Page 231 - Dynamics and Control of Nuclear Reactors

230 CHAPTER 16 Nuclear plant instrumentation

measurements in the nuclear measurements in the nuclear steam supply system are
similar. The steam generator is the U-tube type like the type used in most pressurized
light water reactors and required measurements are pressure, temperature, level, and
flow. Primary coolant temperature is measured with resistance temperature detectors
and neutron flux is measured with ex-core detectors and in-core detectors.
In-core neutron detectors are located in different regions of the core. Approxi-
mately 102 Vanadium self-powered detector are inserted vertically and are used
for flux mapping. These detectors have slow response time and provide excellent
flux measurements at steady-state conditions and are not used for reactor control.
Twenty-eight (2 for each of the 14 zones) platinum-clad Inconel detectors are
inserted vertically from the reactivity mechanism deck and are used in the reactor
regulation system (RRS) [8]. Additionally, 34 platinum-clad Inconel detectors are
inserted vertically and 24 are inserted horizontally at various channels as part of
the reactor shutdown system (SDS). One set of in-core BF 3 ionization chambers
is used for flux measurement during start-up. A set of ex-core BF 3 detectors is used
for power measurement. The in-core BF 3 detectors are withdrawn after the initial
start-up phase.
The reactor core and its operation are more complex in PHWRs than in PWRs,
requiring more measurements than in a pressurized light water reactor. The CANDU
reactor core is large (both physically and neutronically) and on-line fuel insertion and
removal occurs frequently. Local changes in the neutron flux occur at refueling sites.
Therefore, more in-core detectors are needed to monitor the flux shape. CANDU
reactors also include measurement of flow, inlet temperature and outlet temperature
in selected channels, requiring more pressure sensors and resistance temperature
detectors.
CANDU reactors also need tritium tracking because neutron captures in deute-
rium produces tritium.

16.6 High temperature reactor instrumentation

There are three high temperature reactors under consideration for deployment. They
are the liquid metal fast breeder reactor (LMFBR), the high temperature gas cooled
reactor (HTGR) and the molten salt reactor (MSR). Coolant temperatures are high
compared to water-cooled reactors and reach as high as 1000°C in gas-cooled reac-
tors. Many sensors used in reactors with water or heavy water coolant will not work
at the higher temperature experienced in the advanced reactors. Either new sensors
must be employed or developed, or the systems must be designed so as not to need
certain measurements.
Sensors used to measure coolant temperature must operate at temperatures above
the limit for RTDs (661°C). Type-N thermocouples with an upper limit of 1200°C
can tolerate conditions in high temperature reactors. But thermocouples are known to
drift at high temperature. (Type-N was developed as a more stable replacement for