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Tritium in Liquid Releases of Nuclear Power Plants With TRITIUM IN LIQUID RELEASES OF NUCLEAR POWER PLANTS WITH VVER AND PWR REACTORS AND SOME WAYS TO SOLUTIONS OF IT S REDUCTION Dalibor SlMEK, Frantisek DUBSEK, Technical University of Brno, Department of Thermal and Nuclear Power Plants A. Introducing. — Tritium is. as for radioactivity, the dominant radionuclide in liquid releases of nuclear power plants with PWR or VVER reactors (tables 1,2; chart I). Even though the tritium is pure p-emitter with only 18 keV of maximum energy of beta particles, increasing concentrations of it in the environment is objectionable because of it's ability to incorporate to DNK and RNK molecules of organisms and it's activity can cause somatic and genetic impacts, which can reveal after several generations. Some of researches even revealed, that projection effects of tritium causes 1/10 of all impacts of chromosomes in a organism. Chemical effects of beta-activity of tritium are considered as more dangerous, because tritium transmutes to inert helium, which immediatelly interrupts all chemical bonds. It is difficult to evaluate the exact toxicity of tritium, but thanks the theoretical considerings of it's presumable longtime undesirable impacts, the tritium is considered as internally dangerous. It is the reason, why Department of Thermal and Nuclear Power Plants of TU of Brno already for three years focuses at the problem of tritium in liquid releases of NPP. The main tasks are to deeply analyse sources of tritium in core of L WR reactors and to suggest next research aims and concrete measurements leading to particular decrease or complete elimination of releases of tritium from nuclear power plants to environment, as way to their enhanced safety. B. Resources of tritium in LWR reactors. There are three basic phenomena producing tritium in core of a LWR reactor: 1. reactions of neutrons with chemical substances dissolved in reactor's coolant to control reactivity of core (H3BO3) or chemical conditions of primar circuit (LiOH in PWR reactors, VVER reactors mostly use KOH which almost doesn't produce H3) 2. reactions of neutrons with deuterium - H2, which is naturally present in water used as coolant of reactor (0.015%) 3. ternal fission of nuclear fuel, when during fission of Uranium or Plutonium also one light nuclei appears, which often is nuclei of H3 ad 1) Tritium is produced with reaction B10(n, 2a)H3. Fig. 1 shows relation between cross section of this reaction and energy of neutrons. ad 2) The resource of tritium with this way is reaction H2(n, y)H3. Cross sections for fast and thermal neutrons are following: 2 in ac"' = 5.50 E-32 [m ] (E - 0.025 eV) tn ac = 7.08 E-34 [nr] (fission spectrum of neutrons) ad 3) Production of tritium with ternary fission: U235 i- n ~> X, + X2 + H3. or Pu239 + n-»X, + X2 + H3. CZ9727159 and the yields of tritium from these reactions are shown in table 3. This means annual production of tritium approximately 0.4 - 0.9 TBq/MW(e)-yr, and literature resources list average value of the yield 1/10000 to 2.2/10000 per fission. Tritium produced with ternary fission in fuel can get through cladding to coolant. These releases are caused with micro-leakages in the zircalloy cladding, and the concrete values in literatures vary from 0.013% to 1% of overall tritium produced in fuel with ternary fission, and some conservative estimates consider even 10%. As the values are so different, there was carried out a project of experimental facility for determining^of tritium releases from burnt fuel assemblies to reactor coolant with accumulated tritium at our department. Simple scheme of this facility, which also simulates pressure and temperature conditions of primar circuit, is shown at fig.2. All these resources of tritium contribute to it's total activity in primar circuit with approximate rate shown in table 4 (for reactor VVER 440), and table 5 (for reactors PWR 1000). A. Various approaches to reduction of tritium activity in liquid releases ofNPP. These ways to reduction of tritium activity in liquid releases of NPP with LWR reactors are considered: a) to reduce the tritium resources as the most efficient way of solution of this problem b) to use new design of primar circuit with changeless volume of coolant to reduce tritium activity in liquid releases c) conversion of liquid tritiated releases into more acceptable gas form d) separation of tritium from reactor coolant. ad a) Reduction of tritium resources in core of LWR reactors. - The reduction of tritium activity in reactor coolant especially means to substitute boric acid, used for reactivity control, with another control mechanism. Boric acid - H3BOJS which concentration in reactor coolant changes during operation from cca 5g/l to 0g/l in dependence of fuel burning, also minimalizes deformation of neutron flux (which results in longer service life of reactor), and enables negative reactivity of reactor core during fuel exchange, when control rods are out of core. Though use of boric acid also has some undesirable effects: • higher concentrations of H3BO3 in reactor coolant causes, that the absolute value of negative temperature coefficient of reactivity decreases and this coefficient transfers into plus values (approximately from 8g/l of boric acid) • boric acid can, in contempt of all measurements, cause corrosion of components of primar circuit • systems connected with using of H3BO3 rather complicate all the primar circuit • higher concentration of boric acid also causes lower effectivity of control rods So it is necessary to try to solve problem of substitution of homogenous absorbator H3BO3 with another control mechanism (more control rods, uran-gadolinium fuel, mechanical absorbators for fuel exchange process, etc). It would bring simplier control of chemical regime of coolant, simplified primar circuit, decrease of investment and operational costs. On the other side, features of homogenous reactivity control using boron acid are so good, that newly suggested solutions would have to be well comparable, and also the changes would widely change the design of fuel assemblies and all the reactor cores. - Ternary fission, respective the releases of tritium from fuel to reactor coolant, is the second most significant resource of tritium in primar circuit. Tritium gets through micro-leakages of fuel cladding to coolant. Diffusion releases are almost unpossible, as the major element of material of cladding - zirconium (cca 98 %) chemically binds tritium and creates stabil trilides ZrH3, which disables tritium to get through the cladding. It is very useful aside effect of usage zircalloy as material of cladding. If stainless steel would be used, approximately 80% of tritium inventary in fuel elements would release to reactor coolant. It means, the onty-way how to restrict this tritium resource is to improve quality of production of zircalloy cladding to reduce appearing of micro- leakages in it and also to operate reactors without quick power changes, which especially intenzifies initializations of the micro-leakages in material of cladding. ad b) Simplified design of primar circuits of NPP with LWR reactors. Suitable patterns in this problem are NPP with PHWR reactors, which use heavy water as coolant. High costs of heavy water and much higher tritium activity produced in coolant with reaction of neutrons with deuterium demand almost changeless filling of primar circuit (annual losts even less than 1% of primar circuit volume). But it is also enabled with possibility of fuel exchange during reactor operation without depressurizing and opening of primar circuit, which is necessary at LWR reactors. In project of NPP with LWR reactors with simplified primar circuit with changeless filling of water, consequent particular problems were solved: 1) impact of accumulated tritium on neutron-physics characteristics of reactor core 2) assessment of impact of accumulated tritium in primar circuit on operational staff and possible ways how to minimize it 3) considering of possibilities to treat the volume of primar circuit coolant with accumulated tritium after service life of a NPP 4) scheme of simplified primar circuit of a NPP with changeless volume of reactor coolant ad 1) Detail analyse shows that there would be almost no impact of tritium accumulated in primar 3 circuit on neutron-physics characteristics of reactor core, because only 0,625 cm of (H3)2O would be produced in reactor coolant of VVER reactor, considering 30 years of operation of NPP. ad 2) There is a little worse situation when considering impact of primar circuit with accumulated tritium on operational staff, especially during fuel exchange period with depressurized and opened reactor, when tritiated water evaporates into reactor hall. But this situation also can be well solved. After 30 years of operation of VVER 440 reactor 74 TBq of tritium is accumulated in primar circuit, which is cca 392 GBq/m3 of specific activity of coolant, whilst recommended top limit of specific activity in NPP Dukovany is actually 11 GBq/m3, it is 35.6 times lower value. Limits for ingestion are 3.7 GBq/m5 for operational staff, and 0.12 GBq/m3 for other people. Top values for inspiration are 0.185 MBq/m3 for operational staff but annual limit of intaken tritium is 0.444 GBq. This shows, that it would be necessary to devote a lot of attention to fuel exchange period. Considering actual design of primar circuit following measures are possible: - minimize evaporation of tritiated water from water level with moisturizing of air in reactor hall - make air barrier over water level of pools for fuel exchange using draining of vapor, similarly like at pool of burnt fuel - establish using of suitable protective clothing for operational staff participating in fuel exchange or another actions with depressurized and opened reactor 174 All these three possibilities were solved at the department and the results are following: - Activity of evaporated tritium from water level during fuel exchange is cca 1.1 GBq/hour (considered air humidity in reactor hall is 30% and temperature 27°C.
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