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. temperature of water 25°C). If moisturizing of reactor hall air would be used, the activity of evaporated tritium would decrease to cca 0.34 GBq/hour. For example of NPP Dukovany the volume of reactor hall is -160 000 mJ and considering complete exchange of air in the ha]l every 2 hours means specific activity of 6.56 Bq/1. But real situation with the source of tritium - water level - near operational staff would be several times worse. Exact theoretical estimate of it is too complicated. Probably, operational staff would have to use protective clothing.

ad c) conversion of liquid tritiated releases into more acceptable gas form

Gas releases generally are better acceptable, then liquid ones. The reason is much higher attenuation of activity in atmosphere, then in waters. There are two possibilities to convert tritiated liquid releases of NPP to gas form. The first possibility is to lead liquid releases to opened evaporating pools, where tritiated water would evaporate to atmosphere, or to lead liquid releases to tertiary circuit of NPP, when tritiated water would be evaporated in cooling towers. This restriction of negative impact of radioactive liquid releases on the environment is very simple, effective and economical. ad d) separation of tritium from reactor coolant.

Actually no simple, effective and sufficiently economical way how to separate tritium from reactor coolant of NPP with LWR reactors is known. Some ways of separation of tritium from water exist, but the main problem is in very small amount of this tritium. There were used some plants for separation of tritium from coolant and moderator at PHWR reactors in Canada (NPP Darlington), but the specific amount of tritium is approximately 600 times higher, then in LWR reactors. The Darlington plant for separation of tritium uses principle of isotope exchange and separated tritium is stored in form of metal tritides. If such a plant would be constructed in Europe, volumes of water reactor coolant could be treated there after finish of operational life, if idea of NPP with changeless volume of coolant would be realised. But considering actual price of tritium cca 100 000 $/g it wouldn't be economicall.

D. Conclusion.

The problem of restriction of tritiated liquid releases of NPP using LWR reactors can be resumed into two basic questions:

1. Which measurements are possible to be realised at operating NPP with LWR reactors? 2. Which changes could be practised in new projects of NPP with these types of reactors?

Working over these questions at Faculty of Mechanical Engineering, Department of Thermal and Nuclear Power Plants gave following answers:

Ad 1) The most economical and faerly efficient way to restrict negative impact of tritium liquid releases on the environment apparently is convertion of liquid tritiated releases to tertiary circuit of a NPP with much higher attenuation of it in atmosphere. Various methods of separation of tritium from reactor coolant hardly can be realised, because of minimal amount of produced tritium. Also the idea of changeless volume of coolant is not applicable in actual NPP. Particularly it is possible to restrict resources of tritium in core of reactor. For example LiOH used in PWR reactors can be supplied with KOU used in VVER. reactors, or to operate reactors with lower concentrations of H,BO, in coolant and higher usage of mechanical and burning absorbators.

Ad 2) In prospective projects of NPP are, in our opinions, possible following ways, which needs a lot of research and development activities:

- to use uranium-gadolinium fuel to restrict role of boron acid, when it could be used only for fuel exchange process and improve technology of production of zircalloy cladding of fuel elements, which would be more resistent against initialization of micro-leakages - to simplify primar circuit and cohering systems with primar circuit with changeless volume of coolant accumulating tritium (with sufficient protection of operational staff especially during fuel exchange period), when the volume would be transfered into a hermetic basin to let the tritium die off after service life of NPP (also releases of active corrosion and fission products would be eliminated)

Table 1: Typical radionuclides in liquid releases of NPP with VVER reactors [9], Medium specific activity in Radionuclide Half-time T1;2 Specific activity of releases outlet channel (after before attenuation attenuation) (Bq/m3) (Bq/m3) (d - days, y -years)

Q51 27,8 d 1.2E-6 4.1E-11 Mn54 303 d 2.5E-7 8.1E-12 Fe59 45 d 2.8E-8 9.2E-13 Co58 71 d 4.IE-6 1.3E-10 Co60 5,2 y 1.1E-6 3.5E-11 Zr95 65 d 1.3E-8 4.1E-13 Nb95 35 d 6.3E-8 2.0E-12 1131 8d 4.1E-7 4.8E-12 Csl34 2y 6.3E-7 2.0E-H Csl37 30 y 8.9E-7 2.8E-11 Sr89 52 d 8.9E-9 2.8E-13 Sr90 28 y 1.4E-9 4.8E-14 Xel33 5,2 d 4.4E-7 1.4E-11 H3 12,3 y 1.0E-3 3.4E-8

