• PDF / 502,610 Bytes
• 10 Pages / 420.48 x 639 pts Page_size
• 62 Downloads / 216 Views

DOWNLOAD

REPORT

---

EFFECTS OF RADIATION DAMAGE AND RADIOLYSIS ON THE LEACHING OF VITRIFIED WASTE* W.G.BURNS, A.E.HUGHES, J.A.C.MARPLES, R.S.NELSON and A.M.STONEHAM Atomic Energy Research Establishment, Harwell, Didcot, Oxon OX11 ORA,

U.K.

INTRODUCTI ON Attack on the glass by leaching in water is

the only probable process by

which the long-lived radioactive isotopes incorporated in vitrified waste might be returned to the environment after future disposal.

Radiation from the

incorporated waste could increase the leach rate either by damaging the structure of the glass or by radiolysing the water near the glass surface to produce ionic species that attack the glass more readily. A consideration of the various radiation damage processes leads to the conclusion that the most important contribution to damage to the glass comes from the recoil nuclei resulting from a-decay of actinides incorporated in

the waste,

together with the fission products 1-3: these actinides are chiefly curium, americium and neptunium,

together with any small amounts of plutonium and

uranium that have not been removed for future use as nuclear fuel.

The re-

coiling actinide nucleus from an a-decay has an energy of about 100keV and displaces over 1000 atoms in the glass;

although the a-particle has a much

greater energy ("5-6MeV)

lost in ionising the atoms in

and this ionisation is glass.

most of this is

its

path

thought to have mostly only a transitory effect on the

The S-particles

(electrons) emitted by the fission products likewise

lose almost all their energy by ionisation. The conventional way to test the ability of the glass to withstand the effects of a-decays is emitter such as

238

to dope samples of the glass with a short half-life a-

Pu or one of the curium isotopes 1-8 .

glass to precisely the same process as will occur in greatly increased dose rate. increases in

leach rate.

This subjects the

the real waste but with a

These experiments have not shown significant

However,

recent work in which the radiation damage

has been produced with ion beams has suggested that after a critical dose of radiation a large increase in 9-11 fifty

leach rate could occur, by up to a factor of

In addition, leach tests in a flux of y rays have shown that radiolysis 12-:4 effects in the water can increase the leach rates of the glasses *Parts of this work were commissioned jointly by the UK Department of the Environment and the Commission of the European Communities as part of their Radioactive Waste Management Research Programmes. The results may be used in the formulation of UK Government policy but at this stage they do not necessarily represent such policy. The work is described in more detail elsewhere 3 .

340

ION-BEAM IRRADIATIONS Dran et al?-II irradiated glasses and crystalline minerals with a beam of 200 keV Pb ions and showed that an enhanced leach rate occurred above a criti2

cal dose of about 5 x 1012 ions.cm- , which corresponded to the dose at which the zones damaged by each Pb ion overlapped; 100

and a length of 500R. 21