Inhibition of nuclear reactor coolant circuit contamination

Abstract: A method of inhibiting or reducing contamination of out-of-core components of the primary coolant circuit of a pressurised water reactor due to absorption by oxide films of such components of cobalt and other radioactive contaminants generated by neutron irradiation of coolant impurities while in the reactor core, the method consisting of intermittently (eg at reactor shutdown for maintenance purposes), and/or once-off during commissioning of the reactor, holding the temperature of the circulating coolant within a suitable range below normal operating temperature for an extended period while maintaining its boric acid concentration substantially higher than during normal operation. Inhibition of absorption by component oxide films persists long-term and predisposes to preferential and improved removal of contamination by the usual purification equipment which operates continuously during normal operation of the reactor.

Description: This invention relates to the contamination of nuclear reactor coolant circuits by radionuclides, and to a method of inhibiting or reducing such contamination. In particular, the invention relates to the inhibition of radioactive contamination of out-of-core components of the primary coolant circuit of a pressurised light-water-cooled nuclear reactor (PWR). In a PWR of well known kind, the primary coolant circuit, through which heat is removed from the reactor core, contains light water which serves both as coolant and as moderator and is circulated by a pump through the core (in which it absorbs heat) and then through the primary side of a tube-in-shell heat exchanger (in which the heat is transferred to a secondary system in which a separate supply of water is converted into steam) before being recirculated to the core. The primary circuit coolant water leaves the core at a temperature of about 320°C, and to prevent bulk boiling at that temperature it is maintained at high pressure by means of a pressuriser which also forms part of the primary coolant circuit or system. The coolant water reacts chemically with the materials forming the primary circuit, producing oxide films on the stainless steel and other alloy surfaces and releasing soluble metal impurities into the coolant water. The surface oxides become very complex since a number of mass transfer processes contribute to their formation. A dense, adherent oxide layer forms on the alloy surfaces (which has a beneficial inhibiting effect on corrosion rates) and a porous, non-inhibiting layer develops on top of the adherent layer. In addition to the oxidation of the alloy constituents at the oxide/metal interface, metal oxide deposition can occur from saturated coolant water, insoluble material of the porous layer can become resuspended in the coolant flow, and dissolution/precipitation occurs during cooling and heating cycles. Although empirical chemical formulae, such as nickel ferrite, can be given to these surface deposits, they are generally very heterogeneous. Thus corrosion products, both soluble and insoluble, exist within a reactor primary coolant circuit. While the reactor is operating, these corrosion products are transported into the reactor core where they become activated by interaction with neutrons. A wide array of radionuclides is thus produced, but the following Table lists those of major interest. Principal gamma-emitting nuclides formed by neutron irradiation from corrosion products in water reactors. Parent Nuclides Radio-Nuclide Formation Mechanism Half-Life Gamma Emission Energy (MeV) %Parent Natural Metal ⁵⁰Cr ⁵¹Cr n,γ 28d 0.3 4.3 ⁵⁸Fe ⁵⁹Fe n,γ 45d 1.1 0.3 1.1 ⁵⁸Ni ⁵⁸Co n,p 71d 0.8 68 ⁶⁴Zn ⁶⁵Zn n,γ 224d 1.1 49 ⁵⁴Fe ⁵⁴Mn n,p 313d 0.8 5.8 ⁵⁹Co ⁶⁰Co n,γ 5.28y 1.2 100 1.3 Of these radionuclides, cobalt-60 with its long half-life and powerful gamma emission usually accounts for some 80% of the radiation dose received by PWR operations staff. After leaving the reactor core, the radioactive corrosion products sorb or deposit on out-of-core alloy oxide films, yielding radiation levels there which may be as high as tens of millisieverts per hour. It is highly desirable that such radioactive contamination of out-of-core components of the reactor primary coolant water circuit or system (which occurs despite the provision of coolant purification equipment through which water from the reactor primary coolant circuit is constantly circulated during normal operation of the reactor) should be minimised. Methods for reducing such contamination have already been proposed. US Patent No 4 526 626, for example, is directed to a method of reducing such contamination in a PWR by subjecting the primary coolant circuit surfaces, exposed to the coolant water, to an anti-corrosion treatment using chelating agents to build up on such surfaces a corrosion resistant oxide layer containing chromium and iron but substantially free of cobalt and nickel. However, although the corrosion resistant layer so built up may initially be almost free of cobalt, it can prove to have a strong attraction for that element which leads to its becoming progressively more contaminated with the passage of time, due to the unavoidable presence of residual levels of cobalt