Chemical and volume control system (CVCS) The CVCS with its main subsystems, the boron thermal regeneration system (BTRS) and boron recycle system (BRS), supports the RCS in the following areas:

• Control of RCS inventory.

• Maintenance of chemical specification in terms of lithium, boron, hydrogen and anion/cation concen­trations.

• Addition or removal of boron for reactivity control.

• Control of activity in terms of corrosion product (soluble and particulate) and fission product removal.

• Recycling boron as appropriate.

The CVCS can be regarded as a chemical control loop and is shown diagrammatically in Fig 1.64. The flow rate is controlled by two normal letdown ori­fices (each rated at 17 mVh although much higher on some plant) and the excess letdown orifice (typi­cally 10 mVh), which reduce system pressure to the CVCS and may be used in any combination. For a typical coolant volume of 240 m3 and a core flow of

18.5 x 103 kg/s, the CVCS flow would be between 0.025% (normal) and 0.040% (maximum), which re­cycles the coolant inventory in 14.2 h.

The CVCS letdown flow is first treated for ionic impurities (fission and corrosion products) by a flush — able mixed bed demineraliser, with the cation resin (40%) in the (Li-7)+ form and the anion resin (60%) in the hydroxyl (OH)- form. The cation resin removes impurity cations as soluble and active species and the Li-7 form is used rather than H-t — form to minimise any pH change. The anion resin is converted into the borate form in operation, implying a small loss of borate from the system initially, and the borate sub­sequently exchanges for impurity anions. The mixed bed demineraliser is designed to reduce the concen­tration of isotopes (except Rb and Cs) by a factor of 10, and is sized to remove fission products and corro­sion products from one fuel cycle with 1% defective fuel. The resin beds also act as crude mechanical filters for particular material. There are two mixed bed demineralisers with one normally on standby and, if necessary, a resin charge can be replaced with the plant on load.

A flushable cation resin bed in the hydrogen form is located downstream of the mixed bed deminerali­sers and is used intermittently on the letdown flow to control the Li-7 concentration. The separate cation unit is sized on the Li-7 production in the core (from the B-10 (n, a) Li-7 reaction) for base-load operation. It also has sufficient capacity to maintain Cs-137 con­centration in the coolant below 0.03 /iCi/cc with 1% defective fuel. The unit is designed to reduce all cation impurity concentrations by a factor of 10. Downstream of the ion exchange units is a filter de­signed to collect resin fines and particulates of 25 fim or greater.

Control of gaseous fission products, including tri­tium, and maintenance of the dissolved hydrogen lev­el is carried out in the volume control tank of the CVCS. The primary coolant is sprayed into a hydro­gen atmosphere at 1.03 bar in order to strip fission products (noble gases such as krypton and xenon) and produce a 25-50 cc (STP) Нг/kg H2O concentration in the circulating primary coolant. Waste H2 contain­ing fission products is passed to the gaseous waste processing system (GWPS).

The CVCS contains two boric acid tanks for stor­age of sufficient 4-4.5 wt % boric acid solution for refuelling, plus safe shutdown from full power opera­tion with a new core and most of the control rods not inserted. A batching tank is provided for making up this solution and the contents of the boric acid tanks will be oxygenated due to equilibration with air. However, flow into the primary circuit is small compared with the coolant inventory, and the com­bination of oxygen stripping in the VCT, dissolved hydrogen and radiolytic reactions will ensure that the RCS does not become significantly oxygenated by boron injection.

The CVCS also provides preparation and injec­tion facilities for LiOH solutions for pH control, and N2H4 solutions for chemical de-oxygenation prior to

start up. It is noted that N2H4 addition leads to the formation of NH3 and under these conditions the CVCS demineraliser beds would be bypassed in order to avoid adsorption of NH4+ and release of Li + .

Reactor make-up water system (RMWS)

This system supports the RCS chemistry control by providing a source of high quality water to maintain coolant inventory. The system receives water from the degasifier tank of the demineralised water supply sys­tem, and also from other recycle sources in the bo­ron recycle system. The water is stored in a large tank at atmospheric pressure, covered by a movable diaphragm to prevent egress or ingress of gases and protected against freezing by heaters. The water may
be recycled through a degasifier and demineraliser to meet the specification given in Table 1.26 and if ne­cessary can be purified by re-evaporation.