current situation on the compatibility of materials used in the Winfrith SGHWR and possible future versions, and discusses how this may be affected by the choice of water chemistry for the system. Broadly this entails operating the reactor neutral, under oxidizing conditions due to radiolysis, or with an ammonia addition which must be at a sufficient level to produce reducing conditions, l.e., supress oxygen formation and hence nitric acid formation. 

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