Patent Number: 051715155
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The process of the present invention involves the addition of an effective amount of an aqueous solution of zinc borate, Zn(BO.sub.2).sub.2, to the reactor coolant water of a pressurized water reactor. The zinc ions which are thereby added to the reactor coolant water serve to inhibit corrosion within the pressurized water nuclear reactor system, and more specifically, within the primary circuit of a pressurized water nuclear reactor system. An advantage of using zinc borate as the source of zinc ions is that zinc borate is sufficiently soluble in water. Thus, the zinc ions can be added to the reactor coolant water as an aqueous solution, rather than as a slurry. Preferably, the aqueous solution of zinc borate is an aqueous solution of zinc borate in aqueous boric acid. Zinc borate, as shown in the drawing is sufficiently soluble in a boric acid solution so as to provide a solution thereof for addition to the reactor coolant water. An important advantage of using an aqueous solution of zinc borate in boric acid is that boric acid is often conventionally added to the coolant water of pressurized water nuclear reactors as a neutron absorber or "chemical shim". Generally, the concentration of boric acid in the coolant water of pressurized water reactors for such a purpose is up to about 0.1 molar (M). Thus, the borate anion from the zinc borate will not introduce an additional anion to the coolant water that could serve as a corrosion accelerator, or otherwise upset the chemical balance of the reactor coolant water. The aqueous solution of zinc borate can be prepared by adding a 0.1N (normal) zinc nitrate solution, which has been heated, to a 0.15 normal solution of borax while stirring. The resulting white flocculant precipitate is dried, washed with water, washed with alcohol, and dried again at a temperature of between about 60.degree. C. and about 70.degree. C., to form a fine powder which is readily dissolved in boric acid solutions. An alternative method of preparing the aqueous solution of zinc borate is to react an aqueous solution of boric acid with zinc oxide at an elevated temperature. An equal volume of a 96 percent, by weight, alcohol is added to the aqueous solution of boric acid and zinc oxide to cause the formation of a white gelatinous precipitate of the general composition Zn(BO.sub.2).multidot.H.sub.2 O. These methods are described by B. M. Shchigol, in "Some Properties of Zinc and Cadmium Borates", Russian Journal of Inorganic Chemistry, September 1959, pp. 913 to 915. Zinc borate, identified as Zn.sub.3 B.sub.4 O.sub.9 .multidot.5H.sub.2 O, may also be purchased commercially from Alpha Products of Danvers, Mass. The aqueous solution of zinc borate is preferably concentration of zinc ions in the coolant water is from about 10 to about 200 parts per billion (ppb), more preferably, from about 10 to about 50 ppb, and, most preferably, from about 10 to about 20 ppb. EXAMPLE Laboratory testing was carried out in a refreshed autoclave to evaluate the effect of zinc borate addition on the corrosion behavior of four materials exposed in a pressurized water reactor primary coolant environment. These four materials were 304 stainless steel, alloy 600, Alloy 690, and Zircaloy-4. The surface conditions of the specimens were similar to that in a pressurized water reactor: ground surface for the 304 stainless steel (304) the inside of a tube for Alloys 600 (600) and 690 (690), and the outside of a tube for Zircaloy-4 (Zirc-4). The refreshed autoclave system consisted of a four-liter 316 stainless steel autoclave, a 20 liter 304 stainless steel solution reservoir, a pressurizing feed pump, and a back pressure regulator. The solution reservoir was pressurized with 29 psia hydrogen gas to produce a dissolved hydrogen value of 35 cc (STP)/kg water. Two refreshed tests were performed at 300.degree. C. The first test had the "middle-of-life" (MOL) primary chemistry (500 ppm boron as boric acid and 1.1 ppm lithium as lithium hydroxide) and served as a reference. The second test had the MOL chemistry +100 ppb zinc as zinc borate. The initial and final solutions for each of the two tests were analyzed for lithium and boron. For the second test, analyses were performed for zinc before, during, and after the test. Zinc additions, when needed, were made shortly after the analysis to compensate for zinc depletion. The duration of each of the two tests was 300 hours. The chemical analyses of the solutions in the two tests are as follows: ______________________________________ CHEMICAL ANALYSES OF SIMULATED PWR REACTOR PRIMARY COOLANT WITH AND WITHOUT ZINC ADDITION ______________________________________ Tests 1 and 2 - Li/B Analysis Cond. Solution Test Li (ppm) B (ppm) pH (S/cm) ______________________________________ Initial 1 1.3 500 6.1 11 Final 1 1.1 500 6.2 13 Initial 2 1.3 500 5.8 10 Final 2 1.1 500 6.0 11 ______________________________________ Test 2-Zn Analyses Zn Borate Analysis Calculated Hrs. Addition (mg) MUT AC MUT ______________________________________ 0 4.8 92 100 48 78 15 50 1.0 99 144 86 33 146 1.0 107 216 102 38 218 None 300 82 72 ______________________________________ Note that the zinc in the autoclave (AC) did not approach the 100 ppb test conditions until near the end of the test. On the average, the specimens were exposed to a zinc concentration of about 40 ppb. The behavior of the zinc concentrations in the autoclave and makeup tank (MUT) during the first half of the test indicates that zinc was being deposited or incorporated into the surface of the autoclave or sample specimens. As the test progressed, the rate of zinc incorporation was reduced, since the final analysis showed the concentrations in the makeup tank and autoclave were about equal. After the 300 hour exposure, the specimens were removed from the autoclave and photographed. Scanning electron microscope photographs were also taken to determine if the zinc addition had affected the film morphology and for evidence of incorporation of zinc into the corrosion film. Photographs of the four specimen surfaces exposed to normal and zinc added coolant showed slightly darker surfaces for 304, 600 and 690 specimens while there was not much difference between the Zirc-4 specimens. Since darker oxides on specimens exposed under the same conditions are generally thicker, the lighter oxide on the three materials suggest that the zinc borate addition has produced a thinner oxide than was produced without the addition. As a result of the process of the invention, the corrosion within the reactor is inhibited. Thus, the transport of corrosion products and radioactive cobalt ions through the reactor, as well as levels cf radioactivity within the reactor, are reduced.