in the system. Furthermore, the corrosion resistant film so produced suffers, like those resulting from earlier treatment methods referred to in the said patent, from a tendency to flake off the underlying surface and thus fail in its intended function of protecting it against corrosion. It is an object of the present invention to provide a more convenient and satisfactory method of inhibiting or reducing such contamination, and in particular cobalt contamination, in the primary coolant circuit of a pressurised water cooled reactor. According to the invention there is provided a method of inhibiting or reducing radioactive contamination of out-of-core components of the primary coolant circuit, containing light water with a boric acid content, of a pressurised light-water-cooled nuclear reactor by radionuclides generated within the core of the reactor by irradiation of metallic impurities dissolved in and carried by the circulating coolant water, the method comprising the steps of providing for continuous removal of such impurities and radionuclides generated therefrom from the coolant water by continuous diversion of a proportion of the coolant water, during normal operation of the reactor at normal operating temperatures and boric acid concentrations of the coolant water, through coolant-water purification equipment associated with the primary coolant circuit, and, prior to a period of such normal operation of the reactor, circulating the coolant water at lower temperature and higher boric acid concentration through the said out-of-core components, while maintaining the water at a lower temperature in the range from 100°C to 150°C, and maintaining a higher boric acid concentration than during normal operation, during an extended period of time of at least 12 hours, the temperature, concentration and time period being such as to inhibit the uptake of the said radionuclides of such components before and during subsequent normal operation of the reactor. The step of circulating the coolant water at lower temperature and higher boric acid concentration than in normal reactor operation may be carried out intermittently (between successive periods of normal operation of the reactor), notably during an approach to reactor shut-down to allow for routine refuelling and maintenance, or during the subsequent return from the shut-down condition to resume normal operation; but an especially favourable time for carrying out this step is during initial commissioning of the reactor (preferably towards the end of the hot functional commissioning period which is when the boric acid injection system is being proved) before the reactor has been fuelled and thus before its first period of normal operation, thereby to effect removal of impurities from the coolant water before any irradiation thereof has occurred in the reactor core. Preferably the continuous diversion of coolant water through the coolant-water purification equipment is maintained during the period of circulation at reduced temperature and increased boric acid concentration, as well as during subsequent normal operation of the reactor, since during that period (even though it will be relatively short) the availability of impurities to be removed from the system by the coolant-water purification equipment is much higher than during the subsequent period of normal operation. Nevertheless, during the subsequent period of normal operation the concentration of dissolved impurities available to be scavenged by the purification equipment remains at an enhanced level, due to use of method of the invention, compared with what would otherwise be the case. The invention will be more fully explained and understood by reference to the following description of experiments and investigations carried out in connection therewith. In a PWR, the primary coolant water normally contains additions of lithium hydroxide to control or limit metal corrosion rates, as well as boric acid to control the reactivity of the reactor, and dissolved hydrogen in amounts between 25 and 50 cm³/kg to control radiolytic breakdown of the coolant water. The concentration of cobalt in the coolant water at operating temperatures (about 320°C) was found experimentally to be no more than 5-10 parts per trillion (ppt) or 5-10 µg of cobalt per tonne. It was also found (in actual reactors and in an experimental rig simulating a reactor primary coolant circuit but without the associated coolant-water purification equipment) that as the primary coolant is allowed to cool, as occurs during the approach to reactor shut-down for maintenance purposes, higher cobalt concentrations are measurable in the coolant water. With the chemical conditions of the coolant water, in the experimental rig, adjusted as normal for reactor start-up or approach to shut-down, ie a boron concentration of 2000 ppm (parts per million) and less than 0.1 ppm of lithium, and with no hydrogen overpressure but with dissolved oxygen held at 1 ppb (parts per billion) or less by an addition of hydrazine or provision of a nitrogen overpressure, it was found that maximum cobalt concentration in the coolant water was obtained at temperatures in the range of 120-140°C. Doubling the boron concentration to 4000 ppm was found to reduce the