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Content archived on 2024-04-15

STUDIES OF THE GEOCHEMICAL BEHAVIOUR OF ARTIFICIAL AND NATURAL RADIONUCLIDES IN COASTAL WATERS

Objective

THE COASTAL WATERS OF THE NORTHEAST IRISH SEA HAVE RECEIVED, AND CONTINUE TO RECEIVE, A SUBSTANTIAL INPUT OF LONG-LIVED ARTIFICIAL RADIONUCLIDES FROM THE AUTHORIZED DISCHARGES OF LIQUID EFFLUENTS FROM THE REPROCESSING PLANT OPERATED BY BRITISH NUCLEAR FUELS PLC., AT SELLAFIELD, CUMBRIA. THE BEHAVIOUR IN THE ENVIRONMENT OF THESE RADIONUCLIDES, WHICH INCLUDE ISOTOPES OF NEPTUNIUM, PLUTONIUM, AMERICIUM, CURIUM AND TECHNETIUM, GOVERNS THE CONSEQUENTIAL DOSES TO INDIVIDUALS AND POPULATIONS BOTH IN THE SHORT AND LONG TERM. DETAILED INVESTIGATIONS ARE IN PROGRESS OF THE INTERACTIONS OF THE RADIONUCLIDES WITH SUSPENDED AND SETTLED SEDIMENTS BECAUSE THESE PROCESSES ARE SIGNIFICANT DETERMINANTS OF THEIR OVERALL BEHAVIOUR.
THE STUDIES INCLUDE DETERMINATIONS OF THE PARTITIONING OF THE RADIONUCLIDES BETWEEN WATER AND SUSPENDED AND SETTLED SEDIMENTS, THEIR DETAILED DISTRIBUTIONS IN SETTLED SEDIMENTS AND THEIR CHEMICAL SPECIATION. THE PHYSICAL, GEOCHEMICAL AND BIOLOGICAL PROCESSES RESPONSIBLE FOR THE OBSERVED DISTRIBUTIONS ARE BEING INVESTIGATED. THE BEHAVIOUR OF A VARIETY OF NATURAL RADIONUCLIDES IS ALSO BEING STUDIED, BOTH WHERE THESE PROVIDE CHEMICAL ANALOGUES FOR THE ARTIFICIAL NUCLIDES AND, PARTICULARLY, IN THOSE INSTANCES WHERE INFORMATION ON THE RATES OF DYNAMIC PROCESSES, E.G. SEDIMENT MIXING, CAN BE OBTAINED.
The detailed behaviour and distribution of natural and artifical radionuclides in marine sediments in the coastal waters of the northeast Irish Sea were determined and the chemical, biological and physical processes controlling the distributions have been investigated.

The regional distribution of a number of long lived alpha emitting and gamma emitting artificial radionuclides has been examined. The distribution is dominated by the point source Sellafield discharge, modified by the local hydrographic regime, water depth and sediment grain size.

It has been previously confirmed that a principal cause of radionuclide mobility in seabed sediments in the bioturbating activity of certain macrofauna, which can result in the penetration of recently contaminated surface sediment to depths of tens of centimetres. This also has an impact on seabed chemistry. A detailed chronology of artificial and natural radionuclides was determined from cores taken from a site showing little evidence of bioturbation, and was used to attempt to remove the effects of bioturbation from the sediment history of Sellafield discharges.

Naturally occurring radionuclides of the uranium-238 decay series have been used to identify and quantify key biogeochemical processes, including sediment mixing rates and accumulation rates.

A large scale survey of seawater concentrations in United Kingdom waters was conducted, confirming the continuing decrease in radionuclide concentrations in response to the decrease in Sellafield discharges.