Table 2: Radioactivity of H3 stnd of corrosion and fission products in liquic releases of NPP Dukovany [14] Activity of corrosion Specific activity of Electricity Activity of H3 in and fission products Specific activity of corn and fiss. production liquid releases in liquid rel. H3 products Year (GBq) (TWh) (TBq) (TBq/TWh) (Gbq/Twh)

1990 12.585 20.1 0.183 1.60 0.015 1991 12.132 18.3 0.314 1.51 0.026 1992 12.250 19.3 0.100 1.58 0.008 1993 12.627 18.6 0.412 1.47 0.033 1994 12.997 15.6 0.374 1.20 0.029 1995 12.23 14.5 0.170 1.19 0.014 Table 3: Survey of yields of H3 from ternary fission of U and Pu [1|. Fissile nuclide Energy of neutrons Yield of H3 from fission U235 thermal 0.85 -0.99 E-4 160-600keV 2.0 E-4 200 - 800 fceV 2.2 E-4 U233 thermal 0.91 E-4 Pu239 thermal < 1.8 E-4

Chart 1: Specific releases of tritium in liquid releases of several NPPs with LWR reactors.

10,00 X B1990 "5 9,00 8,00 R1991 ictivi l qrr w 7,00 P1992 U w 6,00 P1993 5,00 D1994 4,00 H1995 3,00 1 2,00 111 1 1,00 1 1 ll 0,00 •l 1 1 1II Dukovany, Kosberg, Tomari, Francie1, Francis, Biblis, Biblis, Loviisa, £R JAR JPN bloky900 bloky SRN1146 SRN1240 Rn MWe 1300 MWe MWe MWe

Fig. 1: Relation between cross section of B10(n, 2oc)H3 reaction and energy of neutrons [12].

13 |

B r

2.,,

F \

'LO'IO'IO 10*10 Meutron Znerr.' 'eV)

177 Fig. 2: Simplified scheme of experimental facility for exploring of H3 releases from burnt fuel elements through zircalloy cladding.

Legend: 1 - pressure vessel 2 - burnt fuel assembly 3 - outside insulation 4 - inside insulation 5 - electro heater S - expansion tank ' - nitrogen cylinder 8 - linking pipes • - s»fcty valve 10- closing valve 11 - cooled ouilci 12- pump witt closing fittings

Table 4: Contribution of individual sources of tritium to it's overall activity in primar circuit of NPP with VVER reactor All Sources of tritium Contribution to overall activity (%) Ternary fission 22.2 B10(n, 2a)H3 in coolant 71.6 H2(n, y)H3 in coolant 0.7 Li7(n, na)H3 in coolant 1.6 Li6(n, a)H3 in coolant 3.0 B10(n, 2a)H3 in control rods 0.9

Table 5: Contribution of individual sources of tritium to it's overall activity in primar circuit of NPP with PWR reactor [13]. Sources of tritium Contribution to overall activity (%) Ternary fission 22 BI0(n,2a)H3 57 Li7(n, na)H3 + Li6(n, a)H3 20.6 H2(n. y)H3 0.4

178 Fig. 3: Simplified scheme of simplified primar circuit of NPP with changeless volume of coolant (without emergency systems).

REFERENCES:

1. Hanslik E., Mansfeld A.: Tritium v odpadech, Praha, 1983 2. JakeSova L., Havelka S.: Palivo pro lehkovodnf reaktory s gadoliniovym vyhoh'vajicim absorbatorem, UJV, kei u Prahy, 1986 3. Kepak F.: Separace radionuklidu z plynu, Academia, Praha, 1989 4. Marek J.: Jaderna energetika, Slovak a zivotni prostfedi, Panorama, Praha, 1987 5. Ochrana L, Cetkovsky S.: Provoz, zkouSenf a dozimetrie jadernych central, VUT Brno, 1988 6. Pav T., Saxl I.: Fyzikalni vlastnosti zirkonia a jeho slitin, Rez u Prahy, 1971 7. Phillips J. E., Easterly C. E.: Sources of Tritium, Nuclear Safety 22-5/1981 8. Saro §., Tolgyessy J.: Radioaktivita prostredia, Alfa Bratislava, 1985 9. Seda J.: Dozimetrie ionizujfeiho zafeni, Praha, 1983 10. Vrtilkova V.: Zprava UJP-749: Rozpustnost vodiku v p-zirkoniu, Zbraslav 1994

11. Databank of nuclear data - IAEA, Austria 12. Databanka of nucklear data - JENDL, Japan 13. Preliminary Safety Report of Sizewell B, Nuclear Electric 14. Annual Report of NPP Dukovany, Information centre of NPP Dukovany, 1996