optimum temperature to about 100°C but not to change significantly the maximum concentration of cobalt, which remained at about 5 ppb. This is 500-1000 times as great as the cobalt concentration to be found in the primary coolant water of a PWR in normal operation, so that in a reactor diversion of coolant water through purification equipment during the period of the low-temperature regime will result in a favourably increased rate of removal of the cobalt from the coolant water; and the longer the duration of this low-temperature regime, the greater the amount of removed cobalt may be expected to be, though, of course, the maximum available duration will usually be limited by the requirements of other procedures being carried out on or around the reactor at the same time. The above-outlined results were obtained by means of experiments which involved circulating the borated coolant water round the experimental rig for prolonged periods, at temperatures in a range from about 150°C down to below 100°C. After completion of these experiments the coolant water temperature was raised to 300°C, the boron and lithium concentrations were adjusted to 1200 ppm and 2.2 ppm respectively and a hydrogen overpressure was provided. It was found that the soluble cobalt concentration remained at an enhanced level which stabilised at about 500 ppt, which is 50-100 times that normally present in power reactor coolant water at 300°C. Further studies then showed that, as circulation of the coolant water in the experimental rig was continued over several months under constant chemistry and temperature conditions corresponding to normal rector operation, the soluble cobalt concentration decreased very slowly. Soluble cobalt at such a relatively high concentration would normally be rapidly captured by the oxide films on the surfaces contacted by the coolant water, so rapidly reducing the cobalt concentration, however this acquisitive behaviour of the oxide films is inhibited by the treatment to which they have been subjected, namely exposure over a period of time to coolant water at temperature intermediate between those obtaining during reactor operating and shutdown conditions and with a boron content higher than would be provided during normal operating conditions of a reactor. It follows that if the oxide films in out-of-core components of a reactor primary coolant circuit are inhibited by similar treatment, then, during subsequent normal operation of the reactor (including circulation of a proportion of the primary coolant water through coolant purification equipment, in normal manner), natural cobalt and radioactive cobalt produced in the core by neutron irradiation of coolant water impurities will be acquired by out-of-core oxide films to a lesser extent, and will remain available to be removed by the purification equipment to a greater extent, than would otherwise be the case. Solely for illustrative purposes, if the soluble cobalt concentration in the coolant water in the primary coolant circuit of a PWR were increased to 500 ppt with a flowrate of 20 tonne/hour through the ion exchange purification beds of the purification equipment, 24g of cobalt would be removed over a 100 day period of power reactor operation. Results obtained in a further experiment carried out in the same experimental rig show, in broad terms, the same effects. In this later experiment, water at 120°C and containing 4000 ppm of boron was circulated round the rig for two days, after which very high dissolved cobalt levels, approaching 200 ppb, were measured. When the temperature and boron content of the water circulating in the rig were then adjusted to correspond to normal PWR operation, the soluble cobalt concentration decreased to 150 ppt and then to 50 ppt over a period of 3-4 days. Thus, compared with the earlier results, a very substantially higher maximum cobalt concentration was then followed by a much steeper fall over a much shorter period; but the final cobalt concentration was still greater, by a factor of 5-10, than the concentration which is found, under normal reactor operating conditions, if the coolant water and the oxide-covered surfaces in contact with it are not subjected to the low-temperature high boron treatment. If a similar response to the low-temperature high-boron treatment were obtained in an actual PWR with coolant-water purification equipment, the total removal of cobalt, integrated over the period during and after the treatment, could amount to up to 200g of cobalt. Further investigations using the technique of secondary ion mass spectrometry (SIMS) analysis have confirmed that the low-temperature high boric acid treatment results in increased dissolution of cobalt, nickel and iron from established oxide films in both the experimental rig and the primary coolant circuit of a PWR. It is found that dissolution of these cations persists down to depths of several hundred nanometres. Since chromium is not dissolved, the residual oxide shows an increase in its chromium to cobalt, chromium to nickel and chromium to iron ratios; and this agrees with the findings that the soluble levels not only of cobalt but also of nickel and iron in the coolant water in the experimental rig were all much higher after the low-temperature