A box model (MIRMAID) of the Irish Sea has been developed to predict the distribution of artifical radionuclides released from sellafield.
ELEMENTS OF THE PROGRAMME WILL INCLUDE:
A. THE DETERMINATION OF THE DETAILED DISTRIBUTION AND BEHAVIOUR OF LONG-LIVED RADIONUCLIDES, PARTICULARLY PU-238, PU-239+240, PU-241, AM-241 AND CM-244 IN THE SETTLED SEDIMENTS OF THE IRISH SEA AS A FUNCTION OF BOTH DISTANCE FROM THE EFFLUENT INPUT AND DEPTH IN THE SEABED. SEDIMENT CORES WILL BE COLLECTED FROM RESEARCH VESSELS AND NUCLIDE CONCENTRATION PROFILES DETERMINED BY RADIO-CHEMICAL ANALYSES IN THE LABORATORY. THE PHYSICAL AND CHEMICAL FORM OF THE NUCLIDES WILL ALSO BE EXAMINED. THE DISTRIBUTIONS OF SHORTER-LIVED, PREDOMINANTLY GAMMA-EMITTING RADIONUCLIDES WILL ALSO BE EXAMINED TO PERMIT AN ASSESSMENT OF THE TIME-SCALES OF DIFFERENT PROCESSES.
B. INVESTIGATIONS OF THE PHYSICO-CHEMICAL CONDITIONS OF SEDIMENT INTERSTITIAL WATER AND HOW THESE EFFECT THE CHEMICAL SPECIATION OF THE RADIONUCLIDES AND THEIR BEHAVIOUR, PARTICULARLY THEIR MOBILITY AND PHASE-PARTITIONING IN THE SEABED. METHODS ARE BEING DEVELOPED TO OBTAIN ADEQUATE SAMPLE VOLUMES OF INTERSTITIAL WATER AND TO MAKE THE CHEMICAL ANALYSES UNDER CONDITIONS, AS FAR AS POSSIBLE, UNPERTURBED BY THE SAMPLING PROCEDURE.
C. THE DISTRIBUTIONS OF NATURALLY-OCCURRING RADIONUCLIDES (TN-234, PB-210) BETWEEN WATER, SUSPENDED SEDIMENT AND SETTLED SEDIMENTS WILL BE EXAMINED TO PROVIDE ESTIMATES OF THE RATES OF SCAVENGING AND SEABED MIXING. FOR LONGER TIMESCALES, C-14 CAN BE USED TO PROVIDE VALUES FOR SEDIMENTATION AND MIXING RATES.
D. THE DISTRIBUTIONS OF BENTHIC ORGANISMS IN RELATION TO SEDIMENT TYPE AND DEPTH ARE BEING DETERMINED, AND THE INFLUENCE OF THEIR LIFESTYLE (FEEDING HABITS, BURROWING ACTIVITY, ETC.) ON RADIONUCLIDE DISTRIBUTIONS WILL BE INVESTIGATED.
E. LABORATORY STUDIES WILL BE MADE OF THE FACTORS WHICH CONTROL THE INTERACTIONS BETWEEN RADIONUCLIDES AND NATURAL SEDIMENT PARTICLES.
F. EQUIPMENT WILL BE DEVELOPED TO INVESTIGATE HOW TIDAL CURRENTS AND WAVE ACTION INFLUENCE THE MOVEMENT OF FINE PARTICULATE MATERIAL IN THE IMMEDIATE VICINITY OF THE SEABED-SEAWATER INTERFACE, AND THE EXTENT TO WHICH RADIONUCLIDES ARE TRANSPORTED BY THIS PROCESS.
G. A MATHEMATICAL MODEL WILL BE DEVELOPED ON THE BASIS OF ALL AVAILABLE INFORMATION CONCERNING THE NORTHEAST IRISH SEA TO INVESTIGATE HOW THE DISTRIBUTIONS OF THE SEDIMENT-SORBED RADIONUCLIDES MIGHT CHANGE WITH TIME, AND ALSO TO INDICATE WHERE RESEARCH EFFORT COULD BE MOST USEFULLY APPLIED TO IMPROVE OUR UNDERSTANDING OF THE BEHAVIOUR OF RADIONUCLIDES IN THIS NEAR-SHORE ENVIRONMENT. THE MODELLING EFFORT WILL DEPEND UPON, AND GENERATE, PARTICULAR LINES OF RESEARCH, INCLUDING THOSE IN A - F ABOVE.

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THE MINISTER OF AGRICULTURE, FISHERIES AND FOOD
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