ICENS operates two very powerful complementary nuclear techniques, Neutron Activation Analysis and X-Ray Fluorescence which provide enormous analytical power for a wide range of elements.
Neutron Activation Analysis
Neutron Activation Analysis (NAA) is the flagship analytical method of ICENS. It is probably the most powerful of elemental analytical techniques, and is an important reference method against which other techniques are frequently compared. Analyses are usually done without chemical pre-treatment of the sample, thus avoiding problems related to incomplete dissolution, for example of soils and rocks; loss of volatile; potential combination; and additional expense. Moreover, since neutrons interact directly with the atomic nuclei, the state of chemical combination is not relevant. Theoretically, some 70% of the 88 naturally occurring elements can be determined by NAA.
SLOWPOKE has five available irradiation sites. The neutron flux around the core is radially homogeneous, with less than a 1% difference between the four inner sites. A pneumatic system is used for sample transfer to and from the reactor core. Irradiation times range from seconds to hours depending on the half-lives of the radionuclide involved.
An in-pool irradiation system has been developed for samples up to about 20g. The increased sample size compensates for the 90% reduction in neutron flux and can be advantageous, when as with the analysis of gold, there may be a strong nugget effect and as a result larger samples are desired. ICENS presently routinely measures the concentrations of about 54 elements at parts per million (ppm) or parts per billion (ppb) limits of detection, depending on the element and sample matrix. The limits could be reduced even further in post-irradiation separation techniques were utilized, which currently are not carried out.
Gamma Spectroscopy System
The induced gamma radioactivity is measured with one of four high resolution gamma spectroscopy systems using solid-state germanium detectors. Two systems are dedicated to short-lived radionuclide; the other two, are connected to automated sample changers, for unattended counting over longer periods.
The minimum amounts of elements that can be detected in routine NAA at ICENS are shown:
To date, SLOWPOKE has performed over 30,000 irradiations on some 75,000 samples of rocks, sediments, soils, plants, foods and human and animal tissues.
SLOWPOKE is a tiny nuclear reactor. It was designed by Dr. John Hilborn at the Chalk River Nuclear Laboratory of Atomic Energy of Canada Limited (AECL), specifically for use in universities, hospitals and research institutions. It is a tank in pool reactor i.e. the core is inside a sealed tank containing extremely pure water, which acts both as moderator (slows the fission neutrons for improved fission efficiency) and coolant for heat transfer to the outer pool. The pool is 6.4 m deep, 2.4 m in diameter and 0.3 m thick. It is constructed of specially reinforced waterproof concrete covered with mastic asphalt on the outside, and epoxy resin on the inside. The concrete is separated from the surrounding earth by 0.91 m of packed sand. The pool contains continuously cooled and purified water to a depth of 5.1 meters, for both heat transfer and biological shielding. The maximum rated thermal power output is only 20 kilowatts.
The reactor core consists of an assembly of 296 fuel pins. A 100-mm thick beryllium annulus encases the 23 by 25 cm fuel cage. This, together with a 50mm beryllium disc at the bottom of the core, is the main neutron reflector. The core container is sealed and no internal adjustments are allowed. Long term changes in performance due to fuel burn-up and poison build-up are corrected for by adding top semi-circular beryllium reflector shims, each only a few millimetres thick, to allow very occasional adjustments with usage.
The reactor is simple to operate. The operational features are located on a single console. The power level is controlled to ± 1% of the selected setting on the console by a single cadmium control rod, via feedback from a neutron detector located within the beryllium annulus. This remarkable stability enormously enhances SLOWPOKE's value as an analytical tool.
The reactor power level, cadmium rod position and temperature are continuously monitored, even when the reactor is not in use, and the charts are stored indefinitely. ICENS has an emergency power generator of sufficient size to maintain electricity for all systems during a power loss and in addition emergency power for monitoring is provided by a set of truck batteries.
When a 235U (Uranium 235) nucleus absorbs a neutron, it almost instantaneously splits into two approximately equal parts and produces between two and three neutrons per fission. These neutrons may in turn induce other fissions, be lost by absorption in impurities, or may escape from the fissile mass. The overall behaviour of the chain reaction depends on the multiplication factor, k = (number of fissions in one generation/number of fissions in the preceding generation). If k > 1 the chain reaction grows rapidly with time; if k < 1. the chain reaction cannot be maintained. In a critical system, k = 1 and the number of neutrons is constant with time. This is achieved in SLOWPOKE by the cadmium control rod whose position within the reactor core is set according to the neutron flux desired.
The quantity 100(k-1)/k, known as the excess reactivity, is a convenient measure of reactor behaviour. The very low excess reactivity of SLOWPOKE is an essential element in the safety philosophy of its design. Many research reactors are designed to have very large available excess reactivities (say 2% - 4%) and operators can alter it, vary the positions of the fuel elements, and perform a variety of other operations. Consequently, such reactors must include extensive instrumentation to monitor instantaneous power levels and rates of power increase, and have duplicated and automatically actuated shut-down systems.
SLOWPOKE does not require these because of its large negative temperature coefficient, the low excess reactivity and severely limited operating conditions. When our SLOWPOKE was being commissioned, the excess reactivity was only 0.34% compared with the 0.788% for prompt criticality, and the reactor container was then bolted and sealed. AECL states that no credible operator intervention could increase the reactivity to this value that could produce an uncontrolled chain reaction. The strong negative temperature effect ensure that the reactor efficiency falls off rapidly with temperature, and indeed one impressive commissioning test is to withdraw the control rod completely to demonstrate this self-limiting behaviour. In the UWI test, the core water outlet temperature rose from 18.3°C to a peak temperature of only 68°C, after which the power, that had peaked at about 80 kW, decreased leading to shut down. These two features, low excess reactivity and self-limiting power r excursions, the basis of the intrinsic safety of SLOWPOKE, do restrict reactor operations, but make it possible for laboratories with limited budgets to safely operate the reactor. AECL does not consider the simultaneous loss of both the pool and container water to be a credible event, but, even if coolant were lost with the reactor operating at full power there is no danger of a meltdown.
Due to the simplicity and the inherent safety of the design, regular testing and maintenance of the SLOWPOKE control systems are minimal; the weekly maintenance of the reactor consists of checking out a list of twelve items, including emergency shutdown procedures and backup systems, and generally takes no more than two hours to complete. As part of the Safeguards Agreement to which the Government of Jamaica is a signatory, the reactor has been made subject to inspection by IAEA.
Despite its small size and inherent safety, SLOWPOKE must be operated with care and only according to the agreed protocol. Access to the reactor is strictly controlled and new proposals for its use must be approved by a reactor utilisation committee. These restrictions are adhered to, not only because of safety, but also because SLOWPOKE is an irreplaceable asset.
Since no changes are ever made to the core, there are no fuel cycles or spent fuel storage to be considered. Radioactive waste disposal arises only at the end of the useful life of the reactor, which with the installation of the "big shim", donated to UWI by the University of Toronto, is estimated to be about 30-40 years hence. The contract with AECL specifies that the spent core be returned to Canada.
SLOWPOKE is a remarkable analytical tool. It brings enormous multi-element analytical capability with high accuracy and throughput to ICENS, and is the basis of most of the extensive analytical work being performed.