boric acid treatment had been carried out than before. The residual oxide film, after such increased dissolution from it of cobalt, nickel and iron, is in an unstable state from which, once standard physico-chemical coolant conditions have been restored, it tends to restabilise by re-absorbing cobalt, nickel and iron. Such re-absorption is found, however, to be relatively slow; it appears to be inhibited by changes produced in the form of the film during its depletion. In consequence, a relatively high dissolved cobalt level persists for a considerable period after the low temperature treatment has ceased. At least in the case of a PWR in which, during normal operation, the primary coolant water has a boric acid concentration in the vicinity of 1200 ppm at start of cycle and leaves the reactor core at temperatures close to 320°C, it is preferred that in carrying out the method of the invention the coolant temperature is reduced into the range 100-150°C and its boric acid concentration is increased to 2000 ppm or more, and these conditions are maintained for a period greater than twelve hours. Boric acid concentrations of 4000 ppm or even higher levels, up to say 6000 ppm, can be contemplated. During a normal shutdown or start-up of a PWR, the primary coolant usually passes through the temperature range 120-140°C in less than an hour, which is insufficient to produce in any significant degree the contamination-inhibition which the invention is able to confer. However, the method of the invention can in practice be carried out, during an approach to reactor shutdown or during subsequent start-up, by providing a "hold" in the process, ie deliberately extending to an adequate length the period during which the coolant temperature and boric acid concentration remain within their respective suitable ranges.

Claims: A method of inhibiting or reducing radioactive contamination of out-of-core components of the primary coolant circuit, containing light water with a boric acid content, of a pressurised light-water-cooled nuclear reactor by radionuclides generated within the core of the reactor by irradiation of metallic impurities dissolved in and carried by the circulating coolant water, the method comprising the steps of providing for continuous removal of such impurities and radionuclides generated therefrom from the coolant water by continuous diversion of a proportion of the coolant water, during normal operation of the reactor at normal operating temperatures and boric acid concentrations of the coolant water, through coolant water purification equipment associated with the primary coolant circuit, and characterised by, prior to a period of such normal operation of the reactor, circulating the coolant water through the said out-of-core components, while maintaining the water at a lower temperature in the range from 100°C to 150°C, and maintaining a higher boric acid concentration than during normal operation, during an extended period of time of at least 12 hours, the temperature, concentration and time period being such as to inhibit the uptake of the said radionuclides by such components before and during subsequent normal operation of the reactor. A method as claimed in claim 1 and including the step of continuously diverting a proportion of the coolant water through the said purification equipment simultaneously while circulating it at the said lower temperature and higher boric acid concentration during the said extended period of time. A method as claimed in claim 2 and carried out during initial commissioning of the reactor, before fuelling and before the first period of normal operation thereof. A method as claimed in claim 1 or claim 2, wherein the said extended period during which the coolant water is circulated at lower temperature and higher boric acid concentration than normal is a period between two successive periods of normal operation of the reactor. A method as claimed in claim 4, wherein the said extended period is achieved during an approach to reactor shutdown after a period of normal reactor operation, or to normal operation from the shutdown condition, by deliberately extending to adequate length the period during such approach during which the temperature and boric acid concentration of the coolant water are maintained within suitable ranges respectively lower and higher than those pertaining to normal reactor operation. A method as claimed in any of claims 1 to 5 applied to a reactor in which during normal operation the primary coolant water is heated to at least 300°C and is maintained at a boric acid concentration corresponding to a boron content of 1200 ppm or less, wherein during the said extended period the coolant temperature is maintained at a lower value in the range of approximately 140°C to 100°C and the boric acid concentration is maintained at a higher level corresponding to a boron content in a range of approximately 2000 to 6000 ppm. A method as claimed in any one of the preceding claims wherein the duration of the extended period of time is at least two days.

Inventors: CAMPION PETER, CAMPION, PETER
Applicants: ATOMIC ENERGY AUTHORITY UK, UNITED KINGDOM ATOMIC ENERGY AUTHORITY
Application year: 1988
Main classification: G21C17
IPC codes: G21C17, G21C19, G21D1